HomeMy WebLinkAboutR-2009-01 adopt amendments to Public Improvement Criteria Manual (PICM), relating to drainage criteria and improvement standards
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REQUEST FOR CITY COUNCIL AGENDA ITEM
Appropriation
A:end. Da" Req._: Fob?,! 23, r~,' :hI
Requested By: Tim Tietiens 1, J . '-''-, 1 i,tJ/1
Department: Planning
Source of Funds: NIA
Account Number: NIA
Amount Budgeted: NIA
Report: _Resolution: ~ Ordinance: _
Amount Requested: NIA
Exhibits: Resolution vvith Updated Drainage
Criteria Chapter ofPICM.
Previous Drainage Criteria Chapter ofPICM
Budgeted Item: _YES _NO
SUMMARY & RECOMMENDATION
This item was previously discussed at the February 9, 2009 Workshop Meeting. As a result of the
workshop discussion pertaining to the strikethrough version of Chapter 5, Storm Water Design
Criteria, staff recommends council approve a Resolution adopting the previously submitted
updated version of the Storm Water Design chapter to the City's Public Improvement Criteria
Manual.
Action Required of Council:
Consider approval of the Resolution updating Chapter 5, Drainage Criteria, of the Public
Improvement Criteria Manual.
d \ \~ l'OL
Date
RESOLUTION NO. 2009- 0 I
A RESOLUTION ADOPTING A PORTION OF THE "PUBLIC IMPROVEMENT
CRITERIA MANUAL (PICM) CALLED FOR IN ORDINANCE 1444 (THE
DEVELOPMENT ORDINANCE OF THE CITY OF LA PORTE), RELATING TO
DRAINAGE CRITERIA AND IMPROVEMENTS; MAKING CERTAIN FINDINGS
RELATING TO THE SUBJECT; FINDING COMPLIANCE WITH THE OPEN
MEETINGS LAW; PROVIDING A SEVERABILITY CLAUSE; AND PROVIDING AN
EFFECTIVE DATE.
WHEREAS, the City Council has directed the qualified professionals of the City's
staff to update the Public Improvement Criteria Manual, setting forth City Policy and
guidelines relating to the design and construction of certain Public improvements within
the City of La Porte, by virtue of the passage of Ordinance 1444, otherwise known as
the Development Ordinance of the City of La Porte; and
WHEREAS, City Staff has updated certain portions of said policy, relating to
drainage criteria and improvements; and
WHEREAS, the City of La Porte Drainage and Flooding Committee has reviewed
pertinent drainage studies related to the Public Improvement Criteria Manual; and
WHEREAS, the City Council of the City of La Porte has previously reviewed said
policy (in the form attached hereto) at its meeting held on February 9, 2009, pursuant to
due notice, and
WHEREAS, the City Council recommended the drafting of this Resolution to be
approved by City Council at the next meeting
BE IT RESOLVED BY THE CITY COUNCIL OF THE CITY OF LA PORTE:
Section 1. The facts and matters set forth in the preamble of this Resolution
are hereby found to be true and correct.
Section 2. The portion of the PICM relating to drainage criteria and
improvements, as set forth in the attached exhibit, is hereby adopted by the City Council
of the City of La Porte.
Section 3. The City Council officially finds, determines, recites and declares
that a sufficient written notice of the date, hour, place and subject of this meeting of the
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City Council was posted at a place convenient to the public at the City Hall of the City of
La Porte, Texas for the time required by law preceding this meeting, as required by the
Open Meetings Law, Article 6253-17, Texas Revised Civil Statutes Annotated; and that
this meeting has been open to the public as required by law at all times during which
this resolution and the subject matter thereof has been discussed, considered and
formally acted upon. The City Council further ratifies, approves and confirms such
written notice and the contents and posting thereof.
Section 4. This resolution shall be in effect from and after its passage and
approval.
PASSED AND APPROVED this the 1&& day of ~ruMf
,2009.
