This division establishes policies governing storm drainage
facilities within the city, in order to protect the general health,
safety and welfare of the public by reducing flooding potential, controlling
excessive runoff, minimizing erosion and siltation problems, and eliminating
damage to public facilities resulting from uncontrolled stormwater
runoff.
(2005 Code, sec. 16.1.61)
(a) Preliminary plan and study.
(1) For any property involved in the platting process, the owner shall
provide, at his expense, a preliminary drainage study of the area
proposed for development.
(2) The preliminary drainage study shall be submitted concurrently with
the submittal of a preliminary plat, or plat revision for review and
approval.
(3) The studies shall be prepared by a civil engineer licensed to practice
in the State of Texas and experienced in city drainage work. The studies
shall include the date, seal and signature of the engineer responsible
for the plan.
(4) The study shall include the following:
(A) Existing topography shall be shown by contour lines on a basis of
five feet (5') vertical interval unless the shape of the terrain,
in the opinion of the city engineer or his authorized designee, warrants
two-foot (2') vertical intervals; datum shall be that of the United
States Coast and Geodetic Survey.
(B) Existing and proposed drainage facilities both on-site and on adjacent
affected properties.
(C) Proposed contours plus flow arrows for each lot.
(D) The scale shall not be smaller than one inch (1") = two hundred feet
(200') with contour intervals not greater than five feet (5') unless
a variation is specifically approved by the city engineer.
(E) Sufficient design calculations showing preliminary sizes of drainage
facilities and easement sizes and locations.
(b) Final plans.
The owner shall, at his sole expense, provide
complete final plans and specifications for the drainage facilities,
as follows:
(1) The plans and specifications shall be prepared by a Texas licensed civil engineer, meeting the qualifications described in subsection
(a)(3) of this section.
(2) The plans and specifications shall be submitted to the city engineer
for review and concurrence prior to any construction.
(3) The owner and his engineer shall be responsible for the accuracy
of the information furnished in the design of the storm drainage facilities
as it pertains to both the development in question and properties
adjacent to the development. Concurrence in the design by the city
shall not be construed to relieve the owner or his engineer of any
responsibility referred to herein.
(2005 Code, sec. 16.1.62)
(a) The three different types of basic drainage features are as follows:
(2) Reinforced concrete-lined open channels.
(b) Stormwater runoff shall be carried in stormwater pipe when either
of the following applies:
(1) The runoff can be carried in a pipe of sixty inches (60") in diameter
or smaller; or
(2) Where it is necessary for the protection of adjacent facilities that
the stormwater be carried in an enclosed facility.
Headwalls shall be constructed at the outfall of all storm sewer
systems. When it is mutually agreeable to both the city council and
the owner, a concrete-lined channel may be used in lieu of a closed
pipe.
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(c) Reinforced concrete-lined open channels should be used when the criteria outlined in subsection
(b)(1) of this section is exceeded:
(1) Reinforced concrete-lined channels shall conform to the following:
(A) Channels draining an area with a “CA” factor of 250 or
less shall be lined with reinforced concrete in a manner which will
contain the design frequency storm plus one foot (1') of freeboard
within the concrete lining.
(B) Channels draining an area with a “CA” factor of more
than 250 but less than 500 shall be concrete lined to contain the
runoff from a five (5) year return frequency storm with the balance
of the required design frequency storm contained within grassed slopes
not steeper than three (3) horizontal to one (1) vertical and with
a minimum of one foot (1') freeboard.
(C) Channels draining an area with a “CA” factor of more
than 500 but less than 2,000 shall be constructed with a reinforced
concrete pilot channel not less than twelve feet (12') in width and
having at least six-inch (6") curbs, [with] appropriate edge protection
as accepted by the city engineer. The remainder of the channel shall
consist of earthen side slopes with proper vegetative cover on slopes
not steeper than 3 to 1.
(D) Channels draining an area having a “CA” factor of more than 2,000 shall be governed by the criteria outlined in subsection
(d) of this section.
(2) The maximum permissible average velocities in channels shall conform
to the velocities as shown in table 10.02.273, unless otherwise approved
in writing by the city council, on the recommendation of the city
engineer.
