HomeMy WebLinkAboutDrainage and Flooding Report January 2009niam •a00arr.�ee....®R-.w "•aw=a....:.rr.•
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EXECUTIVE SUMA Y......................................we;;;;;.................................... -1
List of Proposed Improvements for Existing Conditions ..............................
Table ES-1
Significant Drainage or Flooding Problems ..................................................
Table ES-2
List of Proposed Improvements for Future Conditions ........................„.....
Table ES-3
ProposedImprovements (Section 1)... .:.:......................................................
Figure ES-1
ProposedImprovements (Section 2)..............................................................
Figure ES-2
ProposedImprovements (Section 3)..............................................................
Figure ES-3
ProposedImprovements (Section 4)..............................................................
Figure ES-4
Proposed Improvements (Section 5)..............................................................
Figure ES-5
Proposed Improvements (Section 6)..............................................................
Figure ES-6
Channel Cross Section for B 112-00-00.........................................................
Figure ES-7
1.1 Purpose and Scope................................................................................... 1-1
1.2 Authorization ........................................................................................ 1-1
1.3 Prior Reports of Present C WDS Report .................................................. 1-1
1.4 CDWS Report .......................................... ........: ........ .: i-...... 1-3
L5Data Sources......... ... 4 ............................................................................... 1=4
1.6 Acknowledgements................................................................................. 1-4
2.1 Background Information.......................................................................... 24
2.1.1 Land Use...................................................................................... 2-1
2.1.2 Drainage Overview...................................................................... 2-2
2.2 Watersheds and Primary Surface Drainage ............................................. 2-3
2 2.1 Armand Bayou Watershed........................................................... 2-3
2.2.2 Clear Creek Watershed................................................................ 2-4
2.2.3 San Jacinto/Galveston Bay Watershed ........................................ 2-4
2.3 Drainage and Flooding Problems............................................................ 2-5
2.3.1 Base Flood Maps and Regulatory Floodplains ............................ 2-5
23.2 Drainage and Flooding Problem Identification ........................... 2-6
2.3.3 Subdivision Areas with Significant Drainage or Flooding
Problems...................................................................................... 2-7
2.3.4 Problem Areas with Short Term Drainage Project Remedy........ 2-8
_ ________ _
TOC-1
Klotz Associates Project No, 0127.008.000 La Porte Citywide Drainage Study
January 2009 City of La Port,
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FloodingProblems ...................................................................................
2-9
2.4.1
Sheet Flow Ponding and Paths ....................................................
2-9
2.4.2
Flow Path Elevation Adjustment .................................................
2-10
2.4.3
Sewerage Improvements .................................... .... :z .... i ....... :4_.-
2-10
2.4.4
Channel Capacity Increase ..........................................................
2-11
2.4.5
Detention .....................................................................................
2-12
3.1 Purpose and Scope of Evaluation ............................................................ 3-1
3.1.1 Drainage Criteria ......................................................................... 3-1
3.1.1.1 Design Frequencies ....................................................... 3-1
3.1.1.2 Detention Requirements ............................................... 3-2
3.1.1.3 Street Ponding Levels:::: ............................... 3-3
3.2 Classification of Watercourses ................................................................ 3-3
3.
3 Hydrologic Models... ............................................................................... 3-4
3.3.1 HEC-HMS Hydrologic Models for Primary Channel
Drainage Areas ............................................................................ 34
3,3.2 Hydrologic Models for HEC-HMS Sub -Areas ........................... 3-4
3.3.3 Correlation Models ...................................................................... 3-5
3.3.4 Hydrograph for Correlation Models ............................................ 3-5
3.4 Hydraulic Models .................................................................................... 3-6
3.4.1 FEMA Hydraulic Models ............................................................ 3-6
3.4.2 Non-FEMA-Modeled Channels .................................................. 3-7
3.5 Evaluation of Storage Requirement ......................... — ............................. 3-7
3.5.1 Mitigation Storage for Channel Widening .................................. 3-7
3.5.2 Diversion Storage .... ................................................................... 3-8
3.5.3 Mitigation of Excess Runoff Due to Development ..................... 3-8
3.5.4 Correlation Models ............................................... ..................... 3-8
4.1 Approach ................................................................................................. 4-1
4.2 Identified Types of Remedies for Existing Conditions ....... 4-1
4.2.1 Remedies for Insufficient Channel Capacity ............................... 4-1
4.2.2 Mitigation of Channel Improvement ........................................... 4-6
4.2.3 Storm Sewer System Improvements ............................................ 4-7
TOC-2
Klotz Associates Project No. 0127.008.000 La Porte Citywide Drainage Study
January 2009 City of La Porte
k I o t z 41,11 a S S 0 C i a t e s
Relief Using Sheet Flow Paths ............................ 4-8
4.3 Identified Types of Remedies for Future Conditions ..............................
4-9
43 ).1 Assumed Detention Strategies for Development .........................
4-9
4.3.2 Channel Improvements ............... ...................................... m .........
4-11
4.3.3 Storm Sewer System Improvements... : ........................................
4-12
4.3.4 Local Ponding Relief Using Sheet Flow Paths ............................
4-12
4A Identified Improvements ..........................................................................
4-12
5.1 Recommended Drainage Improvements ................................................. 5-1
5.1.1 Costs of Feasible Remedies ......................................................... 5-1
5.1.2 Recommended Improvement Projects ......................................... 5-2
5.1.3 Order of Construction of Recommended Improvements ............ 5-4
5.2 Implementation Issues .............................................................................
5-6
5.2.1
Chance of Information or Details of Projects ..............................
5-6
5.2.2
Pre -Construction Requirements .................:.:......:a:.....................
5-6
5.2.3
Tidal and Tropical Storm Influences ...........................................
5=7
5,2.4
Design Frequencies ............................................................. ........
5-8
5.2.5
Non -City Funding ........................................................................
5-8
5.2.6
Drainage Interaction with Other Cities ........................................
5-10
5.2.7
implementation of Drainage Criteria ................. a ..... ...................
5-11
5.2.7.1 Provision of Sheet Flow Paths .........................................
5=1 1
5.2.7.2 Construction in Special Flood Hazard Areas ..................
5-12
5.23.3 Minimum Low Chord Clearances ...................................
5-12
5.2.7.4 Capacity Improvements in Storm Sewer Systems...........
5-12
5.2.7.5 Minimal Detention Levels and Numbers of Detention
Sites6 ................................................................................
5-13
5,23.6 Enforcement of Drainage Criteria ........I__ .....................
5-13
5.2.8
Issues in Regional Pond Development for Future Development
5-14
5.2.9
Existing Detention Issues ............................................. i4:a._ .......
5-15
5.2.10
Easements, Right -of -Way, and Land Acquisitions .....................
546
5.2.10
Ownership and Maintenance of Drainage Facilities... ...... ___
5-17
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Klotz Associates Project No. 0127.008.000 La Porte Citywide Drainage Study
January 2009 City of La Porte
.I o t s s o c i a t e s
Table 2-1
List of Streams
Table 2-2
Significant Drainage or Flooding Problems
Table 2-3
Policy Weight for Flood Problem Intensity
Table 2-4
Prioritizing Drainage Problems
Table 2-5
Short -Term Projects
Table -6
Sheet Flow Paths
Table 3-1
Channel Analysis Summary
Table 4-1
Identified Improvements for FEMA Channels
Table 4-2
Identified Improvements for Non-FEMA Channels
Table 5=1
Project Units Costs
'fable 5-2
Cost Summary of FEMA Channel Improvements
Table 5-3
Cost Summary of Non-FEMA Channel Improvements
Table 5-4
Cost Summary of Relief Swales
Table 5-5
Cost Estimates Breakdown
Table 5-6
Cost Estimates Percentages
Table 5-7
List of Proposed Improvements (Cost -Sorted)
Table 5-8
List of Proposed Improvements (Loss -Sorted)
Table 5-9
List of Proposed Improvements for Future Conditions (Cost -Sorted)
Table 5-10
Ownership and Maintenance
TOC-4
Klotz Associates Project No. 0127.008.000 La Porte Citywide Drainage Study
January 2009 City of La Porte
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EXHIBITS
Exhibit 2-1 Vicinity Map
Exhibit 2-2 Aerial Map
Exhibit 2-3 Contour Map
Exhibit 2-4 Flood plain Map
Exhibit 2=5 Brookglen Subdivision & Spencer Highway Estate Floodplain Map
Exhibit 2-6 Armand Bayou Structural Damage Map
Exhibit 2-7 Clear Creek Structural Damage Map
Exhibit 2-8 San Jacinto & Galveston Bay Structural Damage Map
Exhibit 2-9 Armand Bayou Intensity Map
Exhibit 2-10 Clear Creek Intensity Map
Exhibit 2-11 San Jacinto & Galveston Bay Intensity
Exhibit 2-12 City Identified Drainage Problem Area
Exhibit 2-13 Armand Bayou Potential Sheet Flow Paths
Exhibit 2-14 Armand Bayou Potential Sheet Flow Paths
Exhibit 2-15 Armand Bayou Potential Sheet Flow Paths
Exhibit 2=16 San Jacinto & Galveston Bay Potential Sheet Flow Paths
Exhibit 3-1 Hydraulically Analyzed Streams
Exhibit 3-2 FEMA Streams
Exhibit 4-1 Armand Bayou Potential Detention Pond Locations
Exhibit 4-2 Clear Creek Potential Detention Pond Locations
Exhibit 4-3 San Jacinto & Galveston Bay Potential Detention Pond Locations
Exhibit 4-4 Armand Bayou Proposed Improvements
Exhibit 4-5 Clear Creek Proposed Improvements
Exhibit 4-6 San Jacinto & Galveston Bay ° Proposed Improvements
Exhibit 5-1 Stream Ownership and Maintenance
Exhibit 5-2 HCFCD Regional Detention Ponds
T'OC-5
Klotz Associates Project No, 0127.008-000 La Porte Citywide Drainage Study
January 2009 City of La Porte.
k I o t z 41,41 associates
Figure 3-1
Correlation of Unit Runoff for 10-Fear Event
Figure 3=2
Correlation of Unit Runoff for 50-Fear Event
Figure 3-3
Correlation of Unit Runoff for 100-Fear Event
TOC-6
Klotz .Associates Project No. 0127.008.000 La Porte Citywide Drainage Study
January 2009 City of La Porte:
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Appendix A Source Data Reports arReferences
Appendix B Sources for Funding Information
Appendix C Potential Detention Sites
Appendix D Strom Sewer Upgrade Identify in Letter Report No. 2
Appendix E Back-up CD
TC-7
Klotz Associates Project No. 0127.008.000 La Porte Citywide Drainage Study
January 2009 City of La Porte,
k10tz associates
i �■
A City Wide Drainage Study (C S) for the City of La Porte (City) was undertaken to identify,
develop and recommend drainage improvements to address drainage problems and lessen
flooding and its impacts across the City. Reasons for existing drainage and flooding problems
include 1) insufficient flow capacity in ditches and channels, 2) ponding of waters in streets and
adjacent properties; 3) undersized storm sewers, 4) temporary blockage of storm water inlets by
debris, 5) backup of storm waters in sewers, and 6) lack of overland or sheet flow paths. Also
contributing to the drainage problems are natural effects common to coastal areas: Relatively
small ground slopes making it difficult to rapidly drain away runoff waters; tides and storm
surges causing rising water levels which impede drainage; and frequent but severe storm events
with large amounts of rain falling in short periods of time. Future drainage problems can, on the
other hand, result if the runoff from future land development is not controlled. Flooding is a fact
of life in coastal areas and control of flooding in coastal areas presents significant challenges.
The strategy used to address drainage and flooding issues had two components: 1) remedy of
current drainage and flooding problems; and 2) mitigation of future drainage problems. The
bases of our recommendations are summarized in the Engineering Summary following this
Executive Summary. Details of the engineering analyses leading to the recommendations are
provided in the main body of this report.
To address the current drainage and flooding problems, 1) channel improvements, 2) detention
ponds for flood flow diversion, 3) storm sewer upgrades, and 4) development of relief swales
(Le., directed sheet flow pathways) options are evaluated. Improvements proposed in prior
studies by others were incorporated into the proposed solutions of this study when appropriate.
Relief swales are a very cost effective (i.e., low cost/benefit ratio) drainage improvement. Relief
swales reduce or limit ponding of runoff waters in streets and low lying areas for small to
....._... _.. ...... ....... __.._.
ES — 1
Klotz Associates Project No. 0127.008.000 La Porte Citywide Drainage study
January 2009 City of La Porte
k I o t z 4,41 associates
moderately large storm events which exceed the City's standard design frequency for storm
sewers. Swales, in effect, enhance local drainage system capabilities. Relief swale projects will
require only limited coordination with the Harris County Flood Control District for
implementation. Twelve relief swales, sometimes constructed in conjunction with sewer system
outfall improvements, are recommended, as follows:
Recommended Relief Swale Projects
Project
Subdivision/ Area
Type of Improvement
Project
COST/BENEFIT: �
ID
Benefiting
Construction
Construction Cost
Cost
Per Loss
Removed'
1
IPinegrove Valley
Relief Swale
$6 000
$140
`
Brookglen
(Relief Swales & Outfall Pipe Upgrade
$95,000
$350
3
(Fairmont Park West
IRelief Swales & Outfall Pipe Upgrade
$57,000
$370
4
;Glen Meadow
'Relief Swales & Outfall Pipe Upgrade
$305000
$540
5
'Meadow Park
IRelief Swales & Outfall Pipe Upgrade
$13,000
$540
_
6
Fairmont Park
Relief Swales & Outfall Pipe Upgrade
$19,000
_
$560
7
Creekmont Section 1
!Relief Swale & Outfall Pipe Upgrade
$34 000
$610
f 8
Fairmont Park East
Relief Swales & Outfall Pipe Upgrade
$65,000
$860
9
1Spencer HighwayRelief Swale
$30,000
$1,360
10
Villa Del Rancho
lRehef Swale
$24 000
11
Battleground EstatesRelief
_.____
Swale
$35,,000
$17,500
4
12
. Old La Porte
Relief Swale & Outfall Pipe Upgrade
$336,000
$21,000
Total 12 Projects
$744,000
...._ ... _ .._.
t Total construction
_ _.. ..... __ _,__
cost divided by the number of structures that have been reported in the
_ mw�
past to have flooded
and for which
the recommended project will alleviate flooding in the future.
Construction costs for the above recommended relief swales projects include right-of-way costs
assuming right-of-way can be placed on existing open land or the property line between adjacent
residential lots. These projects should be given high priority for construction.
Additional details abouts -.o-
shown in Figures ES-1 to ES-6 (these table and figures as -. at the end of this summary).
ES-2
Klotz Associates Project No, 0127.008,000 La Porte Citywide Drainage Study
January 2009 City of la Porn
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Flood control projects are intended to provide a high level of flood protection, with their design
based upon an extreme flood event as defined by a 100-year rainstorm event (i.e., only a 1 in 100
chance of occurring in any one year).
All flood control projects are either 1) channel widening; 2) channel lining with concrete when
right -of way is insufficient for widening; 3) diversion of flood waters to a detention pond
(referred to simply as a "diversion pond"); or some combination of these three. All channel
widening or lining projects also include, as part of the project, a detention pond to mitigate
(hence the name "mitigation pond") the adverse downstream impacts arising from the enhanced
discharge capabilities of the improved channel. Project costs include land acquisition costs.
Because of the high level of protection they provide, channel improvement and detention pond
projects are more expensive than relief swale projects. Based upon their relatively low
cost/benefit values, eight channel improvement and diversion pond projects are recommended to
address eight areas of significant flooding problems; these projects are listed on the following
page The recommended improvement and pond projects are separate projects and can be
individually constructed as funds become available. The various cost/benefit ratios, all of which
are $87,500 or less, can be used to help define priorities for construction.
Projects with higher cost/benefit ratios could be selected to address other flooding problem areas;
these less economically efficient projects are given in Table ES-1 (the Engineering Summary
following this Executive Summary discusses these other projects in more detail). One of these
projects, for example, is a linear detention pond previously proposed by others along channel
F216-00-00 (Project 46 in Table ES-1 and Figure ES-4 at the end of this summary). The
approximate location of all evaluated projects is shown in Figures ES-1 to ES-6. Pond location,
size and configuration are all approximate.
Total expenditures for projects will depend upon the projects selected for construction. 'l otal
cost and cost -benefit as well as availability of construction funding and the opportunities for
construction phasing will have to be considered in project selection. The cost -benefit ratio of
ES-3
Klotz .associates Project No. 01271.008.000 La Porte Citywide [drainage Study
January 2009 City of La Pyle
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recommended relief swales is low ($21,000 or less), but channel improvements and diversion
ponds, while having a higher cost -benefit ratio, provide greater protection against larger floods.
Some of the construction cost impacts on the City can be reduced by using alternative, less
traditional funding sources such as state or federal loans or grants, joint funding of projects in
cooperation with other governmental entities, or establishing a storm water utility as an
independent revenue source.
Project
Subdivision/.
area Benefiting
Type of
Project
�.��
I
Constructions
ID
j Improvement
Construction
Cost Per Loss !
Cost
Removed
13
Fairmont Park, Fairmont Park East
; Channel Widening
$781,000
$23 000
14
C Brookglen
Channel Lining
( $5 494 000
$29 400
1 15
Meadow Park, Villa Del Rancho
Channel Widening
( $1,701,QQ0
$47,000
16
fi Lennox Gardens, L Street
1 Pond for Diversion
$1,05�,000
$50,000
18
Battleground Estates, Pinegrove Valley, P
Channel Lining
$1,032,000
$54,000
Street
19
Shady River
Channel Widening
$361,000
$60,200
1
21
Woods on the Bay, Pine Bluff, Shady
Channel Widening
$600,000
$75,000
River
23
Meadow Crest, Creekmont, Glen
—
Pond for Diversion
...... w
$8,314,000
$87,500
Meadows, Fairmont Park, Fairmont Park
West
-- — - -------
Total 8 Protects
j $19,375,000
1111_1_1111111_]'1____'____'
t
---------
--
Total Construction
cost divided by the number of structures that have been reported in the past to have flooded and
for which recommended projects will alleviate flooding
in the future.
As the cost/benefit ratio rises, projects become less economically efficient. At some point; the
cost/benefit ratio becomes so high as to render a project unreasonably expensive. An evaluation
of the cost/benefit ratio (discussed at greater length in the Engineering Summary) for various
ES-4
Klotz Associates Project No. 0127.008.000 La Porte Citywide Drainage Study
January 2009 City of La Porte
k I o t z I kF1 I associates
potential projects indicates that projects with cost/benefits ratios of $87,500 or less should be
considered for construction while those greater than this amount warrant considerable
justification. All recommended projects meet this criterion.
