HomeMy WebLinkAboutR-1999-31
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RESOLUTION NO. qq_ ~,
A RESOLUTION ADOPTING A PORTION OF THE "PUBLIC IMPROVEMENT
CRITERIA MANUAL ("PICM") CALLED FOR IN ORDINANCE 1444 OF THE CITY
OF LA PORTE (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 prepare a 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 developed a portion of said policy, relating to drainage
criteria and improvements; and
WHEREAS, the Planning Commission of the City of La Porte has reviewed said
policy (in the form attached hereto as Exhibit "A"), and has received public comment on
said policy at its meeting held on August 19, 1999, pursuant to due notice, and
WHEREAS, the Planning Commission recommends approval of said Policy, in
the form attached hereto as Exhibit "A";
NOW, THEREFORE, BE IT RESOLVED BY THE CITY COUNCIL OF THE CITY OF
LA PORTE, TEXAS:
Section 1. The facts and matters set forth in the preamble of this Resolution
are hereby found to be true and correct.
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Section 2. The portion of the PICM relating to drainage criteria and
improvements, as set forth on Exhibit "A" attached hereto, 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
City Council is posted at a place convenient to the public at the City Hall of the city for
the time required by law preceding this meeting, as required by the Chapter 551, Tx.
Gov't Code; and that this meeting has been open to the public as required by law at all
times during which this ordinance 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. In the event any clause, phrase, provision, sentence, or part of this
Resolution or the application of the same to any person or circumstance shall for any
reason be adjudged invalid or held unconstitutional by a court of competent jurisdiction,
it shall not affect, impair, or invalidate this ordinance as a whole or any part or provision
hereof other than the part declared to be invalid or unconstitutional; and the City
Council of the City of La Porte, Texas, declares that it would have passed each and
every part of the same notwithstanding the omission of any such part thus declared to
be invalid or unconstitutional, whether there be one or more parts.
Section 5. This Resolution shall become effective after its passage and
approval.
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PASSED, APPROVED, AND RESOLVED this XlfLday of 5e.f'r, 1999.
CITY OF LA PORTE
BY:~~__
, N RMAN L. MALO ,MAYOR
ATTEST:
APPROVED:
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EXHIBIT A
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5.1 GENERAL PROVISIONS
Drainage Criteria for development within the City of La Porte, and where applicable within La Porte's
Extraterritorial Jurisdiction (E.T.J.) is dependent upon the size and type of development, the conditions
within the individual watershed, the conditions or design of the receiving stream, bayou, channel,
roadside swale, culvert, or roadway.
The basic objective of this policy is to minimize the threat of flooding to areas within the City and its
E. T.J. and to minimize the effect of continued development on individual watersheds.
1. The City believes that the best long-term means of accomplishing its objective is a
continued program of improvement and extension of the Harris County Flood Control
District'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 adjacent to the channels.
2. The City recognizes that both the 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 Harris
County 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 document conflict with the
City's duties under the Flood Hazard Prevention Ordinance, the regulations of the 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 contributing drainage area. In
new developments, the developer shall provide all the necessary easements and rights-
of-way required for drainage structures, including storm sewer and open or lined
channels.
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5.1 CONSTRUCTION PLAN REQUIREMENTS
5.2.1 A drainage map shall be included in the construction plans. The drainage area map shall
include:
A. Drainage areas, including areas draining from off-site onto or adjoining the project.
B. Design storm runoff, based on the type of facility and storm frequency listed in Section
5.4.
C. 100 - year runoff
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 100-year storms in the outfall channel. All
available information will be considered when making this determination.
F. Flow per inlet.
G. Maximum ponding elevation
5.2.2 Detailed drainage calculations shall be submitted with the construction plans.
5.2.3 A lot grading plan should demonstrate that the finished grading plan will drain to approved
collection and discharge points and that the overland flow of water from adjacent properties
will not be impeded.
5.2.4 The hydraulic gradient for the design storm may be shown on the construction drawings.
Calculations for the elevation of the hydraulic gradient shall be provided with the design storm
drainage calculations. The hydraulic gradient must be below the gutter line for the design
storm. The tailwater elevations based on a 25-year frequency in the outfall channel shall be
used for calculations of the hydraulic gradient
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
infrastructure and may require it to be supplemented with detention or other facilities.
