A. This section shall define the methods, performance, standards, and design criteria which shall be used in the preparation and evaluation of the reports and plans required as documentation in Section 110-254.
B. Drainage calculations.
"C" Coefficients According to Description of Drainage Area
Description of Area | Runoff Coefficients |
Description of Area | Runoff Coefficients |
Business | |
Downtown | 0.70 to 0.95 |
Neighborhood | 0.50 to 0.70 |
Residential | |
Single-Family | 0.30 to 0.50 |
Multi-units, detached | 0.40 to 0.60 |
Multi-units, attached | 0.60 to 0.75 |
Residential (Suburban) Apartment Industrial | |
Light | 0.50 to 0.80 |
Heavy | 0.60 to 0.90 |
Parks, Cemeteries | 0.10 to 0.25 |
Playgrounds | 0.20 to 0.35 |
Railroad Yard | 0.20 to 0.35 |
Unimproved | 0.10 to 0.30 |
"C" Coefficients According to Character of Surface
Character of Surface | Runoff Coefficients |
Character of Surface | Runoff Coefficients |
Pavement | |
Asphaltic and Concrete | 0.70 to 0.95 |
Brick | 0.70 to 0.85 |
Roofs | 0.70 to 0.95 |
Lawns, Sandy Soil | |
Flat, 2 percent slope | 0.05 to 0.10 |
Average, 2 to 7 percent slope | 0.10 to 0.15 |
Steep, 7 percent slope | 0.25 to 0.35 |
Acceptable formulas for determining time of concentration are as follows:
Time = K(L.37/S.2)
L = Length of flow in feet
S = Average slope in feet/foot
K = Constant for character of surface
Values of K: |
Values of K: | |
Pavement | .372 |
Bare Soil | .604 |
Poor Grass | .90 |
Average Grass | 1.00 |
Dense Grass | 1.13 |
Commercial | 0.445 |
Residential | 0.511 |
Cultivated Soil | 0.775 |
Timber; thin grass | 0.942 |
Pasture | 1.040 |
An acceptable formula for channel flow is:
Time = K(L 2/S).385
Values of K: |
Values of K: | |
Curbed Street | .0035 |
Concrete Lined Channel | .006 |
Sodded Swale | .008 |
Bar Ditch | .012 |
Straight; clean stream | 0.00592 |
Average Stream | 0.00835 |
Meandering; pooling stream | 0.01020 |
An acceptable formula for intensity is as follows:
I = average rainfall intensity, inches per hour
T = rainfall duration (or time of concentration), minutes
d
STORM RETURN
2YR | 5YR | 10YR | 25YR | 50YR | 100YR | |
a = | 56.43 | 72 | 82 | 95 | 108 | 120 |
11.5 | ||||||
c = | 0.810 | b = 15 | c = 0.80 |
WHERE T = TIME OF CONCENTRATION - d
a. Rational method analysis—Practices in Detention of Urban Stormwater Runoff, American Public Works Association. Special Report No. 43
b. Stormwater flood routing techniques.
2. Flow volumes. The calculation of runoff flow volumes used in the sizing of detention or retention facilities or other flood routing analysis shall be done by the use of accepted stormwater routing principals and the method shall be approved by the City Engineer or other authorized City representative.
3. Structure sizing. The sizing of all drainage facilities or structures shall be done by the use of established available engineering principles and practice and the methods shall be acceptable to the City Engineer or other authorized City representative.
a. The structure sizing shall include an analysis of the existence of inlet or outlet control design, freeboard; spillway design, and a backwater analysis where applicable.
b. Where Manning's Equation is used, the value for coefficients will be taken from the following references:
(1) Open channels.
(A) Open Channel Hydraulics. Chow. Ven T., McGraw- Hill Book Co., New York, N.Y. 1959.
(B) U.S.D.A. Handbook SCSTP-61, W.O. Ree.
(C) Roughness Characteristics of Natural Channels.
U.S.G.S. Water Supply Paper No. 1849, Washington, D.C. 1967.
(2) Closed conduits.
(A) Seelye's Handbook.
(B) Handbook of Hydraulics, King. H.W.
(C) USBR Hydraulic Tables.
(D) Modem Sewer Design, American iron and Steel Institute.
c. Natural open channels with vegetative linings shall be designed considering infrequent mowing unless other conditions are demonstrated and justified, guaranteed maintenance for exceptions must be established.
d. Cover vegetation for channel linings must be selected according to Standards established by The Soil Conservation Service of McClain County and shall be approved by the City Engineer or other authorized City representative.
4. When stormwater is conveyed via streets, flumes, swales, or open channels, consideration will be given to pedestrian and traffic convenience and safety. Special provisions may be required by the City Engineer or other authorized City representative to avoid sheeting of water across arterial intersections, inundation of crosswalks or sidewalks, inundation of driving lanes, and other safety or nuisance problems. These special provisions may include additional easements or rights-of-way, structures, flumes, or pavement width.
C. Performance criteria.
1. Detention or retention facilities shall have the following performance criteria:
a. Detention or retention facilities shall be sized by hydraulic stormwater routing through the detention basin and to accommodate or contain any changed drainage conditions due to the proposed development because of increases in the runoff volume or rate.
b. Detention or retention facilities shall have a release rate which does not exceed:
(1) The pre-development of runoff conditions for all return storms; or
(2) The "safe" capacity of the downstream drainage channel or system if any increase in runoff volume rate is approved.
2. Open channels, flumes, or swales shall have the following performance criteria:
a. All channels, flumes, or swales shall be designed to carry the fifty (50) year return frequency rainfall event.
b. The performance of all channels, flumes, or swales during the one hundred (100) year return frequency rainfall event must be indicated.
3. Storm sewers and appurtenances shall have the following performance criteria:
a. The combination of storm sewer and its overflow outlet shall have the capacity to convey the ten (10) year return frequency rainfall event within the drainage easement(s) or right(s)-of-way.
b. The performance of the overflow outlet(s) for the storm sewer system during the one hundred (100) year return frequency rainfall event shall be indicated under the condition that the storm sewer is plugged or inoperative.
c. The minimum velocity is a storm sewer flowing full shall be three (3) feet per second. Velocities less than three (3) feet per second may be approved if properly designed sediment traps which are accessible for maintenance are provided at the upstream end of the sections where the velocity is to be less than three (3) feet per second.
4. Streets with curb and gutters shall have the following performance criteria:
a. Arterial streets shall maintain two (2) clear ten (10) foot lanes of traffic during the twenty-five (25) year return frequency rainfall event.
b. Drainage on collector streets shall not exceed two (2) inches in depth at the crown of the street during the twenty-five (25) year return frequency rainfall event.
c. Drainage on residential streets shall not exceed three and one- half (3½) inches in depth at the crown of the street during the twenty-five (25) year return frequency rainfall event.
5. Streets without curb and gutters shall have the following performance criteria:
a. Arterial streets shall maintain two (2) clear ten (10) foot lanes of traffic during the twenty-five (25) year return frequency rainfall event.
b. Residential or collector streets shall maintain two (2) clear eight (8) foot lanes of traffic during the twenty-five (25) year return frequency rainfall event.
(Ord. No. 02-08, § 5, 5-6-2002)