TABLE 10.02.273. MINIMUM ROUGHNESS COEFFICIENTS AND MAXIMUM
PERMISSIBLE VELOCITIES FOR CHANNELS
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Channel Description
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Coefficient of Roughness
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Max. Mean
(n)
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Permissible Velocity
(fps)
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I.
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Minor natural streams
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A.
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Fairly regular section
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1.
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Some grass and weeds; little or no brush
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0.030
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3 to 8
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2.
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Dense growth of weeds, depth of flow materially greater than
weed height
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0.035
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3 to 8
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3.
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Some weeds, light brush on banks
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0.035
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3 to 8
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4.
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Some weeds, heavy brush on banks
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0.050
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3 to 8
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5.
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Some weeds, dense willows on banks
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0.060
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3 to 8
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6.
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For trees within channels with branches submerged at high stage,
increase all values above by
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0.010
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B.
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Irregular section with pools, slight channel meander, use 1A
to 5A above, and increase all values by
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0.010
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C.
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Floodplain - pasture
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1. Short grass
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0.030
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3 to 8
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2. Tall grass
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0.035
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3 to 8
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D.
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Floodplain - cultivated areas
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1. No crop
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0.030
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3 to 8
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2. Mature row crops
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0.035
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3 to 8
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3. Mature field crops
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0.040
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3 to 8
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E.
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Floodplain - uncleared
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1. Heavy weeds, scattered brush
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0.050
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3 to 8
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2. Wooded
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0.120
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3 to 8
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Channel Description
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Coefficient of Roughness
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Max. Mean
(n)
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Permissible Velocity
(fps)
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II.
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Major natural streams. Roughness coefficient is
usually less than for minor streams of similar description on account
of less effective resistance offered by irregular banks or vegetation
of banks. Values of “n” for larger streams of mostly regular
sections, with no boulders or brush may be in the range from
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0.028 to 0.033
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3 to 8
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III.
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Unlined vegetated channels
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Clays (Bermuda grass)
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0.035
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5 to 8
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Sandy and silty soils (Bermuda grass)
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0.035
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3 to 5
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IV.
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Unlined non-vegetated channels
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Sandy soils
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0.030
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1.5 to 2.5
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Silts
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0.030
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0.7 to 1.5
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Sandy silts
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0.030
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2.5 to 3.0
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Clays
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0.030
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3.0 to 5.0
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Coarse gravels
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0.030
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5.0 to 8.0
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Shale
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0.030
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6.0 to 10.0
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Rock
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0.025
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15
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V.
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Lined channels
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Neat concrete
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0.015
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15
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Riprap (broken concrete and rubble)
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0.030
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15
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(d) Natural channels may be preserved when the criteria in subsection
(c)(1)(D) of this section are met or when the developer desires to preserve natural channels within his addition for purposes of aesthetics and/or open space. A developer proposing preservation of a natural channel shall submit an application for city council approval, after report and recommendation of the city engineer, prior to approval of the preliminary plat. This application shall contain the following information:
(1) Topographic, hydrologic and hydraulic information sufficient to properly evaluate the proposal and showing that: (i) before any site work has begun, all land having an elevation below the twenty-five (25) year return frequency flood [is] contained within an easement dedicated to the public of [for] the purpose of providing drainage; (ii) the channel easement has a minimum hydraulic capacity to accommodate a twenty-five (25) year return frequency storm based on a fully developed watershed; (iii) that all minor channel improvements, such as reshaping, realignment, etc., are protected with sodding, backsloping, cribbing, or other bank protection that is designed and constructed to control erosion from the twenty-five (25) year return frequency flood. An analysis shall also be made to determine the effects of the 100-year flood as required by section
10.02.274(g)(3).
(2) When the natural channel to be preserved is one which has had a floodplain information report prepared on to [sic] by the Corps of Engineers, the 100-year return frequency storm shall be as shown in that particular report unless otherwise directed by the city engineer. All requirements contained in article
3.06 of this code concerning hazards shall be applicable in addition to the requirements outlined above.
(2005 Code, sec. 16.1.63)
(a) The purpose of this section is to establish standard criteria, principles,
procedures and practices for the design of the storm drainage facilities.
(1) The design factors, formulas, graphs and procedures presented or
referred to herein are intended for use as engineering guides in the
design of drainage facilities and in the solution of drainage problems
involving the quantity, method of collection, transportation and disposal
of stormwater.