Stormwater Criteria 2 of 12
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5.4 STORM FREQUENCY, RUNOFF AND DATUM
A. Storm Frequency
All drainage improvements shall be designed for the following storm frequencies.
Type of Facility
Road Side Ditches and Culverts
Storm Sewers
Ditches & Culverts Draining 100 acres and more
Bridges
Creeks
Detention Facilities
3 year
3 year
25 year
100 year
100 year
Refer to Section 5.5
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 expressed by the following equation:
Q = CIA, where
Q is the storm flow at a given point in cubic feet per second (c.f.s.).
C is a coefficient of runoff (see Table 1).
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, Detenninination of
Time of Concentration)
A 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 rainfall intensity, duration and frequency curves, shown in Figure
1. The storm frequency used for this determination will be according to the facility to
be designed as listed in Section A.
1. Runoff Coefficients and Time of Concentration
Stonnwater Criteria 3 of 12
Zone
R-1
R-2
R-3
MH
NC
GC
BI
LI
m
PUD
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Runoff coefficients, as shown in Table I, shall 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 will be required for a drop of water to flow from the upper limit of a drainage area
to the point of concentration. The time of concentration to any point in a storm
drainage system is a combination of the "inlet time" and the time of flow in the drain.
The inlet time is the time for water to flow over the surface of the ground to the storm
drain inlet. Because the area tributary to most storm sewer inlets is relatively small, it is
customary in practice to determine the inlet time on the basis of experience under
similar conditions. Inlet time decreases as the slope and the imperviousness of the
surface increase, and it increases as the distance 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 shall the inlet
time be more than the time shown in Table I.
Table I
Zoning District Name
Low Density Residential
Mid Density Residential
High Density Residential
Manufactured Housing District
Neighborhood Commercial District
General Commercial District
Business Industrial Park District
Light Industrial
Heavy Industrial
Planned Unit Development District
Runoff
Coefficient
"C"
0.50
0.60
0.80
0.55
0.80
0.85
0.85
0.85
0.85
variable
Maximum Inlet
Time in
Minutes
IS
IS
10
IS
10
10
10
10
10
10 to 15
Miscellaneous 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
Stormwater Criteria 4 of 12
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C. Datum
All drainage plans shall be prepared based on United States Geodetic Survey datum,
1978 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 - "F212
. Upstream of Main Street
Little Cedar Bayou - F216
. Upstream of State Highway 146
Big Island Slough - B 1 06
. All segments
Spring Gully - BI09
. All Segments
Willow Spring Bayou - B 112
· 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
detennined by applying Harris County Flood Control District criteria for small watersheds.
Table 2 (Also, See Figure 4)
Developed Area
o to 3 acres
3 to 10 acres
10 to 50 acres
Storage Coefficient
0.20 acre ft./acre
0.45 acre ft./acre
per HCFCD criteria
Stonnwater Criteria 5 of 12
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For developments, which are larger than 50 acres, Harris County Flood Control District and the City
Engineer shall approve the detention facility criteria.
5.5.2 {}utlet Sizing
1. The outlet structure shall be designed using the orifice equation as follows:
Q = CA X (2ghi12
Where,
C=0.8
A = cross sectional area
g = 32.2 feet / sec2
h = head differential
For head differential use 2' or the 100-year water swface in pond minus the 25-year water
surface in receiving channel, if available.
2. Minimum restrictor shall be 6" diameter.
5.5.3 Additional Standards For Detention Basins
The detention facility shall be designed for easy maintenance. For smaller developments the
designer is encouraged to use parking lots, underground piping, swales, greenspaces, etc. to
achieve the volume required.
For larger developments every consideration shall be given to designing of the facility for
multipurpose use, such as playgrounds, ballfields, miniparks, required greenspace, etc., to assure that
maintenance will be accomplished. The design shall include the following:
I. an earthen detention basin shall have minimum side slopes of 4:1 and a minimum bottom width
of ten feet;
2. the bottom of the 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 the 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 channel may
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 the detention basin and clearly shown on the construction drawings or site plan and
subdivision plat.