By:
~.' C_I.TY..O __FLA~E
~e.\~
Alton Porter, Mayor
ATTEST:
~ tUtIJ~ (~
Martha Gillett, City Secretary
APPROVED:
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/ ....,/ .f: w'~ /4l~,>:r'
.' ,/,'-",{J;i".:,
, /' / _,,".,., -, ..' V'. \,
Clark T. Askins, City Attorney
...7/:C /.
, .~ ,.,1/ .. 4
'''_k'''- y _4 /'f' /.. :",
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CHAPTER 5
STORM WATER DESIGN CRITERIA
5.1 GENERAL PROVISIONS
Drainage Criteria for development within the City of La Porte, and where applicable within La Porte's
Ex1raterritorial Jurisdiction (ETJ.) is dependent upon the size and t)'j)e of development, the conditions
\\1thin the individual watershed. the conditions or design of the receiving stream, bayou, channel, roadside
swale, culvert, or roadway.
The basic objective oftms policy is to minimize the threat of flooding to areas within the City and its E.TJ.
and to minimi:r-c the effect of continued development on individual watersheds.
I. The City believes that the best long-teml means of accomplishing its objective is a continued
program of improvement and extension of the Harris County Flood Control Districl's system of
open channels. This statement recognizes the technical reality that an essential prerequisite to an
effective flood control program is a system of open channels capable of carrying storm runoff of
any type in Harris County to Galveston Bay without adversely impacting existing urban areas
aqjacent to the charmels.
2. The City recognizes that both District's and City's existing open channel system is, in many
instances, inadequate to accomplish the goal of eliminating existing flooding conditions for
existing levels of urban development, or for ultimate development in the watershed.
3. The City therefore recommends that where required, certain additional flood control facilities be
utilized to supplement the open channel system. Such flood control facilities shall be designed to
preclude flooding in areas that do not presently flood and not increase flood levels where flooding
now occurs. Specifically, the City supports the use of storm water detention to supplement the
open channel system until long'-term channel improvements can be completed, or as permanent
facilities where additional open channel improvements are not feasible. The result will be that
new development will limit or restrict the impact downstream.
5.1.1 All the drainage plans and construction shall meet or exceed the requirements of the City of La
Porte, Harris County Flood Control, Harris County, TxDOT, or any other entities having
jurisdiction over a facility (i.e. roadway, channel, etc.).
5.1.2 Unless otherwise provided for in these policies, development shall follow the Hams COWlty Flood
Control District Criteria Manual for the design of Flood Control and Drainage Facilities in Harris
County, Texas.
5.1.3 If application of the policies and criteria contained in this docwnent conflict with the City's duties
under tIle Flood Hazard Prevention Ordinance, the regulations of tile Flood Hazard Prevention
Ordinance shall apply.
5.1.4 Drainage structures shall be constructed in such locations and of such size and dimensions to
adequately serve the development and the contributjng drainage area. In new developments, tIle
developer shall provide all the necessary easements and rights-of-way required for dminage
structures, including stonn sewer and open or lined channels.
5.2 CONSTRUCTION PLAN REQUIREMENTS
5.2.] A drainage map shall be included in the construction plans. 111e drainllge area map shall include:
Stonnwater Criteria Page I of JO
A. Drainage areas, including areas draining from off-site onto or adjoining the project
B. Design stOIm runoff. based on the type of facility and stOIm frequency listed in Section
5.4.
C. 100 - year runoff sheet flow pattern.
D. Route of overland flow including the overflow to a drainage channel or detention facility
E. Water surface profiles for the 25-year and/or HIO-year stoImS in the outfall channel. All
available information will be considered when making this determination.
F. Flow per inlet
G. Maximum ponding elevation
H. For bridge designs the loo-year WSEL must be shown on the plans and the low chord of
the bridge must be a minimum of eighteen inches above the 100-year WSEL.