(2) Methods of design other than those indicated or referred to herein
may be considered in complex and difficult cases where experience
clearly indicates they are preferable; however, these deviations in
design methods shall not be attempted until approval thereof has been
obtained from the city engineer.
(3) The methods outlined or referred to herein include accepted principles
of surface drainage engineering and should be a working supplement
to basic design information obtainable from textbooks and publications
on drainage.
(b) Determination of design discharge.
The two (2) methods
acceptable for use are:
(1) Modified Rational method (Q = CCa IA) to be used on small watersheds
of 1,000 acres or less. Ca values to be used in this formula are shown
in table 10.02.274-1.
TABLE 10.02.274-1. ANTECEDENT PRECIPITATION FACTOR “Ca”
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Recurrence Interval
(Years)
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Ca
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2 to 10
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1.0
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25
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1.1
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50
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1.2
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100
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1.25
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Note: The product of C x Ca should not exceed 1.0.
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(2) Unit hydrograph techniques shall be used for areas greater than 1,000
acres. The technique and the data to be used for the determination
of [the design discharge shall be submitted to] the city engineer
or his authorized designee prior to the calculations being completed.
A complete set of all detail calculations must be submitted to the
city engineer for approval prior to completion of the plans for the
drainage system.
(c) Drainage area.
(1) Area of the watershed shall be determined using the information required by section
10.02.272(a).
(2) Outline of drainage areas must follow natural drainage features in
nonurbanized areas. Consideration shall be given to man-made features
in urbanized areas.
(d) Runoff coefficients.
(1) Storm drainage shall be designed for ultimate development of the
watershed and, therefore, runoff coefficients used shall consider
these fully developed conditions. Master plans, zoning maps and land
use plans shall be used to determine the ultimate development.
(2) Table 10.02.274-2 shall [shows] the values for runoff coefficients
which shall be used in the determination of stormwater runoff.
TABLE 10.02.274-2. RUNOFF COEFFICIENT “C”
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Type Area or Land Use
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Radiologic [Hydrologic] Soil Groups
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A
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B
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C
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D
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Parks and permanent open space
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0.35
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0.37
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0.39
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0.41
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Single-family detached residential zoning
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0.50
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0.52
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0.55
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0.58
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Apartment zoning
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0.65
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0.71
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0.87
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0.90
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Commercial
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0.95
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0.96
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0.98
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1.00
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SOIL GROUP CLASSIFICATION
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Group A Deep sand, aggregated silts
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Group B Sandy loam
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Group C Clay loam, shallow sandy loam
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Group D Heavy plastic clays
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(e) Time of concentration.
(1) Time of concentration is defined as the longest time that will be
required for a drop of water to flow from the upper limit of the drainage
area to the point of concentration.
(2) Time of concentration is a combination of surface flow time and time
of flow in the storm drainage facility. For overland flow times, the
methods are to be approved by the city engineer; Kinematic Flow Theory,
or the SCS Method in table 10.02.274-3 are acceptable formulas for
computing times of concentration.
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Hydrologic Soil Groups
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A
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B
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C
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D
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Parks and permanent open space
|
39
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61
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74
|
89
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Residential
|
77
|
85
|
90
|
92
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Apartments
|
81
|
88
|
91
|
93
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Commercial
|
89
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92
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94
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93
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Paved parking lot
|
98
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98
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98
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98
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(f) Rainfall intensity-duration-frequency curves for this area have been
developed using the most current data available. These curves shall
be from Technical Paper-40 or Hydro-35 and used in the design of all
drainage facilities.
(g) Design storm frequency.
(1) It is general practice to design city storm drainage systems to accommodate
the runoff from a storm which, on the average, is equaled or exceeded
once every five (5), ten (10) or twenty-five (25) years.
(2) Table 10.02.274-4 states the design storm frequencies that shall
be used in the design of drainage facilities.
TABLE 10.02.274-4
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Type of Facility
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Minimum Design Frequency
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Storm sewers (with inlets on grade)
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5 years
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Storm sewers draining low point inlets
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100 years
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Culverts, bridges, channels, creeks
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100 years
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(3) In connection with the design of facilities such as low point inlets,
culverts, bridges, channels and creeks shown in table 10.02.274-4,
the discharge for a 100-year return frequency storm and the possible
consequences therefrom shall be evaluated to determine if such consequences
are sufficient to warrant the enlargement of the planned facility.