When project costs are judged to be too high for the benefits obtained, options to consider
include 1) no action; 2) citizen evacuation when severe flooding is anticipated, 3) extensive flood
proofing of individual buildings; 4) property buy-out and/or building relocation; and 5) reliance
upon insurance or emergency relief funds for cost recovery after damage is incurred.
Subdivisions where storm sewer system improvements are needed and/or more detailed
investigation is needed to determine the extent of needed sewer improvements have been
identified and are listed in Table ES-2 in order of estimated drainage problem severity. Of these
subdivisions, Brookglen, Creekmont Section 1, Glen Meadows, Fairmont Park East, Pinegrove
Valley, and Spencer Highway Estates have high drainage problem severities; and thus should be
given high priority for problem solution. The Creekmont Section 2 Project is already nearing
construction. Initially proposed improvements have been already identified for Brookglen,
Creekmont Section 1, Glen Meadows, Fairmont Park East; these improvements include relief
swales as part of the project but will require further engineering investigation for defining full
project details.
I 1 �
Future drainage problems may arise from land development for residential or commercial
structures which would, without mitigation, result in increased rates of runoff and possibly
overtax drainage facilities. Developers are usually required by the City to provide mitigation of
runoff increases. Two approaches are commonly used to provide necessary mitigation: on -site
detention or regional detention. The choice between the two is typically dictated by economics.
s111111011
only can a detention pond mitigate excess runoff from land development, but it can also provide
ES- 5
KIotz Associates Project No. 0127.008.000 La Porte Citywide Drainage Study
January 2009 City of La Porte
k I o t z I i I associates
detention for flood water diversion and mitigation of channel capacity improvements, as well as
habitat improvement, a community amenity, or, during dry periods, recreational opportunities.
Detention ponds can be constructed in phases, with early phases being used to provide detention
for diversion or mitigation and later phases being used to provide detention of increased runoff
from land development.
Table ES-3 lists potential regional detention ponds which could be used to mitigate future land
development impacts or, in some cases, also be used for diversion or channel improvement
mitigation. Approximate locations of the various detention facilities are shown in Figures ES-1
through ES-6.
On -site detention costs are borne by a developer, while regional detention, which combines the
necessary detention mitigation for several different development sites into one "regional"
detention facility, may be constructed using a variety of funding mechanisms. Costs, all or in
part, for regional detention could be borne by developers; the City; by governmental entities
partnering with the City; or some combination of these. Estimated costs for construction of the
various potential detention facilities, when fully developed, range from $613,000 to $26,752,000.
As an aid to assessing these costs, a cost per acre of estimated area available for development
upstream of the detention pond is also listed. Cost per acre of developable land ranges from a
low of $5,800 to a high of $64,000. Ponds at the higher levels of cost are unlikely to be
economically viable; but even for those ponds which are economically viable, the particular
ponds which should or will be built will depend to a considerable extent upon how city
development patterns evolve over time and the urgency for use of a regional pond.
Concluding Remarks
This CWDS recommends a variety of drainage improvement and flood control projects,
including those that can be relatively easily implemented in the short term, those which are more
ES-6
Klotz Associates Project No. 0127.008.000 La Forte Citywide Drainage Study
January 2009 City of La Port•-
I o t z associates
. . ... ! !-
-. t111211110111RIUM 1,11111111111111111111111
Projects can be implemented individually and are not contingent upon each other; sequencing of
projects can be used to implement a series of projects over time. While guidance has been
provided to assist in deciding which projects should receive priority for implementation, the
decisions as to the priorities for construction of improvement projects is, in the final analysis, the
responsibility of City leaders.
ES-7
Klotz Associates Project No. 0127.008,000 La Porte Citywide Drainage Study
January 2009 City of La Porte
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A City Wide Drainage Study (CWDS) for the City of La Porte (City) was undertaken to identify
and describe existing and future drainage and flooding problems across the City and devise
solutions for the identified problems. Sources of drainage and flooding problems in the City are
several and vary with location in the City. Some channels and major drainage ditches have
insufficient capacity for conveyance of the runoff from severe storm events. Sometimes low
lying developed areas adjacent to but beyond the boundaries of a ditch or channel can become
flooded even if the channel itself is not full because water levels in the channel are above the
level of the adjacent low lying areas. Poinding in low lying areas is sometimes caused by lack of
surface pathways to rapidly drain away storm waters which are not captured and carried away by
storm sewers. Storm sewers draining to a channel or ditch may not always have sufficient
capacity to prevent collection of waters in streets and adjacent properties. Storm sewer system
capacity can be limited by pipe size, insufficient numbers of inlets, debris -blocked inlets, or
backup of water in the storm sewers due to high water levels in receiving channels. Storm
drainage problems in the City are also in part due to natural effects common to coastal areas:
Ground slopes are relatively small, making it difficult to rapidly drain away storm runoff waters.
Tidal effects worsen drainage conditions, with storm tidal rises or storm surges causing rising
water levels in the channels and bayous near the coast and limiting how well water can drain.
The Texas coastal area is also subject to frequent but severe storm events with large amounts of
rain falling in short periods of time, often overpowering drainage systems.
This planning report presents the result of the CWDS and provides recommendations for
improved drainage infrastructure to reduce flooding and its impacts in the City. Three letter
reports have been previously developed as part of the work leading to this CWDS report. Letter
Report No. 1, dated March 10, 2008 described the City's existing drainage infrastructure and
general data collection activities. The City lies in three major watersheds: Armand Bayou, Clear
Creek, and San Jacinto/Galveston Bay. Assembled data and previous reports by others (see
Appendix A in the main report following this summary) were used to provide a preliminary
EN - 1
Klotz Associates Project No, 0127.008.000 La Porte Citywide Drainage Study
Januaty''009 City of La Porte
k I o t zi 1.1 lassociates
identification of apparent critical drainage problems and definition of short term solutions to the
more critical problems. Sources of current drainage problems include inadequate channel
Letter Report No. 2 dated May 7, 2008 also provided a description of flooding conditions using
hydraulic models to provide additional evaluation of critical drainage problems, and determine
potential storm sewer system limitations and possible solutions. Several short term solutions
were also proposed for sewer systems in four subdivisions currently experiencing significant
drainage problems; these are summarized in Table ES-2. Details of these particular solutions are
described in Letter Report No. 2; Appendix C of the CWDS report following this summary
provides summary tables from Letter Report No. 2 describing the proposed improvements.
Other concentrated studies were used to address a current solution in Creekniont Section 2 which
now nearing construction (see Project 45 in Table ES-2).
Letter Report No. 3 examined drainage criteria and standards and made recommendations for
their improvement. Long term flooding problems were identified and prioritized. Conceptual
solutions were identified. Conceptual solutions which are considered practical for use in the City
are channel widening, channel lining, more effective use of overland storm flow relief pathways,
construction of diversion and detention ponds, upgrading of storm sewer systems, and use of
regional detention. Potential funding sources and mechanisms for drainage and flood control
infrastructure were discussed.
Drainage problems arise from high tides and surge induced by Galveston Bay storms; such tides
and surge impacts cannot commonly be mitigated by drainage infrastructure improvement. To
limit storm surge impacts, considerations should be focused upon such options as early flood
warning systems for citizen protection for near shoreline areas, evacuation in severe storm
situations, construction of finished floor slabs and roadways above predicted storm -produced
high tides or surge levels, construction of coastal storage systems in conjunction with tide gates
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mouthsat the ►ayous; flood proofingresidences;
This CWDS report brings together pertinent information developed in the three letter reports as
well as information developed subsequent to the letter reports to document both current drainage
issues and anticipated drainage problems arising from future development. The conceptual
strategy for addressing current and anticipated flooding and drainage problems consists of two
basic evaluations: 1) Solutions to current drainage and flooding problems, and 2) potential
mitigation of future drainage problems. To perform these evaluations, existing Federal
Emergency Management Agency (FEMA) hydrologic (HEC-HMS) models and hydraulic (HEC-
RAS) models for most major channels in the City were gathered -and evaluated; these models
were termed FEMA models. Limited field survey along with other data was used to construct
approximate models for some bayous and tributary channels for which FEMA models had not
been developed; these later models are termed non-FEMA models. Collectively, 16 different
FEMA and non-FEMA models were modified or developed to evaluate flooding conditions and
identify possible improvements to reduce flooding problems.
Solution of Current Drainage and Flooding Problems
To address the current flooding problems, channel improvements, development of relief swales
(i.e., directed sheet flow pathways), storm sewer outfall improvements, limited storm sewer
improvements; and detention ponds for flood flow diversion (termed "diversion ponds") were
considered. Proposed improvements made in prior studies by others were also considered as
appropriate to the drainage problems identified in this study. While detailed storm sewer
network analyses were not included in this planning level study, information on storm sewer
systems was utilized in defining surface drainage systems, identifying the need for storm sewer
system improvements, and in some cases identifying upgrades for sewer systems.
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Recommendations for drainage system improvements to address current flooding problems are
summarized in Table ES-1 (located at the conclusion of the Executive Summary). Various
potential projects are identified by a Project Number. figures E-1 through ES-6 locate where the
improvement projects are proposed. In some cases options for addressing the same drainage
problem are provided (e.g., as in the case of Projects 14 and 20 for the Brookglen area). Table
ES-1 identifies the improvement, the area it benefits, and the channel or channels for which the
improvement is being made. The table provides a basic description of the improvement, its
estimated cost, and its benefit in terms of estimated reduction in number of flooded
(predominately residential) structures. Information for identification of previously flooded
structures included repetitive loss and flooding report data provided by the City. (Flooding
reports are detailed in Exhibits 2-6 through 2-11 at the end of the main text.) The potential
projects are ordered according to their cost/benefit, computed as the ratio of the cost for the
improvement divided by the estimated number of structures removed from flooding. This
cost/benefit can be used by City leaders as a tool for defining priorities for construction of
proposed improvements.
Among the various improvements proposed, surface storm water relief swales are relatively
inexpensive and easily implemented. Relief swales are essentially shallow, wide ditches located
to carry ponded water away to larger drainage ditches or channels and are used to improve the
storm sewer system drainage. Relief swales are recommended when storm drainage conditions
indicate the swale will be effective and construction will be feasible. In the urban areas of La
Porte, where land availability of drainage easements is typically limited, Swale widths can be
kept to a minimum by using concrete swales, as opposed to grass swales. The recommended
swales are assumed to be concrete lined swales.
Recommended relief swales with their basic sizes are given in Table ES-1; locations of the
proposed relief swales are shown in Figures ES-1 through ES-6. The swales are used to improve
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drainage in areas where significant amounts of flooding problems have been -reported and
evaluation indicates lack of overland drainage for sheet flow discharge.
Some proposed relief swales are proposed to be located in a storm sewer system outfall
easement; in these cases, the outfall pipe will require replacement in addition to construction of
the swale (e.g., see Project 2 in Table ES-1). Storm sewer system capabilities can also be
improved by increasing the outfall pipe size and thereby reducing the "chocking" effect it has on
upstream storm sewers during extreme event storms. Such upsizing is identified when review of
storm sewer system data indicated that the existing outfall was undersized.
The following table summarizes the proposed relief swale projects:
EMU=
. ..... — - ---- - - ------- --
Project Subdivision/ Area Type of Improvement Project COST/BENEFIT:
ID Benefiting Construction 'Construction Cost
Cost Per Loss
Removed'
I Pinegrove Valley Relief Swale $6,000 $140
2 jBrookglen Relief Swales & Outfall Pipe Upgrade $95,000 $350
3 Fairmont Park West Swales & Outfall Pipe Upgrade $57,000 $370
— - ------- -- - ----------------------- . .... . -6 .. . ....................... . .
4 IGIen Meadow West
Swales & Outfall Pipe Upgrade $30,000
f. ... ... .. ....... —_
5 jMeadow Park Relief Swales & Outfall Pipe Upgrade $13,000 $540
. . . .............
6 'Fairmont Park lRelief Swales & Outfall Pipe Upgrade $19,000 $560
rsrsne
7 Creckmont Section 1 Relief Swale & Outfall Pipe Upgrade $34,000 $610
- ------------------ ------
8 'Fairmont Park East lRelief Swales & Outfall Pipe Upgrade $65,000
9 Spencer Highway Relief Swale $3 0,000
— ------- ------- — — - -----
10 Villa Del Rancho lRelief Swale $24,000 $2,670
11 Battleground Estates ]Relief Swale $35,000 $17,50 0
- - --------
0 $21,000
12 Old LaPorte Relief Swale & Outfall Pipe Upgrade $336,00
--------------------
Total 12 Projects $744,000
Total construction cost divided by the number of structures that have been reported in the past to have flooded and
for which the recommended project will alleviate flooding in the future.
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Storm Sewer Improvements
Areas where sewer system improvements are needed are listed in Table ES-2. Reasons for the
needed sewer system improvements can be several, including insufficient pipe size, insufficient
street inlet capacity, or high tailwater levels in the receiving channel. High tailwaters are
fundamentally a problem in the receiving channel capacity, while insufficient capacity of storm
sewer pipes and inlets is a true storm sewer deficiency. An identification of the likely source of
the storm sewer system problem, either insufficient receiving channel capacity or insufficient
storm sewer system capacity, is given. In some cases the flooding was judged to a combination
of both causes.
Subdivisions where storm sewer system improvements are needed and/or more detailed
investigation is needed to determine the extent of needed sewer improvements have been
identified and are listed in Table ES-2 in order of estimated drainage problem severity. Of these
subdivisions, Brookglen, Creekmont Section 1, Glen Meadows, Fairmont Park East, Pinegrove
Valley, and Spencer Highway Estates have high drainage problem severities; and thus should be
given high priority for problem solution. The Creekmont Section 2 Project is expected to
constructed in the near future. Some proposed improvements have been already identified for
Brookglen, Creekmont Section 1, Glen Meadows, Fairmont Park East; these improvements
include relief swales as part of the project but will require further engineering investigation for
defining full project details.
Channel Improvements and Ponds for Diversion of Flood Waters
Channel improvements or detention ponds to which some channel flood flow can be temporarily
diverted (i.e., "diversion ponds") are recommended where the out -of -bank flow or high in -
channel water levels. The latter condition may be a primary source of flooding when low lying
areas at less than top -of -channel bank elevations occur beyond the channel banks and channel
flood waters can move from the channel (through low points along the bank or by sewer backup)
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into the low lying areas, as illustrated in Figure ES-7 for Ditch B 112-00-00. Out -of -bank or
adversely high in -channel hooding is evaluated for the 100-year flood frequency. A 100=year
design frequency criterion is used in the City as well as by other municipalities and drainage
districts throughout most of the Houston area for assessing flooding impacts for severe storm
Channel improvements are accomplished by one of following: widening of the channel, lining
the channel with concrete to reduce flow resistance, or a combination of these two methods.
Table ES-1 lists the proposed widening projects while Figures ES-1 through ES-6 show where
channel widening is proposed. Widening is accomplished by generally excavating the soil in
mid and lower portions of the channel to make full use of the channel right-of-way as
approximately defined by the existing approximate top width of the channel. Slopes for non
lined channels are set to a maximum of 3:1 (i.e., 3 horizontal to 1 vertical) as defined by City
design criteria. In many locations, current land use will preclude significant channel widening
and improvement in the channel capacity will require lining of the channel, as currently exists in
many of the major ditches and channels in the City. The data of Table ES-1 indicates which
channels are recommended for actual widening and those channels which, because of limited
space for channel widening, would be recommended for concrete lining. In some instances,
large drainage pipes might be used as an alternative to channel widening (e.g.,see Project 28 in
Table ES-1).
Increase in downstream flows is an adverse impact from channel improvements; this impact can
be mitigated with detention ponds (i.e., "mitigation ponds") specifically designed to capture the
flow increase and temporarily detain the increase until it can be released without adverse impact.
All channel widening or lining projects include as part of the project a detention pond to mitigate
(hence the name "mitigation pond.") the adverse downstream impacts arising from the enhanced
discharge capabilities of the improved channel. Potential sites for the mitigation detention ponds
are shown in Figures ES-1 through ES-6. In some cases, optional locations for a mitigation pond
for one particular channel improvement are shown; in such cases, the actual mitigation pond
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would be either at one of the two sites, or, possibly smaller ponds would be used at each of the
optional sites. Detailed engineering analyses would be required to identify the optimal number,
configuration and sizes of individual ponds. In all situations, the pond configurations, locations,
and sizes shown are only approximate.
In special situations, mitigation may be avoided if it can be demonstrated that adverse impacts or
are absent or of very little consequence. An example of this possibility is provided by the
improvements for Ditch F216-01-00, for which three alternatives have proposed (Projects 26, 28,
and 30 in Table ES-1). Project Options 26 and 28 include a mitigation pond; however; the
channel improvement extends to the confluence of F216-01-00 and F216-00=00, which lies quite
close to Galveston Bay. Thus locations downstream of the channel improvements' potential
impacts are quite limited in extent, and because of the relatively large flows that occur in F216-
00-00, the increases in flow due to improvements along F216-01-00 may be so small, in a
relative sense, that they are inconsequential. Hence, mitigation of the channel improvement may
be unnecessary. Detailed engineering analysis would be required to confirm this speculation.
It is recognized that using channel lining to improve channel conveyance capacity is not
desirable from an environmental or permitting perspective. On the other hand, widening of the
channel with 3:1 or flatter side slopes will often result in a channel width that will significantly
impact adjacent properties, including in some instances actual residences and consequent
requirements for possible buy-out of affected residences.
Therefore, the option of off -channel diversion detention storage is sometimes considered. Table
ES-1 identifies proposed detention ponds for diversion use while Figures ES=1 through ES=6
show approximate locations of the diversion ponds. Pond configurations, locations, and sizes
shown are only approximate. When optional locations are available for a mitigation pond, the
optional locations are shown.
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In developing channel capacity improvements, channel widening is given preference over
detention since widening is typically less expensive than other improvement options. However,
in some instances, diversion to a detention pond may become the preferred choice or part of a
preferred choice because of either its relative cost, the potential use of the detention site to serve
multiple uses, or other special characteristics of the project.
Thus, for example, two alternatives (Projects 23 and 25) are proposed for Ditch B106-00-00
(also know as Big Island Slough); see Figures ES-2 and E -3. Project 25 proposes a concrete
lined channel, mitigation pond, and an approximately 207 acre-foot diversion pond. Project 25
requires diversion detention in order to limit channel lining to between Spencer Highway and the
confluence of 106-00-00 and B106-02-00. Estimated construction costs for this project are
$11.7 million (see Table ES-1). Project 23 proposes a larger diversion pond, with 228 acre-feet
of storage, in approximately the same location as that for Project 25 but no channel
improvements. The cost of Project 23 is only $8.3 million. The larger pond size of Project 23
achieves the same net result as the combined channel improvement and smaller pond of Project
25. Between these two alternatives, the alternative without channel improvement is the preferred
alternative; the Project 23 choice, which does not include the channel lining, is clear because the
diversion ponds of both projects are located at the same approximate location.