5.2.2 Detailed drainage calculations shall be submitted with the construction plans.
5.2.3 A lot grading plan should demonstmte that the finished gmding plan will drain to approved
collection and discharge points and that the overland flow of water from adjacent properties will
not be impacted. A dedicated easement must be provided at the tenninus to allow the loo-year
event to sheet flow to the ultimate outfall. In no instance will plans be approved without specific
detail as to how the lOO-year event will sheet flow from the development to the outfall without
adversely impacting structures.
5.2.4 TI1C hydraulic gradicnt for the design stonn may be shown on the construction drawings.
Calculations for the elevation for the hydraulic gradient shall be provided with the design stonn
drainage calculations. The hydraulic gradient must be below the gutter line for the design stonn.
The tailwater elevations based on a 25-year frequency in the outfall channel shall be used for
calculations of the hydraulic gradient
If the 25-year WSEL is not available then use 80% of the ultimate channel height. In areas which
are tidally influenced the mean high tide elevation will be used unless it is lower than the 25-year
WSEL, in which case the higher value will be used. In instances where the 25-year WSEL in the
receiving channel is lower than the top of pipe then the top of pipe shall be used as the starting
point for HGL calculations.
5.2.5 The loe-dtion of detention facilities required by these standards shall generally be in the vicinity of
the lowest point of the property being developed. The city engineer has authority to make
exceptions in the event that the runoff is conveyed via underground stonn sewer facilities that
accommodate the 100-year discharge.
5.3 USE OF PREVIOUSLY DESIGNED AND INSTALLED INFRASTRUCTURE
Situations where previously installed infrastructure is in place but not yet utilized to its design
capacity will be considered on a case by case basis. The developers engineer shall after
consultation with the City, prepare a report that
a. outline the original design criteria
b. evaluates the impact of the original design on the receiving stream, adjoining
properties and/or the 100 year Floodplain.
Based on the report, the City Engineer may allow full or partial use of the previously installed
infrastmcture and may require it to be supplemented 'with detention or other facilities.
S.4 STORM FREQUENCY, RUNOFF AND DATUM
A. Stann Frequency
Slollnwaler Criteria Page 2 of) 0
All drainage improvements shall be designed for the following storm frequencies,
n,pe of Facility
Road Side Ditches and Culverts
Storm Sewers
Ditches & Culverts Drainage 100 acres and more
Bridges
Creeks
Detention Facilities
5 year
5 year
2S year
100 year
100 year
Refer to Section S.5
B. Storm Runoff
Design flow of storm water runoff is to be calculated using the Rational Method.
The Rational Method is based on the direct relationship between rainfall and runoff, and
the method is e>'1>ressed by the following equation:
Q = CIA, where
Q is the storm flow at a given point in cubic feet per second (c.[s).
C is a coefficient of runoff (see Table I).
I is the average intensity of rainfall in inches per hour for a period equal to the
time of flow from the farthermost point of the drainage area to the point under
consideration. (See figure 1, I-D-F Curves and Figure 2, Determination of Time
of Concentration)
^ is the drainage area in acres
The size and shape of the watershed must be determined for each installation. The area
of each watershed may be determined through the use of planimetric-topographic maps of
the area, supplemented by field surveys in areas where topographic data has changed or
where the contour interval is insufficient to adequately determine the direction of flow.
The outline of the drainage area contributing to the system being designed and outline of
the sub-drainage area contributing to each inlet point shall be determined.
When calculating the peak flow rate of storm water runoff, rainfall intensity will be
determined from the rainfuJl intensity, duration and frequency curves, shown in Figure 1.
The storm frequency used for this determination \'till be according to the facility to be
designed as listed in Section A.
1. Runoff Coefficients and Time of Concentration
Runoff coefficients, as shown in Table I, sball be the minimum used, based on
total development under existing land zoning regulations. Where land uses
other than those listed in Table I are planned, a coefficient shall be developed
utilizing values comparable to those shown. Larger coefficients may be used if
considered appropriate to the project by the City Engineer.
