(A) The purpose of this section is to establish standard criteria, principles, procedures and practices for design of 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 stormwater `quantity, method of collection, transportation and disposal.
(2) Methods of design other than those indicated or referred to herein may be considered in complex and difficult cases where experience clearly indicated they are preferable; however, these deviations shall not be attempted until approval has been obtained from the City Engineer or his authorized designee.
(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.
(1) The two methods acceptable for use in the city are:
(a) Modified rational method (Q=CCaIA) to be used on small watersheds of 1,000 acres or less, where:
Q = Discharge expressed in cubic feet per second
C = Runoff coefficient (Table III (Division (D)(2) below)
Ca = Antecedent precipitation factor (Table II (§ 154.126))
I = Rainfall intensity in inches per hour (Cleburne, Texas Rainfall Intensity Curves)
A = Area in acres
(b) 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 approved by 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 or his authorized designee for approval prior to completion of the plans for the drainage system.
(2) Table II: Antecedent Precipitation Factor Ca.
Table II Antecedent Precipitation Factor Ca. | |
Recurrence Interval (Years) | Ca |
2 to 10 | 1.0 |
25 | 1.1 |
50 | 1.2 |
100 | 1.25 |
Note: The product of Ca should not exceed 1.0. | |
Note: The product of Ca should not exceed 1.0.
(C) Drainage area.
(1) Area of the watershed shall be determined using the information required by § 154.123(A)(4) and final grading plan.
(2) Outline of drainage areas must follow natural drainage features in nonurbanized areas. Consideration shall be given to manmade 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 III below gives values for runoff coefficients which shall be used for the determination of stormwater runoff.
Table III Runoff Coefficient "C" | ||||
Type Area or Land Use | Hydrologic Soil Groups* | |||
A | B | C | D | |
Table III Runoff Coefficient "C" | ||||
Type Area or Land Use | Hydrologic Soil Groups* | |||
A | B | C | D | |
Parks and permanent open space | .35 | .38 | .41 | .45 |
Single-family zoning | .50 | .52 | .55 | .58 |
Duplex zoning | .60 | .63 | .66 | .70 |
Apartment zoning | .65 | .70 | .77 | .80 |
Commercial | .90 | .91 | .90 | .93 |
CBD | 1.00 | 1.00 | 1.00 | 1.00 |
Industrial | Must be proposed by design engineer and approved by City Engineer | |||
*Soil group classification: Group A Deep sand, aggregated silts Group B Sandy loam Group C Clay loam, shallow sandy loam Group D Heavy plastic clays | ||||
(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 over land flow times, the methods are to be approved by the City Engineer or his authorized designee.
(3) Inlet times shall be:
(a) Five minutes for property zoned for multiple-family, local business, central business, commercial or industrial.
(b) Ten minutes for property zoned for parks, schools, single-family residential and duplex.
(F) Rainfall intensity.
(1) Rainfall intensity-duration-frequency curves for the city are set forth on chart reproduced below:
(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 ten or 50 years.
(2) Table IV shows the design storm frequencies to be used in the design of drainage facilities.
(3) In connection with the design of facilities such as low point inlets, culverts, bridges, channels and creeks shown in Table IV set forth in division (J) below, the discharge for a 100-year return frequency storm and the resulting possible damages therefrom shall be evaluated to determine if the damages 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 the possible damages resulting from the above information.
(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 value of 0.015 for asphalt or 0.013 for concrete streets. Streets shall be designed to flow not more than one inch above the curb during a ten-year return frequency storm on residential streets. Collectors and thoroughfares shall maintain a minimum of eight feet of pavement width above the ten-year frequency.
(J) Positive overflow. In areas where positive overflow is not provided within the street, a concrete flume with a minimum width of four feet with six-inch curbs shall be required between lots in a drainage easement, near the low point in the street.
Table IV | |
Type of Facility | Minimum Design Frequency |
Storm sewers (with inlet on grade) | 10 years |
Storm sewers draining low point inlets | 50 years |
Culverts, bridges, channels, creeks | 50 years |
(K) Flow in storm sewers and channels. The capacity of storm sewers, culverts, bridges, and open channels shall be determined by the use of the Manning Formula, and the design of the facility derived accordingly. The Manning Formula is:
Q = 1.486 r 2/3 s 1/2 A
n
n
Where:
Q = Discharge, expressed in cubic feet per second
n = Roughness coefficient, based on condition and type of conduit lining
s = Slope of hydraulic gradient, expressed in feet of vertical rise per foot of horizontal distance
r = Hydraulic radius (area of flow divided by wetted perimeter)
A = Area of flow in conduit, expressed in square feet
('68 Code, App. B, § 7.05) (Ord. 8-1989-34, passed 8-8-89)