For flooding along Ditch B112-00-00 (Willow Springs Bayou) in the western "panhandle" of the
City, two alternative solutions have been identified. Flooding in this panhandle segment of
112-00-00 is due not primarily to over -bank flooding, but, rather, due in large measure to
flooding of low lying areas beyond the channel banks even before the water levels rise to the top
of channel. As illustrated by the representative channel section view in Figure ES-7, some areas
beyond the channel bank are low relative to top of the channel. Rising flood waters in the
channel can move out the channel through low points in the channel bank or by backup into
sewers into these low lying areas. The widening necessary to keep channel flows low enough to
prevent flow from the channel moving into low lying areas is large (some 175 feet if 30-foot
channel maintenance berms are included; see Project 14 in Table ES-1). Therefore diversion of
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flow to detention storage to lessen the maximum flow and lower the water surface in the channel
becomes a potential option to widening of the channel.
Diversion sites of sufficient size in the City that could provide sufficient storage to generally
lower water levels in B112-00-00 to the point where flooding and backup of waters into
residential areas is not a problem are not available. However, some open lands north of the City
boundary not committed to future development are apparently currently available for a diversion
pond (see pond site for Project 20 in Figure ES-1), Because of its location, coordination for
development of this site would have to be pursued in close cooperation with the adjacent city and
other stakeholders. The estimated cost for the channel lining project within the City limits,
Project 14, is approximately $5.5 million, while the estimated cost for diversion pond
construction outside of the City limits, Project 20, is $11.3 million. However, the diversion
project has potential regional benefits (and does not have the undesirable environmentalfeatures
of a lined channel) and thus has consequent possibilities for cost sharing with other parties. Thus;
before a decision is made as to which option to pursue; discussion needs to be undertaken with
the adjacent city as well as other stakeholders such as the Harris County Flood Control District
(HCFCD) as to the possibilities of development of the site for a detention pond.
The estimated costs for constructing the various recommended projects (exclusive of sewer
system improvement costs) to address current flooding and drainage projects are listed in Table
ES-1. The cost elements include land acquisition (assumed to be developed land for channel
improvements and undeveloped land for ponds), site preparation, excavation, lining (when used),
culvert removal and/or installation (when part of the project), and site stabilization after
construction. Because of the nature of home buy-out, buy-out as an alternative to channel
improvements or diversion ponds was not specifically evaluated. The recommended priority for
the various projects is based upon the cost -benefit analysis described above.
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The various channel improvement and diversion pond projects range from a low of $361,000
(Project 19) to a high of $11,752,000 (Project 25). Cost/benefit ratios range from $23,000
(Project 13) to $3,078,000 per loss removed (Project 32).
Total expenditures for projects will depend, of course, upon the projects selected for construction.
Total cost and cost -benefit as well as availability of construction funding and the opportunities
for construction phasing will have to be considered in project selection. The cost -benefit ratio of
relief swales is low, but conveyance improvements, because of the high level of protection they
provide, have larger costs. Some of the construction cost impacts to the City can be lessened by
using alternative, less traditional funding sources such as state or federal loans or grants, joint
funding of projects in cooperation with other governmental entities, or establishing a storm water
utility as an independent revenue source.
As the cost/benefit ratio rises, projects become less economically efficient. At some point, the
cost/benefit ratio becomes so high as to render a project unreasonably expensive. Clearly some
of the higher cost/benefit projects of Table ES-1 fall into this category. Precisely where the
breakpoint lies between an acceptable and an unacceptable level of cost/benefit level is a matter
of policy; availability and source of funds, and competition for funds. However, some guidelines
for selecting a breakpoint can be identified; as described in the following.
If the project (Project 32) with the highest cost/benefit (and a cost of $6.2 million) is removed
from consideration, the largest cost/benefit drops to $935,000. Clearly a cost/benefit of this
magnitude for removal of a loss is unrealistic. If, however, only the most cost efficient options
(i.e., smallest cost/benefit) are considered among the various options (while still excluding
Project options 32 and 33 for Channel F212-00-000), Projects 30, 295 28, and 27 can be removed
and the largest cost/benefit drops to $161,555 (for Project 26). The largest project cost, however,
still remains at $11.8 million, just as it did before any projects were dropped from consideration.
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If, however, Project 26 is removed from consideration, Projects 25 and 24 can also be removed
from consideration because they have more efficient options. Consequently, the largest
cost/benefit drops to only $87,500 (for Project 23) and the largest project cost drops to $8.3
million (also for Project 23). The projects removed from consideration reduce the channels for
which projects remain from 10 to 8. Additional removals require more comprehensive
considerations that would involve City policy and funding considerations.
Project Subdivision/ Area Benefiting Type of Project Construction
ID { Improvement Construction Cost Per Loss
Removed' Cost Rved'
su �.m
13 ( Fairmont Park Fairmont Park East Channel Widening $781,000 ( $23,000
14 Brookglen Channel Lining
$5 494 D00 g g $29,400
11111_
15 Meadow Park, Villa Del Rancho Channel Widening $1,701,000 $47,000
16 Lennox Gardens, L Sheet Pond for Diversion $1,092,000 $50,000
l8 Battleground Estates, Pinegrove Valley; P Channel Lining $1,032,000 $54,000
Street
...� r ,wnm
19 Shady River Channel Widening, $361 000 $6fl 200 1
__---
21 Woods on the Bay, Pine Bluff, Shady Channel Widening $600,000 $75,000
I
River
mm.2� �.v_e
Meadow Crest, Creekmont, Glen Pond for Diversion $8,314,000 $87,500
- I
Meadows, Fairmont Park, Fairmont Park
West
I Total 8 Projects 1 $19,375,000
Total Construction cost divided by the number of structures that have been reported in the past to have flooded and
for which recommended projects will alleviate flooding in the future.
Thus, in view of project cost/benefits and total projects costs, $87,500 per loss removed appears
to be a reasonable breakpoint for deciding whether channel improvements or diversion ponds are
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appropriate to addressing flooding problems. Projects which have higher cost/benefit levels
could: be considered, but warrant considerable justification.
If then the least. expensive iiroect of _the _various otions for a particular channel which meet the
problemsFuture drainage •m land development forcommercial
typically
Development is typically required by the City to provide mitigation of runoff increases, i.e.,
construction of facilities that eliminate the increased runoff. Two policy -based approaches are
generally considered in providing necessary mitigation: on -site detention (possibly coupled with
best management practices which reduce the amount of runoff generated) or regional detention.
In practice, a combination of the two approaches is used. Some areas or developments may rely
upon on -site detention while other areas or developments may rely upon regional detention.
Regional detention facilities wire evaluated for planning purposes. Planning for regional
detention requires an identification of potential detention volumes and locations where detention
facilities might be located. Under the assumption that regional detention is used, Table ES-3
identifies potential regional detention projects which could mitigate future land development
drainage impacts or possibly be used to address yet unidentified current drainage problems.
Possible approximate locations for the various detention facilities are shown in Figures ES-1
through ES-6. For illustration purposes, pond shape is usually assumed to be square. Actual
pond configuration would depend upon site specific details such as property boundaries, site
topography, and necessary characteristics of inlet and outlet works.
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There are a variety of pros and cons in the use of regional detention as an alternative to on -site
detention for runoff control from future development: On -site development costs are the sole
responsibility of the developer but potentially taxable land is lost because of pond development.
On -site detention is easily implemented, while regional detention requires more coordination of
interests to develop. Regional detention allows multiple uses of the detention and consequent
multiple sources for funding, but land must be available in sufficient amount in proper locations.
A prime concern for any detention facility is land acquisition; availability of land can often be a
significant limitation in regional detention pond development. On=site mitigation does not
require the single large tracts of land for a pond that a regional pond requires.
Regional detention ponds lend themselves to a variety of different funding mechanisms. Costs,
all or in part, for regional detention could be borne by the City, by developers through payments
to the City, or some combination of City and developer funds. if the detention system serves
regional purposes beyond just mitigation of land development projects, Harris County Flood
Control District (HCFCD) may participate in the detention pond development and the costs of its
construction. Estimated costs for construction of the various detention facilities are given in
Table ES-3. As an aid (only) for assessing these costs, a cost per acre of developable land is
also listed.
The potential regional detention facilities are ordered according to estimated total cost for full
development of the detention site. Phasing of pond construction to match detention needs as
they develop could be used to spread costs over time. What regional facilities are actually built
first will depend upon how city development patterns evolve over time, costs of on -site vs.
regional detention for specific land development projects, and what detention needs other than
mitigation of development runoff may be served by the pond. The listing of Table ES-3 is not
intended to define which regional detention facilities should be built or which should be built
first.
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To maximize the effectiveness of proposed drainage improvements and minimize future drainage
problems, the City should rigorously enforce drainage criteria and standards. Future finished
floor slab and critical roadway elevations should be established. Provisions for sheet flow relief
pathways and assurance of adequate capacity in new sewers systems which meet upgraded City
criteria should be enforced. Construction in flood hazard zones and flood -prone areas should be
avoided as much as possible. And, to the extent that they are not already in place, agreements
need to be developed between the City and HCFCD to assure adequate levels of channel
maintenance to maintain channel conveyance.
Implementation of recommended projects can initially focus upon very cost efficient, less
expensive projects such as relief swales, which can be relatively easily implemented. Larger,
more complex projects to address flooding problems along channels as well as detailed analysis
of some storm sewer systems should follow. The more complex flood protection projects will
require more effort to implement, but will provide a high level of flood protection. Selection of
the more cost efficient alternatives will usually facilitate project implementation. Impacts of
recommended improvements should be recognized and appropriate mitigation implemented.
Regional detention opportunities should be explored when impacts lie or have their source
beyond City boundaries.
Projects can be implemented individually and are not contingent upon each other; sequencing of
projects can be used to implement a series of projects over time. While guidance has been
provided to assist in deciding which projects should receive priority for implementation, the
decisions as to the priorities for construction of improvement projects is, in the final analysis, the
responsibility of City leaders.
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1.1 Purpose and Scope
Klotz Associates, Inc, has prepared a City Wide Drainage Study! for
PorteLa provides
exlstTng and anticipated drainage and flooding problems in the City and develops both
The general scope of the CWDS includes the assessment and detailing of drainage and
flooding problems; examination of hydrologic and hydraulic conditions as a basis for
characterizing problems and identifying potential remedies; characterization and
prioritization of drainage and flooding problems using various hydraulic modeling
methods; development of remedies for addressing the drainage problems; estimate of the
cost of potential remedies; and recommendations and considerations in implementing
conceptual remedies.
1.2 Authorization
Development of the City Wide Drainage Study by Klotz Associates was authorized by
the City of La Porte by agreement dated January 29, 2008.
1.3 Prior Reports of Present CWDS Report
Pursuant to scope, three prior reports have been prepared as part of the development of
the CWDS.
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Letter Report No. 1, dated March 10, 2008, collected and assembled key data (Tasks 1.1
and 1.4 of scope), generally described the watersheds and drainage system in the City
using various techniques and data including LiDAR data (Task 1.2), and conducted and
documented site visits (Tasks 1.3). Key drainage reports developed prior to the present
study were also summarized (Task 1.6). In addition, the assembled data was used to
assess and provide a preliminary identification of apparent critical existing drainage
problems (Task 1.5). This assessment led to identification of short term solutions (Task
2.3) which were communicated to the City Council and Flooding and Drainage
Committee (Task 2.4).
As part of the data gathering activities documented in Letter Report No. 1; available and
previous prepared hydrologic and hydraulic data and models for various bayous in the
City were obtained (Task 2.1). Review and update of the models continued though
preparation of Letter Report No. 2 and Letter Report No. 3 (Task 2.5). Modeling was
assisted with limited survey for selected bayous (Tasks 2.6 and 3.1) so that models and
identification of drainage and flooding problems could be refined for both existing and
estimated future conditions (Task 2.5).
Letter Report No. 2 provided a description of flooding conditions (Task 2.2) and
provided a detailed description of available hydraulic models (Task 2.5), while update
of the models continued. Letter Report No. 2 also provided additional characterization of
critical drainage problems and refined improvements for addressing short term drainage
problems. Letter Report No. 3 completed the development of hydraulic models for the
various bayous in the City being studied. A report on key aspects of the findings
documented by Letter Report 2 was presented to the City Council and Flooding and
Drainage Committee (Task 2.4).
ReportLetter • drainage design criteria and standards (Task 4. 1) and made
recommendations for modifications to the City's criteria. Long term drainage problems
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were described and prioritized (Task 4.2) with regard to the relative adverse impacts on
the residential areas. Reasons for drainage system deficiencies were categorized as a
preliminary to developing conceptual remedies. Conceptual remedies were generically
described, and preliminary unit cost factors to use in estimating remedy costs were
determined (Task 4.3). Potential funding sources for capital improvement projects
addressing drainage and flooding issues were identified (Task 4.4). Information
developed for Letter Report No. 3 was provided to the City Council and Flooding and
Drainage Committee (Task 4.2).
Management of the execution of work leading to these letter reports as well as the CWDS
Report was performed pursuant to Tasks 1.7, 2.3, 4.4 and 4.5.
1.4 CWDS Report
This CWDS Report brings together (pursuant to Task 4.5 of the study scope) the more
cogent aspects of the three letter reports to quantify the character of the problems as
deduced from both earlier evaluations in the three letter reports and application of the
hydrologic and hydraulic models to specific bayous and channels. This application
provides estimates of channel capacities and identifies where capacities are insufficient to
meet capacity design goals to further define the nature of the drainage problems in the
City (Task 2.2). Various workable remedies for achieving the design capacity are
identified at a conceptual level and approximately sized and located (Task 4.3).
Remedies include possible channel modifications and detention storage. Approximate
dimensions of modified channels are presented. Potential detention pond locations are
identified and storage requirements quantified. Remedies are described for the study
bayous on a bayou=by=bayou basis. Estimated costs of proposed remedies are provided
(Task 4.3) Potential ways to prioritize implementation of drainage improvements are
suggested. Issues in implementing the potential improvements are discussed and
recommendations are presented.
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The data that was gathered was from a variety of sources, including earlier studies and
reports from the City, HCFCD, and Consultants; hydraulic and hydrologic models from
FEMA; plans and profiles of existing storm drainage systems; field reconnaissance; and
from discussions and correspondence with residents' and City Staff input. Appendix A
provides a tabulation of earlier studies and reports reviewed as part of the development of
this CWDS,
1.6 Acknowledgments
acknowledgeKlotz Associates wishes to r help of the following people in
providingdrainage systems and conditions
La Porte, including the City Staff and - Department of PublicWorks,• d
Drainage Committee of the City of La Porte, Harris County Flood Control District
(HCFCD), and the people of La Porte. The City Staff provided important information for
this report and helped significantly in identifying current flood problems and issues
reported by citizens of the City. Coordination with local communities in the area was
facilitated by HCFCD and the City's Drainage Committee; monthly meetings with the
Committee were important to improving, our understanding to issues of concern.
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2.1 Background Information
The City lies entirely within Harris County, Texas, and encompasses approximately 19.7
square miles. It is located on the extreme east side of Harris County on the shores of
Galveston Bay (see Exhibit 2-1). The City is bounded by the City of Deer Park on the
west, the east -west State Highway 225 on the north, and the community of Shore Acres
and properties of the Port of Houston on the south. State Highway 146, extending in a
generally north -south direction approximately bisects the City. Key thoroughfares in
addition to State Highways 225 and 146 are Fairmont Parkway, Spencer Highway -Main
Street, and Broadway (see Exhibit 2-1).
The City has a mixture of residential, industrial and commercial land uses. The area
between Fairmont Parkway and Spencer Highway is composed primarily of small
residential lots. Areas north of Spencer Highway have not only typical residential
medium to small lot developments (with lots commonly in the range of 0.15 to 0.25
acres) but also some large rural lot residential areas (with lot sizes typically in the 3 to 5
acre range). Areas east of SH 146 include residential lot developments, commercial areas,
and industrial areas.
The City has a wide variety of land use: rural, urban, industrial, and commercial. Exhibit
2=2 shows an aerial of the City. The City area lying east of SH 146 and adjacent to
Galveston Bay, often referred to as "(old LaPorte," is predominately residential and
commercial land. The southwest side of the City east of Sens Road is predominately
residential. The northern side of the City is an industrial area composed of primarily
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petrochemical facilities. The northeastern portion of the City is considered to be a "Large
Lot District," composed primarily of large rural residential lots. The south side of the
City is a rural area that is a Planned Unit Development, intermingled with some small,
established residential areas. La Porte Municipal Airport is in the center of the City,
north of Spencer Highway.
2.1.2 Drainage Overview
The City is drained by both storm sewers and open and roadside ditches. Storm sewered
areas are commonly found in the newer developments in the west side of the City and in
the Brookglen subdivision. The topography of the City is generally flat and averages
about 24 feet above sea level (see Exhibit 2-3).
Drainage problems have been reported or identified in many areas of the City, but many
of the drainage problem or flood prone areas are concentrated in the: older areas of the
City. More recently developed areas of the City typically have less reported or identified
drainage problems. Tidal variations in the lower San Jacinto Bayou and Galveston Bay
can significantly affect drainage in the eastern side of the City.
CountyThe city has approximately 35.1 miles of Harris + Control (HCFCD)
+ + channels formthe primary componentof
drainage system in La Porte.also has approximately
contiguous to Galveston Bay. There are seven major channels forming the primary
surface drainage system of the City (see Exhibit 2-). There are also nine major
tributaries to these primary channels. Drainage conditions in and along these primary and
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The City lies in three major watersheds (see Exhibit 2-2): the Armand Bayou Watershed,
the Clear Creek Watershed, and the Lower San Jacinto/Galveston Bay Watershed.
The Armand Bayou Watershed lies on the southern and western side of the City. The
watershed drains in a generally southern direction to Clear Creek. Much of the western
portions of the City watershed lie in this watershed; many of the residential areas of the
City are located in this watershed as well.
The primary drainage channels and their major tributaries in this watershed are listed in
Table 2-1 and shown in Exhibit 2-6. Drainage area sizes and levels of current
development for these channels and tributaries are also summarized. It should be noted
that these watercourses drain highly developed areas, but there are some large
undeveloped tracts especially along Big Island Slough (B 106-00-00). It should also be
noted that the majority of Willow Spring Bayou (B112-00-00), Tributary 1.78 to Willow
Spring Bayou (B 112=02=00), and Spring Gully (B 109-00-00) are outside of La Porte's
city limits, thus affecting the available remedies for these watercourses.
There are several homes in this watershed with severe flood damage and repetitive losses.
However, review of previously estimated flood plains, as published by Federal
Emergency Management Agency (FEMA) flood insurance rate maps (FIRMS) for the
primary channels in this area indicate major channel flood levels are within the banks of
the channels throughout most of the watershed except in the Spencer Highway Estates
Subdivision and the Brookglen Subdivision (see Exhibit 2-5).