The time of concentration is defined as the longest time, without unreasonable
delay, that Mil be required for a drop of water to flow from the upper limit of a
drainage area to tIle point of concentration. The time of concentmtion to any
point in a stonn drainage system is a combination of the "inlet time" and the
time of flow in the drain. The inlet tinle is tIle time for water to flow over the
surface of the groood to the stonn drain inlet. Because tlJe area tributary to most
stonn sewer inlets is relatively small, it is customary in practice to determine the
Slonnwakr Criteria Page 3 of 1 0
inlet time on the basis of experience under similar conditions. Inlet time
decreases as the slope and tlle imperviousness of the smface increases, and it
increases as the distanee over which the water has to travel and retention by the
contact surfaces.
Time of concentration shall be computed from Figure 2 and in no case shaH the
inlet time be more that the time shown in Table 1.
Table 1
Zone Zoning District Name Runoff Ma.ximum Inlet
Coefficient "c" Time in Minutes
R-I Low Density Residential 0.50 15
R-2 Mid Density Residential 0.60 15
R-3 High Density Residential 0.80 10
MH Manufactured Housing District 0.55 15
NC Neighborhood Commercial District 0.80 --- 10
GC General Commercial District 0.85 10
Bl Business Industrial Park District 0.85 ]()
--
LI Light Industrial 0.85 10
HI Heavv Industrial 0.85 10
PUD Planned Unit Development District variable 10 to 15
MisceUaneous Land Uses
Land Use
Runoff Coefficient "C"
Church
School
Park
0.70 to 0.90
0.50 to 0.90
0.30 to 0.70
C. Dahilll
All drdinage plans shall be prepared based on North American Vertical DahUn of 19&8,
200 I adjustment, consistent with National Flood Insurance Program, Flood Insurance
Study for the City of La Porte.
5.5 REQUIRED DETENTION
Detention Basins - Unless otherwise provided for in this Section, Detention Basins will be
required for developments within the following watersheds.
Little Deer Creek - F2I2
. Upstream of Main Street
Little Cedar Bayou - F216
. Upstream of State Highway 146
Big Island Slough - B 106
. All segments
Spring Gully - B I 09
. All segments
Willow Spring Bayou - B I 12
Stonnwatcr Criteria Page 4 of 10
. All segments
Taylor Bayou AI04
. All segments
The listed watersheds are shown on Figure 3.
5.5.1 Design Standards for Detention Basins
Detention requirements for developments less than 50 acres shall be according to the following table. For
developed areas of 10 acres or less, the required volume equals the total development area times the
appropriate storage coefficient. For areas greater than 10 acres and less than 50 acres the volume is
determined by applying Harris County Flood Control District criteria for small watersheds.
Table 2 (Also, See Figure 4)
Developed Area
() to I acres
1 to 10 acres
10 to 50 acres
Storage Coefficient
0.20 acre ft. / acre
0.45 acre 1'1. / acre
per HCFCD criteria
For developments which are larger than 50 acres, Harris county Flood Control District and/or the City
Engineer shall approve the detention facility criteria
5.5.2 Outlet Sizing
1. The outlet structure shall be designed using the orifice equation as follows'
Q = CA x (2gh)V,
Where.
C=O.8
A = cross sectional area
g = 32.2 feet / sec^2
h = head differential
For head differential use 2' or the 100-year water surface in pond minus the 25-year
water surface in receiving channel, if available.
2. Minimum restrict or shall be 6" diameter.
5.5.3 Generally accepted areas of localized runofIto roadways and the ditches adjacent to them consists of
150-ft wide strips adjacent to the roadway right-of-way on each side of the road. Drainage from beyond
this commonly accepted width zone is discouraged and should be directed to areas and/or drainage
pathways to which natural drainage occurs. If such drainage cannot be safely directed to natural drainage
pathways, on-site detention shall be required at a rate prescribed in this chapter to limit the peak rates of
discharge to an acceptable level for discharge either to the roadway or natural drainage paths. Such event is
prescribed by the design capacity of said receiving outfall.