Stonnwater Criteria 6 of 12
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6. the detention basin, slopes, bottom, maintenance benn, and other associated right-of-way shall
be final graded with a minimum of 6" top soil then hydro-mulch seeded 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 with the
City prior to any development approvals.
5.6 ADDITIONAL DESIGN REQUIRMENTS
5.6.1 Discharge Points
The developer shall terminate all drainage improvements at a discharge point approved by the City.
The developer shall design and construct such discharge point, or outlet, to prevent damage to or
overflowing 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 capacity.
Finished elevations of new pavement, parking areas, or other improvements shall be designed so that
each succeeding high point is lower when moving in a downstream direction. This ensures the 100-
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
1. Maximum depth of water to be allowed in local streets during design flow shall be at the
top of crown, or top of curb, whichever is least.
2. Maximum spread of water in collector streets during design flow shall allow for one clear
lane of traffic (12 feet wide).
3. Maximum spread of water in arterial streets during design flow shall allow for two clear
lanes of traffic (24 feet wide).
5.6.3 Drainage Channels and Structures
1. The developer shall install an underground stonn drain on curb and gutter streets beginning
at the point where the calculated stonn water runoff is of such a quantity that it exceeds the
Stonnwater Criteria 7 of 12
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height specified above. The developer shall construct the storm drain system from this
point to an approved outlet.
2. For non-curb and gutter streets, the developer may use open channel (channel or ditch)
methods to dispose of storm water specified above. Such channels may be in dedicated
drainage 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 than the standard
street/ditch section.
3. If the developer locates the channel in a widened street right-of-way, the City shall approve
the right-of-way width and channel configuration. The depth of flow in the channel shall
not exceed one (1) foot as measured from the ditch flowline to the point on the roadway
established as the high water level in this section.
4. The developer shall design and construct all channels to terminate at an approved outlet.
5.6.4 Habitable Structures
The developer shall provide adequate means for storm water run-offin excess of the "design storm"
capacity (i.e., 3, IO-year storm) to flow around habitable structures.
a.) The developer shall provide a grading/drainage plan which shows that all building sites can
provide a finished floor elevation:
(1) At least one foot (1') above the top of the curb using the highest point along the
portion of such curb fronting the building site, or
(2) At least eighteen inches (18") above the crown of road elevation, using the highest
point along the portion of such road fronting the building site.
(3) At least one foot above the ground elevation along all sides of the building site.
b.) In addition to paragraph (a) above, the developer shall provide a grading/drainage 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 fill required to bring
building pads into compliance with this Code.
d.) Alternate methods of building protection may be accepted by the City upon submittal of
detailed information, review and approval by the City Engineer.
5.6.5 Drainage System Criteria
If an underground drainage system is required, and a 60-inch or smaller pipe will handle the
design slow, 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 drainage channel. If pipe is selected, the
maximum allowable velocity shall be 8 fps in the pipe. Lining materials, if used, shall be
approved by the City.
Stonnwater Criteria 8 of 12
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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 stonn sewers provide internal drainage for a reserve or other tract. Private stonn sewer
connections to public stonn sewers shall occur at a manhole or at the back of an inlet as
approved by the City Engineer. All private stonn sewers within the public right-of-way shall
be constructed in confonnance with the Standards.
5.6.7 All construction shall confonn with the City of La Porte Construction Details.
5.6.8 All stonn 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 stonn sewers shall be located within a public street right-of-way or a stonn sewer
easement, dedicated to the public and adjoining a public street right-of-way. Stonn sewers
through side lot drainage easements are highly discouraged. Limited use may be approved at
the discretion of the City Engineer. If approved, 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 the right-of-way. For all stonn 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 stonn sewer unless otherwise
accompanied by an adjacent easement.
C. Alternate locations for a stonn sewer will be pennitted by the City Engineer.
5.7.3 Recommend alignment within an exclusive stonn sewer easement.
A. Stonn sewers placed in easements shall confonn to the requirements of Section 2.4.5
B. Stonn 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 adjoining
another easement or public right-of-way where the distance may be reduced to two feet
(2'). The stonn sewer shall be placed in the center of the easement. When the stonn
sewer easement adjoins a public right-of-way, the easement may be reduced to a
minimum often feet (10') and the stonn sewer may be aligned close to the right-of-way
line, as long as required clearances are met, with specific approval of the City Engineer.