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2.2.2 Clear Creek Watershed
The CIear Creek watershed drains into Clear Lake which eventually outfalls to Galveston
Bay. The central and southern sections of the city lie in the Clear Creek watershed.
The primary drainage channels and their major tributaries in this watershed are listed
Table 2-1 and shown in Exhibit 2-7. Drainage area sizes and levels of current
development for these channels and tributaries are also summarized. To be noted about
these watercourses is the following: Channel A104-07-00 is characterized as a large,
well -maintained, straight channel, while Taylor Bayou (A104=00-00) and A104-12-01 are
mostly natural channels with heavy forested overbanks.
Flooding in some areas of this watershed has been studied in prior FEMA studies, but the
studied areas in these prior studies lie generally to the south of the City. Taylor Bayou
(A104-00-00) and Channel A104-07-00 (see Exhibit 2-7), which are primary City
drainage channels in this watershed, have been previous modeled by FEMA but not
through the City; consequently floodplains in the City within this watershed have not
been previously delineated.
2.2.3 San Jacinto/Galveston Bay Watershed
The San Jacinto/GalvestonJacinto/Galveston Bay Watershed drains into Galveston Bay and encompasses
portionsthe eastern
The primary drainage channels and their major tributaries in this watershed are listed
shownTable 2-1 and # '.' and levels of current
development for these channels and tributaries are also summarized. Little Cedar Bayou
0 0 11 and characterized
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meandering channels with either heavy forested overbanks or dense
residential/commercials areas.
This area has experienced structural flooding. The San Jacinto/Galveston Bay Watershed
has been partially studied by FEMA as part of the Tropical Storm Allison Recovery
Program (TSARP) However, Little Cedar Bayou (F216-00-00), the most significant
channel in the watershed within the City (see Exhibit 2-8), was the only channel in the
watershed that was specifically studied in this program. The City has plans to construct a
linear detention pond on F216-00-00 to reduce flooding in the area. Exhibit 2-8 shows
stream locations, FEMA delineated floodplains, and locations of flooded structures.
2.3 Drainage and Flooding Problems
2.3:1 Base Flood Maps and Regulatory F000dplains
The City's current Base (100=year) Flood Maps (also identified as F1RMs, i.e., flood
insurance rate maps) were developed as a consequence of hydrologic and hydraulic
studies done for the FEMA-sponsored Tropical Storm Allison Recovery Program
(TSARP); these maps became effective on June 18, 2007. These Base Maps were
developed using particular hydrologic and hydraulic models (conveniently referred to
here as the FEMA models since they have been formally adopted by FEMA) to delineate
the extent of potential floodplains along various larger streams; bayous, and channels in a
watershed for base flood conditions in the area drained by the watercourse in question;
these floodplains define regulatory floodplains (i.e., officially recognized by FEMA and
used for defining flood insurance rates). Exhibit 2-6, 2-7, and 2-8 shows the locations of
the floodplains delineated on the FEMA base maps for bayous and streams studied under
the TSARP program that lie in the City. The delineated floodplains identify areas where
potentials for flooding are high during severe storm events.
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Examination of the delineated floodplains in Exhibits 2-6, 2-7, and 2-8 shows that oat -of -
bank floodplain areas, while present in some areas, are not extensively spread across the
City for those bayous and streams studied with the FEMA hydrologic and hydraulic
models under the TSARP program. Recognize, however, not all bayous and streams in
the City were studied under the TSARP program for the purpose of base floodplain
delineation; watercourses for which neither in -channel nor out -of -bank flooding
conditions are shown are watercourses (or portions of a watercourse) for which no FEMA
hydraulic model has been developed.
2.3.2 Drainage and Flooding Problem Identification
While FEMA-delineated floodplains and the models used for such delineation can be
used to estimate or identify areas of potential flooding problems, other techniques can
and were also used to identify flooding and drainage problem areas. These latter methods
included direct inspection and observation; discussion with knowledgeable persons on the
City staff and City Council, citizen reports to the City on severely damaged structures,
drainage and flooding problems arising from Tropical Storm Allison and Tropical Storm
Erin, flooding data documented by citizens; and formal repetitive loss reports (i.e.,
multiple reports of flood damage to structures made for flood insurance claim purposes).
These latter reports define what are termed repetitive loss data.
The City's repetitive loss data were obtained for this study at the request of the
Floodplain Administrator for the City and used to approximately locate where structural
losses (e.g., residential houses) were occurring. Repetitive loss data help not only
identify the location of the flooding problems but are important to identifying recurring
and long term problem areas.
The agglomeration of these locations in combination with other information on flood
damage locations can be used to identify areas (referenced by the subdivision in which
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the flood damage is concentrated) of significant drainage or flooding problems. Exhibit
2-6, 2-7, and 2-8 shows locations of drainage and flooding problems defined by these
damage and flooding reports.
Based upon the data gathered on drainage and flooding problem locations, general
locations (identified by subdivision areas) of significant drainage or flooding problems
were identified. These are listed in Table 2-2.
To assist in identifying the level of severity of the drainage and flooding problems in the
identified subdivision areas, each flood damage report (arising from Tropical Storm
Allison, Tropical Storm Erin, severe structural damage reports, and repetitive loss reports,
discussed above) was assigned a weighting factor (selected in consultation with City staff
and listed in Table 2-3) so that a severity index could be computed from the sum of the
weighted reports it the general vicinity of the subdivision where the flooding problem
was reported. The details of the process of generating this severity index are discussed at
length in Letter Report No. 3. The computed severity index, termed the flood problem
"intensity" is listed in Table 2-4.
The computed flood problem intensities can be used to graphically display the severity of
flooding problems across the various watersheds, as illustrated in Exhibits 2-9, 2-10, and
2-11. Since the intensity values should be considered only as approximate, the exhibits
use only broad classifications to display the problem severities in various subdivision
Note that it is the relative magnitude of the flood problem intensity parameter that is of
importance, not its absolute magnitude. Consequently, for assistance in decision making
about remedies for drainage and flooding problems, the intensity values can be used to
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variousrank the of r 10, 10 reprcsentj���
I• r r * r _MORWHOM
The subdivision ranking of Table 2=4 is a ranking based upon flood problem severity as
reflected in drainage and flooding reports, as discussed above. Improvement in drainage,
as discussed at length in Letter Report No. 3, will improve drainage conditions across a
large area of a subdivision, not just for those residences for which flooding reports have
been made. The estimated number of lots (in lieu of detailed information on number of
people in a residence) generally benefiting from drainage improvements in the general
vicinity of a subdivision have been also estimated; these estimates are listed in Table 2-4.
The number of lots likely to directly benefit from drainage improvements provides an
alternative method by which to judge the relative merit of drainage improvements.
2.3.4 Problem Areas with Short Term Drainage Project Remedy
Initial work in development of the CWDS examined drainage problems and potential
remedies developed by the City because of conditions conducive to severe flooding; these
problem and remedies have been detailed in Letter Report No. 1. Five critical areas were
identified in this initial identification of drainage and flooding problems by the City (for
convenience these problem areas are identified as City-ldentified Drainage Problem
Areas. These are listed in Table 2-5 and shown in Exhibit 2-12.
Potential remedies for reduction or elimination of localized drainage problems in these
City -Identified Drainage Problem Areas had been identified by the City, the distinctive
feature of the remedies being their ability to be quickly and relatively cheaply
implemented. These drainage problems, the proposed remedies, and the evaluation of the
proposed remedies are summarized in Table 2-5. The particular drainage problem
remedies for these initially identified problem areas are localized and consisted of the
following strategies applied in the immediate vicinity of the identified problem area:
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• Improvement of sheet flow paths by reduction of blockages
• Introduction of a new or emergency sheet flow path
• Lowering of street elevations to promote drainage by reducing obstruction to flow
• Adding capacity to existing ditches or introducing drainage swales
• Improving or realigning a key existing storm sewer to provide increased capacity
Addition of street inlets
24 Probable Causes and Potential Remedy of Drainage and Flooding Problems
Subdivision areas (and areas in close proximity) with significant drainage problems have
been identified above (see Table 2-2). Evaluation of these problems based upon
information provided by the City, review of information in previous drainage reports, site
inspection; proximity to potential flooding sources (i.e., bayous and channels), and
hydrologic and hydraulic modeling to identify the probable cause of flooding.
Identified potential causes for reported or otherwise identified drainage or flooding
problems are the following: lack of overland flow path, prohibitive flow path elevations,
insufficient storm sewerage, and lack of channel capacity.
Remedy of problems are intended to address the root cause of the problem but could
consist of a combination of various techniques for remedy.
2.4.1 Sheet Flow Ponding and Paths
Ponding in localized areas (such as at street intersections or at the end of a cul-de-sac)
due to inability of accumulating runoff waters to drain away from the accumulating area
because of the lack of a overland flow path: If a sheet flow pathway (or a drainage relief
structure, such as a relief storm sewer or relief swale, which achieves the same effect) can
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be provided to drain away accumulating runoff waters, a localized drainage problem can
be possibly resolved. Improved sheet flow pathways were identified for use in the five
City -Identified Drainage Problem Areas discussed in Section 2.3.4 above.
2.4.2 Flow Path Elevation Adjustment
Sheet flow drainage is sometimes prohibited by the elevation of a street, parking lot, or
other large open area. Lowering of such areas can promote more effective sheet flow.
This strategy was used in addressing some of the drainage problems in the City -Identified
Drainage Problem Areas listed in Table 2-2. Table 2=6 and Exhibit 2=13, 2-14, 2-15 and
2-16 shows the cost and locations of possible sheet flow paths. Detail information on
elevation and flow path direction is required to implement this improvement strategy.
2.4.3 Storm Sewerage Improvements
Storm Sewerage, the set of pipes and appurtenances composing a system, may be
inadequate because of several causes:
• Insufficient inlet capacity, which may be the result of insufficient size or type of inlet,
insufficient number or spacing of inlets, inadequate street grading toward the inlet; or
shifting of the elevation of a street or inlet which inhibits flow into the inlet.
• Inadequate sewer pipe capacity, which may be the result of insufficient size, too
shallow of a pipe slope, or excessive energy losses due to pipe material or junction
conditions,
4utfall limitations which limit the maximum discharge from a sewer, due to outfall
pipe material, size, or slope.
• Incorporation of sheet flow options to provide addition drainage capabilities and
prevent excessive ponding can be considered a particular type of sewerage
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improvement since it has the net effect of moving runoff away to a point of discharge
just as an inlet into sewer pipe does.
Any one or more of these adequacies can be addressed by sewer or sewer system
appurtenance replacement or upgrade. In addition to storm sewer or storm sewer
appurtenance replacement, capacity improvement options include addition of parallel
sewers, addition of new inlets, realignment of sewers; and introduction of bypass sewers.
For planning purposes, sewerage improvements are only categorically identified as
sewerage improvements since the choice of a particular improvement technique will
depend upon the details of the current sewerage system which are not available for the
planning purposes of the study.
2.4.4 Channel Capacity Increase
Inadequate capacity of an open channel (ditch, bayou or other watercourse) has two
important consequences: potential overflow of the channel during flood conditions, and
creation of high tailwaters at storm water sewer outfalls, which in turn reduces storm
sewer capacity. Potential methods for channel capacity improvement include the
following:
• Deepening of a channel: Deepening possibilities will generally be limited,
because of flat topography and flowline connection levels at junctions and culvert
crossing structures.
Adjustment of the channel bottom slope Channel slope adjustment will typically
be limited for the same reasons as those limiting deepening of a channel.
• Channel straightening to reduce energy losses and increase bottom slope:
Straightening has limited application in the City because most channels are
already quite straight.
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• Lining of channel bottom and side slopes to increase channel conveyance by
reducing frictional losses: Lining, while sometimes necessary as the only viable
option, is not a preferred technique because of costs and environmental impacts.
• Channel berming at low points: Berming of a channel to form, in effect, a local
levee at locals where the top of bank elevation is low can be used to help contain
high flood waters, but in fact may not function as intended in an urban
environment because of crossings beneath the berm by sewer pipe.
• Widening of a channel: Widening a channel to improvement its capacity is a
preferred feasible technique provided adequate right-of-way is available so that
adjacent properties are not adversely impacted.
• Improvement of hydraulic structure capacity: Bridges, to some extent, and
culverts, often to a considerable amount, can significantly reduce the capacity of a
channel. If data are available for estimating the impact of hydraulic structures on
channel capacity, improving structure capacity, or at least significantly reducing
the constrictions that hydraulic structures like culverts introduce, can be
considered.
2.4.5 Detention
Detention is used for several primary purposes in drainage and flood control:
• On -site Mitigation of Runoff: On -site detention for mitigation of increased runoff
due to development of an area.
• Diversion of Channel Flow: Off-line detention for diversion purposes to reduce
existing channel flows by diverting flow to the diversion pond and reducing flow
and stages in the channel from which the diversion occurs.
• Off -site Mitigation of Runoff: Off-line regional detention for mitigation due to
increased runoff from several sites simultaneously.
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Mitigation of Flow Impacts from Channel Improvements: Off-line or in -line
detention for mitigation of increased discharge from other improvements, such as
channel widening.
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Various methods are used to determine the characteristics of various remedies, i.e.,
potential solutions, proposed to address identified drainage and flooding problems. The
previous section identified problem areas; the following section applies the methods of
this section to size, dimension or otherwise define the key features of proposed remedies.
Basic characteristics of remedies to be determined include channel geometries and
detention pond sizes. The information required to determine these characteristics are
stormy water runoff and channel flow depth, i.e., hydrologic and hydraulic behavior.
3.1.1 Drainage Criteria
Characteristics of remedies are determined in light of City drainage criteria. Key
drainage criteria from the City of La Porte Design Criteria Manual, Chapter 5 used in the
development of drainage and flooding problem remedies are the following:
3.1.1.1 Design Frequencies
Design frequencies, expressed as return period, for various infrastructure elements are the
following:
Sewers: 3-year
Road side ditches and Culverts: 3-year
Ditches and Culverts draining more than 100 acres: 25-year
Bridges: 100-year
Creeks: 100=year
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Since drainage improvements proposed for watercourses are all major drainage ways
(bayous, channels, and creeks), the 100-year design frequency was used to determine
needed infrastructure improvements. Currently, the City staff is in the process of
updating the storm sewers design frequencies requirements to a 5-year design. While the
3-year design is consistent or even slightly more stringent than some other surrounding
communities, the City does experience continuing flooding and drainage problems with
smaller storm events so a 5-year level of protection is warranted.
t
When storm water detention is required, the following criteria are used to specify
minimum detention:
0 to 3 acres
3 to 10 acres
10 to 50 acres
Greater than 50 acres:
0.20 ac-fl/ac
0.45 ac-ft/ac
Per HCFCD criteria
City and HCFCD approval
Since the detention systems considered for addressing drainage and flooding problems in
this CWDS deal with areas typically greater than 50 acres, detention volumes were
determined using hydrologic methods rased upon comparison of hydrograph volumes. It
should be noted that the City staff is currently in the process of updating the detention
requirements for the City. The staff is recommending that a minimum 0.2 ac-ft/ac
detention rate be applied to development areas ranging in size from 0 to 1 acre, while the
0.45 ac-ft/ae detention rate is to be used for areas ranging from 1 to 10 acres. These
detention rates should provide an additional levelof protection to the City.
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Ponding levels in local public streets used as drainage facilities when the 1 OD -year storm
event occurs are not to exceed the minimum of top curb and top of roadway crown for
new roadways or new developments.
3.2 Classification of Watercourses
To conduct various hydraulic and hydrologic analyses with the available data, three types
of watercourses were recognized, as follows:
• Primary Channels: Larger channels and bayous and similar watercourses which
form the backbone of the City's drainage system. These channels are all owned
by HCFCD.
• Major Tributaries Channel; ditches, bayous and similar watercourse of moderate
size which are tributary to primary channels. These tributaries are all owned by
HCFCD
• Other Channels: Small to moderate channels and ditches which are tributary to
primary channels or major tributaries. All channels and ditches which are not
classified as primary channels or major tributaries are classified as "other
channels."
1-lydraulic analysis in the development of this CWDS was performed only for primary
channel and major tributaries. Primary channels and major tributaries are identified in
Table 34 and shown in Exhibit 3-1
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Hydrologic models were used to describe runoff magnitudes and flow in channels. For
evaluation of primary channels and major tributaries, hydrologic models were available
from TSARP studies or were developed for evaluation of primary channels and major
tributaries.
HFC-HMS hydrologic models for entire drainage areas defined by most of the primary
channels are available as a consequence of the TSARP program (these models are
available online from HCFCD). These models use the Tc+R method (a specialized
version of the Clark unit hydrograph method) developed and used by HUM Key
parameters in these models are drainage area, time of concentration Tc, storage parameter
R, channel length L, and level of development as characterized by the DLU parameter,
the latter affecting Tc, R, and imperviousness values. Each watershed for which a HEC-
HMS model was available has a separate HEC-HMS model with its own set of
parameters.
These models are used to define not only peak discharge for the storm event frequency of
interest but also the entire shape (i.e., discharges) of the runoff hydrograph.
r ri WIT, .
For drainage areas within a drainage area with a HEC-HMS model but of lesser size than
the full drainage area for which a fIEC`.-HMS model is available, the peak discharge and
hydrograph discharges were, determined by prorating discharges according to drainage
area size.
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33.3 Correlation Models
For drainage areas not covered by nor included in the available HEC-HMS models, a
correlation model was developed. Data from the HEC-HMS models were used to
compute the unit runoff (peak cfs per acre of drainage area) as a function of channel L
and level of development DLU; see Figure 3-1, 3-2, and 3-2 for the 10-, 50-, and 100-
year event correlations developed, respectively.
These correlations, with a different correlation for different storm event frequencies (e.g.,
10 , 50-, and 100-year storm events), were used to compute peak discharges for areas
which did not have a HEC=HMS model. Discharges at frequencies other than 10-, 50-, or
100-year levels were computed by interpolation.
3.3.4 Hydrograph for Correlation Models
The complete hydrograph for a drainage area with peak discharge described by a
correlation model was approximated by the Malcom hydrograph used in the HCFCD
small watershed method. The discharges defining the Malcom hydrograph are
determined by three parameters: time to peak Tp, total volume of runoff V, and peak
discharge Qp. Only two of these three parameters are independent, the three parameters
being defined by the following equation:
Qp � V/(1.39 Tp)
In application of this method for the present study, Qp is determined by the peak
discharge correlation (discussed above) and the volume of runoff. The volume of runoff
is determined from the intensity -duration -frequency behavior for a 100-year 24-hour
duration rainstorm event and an imperviousness determined by the land development
parameter DLU: The fraction of rainfall which becomes runoff is set equal to the
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imperviousness, while the percentage imperviousness is approximated by the DLU
percentage.