5.5.3 Additional Standards for Detention Basins
nle detention facility shall be designed for easy maintenance. For smaBer developments the designer is
encouraged to use parking lots, underground piping, swales, green spaces, etc. to achieve the volume
required.
l>'tormwater Criteria Page 5 of 10
For larger developments every consideration shall be given to designing of the facility for multipurpose
use. such as playgrounds, ballfields, miniparks. required green spaces, etc. to assure that maintenance will
be accomplished. The design shall include the following:
1. an earthen detention basin shall have minimum side slopes of 4: 1 and a minimum bottom width
of ten feet;
2. the bottom ofthe detention basin shall have a minimum 0.50% cross slope to facilitate quick
drainage.
3. a v-shaped trickle channel a minimum of 5' wide, six-inch thick, reinforced concrete shall be
constructed through tile detention basin at a longitudinal slope of 0.20% to accommodate low flow
and facilitate rapid drainage. For developments less than 3 acres, the trickle channellnay be 2 feet
wide and 4" thick.
4. a minimum 12-foot wide maintenance berm shall be provided around the perimeter of the
detention facility.
5. ingress and egress for maintenance including a dedicated right-of-way if required, shall be
provided to tile detention basin and clearly shown on the construction drawings or site plan
subdivision plat.
6. ilie detention basins, slopes, bottom. maintenance berm, and oilier associated right-of-way shall
be final graded with a minimum of 6" top soil then hydro-mulch or drill-seeded and watered to
facilitate full grass coverage.
7. parties responsible for maintenance of the detention facility must be shown on the plat and or
plans.
8. Pumped detention systems may be allowed with specific approval from the City Engineer.
5.5.4 Ownership and Maintenance of Facilities
The City will not accept maintenance of on site facilities that serve only one tract or development, unless it
is determined to have other public benefits. is recommended by staff and approved by the Planning and
Zoning Commission.
Harris County Flood Control District may, at their discretion, accept maintenance of facilities, provided
they are designed in accordance with the District's criteria manual. Requests for Harris County Flood
Control District to assume maintenance of any facility should be coordinated 'l-vith the City prior to any
development approvals.
5.5.5 Plat Review of Facilities
TIle mtes of detention applicable to any development approved and authorized for construction shall be
subject to the detention mte applicable to the size tract ofland as configured on February 23,2009. As part
of the plat review process, the Planning Department shall utilize the configuration as shown in the Harris
County Apprdisal District maps dated, printed and kept on file as of ilie same date described herein. Any
subdivision of land thereafter, wheilier or not by recorded plat, shall be subject to ilie detention mte as
specified for ilie previously undivided tract
5.6 ADDITIONAL DESIGN REQUIREMENTS
5.6.1 Discharge Points
The developer shall terminate all drainage improvements at a discharge point approved by the City. TIle
developer shall design and construct such discharge point, or outlet, to prevent damage to or overflo",ing
into adjacent property. The City may require creek improvement, channel lining, energy dissipaters or
other improvements for such outlet to prevent erosion or increase the flow eapacity. All piped outfalls into
a rt."Ceiving stream shall be at a 450 to the stream if pipe diameter is less than 42-inches and 300 to the
stream is pipe is greater than 42-inches.
Stol1llwatcr Criteria Page 6 of 10
Finished elevations of new pavement pading areas, or other improvements shall be designed so that each
succeeding high point is lower when moving in a downstream direction. This ensures the IOO-year
discharge has an unobstructed path to the discharge point whether discharging to a channel or detention
pond.
5.6.2 Public Streets as Drainage Facilities
I. Maximum depth of water to be allowed in local streets during design flow shall be at the top
crown, or top of curb, whichever is less.
2. MID..;mum spread of water in collector streets during design flow shall allow for one clear lane
of tmffic (12 feet ",ide).
