09-17-07 Meeting of Drainage and Flooding Committee

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Q = CIA, where Q is the storm flow at a given point in cubic feet per second (c.f.s). C is a coefficient of runoff (see Table 1). [ is the average intensity of rainfall in inches per hour for a period equal to the time of flow from the farthermost point of the drainage area to the point under consideration. (See figure 1, I-D-F Curves and Figure 2, Determination of Time of Concentration) A is the drainage area in acres The size and shape of the watershed must be determined for each installation. The area of each watershed may be determined through the use of planimetric-topographic maps of the area, supplemented by field surveys in areas where topographic data has changed or where the contour interval is insufficient to adequately determine the direction of flow. The outline of the drainage area contributing to the system being designed and outline of the sub-drainage area contributing to each inlet point shall be determined. When calculating the peak flow rate of storm runoff, rainfall intensity will be determined from the rainfall intensity, duration and frequency curves, shown in Figure 1. The storm frequency used for this determination will be according to the facility to be designed as listed in Section A. I. Runoff Coefficients and Time of Concentration Runoff coefficients, as shown in Table 1, shall be the minimum used, based on total development under existing land zoning regulations. Where land uses other than those listed in Table 1 are planned, a coefficient shall be developed utilizing values comparable to those shown. Larger coefficients may be used if considered appropriate to the project by the City Engineer. The time of concentration is defined as the longest time, without unreasonable delay, that will be required for a drop of water to flow from the upper limit of a drainage area to the point of concentration. The time of concentration to any point in a storm drainage system is a combination of the "inlet time" and the time of flow in the drain. The inlet time is the time for water to flow over the surface of the ground to the storm drain inlet. Because the area tributary to most storm sewer inlets is relatively small, it is customary in practice to determine the inlet time on the basis of experience under similar conditions. Inlet time decreases as the slope and the imperviousness of the surface increases, and it increases as the distance over which the water has to travel and retention by the contact surfaces. Time of concentration shall be computed from Figure 2m and in no case shall the inlet time be more that the time shown in Table 1. Stonnwater Criteria Page 3 of 10