3.4 Hydraulic Models
3.4.1 FEMA Hydraulic Models
Hydraulic models (specifically HEC-RAS models) for most primary watercourses are
available as a result of the TSARP program. These hydraulic models were used to
delineate what are now officially designated as FEMA rase floodplains along major
watercourses, such as most of the watercourses defined as primary watercourses in this
CWDS report. These 14EC-RAS models are termed for his study as FEMA models and
channels for which such FEMA models are available are termed FEMA channels. Table
3-1 tabulates the FEMA channels; these channels are also shown in Exhibit 3-2.
The FEMA models are used in the following ways:
To determine existing channel capacity in a FEMA channel: This is accomplished by
executing the FEMA model and by determining at what points the 100-year storm event
rises above the lower of 1) the top of bank or 2) the ground levels in areas immediately
beyond the berms forming the banks of the channel. This latter condition may occur
when the natural ground is below the bank level due to depressed topography beyond the
channel. Top of bank elevations were determined from the IIEC-RAS cross section data
(which in turn are based upon survey data) while ground levels beyond the channel banks
were determined from LiDAR-determined topography.
To determine necessary channel widening to increase the channel conveyance to handle
the 100-year flood: This is accomplished by a trial and error process which gradually
increases the channel width until the required discharge is conveyed within a channel
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water level which is the lower of the top of bank or ground levels in areas immediately
beyond the berms forming the banks of the channel.
3.42 Non-FEMA-Modeled Channels
When a FEMA model is not available for a primary channel or major tributary, an
approximate model is created by assuming the flow in the channel to be at normal depth.
Available data, including field survey data collected for this study; are used to define the
in -channel cross sectional shape. of the channel in question. Representative bottom slope
is obtained from field survey data. With representative channel shape and slope
determined, the depth versus discharge relation for the channel can be determined using
the Manning equation.
The hydraulic model, therefore, for a non-FEMA models station is the set of data
defining channel shape and slope and the Manning equation which uses these data.
3.5 Evaluation of Storage Requirement
Storage is determined when a detention pond is to be sized to reduce peak discharge in a
Depending up•
to compute storage, and the purpose of the detention storage, somewhat different methods
are used to compute required storage volumes.
3.5.1 Mitigation Storage for Channel Widening
For mitigation of channel improvements involving channel widening, the mitigation
storage was estimated as the volume of excavation for the widening.
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The volume of diversion storage for FEMA channels was estimated as the volume of the
hydrograph before diversion for those values of discharges in excess of the peak
discharge after diversion. For non-FEMA channels, diversion volume was estimated as
the difference in bcfore= and after -diversion hydrographs assuming hydrographs were
described by the Malcom hydrograph (see Section 3.3.4) with peak discharge determined
from the runoff calculations using a runoff correlation (see Section 3.5.4 below).
3.5.3 Mitigation of Excess Runoff Due to Development
The volume of runoff for FEMA channels before- and after -detention is computed as the
difference in the IIEC-HMS hydrographs as predicted by the HEC-HMS model with a
peak discharge equal to the peak discharge before and after detention is used. The volume
for non-FEMA channels is computed in the same manner except that the hydrograph is
described by the Malcolm hydrograph model (see Section 3.3.4).
3.5.4 Correlation Models
For drainage basins with correlation models (basins with non-FEMA model channels),
the hydrographs before and after detention are determined by the Malcom hydrograph
with peak discharge determined from the correlation of runoff with area and level of
development (see Section 3.3.3) and runoff volume equal to the design rainfall multiplied
by the estimated imperviousness fraction for the land development condition being
evaluated.
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SECTION 4
IDENTIFIED DRAINAGE AND FLOOD CONTROL REMEDIES
4.1 Approach
Remedies for addressing identified drainage and flooding problems incorporated
improvements identified directly as a consequence of the modeling and analysis methods
discussed in Section 3 of this CWDS report, review of prior drainage studies for the City
and abstraction of recommended drainage improvements from those studies that are
appropriate to the problems identified in this study, and inclusion of some term remedies
identified and described in Letter Report No. 1.
4.2 Identified Types of Remedies for Existing Conditions
Within the limits of information available for the development of the CWDS, identified
drainage and flood control problems fall into three broad categories: Problems arising
from flooding of primary or main tributary channels, as evidenced by estimated out -of -
bank conditions; significant numbers of reports (of different types; as shown on Table 2-
2) on flooding when nearby out -of -bank conditions do not occur or a channel is at
considerable distance from the problem area; or various reports, site inspection, and
review of topographic information indicates a condition conducive to or arising from
significant Iocalized ponding because lack of drainage pathways.
4.2.1 Remedies for Insufficient Channel Capacity
Primary or major tributary channel flooding was concluded to be the primary flooding
problem source r?vhen either delineated f7oodplains more or less encompassed the
drainage problem area or the existing channel in the vicinity of the problem area did not
have the capacity to convey the design discharge. While this evaluation used the channel
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design flow (of 100=year frequency; see discussion of Section 3 regarding design criteria),
the evaluation of extreme event flows was used as the guideline to assess the likelihood
of channel overflow as a significant problem source. For FEMA channels, the 100-year
flow was evaluated to determine in- or out -of -bank conditions, while non-FEMA
channels were examined for the 100-year flow and less severe flood levels as well.
Four basic options were considered in addressing flooding due to channel overflow:
• Channel widening, selected as the most feasible and desirable channel
modification technique if channel modification is to be used to increase channel
capacity. The channel reach length where widening was proposed could be
limited in channel length to areas where actually needed. For planning purposes,
the widening was assumed, when based upon FEMA model analysis, to generally
extend the length of the channel reach between FEMA model sections in the
vicinity of where the current floodplain was out of banks and structures were
being adversely impacted by flooding, such that the reaches to be improved were
contiguous. For non-FEMA model analysis dealing with tributaries, the widening
was assumed to occur in the channel reaches between model sections (with
section locations based upon field survey) where the downstream end of a reach
did not have adequate existing capacity to carry the 100-year discharge. The
upstream end of non-FEMA model channel improvements were based upon
professional judgment, sometimes not included because improvements in the
excluded reaches were judged not to have significant potential for reduction in
structural flooding.
In addition, the widening for which cost estimates were made was done so as to
approximately 1) maintain the widened channel within its current top width and 2)
using side slopes which approximated existing side slopes. In many instances, the
existing slide slopes were steeper than a 3:1 horizontal to vertical side slope. The
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width of right-of-way for this channel improvement thus consisted of two parts: 1)
the approximate existing top width (i.e., bank -to -bank); and 2) an assumed
maintenance berm of 20-feet on each side of the channel.
Widening of channels to the point of significant encroachment upon adjacent
properties was not considered to be a viable option. Table 4-2 shows the
estimated additional channel width that would be required if channel widening
with using a 3:1 side slope were used rather than a slope approximately matching
the existing side slope It is noted that significant encroachment upon adjacent
properties could be expected if a 3:1 channel slope were consistently used for
channel widening.
• Channel lining, which is used as a channel modification when widening, because
of adjacent structures or similar limitations, was not feasible. The lining was
evaluated as being concrete, which would allow steeper side slopes (1 tol) as well
as reduced channel roughness. It is recognized that concrete lining is not a
preferred option from an environmental perspective, but it was found to be
necessary in some situations. Furthermore, detailed design could consider
alternative materials, such as flexible concrete mats of interlocking blocks, partial
lining, and geo-cell systems, which typically are more aesthetically pleasing than
solid concrete lining (but note, side slope steepness on some of these types of
alternatives are quite limited). Right-of-way requirements for the lined channels
are similar to widened channels, i.e., existing width at top -of -bank plus a 20-foot
maintenance berm of each side of the channel.
• Hydraulic structure modification, in which bridges and culverts are modified to
reduce significant channel restrictions and consequent water level impacts: For
FEMA-modeled channels, the structures along the channel are virtually all
bridges. For bridges along the FEMA-modeled channels, review of computed
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water surface profiles for the design discharge for the FEMA-modeled channels
indicated no significant (in comparison to channel widening) impacts on water
surface levels. Thus for FEMA-modeled channels, reduction of structure impact
by bridge or culvert improvement was not considered as an improvement option.
For non-FEMA-modeled channels for which channel and structure information
was limited, it was concluded that bridges, if any, as in the FEMA-model
channels, were not a significant factor in estimating channel capacity.
On the other hand, culverts might be a significant constriction in the non-FEMA=
modeled channels. To assess whether culverts might or might not be a significant
factor in the capacity of the channel, the estimated capacity of the existing
channel was determined using the approximate methods discussed in Section 3.
`l'he capacity of any culvert system along the channel was assumed to be roughly
the same as the capacity of the channel. If, then, the estimated existing channel
capacity were in excess of an approximately 100-year frequency storm event, it
was concluded that the capacity of any culvert system in the channel would not be
a significant limitation on the capacity of the channel if the channel were to be
improved by widening or lining.
On the other hand; if the estimated existing channel capacity was less than that for
an approximately 100-year storm, it was assumed that the existing culverts would
have a significant impact on the capacity if widening or lining were undertaken;
and, therefore, culvert improvements would be required at all culvert systems
along the channel where the widen or lining was to take place. For planning
purposes (and specifically for costing purposes), the nominal improved culvert
system was assumed to be a 5-foot square concrete box culvert with a typical
velocity (at the design flow) of 6 feet per second, so that the capacity of a single
culvert would be approximately 150 cubic feet per second. The number of barrels
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required would be the design discharge divided by 150 cubic feet per second
(rounded upward to the nearest whole culvert number).
• Diversion ponds are off=line ponds potentially used when widening or lining, as
discussed above., is 1) inadequate to lower water levels sufficiently to carry the
design flow without bank -overflow, 2) considered unacceptable because of
construction or environmental limitations; or 3) inadequate to prevent flooding in
low-lying areas beyond the channel (low lying in comparison to the top=of-
channel bank elevation) would require, for flooding of such areas not to occur
(either directly or by backflow into sewer outfalls or bank cuts), that the water
surface elevation for the design flow condition to be dropped to a very low level,
so low that it could not be readily accomplished with widening or lining. A
diversion pond, in essence, diverts some of the design flow out of the channel and
temporally stores it so that the peak flow to be carried in the channel is lowered,
and, a result, the maximum water surface is lowered. The procedure for
determining the necessary storage volume to accomplish this is described in
Section 3.
It is more hydraulically -efficient that diversion ponds be located in the vicinity of
areas where the flooding problems they are intended to remedy are located.
Potential pond locations area discussed in regard to regional detention ponds
discussed below; for effective diversion, ponds would be located in middle or
downstream reaches of a channel at locations were open land were available.
However, due to limited open land near the affected areas, some of the diversion
ponds will have to be located in the upstream reaches of certain channels.
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.2.2 Mitigation of Channel Improvements
Channel improvements, such as widening, will usually result in increases in the peak
discharge that occurs downstream during the design flood condition because the
floodwaters move more effectively in the improved channel. If these increases are
significant, an adverse impact to the downstream regions can result. To prevent such
adverse impacts, mitigation of increases is required. In addition, if these increases are
seen at downstream points outside the City, controversy over the proposed improvements
might result. Consequently, construction of mitigation ponds associated with channel
improvements should be expected.
For planning purposes, mitigation ponds would be expected to be located in the
approximate vicinity of where the channel improvements occur. However, the mitigation
volume would not necessary have to be provided in a single pond, but could be divided
among several smaller ponds. Underground detention might in part be used to provide
some of the required mitigation (though typically, underground detention is more
expensive than surface detention unless land acquisition costs for surface detention are
quite high and the surface above the underground detention can be used for high -valued
purposes; such alternatives could be examined in detailed design).
For planning purposes, the required mitigation volume is estimated as the volume of
excavation needed to construct the channel capacity improvement. Construction of
mitigation will introduce additional cost for channel capacity improvement. Locations
where mitigation ponds (or detention or diversion ponds might be located) are discussed
below in regard to regional detention issues.
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4.2.3 Storm Sewerage System Improvements
When the source of the drainage problem for a particular area of identified significant
drainage problems is not considered to be channel flooding or a flood -related source,
storm sewer system improvements should be considered. Areas of drainage problems for
which inadequate storm sewerage was identified as the likely source of the drainage
problems were listed in Letter Report No. 2. Sewerage improvements could involve
sewer pipe replacement (with larger size pipe), addition of supplemental sewer lines,
sewer pipe replacement with alternative materials, or, perhaps, sewer pipe Iining.
Inadequate sewer inlet capacity, because of inlet size, number, or location, may also be a
root cause of inadequate sewerage. Letter Report No. 2 noted potential deficiencies in
inlet spacing. Subdivision areas where conditions suggest that the underground sewers
systems and/or inlets may be inadequate are listed in Table 2-2.
The CWDS is intended to address surface drainage issues. Where deficiencies in
underground sewer systems or surface inlets are suspected, focused detailed study on
such areas will be required. This CWDS report does not specifically address remedy of
underground sewer system deficiencies.
One important factor in regard to the local drainage provided by storm sewer systems
needs to be borne in mind when addressing potential sewer system improvements. The
system of streets drained by a sewer system is in fact part of the sewer system. Limited
accumulation of storm waters in the streets for the events Iarger than the design storm
event (3-year storm for the City) is a planned behavior. Water accumulations at shallow
depths for larger storm events do not inherently imply that a sewer system is inadequate
or that there is a drainage problem to be remedied. However, if the City prefers to reduce
these areas of ponding, then improving the sewerage (e.g., adding additional inlets,
increasing the storm sewer pipe sizes, building a parallel line) would provide an
additional level of flood -protection. Detailed storm sewer analysis is beyond the scope of
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this planning study, but the City should pursue more detailed sewerage drainage studies
for the various subdivisions where sewer system improvements are apparently the
necessary drainage problem remedy to confirm the problems with the sewerage systems
(if a system exists already) and design sewerage system improvements.
Using4.2.4 Local Ponding Relief Sheet Flow Paths
Excess surface ponding in localizes areas can arise because of the interaction of two
effects: lack of adequate sewer capacity (as discussed above) and the lack of a pathway
that allows excess accumulated waters to drain away from the area of accumulation. The
accumulation of runoff water will occur in a low spot. If the sewer system capacity (for
whatever reason) is insufficient to receive and convey away the accumulating water, the
water accumulates in the low spot Similar remarks hold for drainage systems which use
surface ditches rather than underground sewers. When improvement of the sewer system
is not considered appropriate or feasible (because of, for example; high cost; long term
delay before sewer improvements can be made, or, as is commonly the case, the rate of
accumulation exceeds the properly determined=design capacity of the sewer system); then
relief of the accumulated waters using either a sheet flow relief swale or an underground
relief storm sewer (i.e., another sewer to increase sewer system capacity) can be
considered:
Letter Report No. 1 described certain situations where new sheet flow paths were
proposed to relieve excessive ponding. Because of the localize nature and consequent
relatively small drainage areas in question, proposed construction of sheet flow pathways
was considered as an option to alleviate drainage problems for areas drained by non-
FEMA-modeled tributaries when topographic and other conditions suggested that lack of
sheet flow paths was a significant contributor to drainage or flooding during large storm
events. Underground relief sewers could be considered as an alternative to the surface
sheet flow path, but for planning purposes all ponding relief was assumed to be provided
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by a surface relief swale or a combination relief swale and storm outfall pipe
improvement, with typical width of 8 feet and depth of 3 feet. Detailed drainage analysis
should be done before the construction of sheet flow paths and land acquisition is started.
r
Future conditions are characterized by more or less maximum development of land;
details for evaluating runoff from such lands are described in Section 3. The present
discussion focuses upon the rationale for selection of potential remedies for drainage and
flooding when future development increases the runoff from various areas draining to
either FEMA-modeled channels or non-FEMA-modeled channels.
Future development in a watershed will, generally, increase imperviousness and
consequent runoff beyond that which currently exists. The remedies for existing
conditions (discussed above) are intended to resolve current drainage problems under the
broad -based assumption that significant increases in runoff due to development do not
occur. Increases in runoff due to development, on the other hand, can be dealt with in
two ways: on -site mitigation or regional mitigation.
On -site mitigation is control of runoff from a development site in such a way that the
peak discharge does not increase above the level that existed before the development of
the site. City drainage criteria require that such control is achieved as part of future
development. The on -site mitigation is typically accomplished using on -site surface
detention, but other methods such as subsurface detention and low -impact development
techniques can be used. The key factors in use of on -site mitigation are 1) that the
mitigation is accomplished before runoff leaves the site so that peak discharges in
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channels not on the developed tract are not increased; and 2) the cost of implementing the
mitigation is borne by the developer (in some manner) of the site.
Regional mitigation provides for a facility, almost invariably an off-line surface detention
pond, to be constructed in the watershed where runoff control is needed, with the design
of the detention pond being such that mitigation is achieved for a number of sites
simultaneously. This mitigation is accomplished by two effects: 1) reduction of flows
downstream of the detention site because during a storm event some flow is diverted
from a channel into the pond, and 2) reduction of downstream flows lowers the tailwater
effects on upstream water levels, resulting in lower maximum water levels at upstream
points for similar discharges, which in net effect is as if runoff is reduced from upstream
sources. Because of this latter reduction, the runoff from an upstream site does not, at
least conceptually, have to be mitigated; the mitigation is provided by the regional
detention pond.
In estimating needed detention, currently undeveloped land areas were assumed to
become fully developed with dense residential lots of a typical size of 0.25 acre. The
increased flows and runoff volume caused by the increase in development for the future
conditions would be mitigated with the construction of regional detention ponds. Also,
the construction of regional detention ponds assumes that the receiving streams have the
needed improvements to efficiently convey the flows out of and into the receiving
Several factors affect the operation of -
r 1,11 111 111 .plill
11 .
the runoff is to reach the detention site, then the channel must be sufficiently large that
the increased runoff from the site does not exceed channel capacity (or, from an
alternative perspective, the channel must be modified to allow extra flow because there
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hydraulic inefficiencies which could cause the required detention for the regional pond to
be larger than the sum of the on -site detention. Advantages of regional detention include
1) the ability to mitigate for areas where on -site mitigation is not feasible; 2) the increase
in developable land on a site; and 3) utilization of economy of scale so that the cost to
developer for accessing a pro -rated portion of the regional detention is less costly than
development of on -site detention by the development.
Procedures for determining necessary storage volumes for regional detention are
discussed in Section 3. When applying these procedures, it is assumed that the mitigation
necessary for the entire drainage basin is served by the regional detention pond. (Detailed
design for the various developments in a regional drainage area might, and likely would,
have a mixture of on=site and regional detention.) Exhibits 4-1, 4-2, and 4-3 shows
potential detention sites. The required regional detention can be divided among various
sites in view actual detailed sources of runoff and site availability.
4.3.2 Channel Improvements
For future conditions, channel improvements are not proposed as an alternative to on -site
or regional detention. Development is presumed to be regulated such that increased
runoff does not require additional capacity in channels, unless such capacity increase
arises because of conveyance of site runoff to a regional detention pond. Channel
improvements for the sole purpose of allowing increased development site runoff without
use of detention is an inappropriate allocation of City resources to a single entity; Thus,
channel improvement is not considered as alternative (by itself) to address future
development for planning purposes.