3. Maximum spread of water in arterial streets during design flow shall allow for two clear lanes
of traffic (24 feet wide).
4. TIle street is to be designed to convey the IOO-year event to the outfall point with successively
lower high points. A dedicated easement must be provided at the tenninus to allow the IOO-year
event to sheet flow to tile ultimate outfall. In lieu of sheet flow it is allowable for tile developer to
construct an underground system to convey the IOO-year event.
5.6.3 Drainage Channels and Structures
I. The developer shall install an underground stonn drain on curb and gutter streets beginning at
the point where calculated storm water nllloff is of such quantity that it exceeds the height
specified above. The developer shall construct the stoml drain system from the point to an
approved outlet.
2. For non-curb and gutter strcets, the developer may use open channel (channel or ditch) metllods
to dispose of storm water specified above. Such channels may be in dedicated draining easements
outside the standard street right-of-way upon City approval of the location and alignment of such
easements. Alternatively, the developer may widen the street right-of-way to accommodate an
open channel of greater capacity tI1an ilie standard street/ditch section.
3. Iftl1e developer locates the channcl in a widened street right-of-way, the City shall approve the
right-of-way width and channel configuration. The depili of flow in the channel shall not exceed
one (I) foot as measured from the ditch flowline to the point 011 ilie roadway established as tile
high water level in this section.
4. The developer shaU design and construct aU channels to tenninate at an approved outlet.
5.6.4 Habitable Structures
TIle developer shall provide adequate means for storm water run-off in excess of the ~design stonn"
capacity (i.e., 3. lO-year stoml) to flow aroUfld habitable structures.
a.) TIle developer shall provide a grading/drainage plan which shows that aU building sites can
provide a finished floor elevation:
(1) At least one foot (l ') abovc the top of tile curb using the highest point along the
portion of such curb fronting the building site, or
(2) At least one foot (1 ') above tile crown of ilie road elevation, using tIle highest point
along the portion of such road fronting thc building site.
(3) At least one foot above the grOlmd elevation along aU sides of the building site.
b.) In addition to paragraph (a) above, the developer shall provide a gradillgldminage plan which
meets or exceeds the provisions of Chapter 94, Code of Ordinances, Flood Hazard Reduction.
c.) The developer shall design and construct all streets to minimize any fiU required to bring
building pads into compliance wiili this code.
d.) A1temate methods of building protection may be accepted by the City upon submittal of
dctailed infonnation, review and approval by the City Engineer.
S10nnwaler Criteria Page 7 of 10
5.6.5 Drainage System Criteria
If an underground drainage system is required, and a GO-inch or smaller pipe will handle the
design flow, pipe shall be used. If a 60-inch pipe is not adequate, the developer has the option to
use concrete pipe or natural and/or a lined open drdinage channel. If pipe is selected, the
maximmn allowable velocity shall be Sfps in the pipe. Lining materials, if used, shall be approved
by the City.
5.6.6 Public stonn sewers are defined as sewers and appurtenances that provide drainage for a public
right-of-way, or more than one private tract, and are located in public right-of-way or easement,
private storm sewers provide internal drainage for a reserve or other tract. Private stonn or sewer
connections to public storm sewers shall occur at a manhole or at the back of an inlet as approved
by the City Engineer. All private storm sewers within the public right-of-way shall be constructed
in confomllUlce with the Standards.
5.6.7 All construction shall conform with the City of La Porte Construction Details.
5.6.8 All storm sewers shall meet or exceed the requirements of the "Drainage Criteria Manual for
Harris County. Texas" and the requirements of the City of La Porte.
5.7 LOCATION OF STORM SEWER
5.7.1 Public storm sewers shall be located within a public street right-of-way or storm sewer easement,
dedicated to the public and adjoining a public street right-of-way. Storm sewers through side lot
drainage easements are highly discouraged. Umited use may be approved at the discretion of the
City Engineer. lfapproved, a minimum twenty-foot (20') wide easement is required (10' on each
lot).
