A. Rate Of Flow: The calculation of runoff rates shall be by the rational formula using antecedent precipitation factors. The rational formula and acceptable values shall be as follows:
Formula: Q = C Ca I A
Where:
C | = | Runoff coefficient |
Ca | = | Antecedent precipitation factor |
I | = | Intensity of rainfall in inches per hour |
A | = | Drainage area in acres |
Acceptable values of Ca are as follows:
2 - 10 years | 1.0 |
25 years | 1.1 |
50 years | 1.2 |
100 years | 1.25 |
The product of C Ca must not exceed 1 | |
Acceptable values for C according to the description of the drainage area shall be within the following ranges:
DESCRIPTION OF AREA
Business: | |||
Downtown | 0.70 | 0.95 | |
Other business | 0.50 | 0.70 | |
Residential: | |||
Single-family | 0.30 | 0.50 | |
Detached multi-family | 0.40 | 0.60 | |
Attached multi-family | 0.60 | 0.75 | |
Suburban | 0.25 | 0.40 | |
Apartments | 0.50 | 0.70 | |
Industrial: | |||
Light | 0.50 | 0.80 | |
Heavy | 0.60 | 0.90 | |
Parks, cemeteries | 0.10 | 0.25 | |
Playgrounds | 0.20 | 0.35 | |
Unimproved areas | 0.10 | 0.30 | |
Acceptable values for C according to the description of surface characteristics shall be within the following ranges:
CHARACTER OF SURFACE
Asphalt | 0.70 | 0.95 | ||
Concrete | 0.80 | 0.95 | ||
Roofs | 0.75 | 0.85 | ||
Lawns: | ||||
Sandy soil: | ||||
<2 percent slope | 0.05 | 0.10 | ||
2 - 7 percent slope | 0.10 | 0.15 | ||
>7 percent slope | 0.15 | 0.35 | ||
Heavy soil: | ||||
<2 percent slope | 0.13 | 0.17 | ||
2 - 7 percent slope | 0.18 | 0.22 | ||
>7 percent slope | 0.25 | 0.35 | ||
Agricultural land: | ||||
Bare, packed, smooth | 0.30 | 0.60 | ||
Bare, packed, rough | 0.20 | 0.50 | ||
Cultivated: | ||||
Heavy soil, no crop | 0.30 | 0.60 | ||
Heavy soil, with crop | 0.20 | 0.50 | ||
Sandy soil, no crop | 0.20 | 0.50 | ||
Sandy soil, with crop | 0.10 | 0.25 | ||
Pasture: | ||||
Heavy soil | 0.15 | 0.45 | ||
Sandy soil | 0.05 | 0.25 | ||
Woodlands | 0.05 | 0.25 | ||
B. Rainfall Intensity: The following formula is acceptable for calculation of rainfall intensity for various storm recurrence intervals using the factors provided:
Formula: | I = | a (Td + b)c |
Where:
I | = | Intensity of rainfall in inches per hour |
T
d | = | Duration or time of concentration |
a, b and c | = | Values taken from the table below for specified recurrence intervals |
STORM RECURRENCE INTERVAL
Factors | 2-Year | 5-Year | 10-Year | 25-Year | 50-Year | 100-Year |
a | 56.43 | 72.0 | 82.0 | 95.0 | 108.0 | 120.0 |
b | 11.5 | 15.0 | 15.0 | 15.0 | 15.0 | 15.0 |
c | 0.81 | 0.80 | 0.80 | 0.80 | 0.80 | 0.80 |
C. Time Of Concentration: The following formulas are acceptable for calculation of time of concentration as applied in the rational formula:
Overland flows: | Formula: | T = | K(L0.37) S0.2 |
Channel flows: | Formula: | T = | K(L0.77) S0.385 |
Where:
K | = | Surface coefficient |
L | = | Length of flow in feet |
S | = | Average slope in feet per foot |
K values for overland flow: | |||||||
Surface | K Value | Surface | K Value | ||||
Pavement | 0.372 | Commercial | 0.445 | ||||
Bare soil | 0.604 | Residential | 0.511 | ||||
Poor grass | 0.900 | Cultivated soil | 0.775 | ||||
Average grass | 1.000 | Timber, thin grass | 0.942 | ||||
Dense grass | 1.130 | Pasture | 1.040 | ||||
K values for channel flow: | |||||||
Surface | K Value | Surface | K Value | ||||
Curbed street | 0.0035 | Straight clean stream | 0.00592 | ||||
Concrete channel | 0.006 | Average stream | 0.00835 | ||||
Sodden swale | 0.008 | Meandering stream | 0.01020 | ||||
Bar ditch | 0.012 | Pooling stream | 0.01020 | ||||
D. 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.
1. The structure sizing shall include an analysis of the existence of inlet or outlet control conditions and a backwater analysis where applicable;
2. Where Manning's equation is used, the value for coefficients will be taken from the following references:
a. Open Channels:
(1) "Open Channel Hydraulics", Chow, Ven T., McGraw-Hill Book Co., New York, N.Y., 1959;
(2) "USDA Handbook SCS-TP-61", W.O. Ree; and
(3) "Roughness Characteristics Of Natural Channels", USGS water supply paper no. 1849, Washington, D.C., 1967;
b. Closed Conduits:
(1) "Seelye's Handbook";
(2) "Handbook Of Hydraulics", King, H.W.;
(3) "Eschbach Handbook"; and
(4) "Modern Sewer Design", American Iron And Steel Institute.
E. Vegetative Linings:
1. 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; and
2. Cover vegetation for channel linings must be selected according to the "Stephens County Best Management Practices For Erosion Control" and shall be approved by the city.
F. Design Standards And Practices: Where the provisions of this chapter are insufficient to prescribe best design principles and practice, the design standards and practices of the Oklahoma department of transportation shall be used. (Ord. 1387, 1-30-1996)