I • •ing of new structures, the finished floor slab elevation
structures will have to be set at levels which will place the structures above anticipated
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F
/' { 1: ' a �..
elevations which can only avoid inundation by channel improvements.
4.3.3 Storm Sewerage System Improvements
Future development will be required, by City criteria, to provide adequate drainage for
subdivision and similar developments via surface or underground sewer systems. Storm
sewer system improvements are consequently not addressed as part of the CWDS for
future conditions (other than to say the sewer systems will have to be designed according
to City criteria).
4.3.4 Local Ponding Relief Using Sheet Flow Paths
Future development should proceed under the condition, as reinforced by City drainage
criteria that appropriate ponding relief using sheet flow paths be present in the design of
new development. Consequently, sheet flow path relief is not addressed as part of the
CWDS for future conditions (other than to say that such relief should be incorporated into
development design).
4.4 Identified Improvements
Proposed improvements for both existing and future conditions are summarized in Table
4-1 for each FEMA primary channel and major tributary channel, while Table 4-2 shows
recommended improvements for the non=FEMA primary channel and major tributary
channels. The channels for which improvements are proposed are the following:
• A104-00-00 (Existing and Future Conditions)
• A 104-07-00 (Existing and Future Conditions)
• Al04-12-01 (Existing and Future Conditions)
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• B 106-00-00 (Existing and Future Conditions)
• B 106-02=00 (Existing and Future Conditions)
• B 106-05-00 (Existing and Future Conditions)
• B 109-00-00 (Future Conditions)
• B 112-00-00 (Existing and Future Conditions)
• B 112-02-00 (Future Conditions)
• FIOI-00-00 (Existing and Future Conditions)
• F101-03-00 (Existing and Future Conditions)
• F 101-05-00 (Existing and Future Conditions)
• F 10 1 -06-00 (Future Conditions)
• F212-00-00 (Existing and Future Conditions)
• F216-00-00 (Future Conditions)
• F216-01-00 (Existing and Future Conditions)
Improvements for existing condition channels are predominately channel widening.
Exhibit 4-4, 4-5, and 4-6 shows where widening or channel lining is proposed. Widened
channels are presumed to be trapezoidal in section; basic dimension are given in the
Tables 4-1 and 4-2. Estimated excavation for the proposed channel widening has been
determined from the difference in cross sectional area of the existing channel and the
proposed channel. Land acquisition acreage for channel widening assumes that only
right-of-way for maintenance berms along an existing channel must be acquired.
Representative land acquisition costs for undeveloped and developed land are described
in Section 5. It should be noted that no channel improvements are proposed for Little
Cedar Bayou (F216-00-00) for existing conditions. After reviewing the report titled
"Hydraulic Analysis for Little Cedar Bayou Watershed: HCFCD Unit F216-00-00"
submitted by Binkley & Barfield on January 2000, the improvements detailed in that
report were determined to be adequate for planning purposes. It is recommended that the
City pursue the improvements as specified in the aforementioned report to reduce the
current flooding due to lack of channel capacity.
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Note that regional detention for existing conditions is not proposed to remedy current
conditions because of the high cost (cost estimates are discussed in greater detail in
Section 5). This relative cost comparison can be readily seen if it is realized that regional
detention for a channel would have to be much larger than the channel widening
excavation volume. Thus, for example, for Taylor Bayou (A104-00-00), the regional
detention requirement would be about 124 acre-feet (i.e., about 20 times larger than the
mitigation volume). The ritigation pond has an estimate cost of about $250,000, which
is approximately ten times the widening cost.
Regional detention requirements, assuming full development, with all runoff mitigation
to be provided by regional detention, is provided for future development conditions.
Exhibit 4=1, 4=2, and 4-3 shows undeveloped land where ponds might be located. Note
that Exhibits 4-1, 4-2, and 4-3 show numerous detention sites for the watersheds;
however all the identified sites are not needed. Estimated detention volume requirements
identified in Tables 4-1 and 4-2 can be met by multiple sites shown on the
aforementioned exhibits. A list of these potential detention sites is shown in Appendix C.
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SECTION 5
COST AND IMPLEMENTATION
A key factor in implementing potential drainage remedies is the cost for construction of
the project envisioned by the proposed remedy; estimated costs are presented below.
Other factors, however, are important to the implementation and continued effectiveness
of various drainage improvements; these are also discussed below.
51 Recommended Drainage Improvements
5.1.1 Costs of Feasible Remedies
Realistically feasible potential drainage improvements for addressing the underlying
cause of the identified drainage problems were identified in preceding sections of this
CWDS. Approximate construction costs for implementing these problems were
estimated using the unit cost data of Table 5=1; these data are revisions to data developed
in Letter Report No. 3.
Table 5-2 summarizes for FEMA-modeled channels the estimated costs of potential
channel capacity and diversions to address current drainage and flooding problems and
regional detention remedies to address future development issues. Table 5-3 provides
costs for the non-FEMA channels. Non=channel solutions, i.e. relief sheet flow swales,
are presented in Table 5-4 for various subdivisions in the City: It should be noted relief
swales were not considered for a number of subdivisions in the City since these
subdivisions are located too far away from the receiving stream to be an optimal solution.
It has been noted in prior discussion that channel improvements included in some
instances channel lining when widening of a grass lined channel within estimated
availableto provided necessary channel capacity. The
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drawbacks of channel lining have already been noted. Consequently, as an alternate to
channel lining, diversion of flood waters was considered. Diversion alternatives are
listed in Table 5-2 and 5-3. To be noted is the relative high cost of the diversion
alternatives.
In making all cost estimates, a conservative but realistic approach (i.e., estimated costs
were purposely overestimated rather than being underestimated) was taken because of the
preliminary nature of the projects for which costs were being estimated and the fact
considerable time may likely pass before actual construction of proposed improvements.
Table 5-5 presents project costs with a breakdown according to actual construction of
major cost components of channel improvements (which is predominately excavation
costs), detention storage, and Iand acquisition. Table 5-6 shows this same breakdown as
a percentage. It is to be noted that detention storage, whether for mitigation or diversions
to address current flooding problems, is a significant component of total cost.
Cost for regional detention to address future drainage concerns,arisin- from development
are, likewise,itte snifc_ant. It is recognized, however, that while detention for
mitigation or diversion purposes would typically be a cost to be borne by the City, costs
for regional detention to address future drainage concerns _arisin.,from development
would tvnicaliv_ not be borne by the City: such costs would be typically recovered by sale
of detention storage to developers seeking detention to mitigation excess site runoff.
Because of the recognized significant magnitude of the estimated costs of the various
improvements, particularly the infrastructure improvements to address existing drainage
and flooding problems; it is preferable that potential priorities be identified to help
decision makers distinguish between C1P projects for near term construction and those
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projects that should be delayed for consideration to some future time when more funds
become available or the need for a project can be more adequately justified.
Cost of a project is one consideration in establishing priorities for projects; if two projects
accomplish essentially the same result, then the lesser cost project would normally be the
preferred project for implementation. However, it is seldom that two projects have
sufficiently similar results that would allow such a clear choice to be made: Alternatives
are needed to assist in defining project priorities.
The previous discussions in Section 2 looked at drainage improvement needs for various
subdivision areas from two perspectives, as summarized in Table 2=4: 1) the estimated
flooding severity (as estimated from flooding reports), re -expressed in terms of flooding
problem severity rank (10 being highest level of severity and 1 being the lowest level);
and 2) estimated beneficial impact, as estimated by the number of properties judged to be
beneficially impacted by proposed drainage improvements.
On the other hand, it is recognized that particular projects which have been proposed are
intended to eliminate significant flooding in particular areas; these areas where flooding
will be largely eliminated may and generally do cut across portion of different
subdivisions. Exhibits 4-4, 4-5, and 4-6 show, for each of the channel improvement or
diversion project needed in order to address current drainage and flooding problems,
those subdivisions that will, all or in part, be beneficially impacted by a particular project.
If prioritization interest were to be focused upon addressing problems in a particular
subdivision, these exhibits along with Table 2-4 can be used to identify those projects
which should be given a higher level of importance in project prioritization.
A more quantitative basis for establishing a potential prioritization of projects is to count
the number of residents (as reflected in the total number of flooding reports) for which
flooding would be generally eliminated. These flooding report numbers are listed in
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Tables 5-7 and 5-8. Projects which address areas with a large number of flooding
reports are projects which would be recommended as having a high priority for
implementation; projects affecting areas with small numbers of flood reports should be
considered to have low priority and be considered for construction only after other major
problems are addressed. Generally, the lower the project cost and the higher the number
of flood reports being addressed by a project, the higher the priority for construction. It
should be noted that Table 5-7 orders the improvement projects based on cost, while
Table 5-8 orders the improvement projects based on number of flood reports.
Depending upon the cost of projects and the number of drainage reports in an area that
particular projects would address, a particular level of cost per number of reports might
be considered as a basis for defining a cut-off for consideration of a project to be
implemented: Such a cut-off criteria might dictate that some projects never be built; such
a conclusion should be recognized as an issue in setting priorities and selecting various
projects for implementation.
5.1.3 Priorities for Future Regional Detention Projects
In estimating needed detention storage, it should be noted that in order to determine the
benefits of existing developable land that might become developed in the future,
currently undeveloped land areas were assumed to become fully developed with dense
residential lots of a typical size of 0.25 acre. The increased flows and runoff volume
caused by the increase in development for the future conditions will be mitigated with the
construction of regional detention ponds. These regional detention ponds are intended to
address future problems. Also, the construction of regional detention ponds assumes that
the receiving streams have the needed improvements to efficiently convey the flows out
of and into the receiving streams.
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If and when construction of regional detention projects occur, the order of their
construction will be dictated by several factors which cannot at the current time be
determined:
+ Where and at what level development is occurring in the City and thus the
magnitude of the need for having regional detention
• Whether all needed detention is to be provide by regional detention or only a
portion of the needed detention would be provided by regional detention; this
factor may be significant if deciding the number of actual detention projects to be
developed to meet an overall detention need (because as has been previously
noted, all the projected regional detention for a particular channel system does not
have to constructed at one location).
• What type of funding arrangements are to be used to pay for cost for construction
of a regional detention pond
• To what extent joint regional detention projects developed in conjunction with
HCFCD or other cities can be relied upon to meet regional detention goals
• Whether regional detention or on -site mitigation is to be used by developers in the
area potentially serviced by a regional detention facility.
In regard to use of regional detention versus on -site detention (the last issue in the above
list), decisions about use of regional or on -site detention will likely in large measure be
determined on a cost basis: Is it more cost efficient for a developer to pay for
participation in regional detention or to develop on -site mitigation but lose developable
land because of on -site pond construction? To gauge the potential choice to be made,
there is included in Tables 5-2 (for FEMA modeled channels) and 5-3 (for non=FEMA
modeled channels) data which presents the cost of regional detention per acre of
developable land (i.e., currently open land presumed to be fully developed in the future)
tributary to the regional detention site. The smaller the value of this dollars per acre of
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developable land, the more likely would it be that regional detention would preferred
! !" • • - !!
.2 Implementation Issues
In addition to construction costs and prioritization of construction projects; other factors
which are important to constructing or implementing various projects are the following.
5.2.1 Change of Information or Details of Projects
The descriptions of projects identified in this CWDS for possible implementation are
based upon available information at the time the CWDS was prepared. The level of
detail used in the project descriptions is appropriate to the planning level focus of this
study. More detailed analysis will be required for the design of particular drainage
remedies prior to actual construction. Some features of the proposed remedies may
change as more information is developed as part of detailed design. In addition, changes
in development patterns, land use, effects on drainage from out -of -city sources, or other
similar factors affecting drainage behavior may also occur. Because of such possible
changes, priorities for order of construction project may well change over time.
Consequently, drainage conditions in the City should be periodically reviewed to assess
whether conditions have changed sufficiently to significantly affect the character or
priority of construction of recommended drainage improvements as given in this CWDS.
'
Prior to development of a final design and construction of a particular project, various
specialized studies in addition to detailed hydraulic study will be required, including
detailed survey, geotechnical investigation, right-of=way investigation and delineation.
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Since only preliminary estimates of available right-of-way were used in development of
channel improvement alternatives, detailed right-of-way investigation will be required as
part of detailed project design for some projects. Some modifications to proposed
improvements may be required as a consequence of such detailed investigation.
Environmental evaluation should also be anticipated to demonstrate the absence of
adverse environmental impacts for a particular project. Environmental review for
potential impacts on existent wetlands will be required if wetlands are identified within
the proposed project area. Furthermore, since channels may be modified by cut and fill
activities, US Army Corps of Engineers (USACE) Section 404 permit is required for
channel -affecting projects. In addition, for modification of channels (or portions of a
channel) which are tidally affected, USACE review and approval will be required
because tidal -affected waters are, by definition, waters of the United States and under the
jurisdiction of the USACE: The need for significant coordination and review by the
USACE to address potential environmental impacts should be expected.
Proposed modifications to HCFCD channels or modification of discharges to HCFCD
channels will require review by HCFCD; of particular interest to HCFCD will be possible
increases in discharges and potential resulting downstream impacts. Mitigation of
discharges increases should be expected to be an approval requirement.
This CWDS does not specifically address surge impacts in Galveston Bay arising from
tropical storms or hurricanes. Unusually high tides induced by bay storms can induce
nearby shore flooding which drainage infrastructure cannot mitigate. High bay water
levels will propagate up primary and major tributary channels and, depending upon their
magnitude, may induce flooding. To limit adverse impacts
available:options are 1 f floor slabs
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(for new development or redevelopment) above anticipated surge levels; 3) raising of
existing building; 4) construction of tide gates at the mouths of channels draining to the
bay; S) requiring new development to have appropriate flood proofing; and 6) property
buy-outs of affected homes. However, it is to be recognized that even when tropical
storms are the initial cause of flooding, accompanying rainstorms can often induce more
flooding than the surge from a tropical storm. The proposed infrastructure in this CWDS
is intended to address such severe rainstorm events.
5.2.4 Design Frequencies
Pursuant to City drainage criteria, proposed improvements to primary and major
tributaries have been designed for 100-year storm events, depending upon the type of
improvement and the drainage area of the proposed improvement. However, storm
sewer systems, which are not specifically. except for identifying them as problematic in
some areas as addressed in this CWDS, are designed; if City drainage criteria are
followed, for the 3-year storm frequency event. Consequently, limited flooding of streets
is to be expected for some storm events. In responding to citizen complaints about
localized flooding, which can sometimes be relieved by development of sheet flow paths
(as is recommended in this CWDS for some areas), it should be borne in mind that City
drainage system design are intended to use the City streets for drainage for storm events
more severe than a 3-year frequency.
.22. Non -City Funding
Potential funding sources for drainage and flood prevention projects have been
previously discussed (see Section 6 in Letter Report No. 3). In addition to alternative fee
or tax -based methods for generation of additional revenue, the City should consider
seeking grants or loans interest lows from either Federal or State sources. Appendix B
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provides a listing of websites where information on various funding source for drainage
and flood protection projects can be found.
Of particular interest to the City for funding of drainage or flood protection projects
would be both 1) loan programs and 2) grant programs administered by the Texas Water
Development Board. Of particular interest in the loan program category is the State Loan
Program (Development Fund II), which can provide low interest funds for construction of
storage facilities and enlargement of channels, both of which are some of the options that
have been identified for addressing some of the City's drainage problems. 1n the latter
category are grants administered by TWDB for drainage facilities and related activities
such as focused drainage studies. While the competition for such grant funds is
significant, demonstration of a clear need for the funds and a sound technical approach
for use of the funds can go a long way toward being selected for distribution of such
funds.
Also of significant interest are grant funds from the Governor's Division of Emergency
Management which administers grants for hazard (e.g., flooding) mitigation and pre -
disaster mitigation. Grants under these programs can be used for acquisition of flood -
prone structures, retrofit of facilities to increase the flood protection, small scale
structural hazard control projects; and preparation of mitigation action plans.
Another important source of funding for drainage projects could be joint development of
a project between the City and other parties, such as the Harris County Flood Control
District. Such joint funding would typically be contingent upon demonstration of the
mutual benefit of the propose project to both parties.
One such potential joint project is the detention pond option identified for Big Island
Slough (B106-00-00) and Willow Spring Bayou (B112-00-00) (see Table 4-1). Due to
the highly developed nature of the areas in the vicinity of the proposed channel
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improvements, the City might be hard pressed to acquire the needed acreage for building
the detention ponds. However, there are available undeveloped tracts south of the City
The recommended drainage improvement projects are within the boundaries of the City.
This constraint on selection of projects was purposely made in order that implementation
of projects could be pursued at the City's own discretion and without potentially delaying
or undesirable encumberment. However, two factors should be borne in mind in dealing
with drainage issues extending across city boundaries.
Mitigation of increased runoff due to drainage improvements within the City may often
be required to prevent adverse impacts upon downstream locations beyond the City's
boundaries. Increasing the capacity of a channel will typically result in increased
discharges to downstream reaches. To determine whether such increases will be
significant will require detailed engineering analysis done as part of detailed design for a
particular project. If such increases are found to be significant, detention or flow
impediment will typically be required to prevent such downstream increases. If such
increases are significant at points beyond the City's boundaries; special caution and
application of appropriate mitigation will need to be exercised to avoid adverse impacts
on downstream areas beyond the City boundaries.
# - • •rainage improvements• • in this CWDS assum-e that
sourcesdrainage from outside of and upstream of be
actions of become aware of • •
upstream areas which might adversely impact drainage or flooding conditions, the City
should coordinate with the upstream cities to address such anticipated impacts. This
coordination should provide a clear and descriptive enunciation of why adverse impacts
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are expected and identify potential means to mitigate the adverse impacts; either by
remedy within the boundary of the upstream City or by remedy within La Porte with
appropriate contribution to implementation of the remedy by the upstream City. Such
coordination should be pursued where necessary.
If the City suspects that a current flooding problem within the City is the result of past
unmitigated changes in an upstream, non City area; the City could undertake discussions
with the upstream entity believed to be contributing to the current flooding problems.
These discussions and the actions taken in support of the discussions should 1) delineate
the character, location, and extent of the flooding problem believed to be the result of the
upstream conditions; 2) identify, describe and demonstrate the change or set of upstream
conditions which are believed to be the source of the flooding problem in question; 3)
suggest potential remedies for the problem, and 4) propose a recommended plan to
implement actions to institute the remedy.
5; :7 Implementation of Drainage Criteria
Recommendations for upgrades in drainage criteria and standards currently used by the
City were made in Section 2 of Letter Report No. 3. Particularly important among those
recommendations in regard to recommended drainage projects are the following;
Sheet flow path identification and inclusion should be a required consideration in all new
or redevelopment as a typically efficient means to control excess street ponding.