5.7.2 Recommended alignment within a public street right-of-way.
A. Boulevard pavement section with median - along centerline of the right-of-way.
B. Undivided pavement section five feet (5') inside tbe right-of-way. For storn1 sewer
located in a public street right-of-way, a minimum of two-foot (2') shall be maintained
inside the right-of-way line to the outside edge of the storm sewer unless othenvise
accompanied by an adjacent easement.
C. Alternate locations for a storm sewer may be permitted by the City Engineer.
5.7.3 Recommended alignment within an exclusive storm sewer easement.
A. Storm sewers placed in easements shall conform to the requirements of Section 2.4.5
B. Storm sewers within easements shall be placed no closer than five feet (5') measured
from the outside edge of the pipe to the edge of an easement. except when aqioining
another easement or public right-of-way where the distance may be reduced to two feet
(2'). TIle storm sewer shall be placed in the center of the easement. When the storm
sewer easement adjoins a public right-of-way, the easement may be reduced to a
minimum often feet (10') and the storm sewer maybe aligned close to the right-of-way
line, as long as required clearances are met with specific approval of the City Engineer,
5.8 STORM SEWER MATERIALS
5.8.1 Storm sewer and culvert pipe shall be precast reinforced concrete pipe, unless specifically
approved by the City Engineer. Concrete pipe sllall be manufactured in conformance with the
Stoffilwater Criteria Page II of 10
requirements of ASTM C 76, "Reinforced Concrete Culvert, Storm Dmin. and Sewer Pipe,"
current revision. Reinforced concrete pipe shall be Cla.'ls III or stronger. The design engineer
shall provide for increased pipe strength when conditions of the proposed installation exceed the
allowable load for Class III pipe. All concrete pipe constructed in water-bearing soil or forty-two
inches (42") in diameter or larger, sbalI have rubber gasket joints meeting the requirements of
ANSI/ASTM C 443, "Joints for Circular Concrete Sewer and Culvert pipe. Using Rubber
Gaskets", current revision. Concrete pipe with diameter of less than forty-two inches (42") may
be installed using pipe with tongue and groove t)'pe joint and Ram-uelt, or approved equal, as a
joint filler. When specifically approved by tbe City Engineer, reinforced concrete arch and
elliptical pipe conforming to ASTM C506 and ASTM C507, respectively, current revision. may be
installed in lieu of circular pipe. Reinforced concrete box culverts shall meet the minimum
requirements of ASTM C789, "Precast Reinforced Concrete Box Sections for Culverts, Storm
Drains, and Sewers", current revision. Pipe joints for arch and elliptical pipe and box culverts
shall be sealed using Ram-nek or approved equal.
5.8.2 Storm sewer outfalls into open channels shall be constructed using corrugated steel pipe.
Corrugated steel pipe sball be manufactured in conformance with the requirements of
AASHTO Designation M-36-82, current revision. Pipe material shall be Aluminized Steel Type
2, meeting the requirements of AASHTO Designation M-27-791, current revision, or Pre-coated
Galvanized Steel, AASHTO M-246, 10 mil coating on both sides. All pipe shall have a full
double coating, Type A, in accordance with AASHTO Designation M-190, current revision. Pipe
joints and fittings shall meet the minimum requirements of these specifications and shall have an
O-ring gasket seal meeting the requirements of AASHTO C-361, current revision. (See City of La
Porte Construction Details).
5.8.3 Stonn sewer outfalls shall have a slope protection to prevent erosion. Slope protection may be
constructed of slope paving or rip rap. Slope paving shall be four-inch (4") five (5) sack concrete
with six-inch by six-inch (6" x 6") welded wire mesh (W14 x W14) or three eighths inch (3/8")
steel rebar on twenty-four-inch (24") centers, each way. Rip rap shall be a minimum of six-inch
(6") broken concrete rubble with no exposed steel or well-roUllded stone and shall be a minimum
of eighteen inches (18") thick. Slope protection te:\turing shall be required where public access
likely. Refer to the Construction Details for minimum dimensions.