Stonnwater Criteria 9 of 12
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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 shall be manufactured in conformance with the
requirements of ASTM C 76, "Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe",
current revision. Reinforced concrete pipe shall be Class ill or stronger. The design engineer
shall provide for increased pipe strength when conditions of the proposed installation exceed
the allowable load for Class ill pipe. All concrete pipe constructed in water-bearing soil or
forty-tow inches (42") in diameter or larger, shall 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 a diameter of less than forty-two
inches (42") may be installed using pipe with tongue and groove type joint and Ram-nek, or
approved equal, as a joint filler. When specifically approved by the City Engineer, reinforced
concrete arch and elliptical pipe conforming to ASTM C506 and 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.
Conugated steel pipe shall 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-274-79I, 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 the City of La Porte Construction Details).
5.8.3 Storm sewer outfalls shall have 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-rounded stone
and shall be a minimum of eighteen inches (18") thick. Slope protection texturing shall be
required where public access likely. Refer to the Construction Details for minimum
dimensions.
5.9 ADDITIONAL REQUIREMENTS
5.9.1 Minimum depth of a storm sewer (measured to the top of pipe) shall be twenty-four inches
(24") below the top of curb or finished grade, whichever is lower. Minimum size storm sewer
for main and inlet lead shall be eighteen inch (18").
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.
Stonnwater Criteria 10 of 12
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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", Maximum cover on the pipe shall be measured
from the top of pipe to the ultimate finished grade or natural ground, whichever is greater.
C. Corrugated steel pipe shall have a minimum thickness as follows:
PIPE SIZE
(Inches) Corrugations (Inches
242-2/3" X 1/2"
30- 482-2/3" X 1/21
54- 723 't XI" or 511 Xl"
78- 102311 X 1" or 5" X 1"
MINIMUM nnCKNESS
0.052
0.064
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 shall have the minimum thickness given above.
D. Design storm flow in a street shall not exceed the capacity of the street, for the water
surface equal to the top of curb and shall not exceed the inlet capacity. Design storm
flow shall meet Harris County criteria.
E. All bridges must be a minimum of eighteen inches (18") above the toO-year water
surface elevation or in accordance with the Federal Emergency Management Agency
(F. E. M. A.) 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. Storm sewers greater than or equal to
forty-two inches (42") in diameter may be laid along a curve using manufactured bends ofless
than or equal to 11 1/4'.
5.10 APPURTANCES
5.10.1 Manholes
A. Manholes shall be placed at all changes in alignment, grade and size of the storm
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 six hundred feet (600')
C. Manhole covers shall be cast iron, traffic bearing, type ring and cover with the words
"storm sewer" cast into the cover.
Stormwater Criteria 11 of 12
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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 top of
curb for the design storm flow.
B. Maximum travel distance of water in the street to a curb inlet shall be three hundred feet
(3001) on a major thoroughfare and in a commercial area. The maximum travel distance
of water in the street permitted in a single-family residential area shall be four hundred
feet (400').
C. No Valley Gutter without prior approval.
D. Curb inlets should be located on the intersecting side street at intersections with a major
thoroughfare. The City Engineer 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 the City Engineer.
H. Backfill around inlets and to top of first stage inlet with cement stabilized sand.
Stormwater Criteria 12 of 12
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City of La Porte
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Intensity vs. Time of Concentration vs. Rainfall Frequency
(IDF Curves)
Source: Hydro 35ffP-40
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100
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, :::::: ;: ...... .... ... ...
, ...... .... """ ...
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......
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1
1
10
Time of Concentration, TC (minutes)
Intensity,i= b
(d+ TC)9
Rainfall Frequency
2-year
3-year
5-year
10-year
25-year
100-year
b
75.01
77.27
84.14
93.53
115.9
125.4
d
16.2
17.1
17.8
18.9
21.2
21.8
/100-year
/5-year
.,......,1 O-year .
-5-year
.............3-year
" 2-year
100
FIGURE 1
e
0.8315
0.8075
0.7881
0.7742
0.7808
0.7500
. .
.... ..... ......
. ,':.
. 00:,"
.