Proposed future development should be critically reviewed in regard to provision of
ponding relief using sheet flow pathways.
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While the City does allow under certain conditions construction in known floodplain
areas (i.e., special flood hazard areas), the City should allow such construction only under
very special circumstances. Experience has shown construction in special flood hazard
areas is an invitation to flooding if such construction is not carefully designed with
flooding issues fully recognized. Critical to allowing such construction, if the City feels
it essential to proceed with such construction, is 1) requiring finished floor slabs to be
above the I00-year flood level; 2) requiring flood proofing of the ground floor structures;
and 3) allowing such construction only in storage areas well beyond the floodway so that
flood flow velocities are quiet small; 4) mitigation of fill in the floodplain along with any
needed detention.
Also to be recognized is that detailed flood flow analysis and floodplain delineation has
not been accomplished for many of the smaller creeks, streams, and channels in the City.
Thus locations of anticipated flooding when severe storm events occur are not accurately
known at the current time. It would be to the City's benefit to conduct flood analyses that
would delineate floodplain areas not currently known.
5.2.7.3 Minimum Low Chord Clearances
New bridge or bridge modifications in the future should be sure to have low chord
clearances in accordwith City criteria.
5.2.7.4 Capacity Improvements in Storm Sewer Systems
Review of storm sewer capacity should be considered in areas where this CAIDS report
has identified inadequate capacity as the likely key source of reported flooding in a
particular area. In making such review, several factors should be considered: 1) what is
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the appropriate design frequency for the sewer system; 2) the inclusion of sheet flow
relief paths to address storm events in excess of the design capacity, and 3) the capacity,
spacing, and size of inlets. Temporary reduction in street inlet capacity due to inlet
clogging by debris is often a root cause of localized flooding. Frequent removal of trash
and debris from streets can be a key element in maintaining the capacity of storm sewer
systems.
11%
The detention requirements identified for mitigation of proposed improvements;
diversion of flood waters, and storage for regional detention are estimated minimal
detention amounts to accomplish the intended purpose of the detention. Actual detention
volumes will be somewhat in excess of these amounts in order to provide potential
freeboard; estimated land acquisition for the pond will require inclusion of land for
surrounding maintenance berms and inlet and outlet structures. Detailed engineering of
the detention system in question will be necessary to refine these features.
One of the key issues in providing the required detention will be determination of
whether one site or more than one site will be used to meet detention requirements.
Detention does not necessarily have to always be provided by a pond at one site only.
The present CWDS report identifies, based upon apparent availability of open land,
potential detention sites. Detailed engineering can evaluate the feasibility of using one or
multiple sites for meeting detention requirements.
5.2.7.6 Enforcement of Drainage Criteria
Drainage criteria as defined by City policy should be rigorously enforced. Letter Report
No. 2 recommended certain modifications to the City's drainage criteria manual.
Irrespective of whether these recommendations are adopted, the criteria in the drainage
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manual should be, for the purpose of preventing the development of future drainaz
s Ms • � * � r
n Fill
[M r
Regional detention pond development has been proposed as a primary strategy for
addressing impacts of future development. Regional detention is one component of a two
prong strategy: On=site detention versus regional detention. If carefully designed and
implemented, either method, or a combination of both, can effectively address potential
future drainage impacts arising from development or redevelopment.
Several factors affect, however, which may make regional detention less attractive than
on -site mitigation: 1) The runoff from an upstream development site must get to the
regional detention site; if the runoff is to reach the detention site, then the channel
conveying the site runoff must be sufficiently large that the increased runoff from the site
does not exceed channel capacity; and 2) off -site detention has hydraulic inefficiencies
which would typically cause the required detention for the regional pond to be larger than
the sum of the on -site detention.
On the other hand, the advantages of regional detention include 1) the ability to mitigate
for areas where on -site mitigation is not feasible; 2) increase in the developable land on a
site; and 3) utilization of economy of scale to lower overall construction costs for
detention.
Other benefits to regional detention, less apparent, are the following:
• Regional detention can be constructed to address some current drainage problems.
Early construction of regional detention to address future drainage could be
combined with detention to remedy current drainage.
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® Regional detention can be constructed in stages (with staged construction at a
single site or by use of multiple sites), thus lessening initial construction costs.
® Regional detention provides a mechanism for cost recovery; sale of detention can
be established to either recover incurred costs or incurred costs with additional
surcharge to recovery other drainage improvement costs.
The availability of regional detention can be an attractor for new development.
If regional detention were to be employed; a cost recovery strategy could be based upon
1) initial funding being provided by the City to provide some detention to address certain
existing drainage problems; 2) allowing early purchase of surplus detention by
developers to reserve detention space in the detention system so that it is present when it
is needed for the development; 3) establishing a fee for purchase which is sufficient to
ultimately recovery both upfront costs and expansion costs; and 4) encouraging use of
regional detention by establishing a development fee on development which uses on -site
detention rather than purchasing storage in a regional facility.
5.2.9. Existing Detention Issues
Detention for remedy of existing flooding problems will generally be required as part of
the mitigation of increased channel conveyance when channel improvements are made to
address current flooding problems if downstream impacts are to be avoided. Potential
detention pond sites have been identified for such mitigation (see Exhibit 4-1, 4-2, and 4-
3).
Detention can be used to address anticipated future increases in runoff due to
development, as discussed in the preceding section. Some of this regional detention
could be also be constructed to alleviate some current drainage problems, but at costs
which are anticipated to be more than other options selected (i.e., channel improvements).
One large site currently available but undeveloped is a site located on property south of
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the City along B109=00=00 on land owned by HCFCD and the city of Pasadena (see Pond
Site 43 in Exhibit 4-1). Joint funding arrangements could be possibly made with HCFCD
which could speed the development of this site under financial conditions favorable to the
City.
There is, in addition, currently a HCFCD regional detention facility (see April 1997,
Wilbur Smith Associates, Consulting Engineers and Planners reference in Appendix A)
along B112-02-00 in Deer Park about 2,000 feet north of Spencer Highway and north of
the La Porte city boundary (see Exhibit 5-2). This pond provides mitigation of channel
improvements in Deer Park north of the pond site as well as possible mitigation of runoff`
from future development north of the pond in Deer Park. There is a potential, albeit
limited, that some surplus detention is available in this existing pond which could be
allocated for regional detention needs in La Porte; discussions with HCFCD and Deer
Park could be undertaken to assess this potential.
There are also four potential regional detention sites (Harris County Flood Control
District, Armand Bayou, HCFCD webpage) located south of La Porte (see Exhibit 5-2)
being considered for development by HCFCD. The most northern of these has a
potential to provide regional detention (to address current conditions or future
development impacts) that may be beneficial to La Porte. Discussions with HCFCD need
to be undertaken to assess the possibility of storage allocation in these reservoirs for the
benefit of La Porte drainage and flooding mitigation.
5.2.10 Easements, Right -of -Way, and Land Acquisitions
The CWDS anticipates that some easements and right-of-ways will have to be acquired.
Because the CWDS is a planning level study; specific acquisition requirements were not
identified. Channel widening, however, was keyed to stay within the existing bank line,
thus possibly requiring only land for maintenance berms, if such easement does not
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already exist. Proposals for sheet flow pathways, likewise, did not address whether the
City had the easement necessary to utilize the proposed sheet flow path; the sheet flow
path identification was based only upon apparent need and availability of open land for
the pathway. Detention ponds, likewise, assumed that acquisition of land could be
accomplished at locations where ponds might be located.
However, when making cost estimates for proposed improvements, account was taken of
reasonably likely cost for land acquisition. For channel widening, land acquisition for
maintenance berms (20-feet on each side of the channel) was assumed. Sheet flow
pathways assumed land acquisition would be required for a path 20 feet wide the full
length of the proposed pathway. For ponds, estimates of pond area were provided based
upon detention requirement and representative pond depth with 30-foot maintenance
berms; these areas defined minimum land acquisition requirements for the ponds.
Buy-out of lands with homes was not specifically identified as alternative for addressing
flooding problems. Channel widening was limited to estimated existing channel widths,
sheet flow paths were selected to avoid existing residents, and detention sites were
limited to currently undeveloped lands. Thus buy=out of homes is not being proposed as
part of this CWDS though some solutions will require that some portion of selected
nearby properties be acquired.
It is recognized that home buy-out is sometimes proposed as a flood control remedy; such
buy-out may become necessary in the future, but the current plan is intended not to rely
upon this strategy.
5.2.11 Ownership and Maintenance of Drainage Facilities
Within the. three watersheds (Clear Creek, Armand, and San Jacinto/Galveston)
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Creek Watershed, eight are in the Armand Watershed, and fourteen in the San
Jacinto/Galveston Watershed; these are listed in Table 5- . Critical to effective function
of these as well as small ditches and channels is proper maintenance. Ditches, channels
and bayous need to be kept reasonably free from obstructions, debris and excessive
vegetation (e.g., trees, brush, high grass) for the watercourses to function as planned.
Maintenance responsibilities, in absence of agreements stating otherwise, are normally
considered the responsibility of the owner of the watercourse. Of the 29 named
watercourses in the City, only nine are believed to be owned by the City. Four others
have unclear ownership or joint ownership with HCFCD. The remaining are owned by
HCFCD (see Table 5-9 and Exhibit 5-1). The City should be aggressive in proper
maintenance of watercourses under their control.
On the other hand, if a watercourse is not owned or under the control of the City but
maintenance is inadequate, the City faces a dilemma. Inadequate maintenance in such
non -City watercourses adversely impacts the function of the watercourse (thus adversely
affecting City drainage and .Hooding),. but the City does not legally have the
responsibilities or liabilities associated with ownership.
It is recommended that the City pursue written inter -local agreements with HCFCD (for
watercourses for which such agreements may not already be in place) regarding
maintenance of watercourses owned by HCFCD lying within the City. For watercourses
of critical importance to adverse effects on flooding in the City, an interlocal agreement
should be developed between the City and HCFCD to assure that if HCFCD does not or
seeks not to maintain the watercourses in question at levels the City thinks appropriate,
the City has permission under appropriate limitations and constraints to perform such
maintenance for an agreed upon compensation or other considerations.
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ToMMTHIM. WN
Stream
Total Drfanage
Type Area
Clear Creek Watershed
Drainage Area
within the Cit
Modeled
A104-00-00 (Taylor Bayou)
1999
13888 1
Yes, TSARP
A104-i}7-00 ('Tributary 3 93 toraylor Bayou)
Tributary
1445.1
370
Yes
A 104-07-01
Tributary
495
495
No, Not enough data
A 104-10-00 (Buggy Gull' Ba+ou)
Trebutary
128 -_
_ 59
No, Not enough data
A104-10-02
Tributary
22
l8
No, Not enough data
A104-12-00
Tributary
526
626
No_ Not enough, data
A10442-01
Tributary'
48
48
Yes
Armand Bayou Watershed
-
B 106.00-00 (13i island Slough)
Primary
2812
2812
Yes
I3106-02-00
Tributary
598
598
Yes
B106-05-00
Tributary
_ 155
i 155
Yes
13106-06-00
Tributary
268
268_
No, Not enough data
B109-00-00(SormgGull'q`)
Primar`A
3452
646
Yes
B109-03-00 (B112-02-00 Interconnect)
Primary
203
203
Yes
B1 12-00-00 (Willow Springs Bayou)
Primary
3259
348
Yes
BI 12-02-00 (Tributary 1.79 to Willow S yrin•T Baroul
Tributa
143
128
Yes
San Jancinto/Calveston Bay Watershed
F101-00-00
Primary
971
871
Yes
F101-01-00
Tributa,
_®.
641
290 41
No, Not enough data
F101-03-00
Tributary
451
451
Yes
F101-06-00
Tributary
245
245
Yes
F 10 1 -06-02 -_ -_ -_ -_
- Tributary
19.5i
19.5
No, Not enough data
F101-06-03
-_ rributary
16.4
16-4
No Not enoustitdata
F101-07-00
Tributary
53
53
No, Not enough data
F 101 08 00
Tributary
78
78
No, Not enough data
F210-00-00
Prim erk'
539
241
No, Not enough data
F212-00-00 (Deer Creek)
Primary
430
430
Yes
F216-00-00 (Little Cedar Bayou)
Primary
1869
1968
Yes
F216-01-00
Tribuvir)
364
364
Yes
F216-02-00
Tributal'v
5
5 _
No, Not enough data
F216-04-60
Tributary
73
73
No. Nat enoa data
Klotz Associates Project No. 0127.008.000 City of La Porte
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DRAINAGELE 2-2
` !
SIGNIFICANT OR PROBLENJ
Name of Flood -Affected Area
Rank(10 is 1iighest)
Number of Tots Likely
(See Exhibit '7, 8, & 9 for
Assuming Flooding
;
to Directly Benefit from
Likely Significant Source/Cause of
location)
Retorts have
"Weight
Drainage Improvement
Flooding
Fifferent
Brookglen
10
600
lVlixture of inadequate Sewerage and Insufficient
Channel Capacity (B112-00-00)
Creckmont Section 1
10
110
Inadequate Sewerage
La Porte
9
375
Insufficient Channel Capacity (F216-00-00)
Glen Meadows
9
160
Mixture of Inadequate Sewerage and Insufficient
Channel Capacity (B 106-00-00)
Fairmont Park Last
8
500
Inadequate Sewerage
Pinegrove Valley
8
220
Mixture of Inadequate Sewerage and Insufficient
Channel Capacity (F101-00-00)
Spencer I fighway Estates
8
100
Inadequate Sewerage
Fairmont Park West
7
390
Inadequate Sewerage
Shady River
7
154
Mixture of Inadequate Sewerage and Insufficient
Channel Capacity (A104-12-00)
Bay Colony
7
128
Inadequate Sewerage
Fairmont Park
6
330
Inadequate Sewerage
Creekmont Section 2
6
30
inadequate Sewerage
Bayside Terrace
5
252
Inadequate Sewerage
Lomax Garden
5
160
Mixture of Inadequate Sewerage and Insufficient
Channel Capacity (F 101-03-00)
Meadow Park
5
91
Mixture of Inadequate Sewerage and Insufficient
Channel Capacity (BI06-05-00)
Old La Porte
4
150
Mixture of Inadequate Sewerage and Insufficient
Channel Capacity (F216-00-00)
Battle Grounds Vista
4
10
Inadequate Sewerage
189
Mixture of Inadequate Sewerage and Insufficient
Pine Bluff
3
Channel Capacity (A104=12=00)
Bay Shore Park
3
50
Data hnsufficient
Beach Park
3
50
Inadequate Sewerage
76
Nxture of Inadequate Sewerage and Insufficient
Woods On The Bay
2
Channel Capacity (A104-12-00)
21
Mixture of inadequate Sewerage and Insufficient
Villa Det Rancho
2
Channel Capacity (B 106-05-00)
Spencer Landings
2
10
Inadequate Sewerage
Meadoweresl
1
50
Inadequate Sewerage
Battleground Estate
------------------------
1— —
20
Inadequate Sewerage
Bay Front To La Porte
0
40
Uata Insufficient
San Jacinto Homes
0
10
Data Insufficient
Klotz Associates Project No.: 0127.0.08.000 City of La Porte
January 2009 City Wide Drainage Study
TABLE
POLICY WEIGHT FACTORS FOR FLOOD PROBLEM INTENSITY
Type 1: Reports on severely damage residences
Type 2: Repetitive loss reports on structural (residential) flooding
Type 3: Tropical Storm. Allison flooding in 2001
Type 4: Tropical Storm Erin flooding in 2006
Type 5: Miscellaneous but reliable data
Klotz Associates Project No.: 0 127.00.000 City of Ea Forte
January 2009 City aside Drainage Stud:
0
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TABLE r
►:; ..
Project Number Nate Progress
Short Term Project I Catlett lane .Pavement Replacement Completed and Constructed
Short Tenn Project 2 Creekmont Park Overilovo, Path Design Completed
Short Tenn Project 3 Drainage Improvements Along Dri ltwood Drive Proposed Relief Swale Path
Short Terse Project 4 Fleetwood Drive Outfall Pipe Upsizing Proposed Outfall Pipe Upsizing
Short Terre Project 5 Glen Meadow Subdivision Interceptor Inlet Proposed tnterceptor Inlet with Additional Outlets
Klotz Associates Project No.: 0127.0W000 City of La Porte
January 2009 City wide Drainage Study
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Item Description
Unit
Total
Quantity
Unit
Price (
'Total
Cost ($)
Land Costs
Land Acquisition (Undeveloped Tract)
ACRE
1
40,000
40,000
Land Acg uistion (Developed)
ACRE
1
145,000
145,000
Channel Costs
Clearing and Grubbing
LF
1
2.50
2.50
lExcavation of Clay with 2.5 CY Back -Hoe and 4 20 CY Dump
Trailers
CY
1
5.70
5.70
Seeding, General, Mechanical Seeding, 2151bs/Acre
ACRE
1
1,000
1,000
Concrete -Lining
SF
l
2.50
2.50
Ri p-rap
CY
1
21
21
Detention Costs
Tical Cost including Excavation, Seedinw, and Outfall
AC -FT
1
25,000
25,000
Culvert Costs
Removal of 24 inch Culvert
LF
1
$ 16
$ 16
Removal of 36 inch Culvert
LF
1
$ 22
$ 22
Removal of 60 inch Culvert
LF
1
$ 30
$ 30
Removal of 10'x8' Box Culvert
LF
1
$ 36
$ 36
Installation of 24 inch Culvert*
LF
l
$ 160
$ 160
Installation of 36 inch Culvert*
LF
1
$ 240
$ 240
Installation of 60 inch Culvert*
LF
l
$ 410
$ 410'
Installation of 10'x8' Box Culvert*
LF
1
$ 845
$ 845
* Installation includes Trench Safety, Pavement Demolition, Pavement Restoration
Excavation, Selected Backfill, and the Culvert.
Klotz Associates Project No. 0127.008.000 City of La Porte
January 2009 City Wide Drainage Study
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ai'
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M
,ar A (IA r - _ AH�� •J rn -� x iw w(( (x c�w� �n +• w
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OI j1Fb Oi�0 0 O O C O j0 O is c (O
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it I s¢ 6
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aj
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p o -a a o 0 0 o g s o c o o o a r a o
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'ju` i
i
z L L ✓ 2 7.r "L <, .r� a { .G .e .f �r �r -'.. ( z 7; 7 7
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I U� I [l� I I fly' ✓� fh f!� ✓� 1F
i
R rLe n a R N� N.. � N R N
_ 0 D O C 1 O :C G!
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JJ o m
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f
n
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7.
� t
tttf r,
{
1
s }
s
c� .