5.9 ADDITIONAL REQIJIREMENTS
5.9.1 Minimum depth of storm sewer (measured to the top of pipe) shall be twenty-four (24") below the
top of curb or finished grade, whichever is lower. Minimum size storm sewer for main and
inlet lead shall be twenty-four inches (24").
5.9.2 Storm sewers shall be bedded using cement stabilized sand (see specification in Section 4.2.3) as
shown in the City of La Porte Construction Details.
5.9.3 Pipe requirements.
A. Reinforced concrete pipe installed at a depth greater than thirty feet (30') shall be designed by the
engineer for the specific installation and approved by the City Engineer, Reinforced concrete pipe
shall be designed in accordance with the American Concrete Pipe Association, "Concrete Pipe
Design Manual", Ma.~mum cover on the pipe shall be measured from the top of pipe to the
ultimate fmished grade or natural gr01md, whichever is greater.
B. Corrugated steel pipe shall have a minimum thickness as follows:
PIPE SIZE
(Inches) Corrugations (Inches)
242-2/3" X 112"
)0- 482-2/3" X 1/21
MINIMUM THICKNESS
0.052
0.064
Stonnwaler Criteria Page 9 of 10
54- 723'1 X 1" or 511 X 1"
78- 102311 X 1" or 5" X 1"
0.064
0.079
Bedding for corrugated steel pipe shall be cement-stabilized sand. Corrugated steel pipe less than
or equal to fifty-four inches (54") in diameter and less than thirty feet (30') deep sllall have the
minimum thickness given above.
C. Design storm flow in a street shall not exceed the capacity of the streel for the water surface equal
to the top or curb and shall not exceed the inlet capacity. Design storm flow shall meet Harris
County criteria
D. All bridges must be a minimum of eighteen inches 08") above the LOO-year water surface
elevation or in accordance with the Federal Emergency Management Agency (FEMA)
regulations, latest revisions, or HCFCD requirements, whichever is greater.
5.9.4 Storm sewers less than forty-two inches (42") in diameter shall be constructed on a straight
horizontal and vertical alignment between manholes. Stoml sewers greater than or equal to forty-
two inches (42") in diameter may be laid along a curve using manufactured bends of less than or
equal to 11 Y4 degrees.
5.10 APPURTENANCES
5.10.1 Manholes
A. Manholes shall be placed at all changes in alignment, grade and size of the stonn sewers;
at the intersection of two or more storm sewers; at all inlet leads; and at the end of all
storm sewers.
B. Maximum spacing between manholes shall be four hundred feet( 400 ').
C. Manhole covers shan be cast iron, traffic bearing, type ring and cover with the words
"storm sewer" cast into the cover.
5.10.2 Inlets
A. Curb inlets shall be spaced and sized to intercept the calculated runoff for the design
storm. The water surface elevation at the inlet shall be less than or equal to the gutter for
the design stoml flow.
B. Maximum travel distance of water in the street to a curb inlet shall be three hundred feet
(300') on a major thoroughfare and in a commercial area The maximum travel distance
of water in the street pennitted in a single-family residential area shall be three hundred
feet (300 ').
C. No Valley Gutter without prior approval.
D. Curb inlets should be on the intersecting side street at intersections with a major
thoroughfare. The City Enginecr shall specifically approve locations at intersections.
E. Grated inlets will not be permitted in an open ditch.
F. Backslope swale interceptors shall be placed in accordance with the requirements of
Harris County.
G. Curb inlets shall have solid inlet lids. Grate or Curb and Grate inlets shall not be allowed
for residential subdivisions. Curb inlets shall be recessed. unless otherwise directed by
tlle City Engineer.
H. Backfill around inlets and to top of first stage inlet with cement stabilized sand.
Stonnwaler Criteria Page 10 of 10