R-2009-01 adopt amendments to Public Improvement Criteria Manual (PICM), relating to drainage criteria and improvement standards

Browse Search

All drainage improvements shall be designed for the following storm frequencies, n,pe of Facility Road Side Ditches and Culverts Storm Sewers Ditches & Culverts Drainage 100 acres and more Bridges Creeks Detention Facilities 5 year 5 year 2S year 100 year 100 year Refer to Section S.5 B. Storm Runoff Design flow of storm water runoff is to be calculated using the Rational Method. The Rational Method is based on the direct relationship between rainfall and runoff, and the method is e>'1>ressed by the following equation: Q = CIA, where Q is the storm flow at a given point in cubic feet per second (c.[s). C is a coefficient of runoff (see Table I). I 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) ^ 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 water runoff, rainfall intensity will be determined from the rainfuJl intensity, duration and frequency curves, shown in Figure 1. The storm frequency used for this determination \'till be according to the facility to be designed as listed in Section A. 1. Runoff Coefficients and Time of Concentration Runoff coefficients, as shown in Table I, sball be the minimum used, based on total development under existing land zoning regulations. Where land uses other than those listed in Table I 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 Mil be required for a drop of water to flow from the upper limit of a drainage area to tIle point of concentration. The time of concentmtion to any point in a stonn drainage system is a combination of the "inlet time" and the time of flow in the drain. The inlet tinle is tIle time for water to flow over the surface of the groood to the stonn drain inlet. Because tlJe area tributary to most stonn sewer inlets is relatively small, it is customary in practice to determine the Slonnwakr Criteria Page 3 of 1 0