A104-00-00
_
FEMA
Conveyance
56,000
5 7, 000
188,000 60,000
361,000
B166-00-00
FEMA
Conveyance
8,399,000
1,585,000
1,500,000 268,000
t1,752,000
B112-00-00
FEMA
Conveyance
2,461,000
698,000
2,031,000 304,000
5494,000
A104-12-01
non-FEMA
Conveyance
t23,000
_229.000
188,000 604000
600,000_
B106-02-00
non-FEMA
Conveyance
341,000
294,000
125000 21000
781,000
B106-05-000
non-FEMA
Conveyance
729,000
576,000
313,000 83,000
1,701,000
F 10 1 -00-00
non-FEMA,
Conveyance
328,000
511,000
156 000 37,000
1,032,000
F101-05-00
non-FEMA
Conveyance
254,000
703,000
-
_ 957,000_
F101-03-00
non-FFMA
Conveyance
716,000
320,000
313,000 72,000
1,421,000
1'212-00-00
non=FEMA
Conveyance
345,000
639,000
597,000 106,000
1,687;000
F206-01-00
non-FEMA
Conveyance
3 335.000
768M0
347 000 . 67,000
4 517-000
Total Convevance Cost =
30303,000
106-00-0
FEMA
Diversion
7,142,000
1,172 000
8,314,000
�Bl��12-00-00
FEMA
Diversion
9.796,000
1,528,000
11,324,000
A 104 12 Ol
anon-FEMA
Diversion
3,063,000
573,000
3,636.000
IIlfl6-02-00
non=FEMA
Diversion
2,344,000
450,000
2,794,000
B106-05-000Inon-FEMA
Diversion
1,625,000
320,000
1,945,000
F101-00-00
inon-FEMA
Diversion
5,219,000
937,000
6,156AOO
F IOI-03-00
9non-FEMA
Diversion
906,000
186,000
1,092,000
1`101-05-00
non-FEMA
Diversion.
719,000
154,000
873-000
F212 00 00
lnon 1 LMA
Diversion
5,219.000
931,000
61156,000
F206 O1 OO
non l LMA
Diversion
5,219,000
1,257,000
6,4"16,000
Total Diversion Cost =
48,766,000
13106-000
FEMA
Sheet Flow
55,000
85,000
140,,000
B106-02-00
non-FEMA
Sheet Flow
25,000
40,0on
65,000
B106-05-000
non-FEMA
Sheet Flow
14,000
23,000
37,000
B 109.00-00
FEMA
Sheet Flo6
11,000
19,000
30,000
B 1 12-00-00
FLMA
Sheet Flow
36,000
59,000
95,000
FIOI-00-00
non-FL:MA
Sheet Flow
13,000
26,000
39,000
F216.00 00
FEMA
Sheet How
128,000
208,000
336,000
Total Sheer Flow Relief Cost
742,000
A104-00-00
FEMA
Regional Detention
6,782,000
1,592,000
87374.000
A104-07-00
FEMA
Regional Detention
15,938,000
2,440,000
-
18-178,000
B 106-00-00
FEMA
Regional Detention
6,344,000
1,012,000
7,356,000
B109-00-00
FEMA
Regional Detention
1,219,000
308,000
1,527,000
B l 12.00-00
FEMA
Regional Detention
1,28 C,OOO
204,000'
1,485,000
B 112-02-00
FEMA
Regional Detention
500,000
140,000
640,000
F216 00-00
FEMA
Regional Detention
4,156,000
1028,000
-
5,184,000
A104-12-01
non�FEMA
Regional Detention
4,250,000
993,000
B l06 02-00
non-Fi MA
Regional Detention
525,00Q
8$,ODO
613,000
13106-05-00
non-FEMA
Regional Detention
5,019.000
1,012,000
6,031,000
FI OI 00 00
non FEMA
Regional Detention
3,359;000
79,400
3,438,400
FIOI-03-00
non-FEMA
Regional Detention
666,000
178.000
844.000
Ft06-06 00
non-FEMA
Regional Detention
984,000
230,000
1,114,000,
F212-00-00
non-FEMA
Regional Detention
2,603,000
388,000 1
2,991,000
F216-01-OQjnon-FEMA
Re-ional Detention
9 OO(LI
70.000
939000
Total Regional Detention Cost-
64.157.400
Klotz Associates Project N..:0121 008.000 City of La Porte
January 2009 City Wide Drainage Study
Channel
i Solution
Chan nel/Mention Costs
Channel Mitigation, Costs
dotal
Channel
Type
li Type
Construction
rand
Construction
Land
Costs
FENL4
Conveyance
16
16
52
17
100
B106-00-00
FEMA
tC'onveXance
71
13 _
13
2
100
1A104-00-00
11112-00-00
FEMA
Conveyance
45
13
37
6
100
A104-12-01
i non-FEMA
Conveyance
21
38
31
In
100
13106-02-00
non-FEMA
,Conmance
44
38
16
3
t00
B106-05-000 non-FEMA
'Conveyance
43
34
l8
5
100
FI01-00-00
non-FEMA
,Conveyance
32
50
15
4
100
FIOI-05-00
non-FEMA
Conveyance
27
73
L00
FIOI-03-00
non-FENLA
Conveyance
50
23
22
5
100
F212-00-00
non-FEMA
Conveyance
20
38
35
6
100
F206-01-00
non-FEMA
Conveyance
74
17
8
1
too
Conveyance Averages
40
32
23 _
5
zoo
B106 00-00
'F..MA
Diversion
86
14
too
B 1 12-00-00
FEMA
Diversion
87
13
100
A 104-12-01
non-FEMA
Diversion
84
16
I00
B 106-OMO
non-FEMA
' Diversion
84
16
100
€3100-05-000 non-FEMA
Diversion
84
16
100
F101-00-00
non-FEMA
Diversion
85
15
l00
FI01-03-00
non-FEMA
Diversion
83
17
100
FIOI-05-00
non-FEMA
Diversion
82
18
100
F212-00-00
non-MMA
Diversion
85
Is
100
F206-01-00
non-FEMA
Diversion
81
19
100
Diversion .4 verages ='
84
16
0
0
900
13106-000
FEMA
Sheet Flow
39
61
100
8106-02-00
non -I EMA
Sheet Flow
38
62
100
B106-05-000
non-FEMA
Sheet Flow
38
62
1.00
B109-00-00
FEMA
Sheet Flow
37
63
100
B l I2-00 00
FEMA
Sheet Flow
38
62
100
FIOI-00-00
non-FEMA
Sheet Flow
33
67
100
F216-00-00
iFEMA
ISlicet Flow
38
62
--
too
Sheet Flow Averages = .
37
63
fi
®
lOf1
A104-00-00
FEMA
'Regional Detention
81
19
l0U
A104-07-00
FEMA
,Regional Detention
87
13
100
B 106-00-00
FEMA
Regionai Detention
86
14—
l0U
13109-00-00
FEMA
Regional Detention
80
20
100
B 112 00-00
FEMA
Regional Detention
86
14
l0U
B t 12-02-00
FFMA
Regional Detention
78
22
100
1`216-00-00
FEMA
Regional Detention
80
20
100
A104-12-01
non-FEMA
keg-tonalDetention8I
19
- -
100
B 106-02-00
non-FEMA
Regional Detention
86
14_
_
100
B 106-05-00
non-FEMA
eponal Detention
83
17
100
F 101-00-00
non-FEMA
Rgional Detention
98 f
,
100
F 101-03-00
noti-FEMA
Regional Detention
79 f
2l
-
100
F106-06-00
'non-FEMA
Regional Detention
79 1
71
100
F21) 00 00
non FEMA
Regional Detention
87
13
L00
F216-01-00nnn
FEMA
Reional Detention
82
18
100
�. .m
egiontri Detention Averages =
84
16
0
0
2dl
Ktolz Associates Projcot N. o:: 0127.008.000 Ci(y of La Porte
January 21)09 City Wide Drainage Stud¢''
j u i
0 p i CD i a C> (D' 0 0
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TABLE
OWNERSHIP
Clear Creelk !'d
A104-00 00 (Taylor Bayou)
HCFCD
HCFCD
A104-07-00 (Tributary 393 to Taylor Bayou)
HCFCD/City
HCFCD/City
A 104-07-0 I
City
City
A104-10-00 (Bo; gy Gully/Bayou)
HCFCD
HCFCD
A 104-10-02
HCFCD
HCFCD
A 104-12-00
HCFCD
HCFCD
A104-12-01
HCFCD
HCFCD
_
Armand ayou Watershed
B106-00-00 (Bi Island Slough)
HCFCD
HCFCD
B 106-02-00
HCFCD
HCFCD
8106-05-00
HCFCD/City
HCFCD/City
B 106-06-00
City
City
B 109-00-00 (Spring Gully)
HCFCD
HCFCD
B 109-03-00 (B 1 12-02-00 Interconnect)
HCFCD
HCFCD
B112-00-00 (Willow Springs Bayou)
HCFCD
HCFCD
B 112-02-00 ('Tributary 1.78 to Willow Spring Bayou)
HCFCD
HCFCD
B 112-6-66
HCFCD
HCFCD
- San Jane into/Galveston _ ay ._.. -aters e
F101-00-00
HCFCD i
HCFCD
F 101=01-00
City
City
FI01-03-00
City
City
F101-06-00
HCFCD
HCFCD
F 10 1 -06-02
HCFCD
HCFCD
F 10 1 -06-03 _
HCFCD
HCFCD
F 101-07-00
City
City
F 101-08-00
City
City,
F210-00-00
HCFCD
HCFCD
F212-00-00 (Deer Creek)
HCFCD/City
HCFCD/City
F216-00-00 (Little Cedar Bayou)
HCFCD/City
HCFCD/City
F216-01=00
City
City
F216-02-00
City
City
17216-0440
City
City
Klotz Associates Project No. 0127.008.000 City of La Porte
January 2009 City Wide Drainage Pltw
J'Ql2i:006,000 06.O® Work P.duK %CACS Final R®po,tEvhibiix,Frh'rbt PA Fl-d Pia-, m d
J.'JG 127,008.00DOe.00'✓1ork Pf duCsl;W�S Fmw 2 U Gry Itl..Mad ]-m.®e Pr hl—A-A&
ID
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ctl
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(31012I643) a's1243slu
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Im
k I o t z s s o c i a t e s
022molum.
City of La Porte Master Drainage Plan (December 1982, O'Malley & Clay, Inc.)
Master Drainage Plan and Interim Improvement Recommendations for Unit FI01-00-00
(September1987, Landev Engineers, Inc.)
Clear Creek Watershed Regional Control Plan (February 1992, Dannenbaum Engineering Co.)
Hydrologic and Hydraulic Study_for Interconnect of HCFC Unit BI12-02-00 to Unit B109-00-00
and HCFC Regional Detention Site Unit Bj12-01-00 (April 1997, Wilbur Smith
Associates, Consulting Engineers and Planners)
Hydraulic Analysis for Little Cedar Bayou Watershed HCFCD Unit F216-00-00 (January 2000,
Binkley & Barfield, Inc. Consulting Engineers)
Hydrologic and Hydraulic Analysis of proposed Channel Improvements to Fairmont Ditch
(B112=05�00) (January 1989, Dodson & Associates, Inc.)
Master Drainage Plan For The City of La Porte (July 1977, Espey, Huston & Associates, Inc.
Engineering & Environmental consultants)
Master Drainage Plan Hargis County Flood Control District Unit FIOI-00-00 (September 1987,
Landev Engineers, Inc.)
Clear Creek Regional Drainage Plan (July 1989, Dannenbaum Engineering Co.)
Regional Flood Control Plan for Tributaries to Armand Bayou (May 1999, Klotz Associates,
Inc,)
City of La Porte Comprehensive Plan (April 1984, Vernon G. Henry & Associates; Inc.)
Taylor Bayou Watershed Master Drainage Plan; Tax Increment Reinvestment Zone 91 (August
2003, CivilTech Engineering, Inc.)
Appendix A- I
Klotz Associates Project No. 0127.008.000 La forte Citywide Drainage Study
January 2009 City of La Ports:
kfotz associates
Hydraulic Analysis.for Sens Road From 300'Norih of Spencer HPVY to 300'North qf Avenue
! 111r • r- - - s
Consulting Engineers)
Engineering, Inc.)
Preliminary Analysisfior F216-00-00 Linear Detention (Nlovember 2004, Binkley & Barfield,
Inc. Consulting Engineers)
Proposed Fairmont Park -way Improvements Frow #(December
Hydraulic AnIalysisfior Sens Road From 300' Alorth qf Spencer HTYY to 300' North of Avenue
Hydrologic & Hydraulics Analysis Pori Crossing Development (June 2006, Goldston Engineering,
Inc.)
Appendix A=2
Klotz Associates Project No, 0127.008.000 La Porte Citywide Drainage Study
January 2009 City of La Porto
klutz 1.11 a s s o c i a t e s
APPENDIX B
SOURCES FOR FUNDING INFORMATION
Harris County Flood Control District Partnerships: wA-,v.hcfcd.org/partnerships.html
Texas Parks and Wildlife: www.tlwwrd.state tx tis/business/gran ts/
Texas Water Development Board. www.twdb.state.tx.us/assistance/assistance main asp
Governor's Division of Emergency Management: www.txdi}s.state.tx.us/deir/i-,a,Jes/tndex.htm
Federal Emergency Management Agency: Repetitive Flood Claims Grant Program
www.fema. Qvl ®vernment/ rant/rfc/index.shtm
"Financial Assistance." 2005. "Texas Water Development Board: March 14, 2007.
<htti)://www.twdb.state.tx.us/assistance/financial/financial main. >
Colley, Jack.. Lettcr to Emergency Management Colleagues. January 29, 2007. Repetitive Flood
Claims Grant Program Guidance for FY 2007. Emailed to firm 3.1.07.
Colley, Jack.. Letter to Emergency-` Management Colleagues. January 29, 2007. Pre -Disaster
Mitigation (PDM) Grant Program Guidance for FY 2007. Downloaded from website
3.14.07. < http:/Iwww.txdps.state.tx.us/dem/pages/downloadableforms.htm#hmgpgrants>
"Hazard Mitigation Grant Program (HMGP)." August 30, 2005. Texas Division of Emergency
Management. March 14, 2007
<hitl //www_„ixd s,state.,t ,us/de /pages/dovvliloadahief_``orins,h_tm# rrra t#rants>
�Appendix -1
Klotz Associates Project No. 0127.008.000 La Porte Citywide Drainage Study
January 2009 City of La Port
k I o t z 4.11 associates
2001. < ttl- 8fl02,tIc.state.tx us/statutes/]
Appendix B®2
Klotz Associates Project No. 0127.008.000 La Porte Citywide Drainage Study
January 2009 City of La Ports
APPENDIX C
Pond 1
1389368
31.9
159.5
118.8
Pond 2
884527
20.3
101.6
757
Pond 3
136547
3. t
15.7
11.7
Pond 4
776445
1.7.8
89.1
66,4
Pond 5
633733
t4.6
72.8
54.2
Pond 6
314673
7.2
36.1
26.9
Pond 7
55348
1.3
6A
4.7
Pond 8
580338
13.3
66.6
49.6
Pond 9
170270
3,9
19.6
14.6
Pond 10
71651
1.6
8.2
6.1
Pond 11
148457
3.4
17.1
12,7
Pond 12
204209
4.7
23.5
17.5
Pond 13
135920
3.1
15.6
1 t.6
Pond 14
182605
4.2
21.0
15.6
Pond 16
380896
8.7
43.7
32.6
Pond 17
9625848
221.0
1104.9
823.2
Pond 18
1176183.3
270.0
1350.1
100M
Pond 20
4069052
93.4
467.1
348.0
Pond 21
506749
11.6
58.2
43.3
Pond 25
458129
10.5
52.6
39.2
Pond 27
281953
6.5
32,4
24.1
Pond 28
2089535
49.0
239.9
178.7
Pond 29
209962
4.8
24.1
18.0
Pond 30
221175
S.1
25.4
18.9
Pond 3 t
564684
13.0
64.8
49.3
Pond 33
1054877
24.2
121:1
90.2
Pond 34
569349
13.1
65.4
48.7
Pond 38
1344930
30.9
154.4
115.0
Pond39
1928957
44.3
221.4
164.9
Pond 40
787407
18.1
90.4
67.3
Pond 43
9863737
226.4�
_
1 132.2
843.5'
Pond 46
467361
10.7
53.7'
40.0
Pond 48
1477836
33.9
169.7
126.4
Pond 49
27300
0.6
3.2
2.3
Pond 50
123901
2.8
14.2
10,6
Pond 51
129252
3.0
14.9
11.1
Pond 52
37145
0.9
4.3
3.2
Pond.53
346481
8.0
39.81
2906
Pond 54
476617
10.9
54.7
40.8
Pond 55
130723
3.0
15.0
11.2
Pond 56
286803
6.6
32.9
24.5
Pond 57
170004
3.9
19.5
14.5
Pond 61
781202
17.9
89.7
66.8
Pond 62
187784
4.3
21.6
16 1
Pond 63
363486
8.3
41.7'
31.1
Pond 64
372049
8.5
42.7
31.8
Pond 65
350345
8.0
40.2'
29.9
Pond 66
540400
12A
62.1
46.2
Pond 67
1132392
26.0
130.0
96.9
Pond 68
306196
7.0
35.2'
26.2
Klotz Associates Project No.: 0127.008.000
City of La Parts
January 2009
City Wide Drainage Study
APPENDIX
Potential Detention
Identification
Area
Area
Volume square
Volume with SS
Number;
(ft2)
(acre)
(acre-feet)
(acre-feet)
Pond 69
261528
6.0
30.0
22.4
'Pond 70
280733
6.4
32.2
24.0
Pond 71
159129
3.7
18.3
13.6
Pond 72
564068
13.0
64.8
48.2
Pond 73
394306
9.1
45.3
33.7
Pond 74
221538
5.1
25.5
19.0
Pond 75
135911
3.1
15.6
11.6
Pond 76
436912
10.0
50.2
37.4
Pond 77
533094
12.2
61.2
45.6
Pond 78
427067
9.8
49.0
36.5
Pond 79
103915
2.4
12.0
8.9
Pond 80
29140
0.7
3.4
2.5
Pond 81
181381
4.2
20.8
15.5
Pond 82
18516
04
2.2
1.6
Pond 83
123037
2.8
14.1
10.5
Pond 84
30119
0.7
3.5
2.6
Pond 85
103245
2.4
11.9
8.8
Pond 86
161328
3. '1
18.5
13:8
Pond 87
91992
2.1
10.6
7.9
Pond 88
55094
1.3
6.3
4.7
Pond 89
196168
4.5
22.5
16.8
Pond 91
97307
2.2
11.2
8.3
Pond 92
201082
4.6
23.1
17.2
Pond 93
245530
5.6
28.2
21.0
Pond 94
141337
3.2
16.2
121
Pond 95
703239
16.1
80.7
60.1
Klotz Associates Project No.: 0127.008.000 City of La Porte
January 2009 City !Fide Drainage Study
cummm
�-ktorm Sewer Upgrade Identify in Letter Report
Appendix E
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