In any areas where stormwater runoff concentrates at low points of
grade or where discharge in excess of the design discharge flows across
private property, the following information shall be shown:
(A) The 100-year design discharge.
(B) The depth of inundation of this discharge.
(C) An evaluation of possible consequences resulting from the above.
(h) Head losses.
The design techniques and methods used
in the determination of all head losses shall be approved by the city
engineer or his authorized designee.
(i) Flow in gutters.
The drainage capacities of streets
and gutters shall be determined by Manning’s formula using an
“n” value of 0.015 for asphalt or 0.013 for concrete streets.
Streets shall be designed to flow not more than curb deep during a
five (5) year return frequency storm.
(j) Positive overflow.
In areas where positive overflow
is not provided within the street, a concrete flume with a minimum
width of four feet (4') shall be required between lots in a drainage
easement, near the low point in the street.
(k) The city engineer, due to particular drainage characteristics of a subdivision, may require minimum finish floor elevations on certain lots contained within said subdivision to be shown upon the plat. The elevations should be calculated based on the most current floodplain management criteria or other criteria as necessary to avoid damages, as specified in subsection
(g)(3) of this section. The elevations shall be shown on the plat prior to filing with the plat records of Tarrant County, Texas. The following note shall be added to any plat upon which the city engineer requires the establishment of minimum finish floor elevations: “The City of Dalworthington Gardens reserves the right to require minimum finish floor elevations on any lot contained within this addition. The minimum elevations shown are based on the most current information available at the time the plat is filed and is subject to change. Additional lots, other than those shown, may also be subject to minimum finish floor criteria.”
(2005 Code, sec. 16.1.64)
All construction shall be in accordance with the Standard Specifications
for Public Works Construction, and all amendments thereto, as adopted
by the North Central Texas Council of Governments.
(2005 Code, sec. 16.1.65)
(a) The owner or developer of property to be developed shall be responsible
for all storm drainage flowing on his property. This responsibility
includes the drainage directed to that property by prior development
as well as drainage naturally flowing through the property by reason
of topography.
(b) Adequate consideration shall be given by the owner in the development
of property to determine how the discharge leaving the proposed development
will affect downstream property.
(c) When any proposed development requires off-site grading or includes
areas of two acres or more where stormwater runoff has been collected
or concentrated, whether it be by permanent drainage systems or streets,
no drainage onto adjacent property shall be permitted except in existing
creeks, channels, storm sewers, or street, unless the following is
provided:
(1) Notarized letter of permission from the affected property owner;
or
(2) Proper drainage easements are obtained; or
(3) If the owner is unable to acquire the necessary off-site easements,
he shall provide the city with documentation of his efforts, including
evidence of a reasonable offer made to the adjacent property owner.
Upon such a written request for assistance, the city shall attempt
to acquire these easements through negotiations. If the negotiations
are unsuccessful, the request may, at the developer’s option,
be submitted to council for consideration of acquisition through condemnation.
In either case, the total cost of the acquisition and cost of the
easements shall be paid by the owner (developer).
(d) The subdivider shall pay for the cost of all drainage improvements
required for the development of the subdivision, including any necessary
off-site channels or storm sewers and acquisition of the required
easements, in which (b)(2) and (b)(3) above apply. In areas where
the proposed off-site improvements are to be made within existing
city right-of-way, an estimate of these off-site costs shall be subject
to council approval. Costs for such off-site improvements shall be
prorated to the extent that the owner pays for a percentage of the
off-site cost based on the increase of the discharge originating within
the limits of the owner’s addition.
(e) Where it is anticipated that additional runoff incident to the development
of the subdivision will overload an existing downstream drainage facility,
whether natural or man-made, and results in hazardous conditions,
the planning and zoning commission and/or council may withhold approval
of the subdivision until appropriate provision has been made to accommodate
the problem, and plans shall be provided which include all necessary
off-site improvement, including storm sewer systems, channel grading,
driveway adjustments, culvert improvement, etc.
(2005 Code, sec. 16.1.66)