-
c:
z~
~g ~6
&&. u-'
50 G'=&&"
L4.;:: 90
<: &...w
~~I <0 l-4
~ l:J Of
3~H' ~.
&&. h.1-- 0.01
.bl 30" 3"""
o CL . ". .'
~ ULc ?VMT.~ ,
<v. 0.02 .
..J .,.,1 .
::: ~Llr - 2S 2.S
~~r-o.e3 Ill,
Oc.,t- II
:)r--C.o~ ...
o~~~_ Z
....l.;, ~ I
ul
-: 3~O' ~~. ~Q'o ~l- " ./ ~.t-.
.tf " :t t:~~~ F':C<EO SOIL) ~ f= ,// I r=
~ ~t'''''Q;'2 ILlEr 0,..0:.0 ~
u..... c../....
~ ~ 200 en ......F 0.20 " g~ ",.00 ~
~f- - ~ - " .. " /U) . 2
-- ~._ 0.0,). ~~.30l~R G~~SSJ .7 ~ ~ ~
~ 150 gJ.-ciO';-g:'.Q.~O(A~.c~:'sSJ .,,/ ~J=
I/; c L . o~v- _ " AI' c:: E" 5.0 ~
o :ro.06 E' . -~~~' ~~ 10.0 ~
l.:.I o.eo (CE~Si:: GRASSi ~
100 '" 20.0 2: 10. 1.0
90 2! rl.o:; . , t-
2' 00 .
eo ::/..1 ~. ~
70 . ~i ~l=-s
60 '-! L
ij ~ 7
. ". I ·
I
e
-
.
ffi::
.
'L~
see
aGO
~cl
IS
1..5
I
0.9
as .
0.7
l'"Ex /\.\0: ,":" 2:':
O'/E;::!..A~~O FLOW
L = 200. .
f" = 0.40 (;"./. G~}\SS;
:. = J.t.) ~.;
I r = 20 ~,::~!.
I "'0"'''' ......- 0- CQ:\.CI:"........ .....'C... =
<t.' . ""':'., .I:'.-=. :- ~..._:~.n___lt:.
I ? n. ~., 0 r/;I~'
-.\.: - ..-. Ii If.
GUTTER FLOW
L = ~oo' .
n = 0.;:'.75
s = to 0/.:.
f = 2.0 t.~!~.
0.6
30
I
J
5
0.5
20
4
0.4.
::
2 G.O +
3
0.3
:
FIGURE 2
TIME OF CONCENTRATION FOR SURFACE FLOW
---------
---..---
,.
I
MAJOR LA PORTE WATERSHEDS
~
~
N.T.S.
WATERSHED BOUNDARY
CJ
~ r
..
I ~
~
t
n == VESTIl/I
1 J BAY
H
r
,---- -
-
I
~
I
I
~
,
I
\
~
~~
L
l
~
"'TI
......
Gl
c:
~
m
w
0.50
,.-..
~
p::
u
<
..........
E-o
~ 0.40
~
~
I
~
p::
u
<
'-'
E-o rn 0.30
Z u
~
......
u
......
~
~
~
0
u
~ 0.20
~
<
p::
0
E-<
{I'J
0.10
CURVE FOR DETERMINING
STORAGE COEFFICIENT
0.60
-
-
- - - -.
-
I I
tt - .'f"" - .. . .- ... . . .. - -.
-. +. - .+
. -I- -- -/.-. .0. J - + .. -. -
I I I
- - ! -.
I :j::
.
I I I
I I I I I
!
I -.
-
I .
I
- - -
- .- -
i
.+' -
- I - -H-- -
I I I - I
I I ! --~. I
.--~. +.. -f.. _.t-. ,-' - .- .+- .t- .-.
.. .-t-- .. tt- --I-' .- -- ..+ I -
I I I
I I !
. - - .. - . - -. -.
I - I ..
I ! I
-
I -
. - I
I
I I
- t - .. -+ r' - -
. - -+ - .. -. .. -. ... - . -. . ..t-
- :it I I
.. .- - . . t.tr
- - --
j I
I I I I I
I I . I - - - .+
I I I
o
20
30
40
50
3
10
DRAINAGE AREA (ACRES)
S = CsA
FIGURE 4