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(1) The design of storm drainage improvements in the city shall be based on flood discharges determined from the Rational Formula. The formula for calculating storm flows in this manner is:
Q = CIA
Q is the storm flow at a given point;
C is a coefficient equal to the ratio that the peak rate of runoff bears to the average rate of rainfall;
I is the average intensity of rainfall for a period equal to the time of flow from the farthest point of the drainage area to the first inlet point on the storm sewer; and
A is the area tributary to the design point.
(2) The subdivider shall pay for the cost of all drainage improvements connected with development of his or her subdivision, including any necessary off-site channels or storm sewers and acquisition of the required easements, with the following exceptions:
(a) An enclosed storm sewer shall be provided in all areas where the quantity of the accumulated storm run-off does not exceed 200 c.f.s.; and
(b) In drainage courses where the accumulated storm run-off is more than 200 c.f.s., and less than 500 c.f.s. either an enclosed storm sewer system or a concrete line channel shall be constructed.
(3) (a) In drainage courses where the accumulated storm run-off is more than 500 c.f.s., the drainage improvements may be either an enclosed storm sewer, a concrete line channel, or an earthen channel.
(b) Earthen channels shall be designed according to the criteria as set forth by the following typical channel section.
(B) Runoff coefficient. The runoff coefficient which considers the slope of the terrain, the character of the land use, the length of overland flow, and the imperviousness of the drainage area shall be determined from the ultimate land development as shown on the Master Plan of the city. The runoff coefficient for the appropriate land uses shall be as follows:
(C) Rainfall intensity-frequency.
(1) The rainfall intensity-frequency curves which are shown on Plate 1 are plotted from data by the United States Department of Commerce Weather Bureau in Technical Paper No. 40. See Appendix A.
(2) The intensity, I, in the formula Q=CIA is determined from these curves by arriving at a time of concentration and adapting a storm frequency upon which to base the drainage improvements.
(a) Time of concentration. The time of concentration, which is the time of flow from the farthest point of the drainage area to the first inlet in the system, consists of the time required to flow in the gutter to the inlet. A minimum time of concentration of 15 minutes shall be used for all areas except commercial areas, and a minimum time of concentration of ten minutes shall be used in commercial areas. A nomograph, shown on Plate 2, is attached for estimating the time of concentration. See Appendix A.
(b) Storm frequency. Recommended design storm frequencies for the storm drainage improvements in the city are shown in a table as follows:
Type of Facility
Description of Area to Be Drained
Maximum Time of Concentration (Minutes)
Design Frequency (Years)
Residential, commercial and industrial
*Culverts, bridges, channels and creeks
Any type of area less than 100 acres
**Culverts, bridges, channels and creeks
Any type of area greater than 100 acres but less than 1,000 acres
Culverts, bridges, channels and creeks
Any type of area greater than 1,000 acres
* When the maximum time of concentration or area to be drained is exceeded, the design shall be based on a 10- year frequency.
** When the maximum time of concentration or area to be drained is exceeded, the design shall be based on a 50-year frequency
*** Whenever, in a storm sewer system, an inlet is located at a low point so that flow in excess of the storm sewer capacity would be directed onto private property, the design frequency shall be increased beyond 10 years if the inlet location is such that overflow could cause damage or serious inconvenience, it may be desirable to increase the design frequency to as much as 25 years.
(D) Area. The area used in determining flows by the Rational Formula shall be calculated by subdividing a map into drainage areas within the basin contributing storm water runoff to the system.
(E) Spread of water. During the design storm, the quantity of storm water that is allowed to collect in the streets before being intercepted by a storm drainage system is referred to as the spread of water.
In determining the limitations for carrying storm water in the street, the ultimate development of the street shall be considered. The use of the street for carrying storm water shall be limited to the following:
Spread of Water
Major thoroughfares (divided)
One traffic lane on each side to remain clear
Thoroughfares (not divided)
Two traffic lanes to remain clear
One traffic lane to remain clear
Six-inch depth of flow at curb or no lanes completely clear
Curves are provided on Plate 3 for determining the spread of water for certain gutter slopes, gutter discharge and pavement crown. Also provided are curves for determining the depth of gutter flow (Plate 4) and curves for determining the curb inlet opening length (Plate 5); see Appendix A
(F) Storm sewer design. Storm water in excess of that allowed to collect in the streets shall be intercepted in inlets and carried away in a storm sewer system. Storm sewer capacity shall be calculated by Manning’s Formula:
Q = A *1.486/n * R2/3 * S1/2
Q is the discharge in cubic feet per second;
A is the cross-sectional area of flow in square feet;
R is the hydraulic radius in feet;
S is the slope of the hydraulic gradient in feet per foot; and
n is the coefficient of roughness (n=.013 for new concrete pipe)
(1) Sample computation sheets are included for inlet and storm sewer design calculations.
(2) In the design of the storm sewer system, the elevation of the hydraulic gradient of the storm sewer shall be a minimum of one and one-half feet below the elevation of the adjacent street gutter.
(3) Storm sewer pipe sizes shall be so selected that the average velocity in the pipe will not exceed 15 feet per second nor less than three feet per second.
(4) Storm sewer systems shall be installed in all areas where the quantity of storm run-off is 200 cubic feet per second or less. A storm sewer system may be constructed when the quantity exceeds 200 c.f.s. at the discretion of the subdivider.
(G) Open channel design. Storm water run-off in excess of that allowed to collect in the streets in developed areas and run-off in undeveloped areas may be carried in open channels (not in the street right-of-way). Open channel capacity shall be calculated by Manning’s Formula and roughness coefficients shall be as follows:
Type of Lining
Roughness Coefficient “n”
Maximum Permissible Mean Velocity
Earth (Bermuda Grass)
8 feet per second
Earth (non Vegetated)
5 feet per second
15 feet per second
15 feet per second
(1) Open channels shall be constructed with a trapezoidal cross-section and shall have side slopes no steeper the three to one in earth and one and one-half to one when lined with concrete.
(2) The subdivider shall dedicate a right-of-way on all concrete line channels of sufficient width to provide for excavation of the open channel of proper width, plus ten feet on each side to permit ingress and egress for maintenance.
(H) Culvert design.
(1) At locations of creek crossings with proposed roadway improvements, it is sometimes necessary to receive and transport storm water under the roadway in culverts. The quantity of flow shall be determined by the Rational Formula, and the capacity of the culvert shall be calculated by Manning’s Formula.
(2) Design of culvert shall include the determination of upstream backwater conditions as well as downstream velocities and flooding conditions. Consideration shall be given to the discharge velocity from culverts, and the following limitations are allowed.
Culvert Discharge - Velocity Limitations
Culvert Discharging On To
Maximum Allowable Velocity (f.p.s.)
Paved or riprap apron
(3) Generally, all culverts shall be designed with a free outfall and the following head losses shall govern the design of the culvert:
(a) Frictional head loss.
hf = SfL where:
Sf = Slope of frictional gradient in feet per foot; and
L = Length of culvert in feet.
(b) Head loss due to change in velocity.
hv = v22/2g - v12/2g
v2 = Velocity in culvert;
v1 = Velocity in channel above culvert; and
g = Acceleration due to gravity.
(c) Head loss at upstream entrance to culvert due to entrance and change in section.
he = v22/2g
Where: v1/2g is equal to or less than six feet per second.
he = v22/2g - 0.5 v12/2g
Where: v1 is greater than six feet per second.
(Ord. 459, passed 12-17-1985)