ARCHIVED REPORTS_1995_2 - Laserfiche WebLink

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SAMPLE DRAINAGE CALCULATION FOR AREA M The surface area of area M, 31. 6 acres, was determined by an AutoCad routine. The slope for the area as shown on drawing 2 (Site Development Plan) is 6%. The runof f coef f icient assumed by the County (0. 6) , is conservative according our experience. For purposes of confirmation, the County used the Hydrologic Evaluation of Landfill Performance (HELP) version 3, which is produced by the Environmental Protection Agency. The HELP program calculated a value of . 54 based on properties of a typical SM soil type as per the Universal Soil Classification System (USCS) with regard to total porosity, field capacity, wilting point, and saturated hydraulic conductivity, a fair stand of grass, and average slope length of 2,450 feet, and an assumed velocity of 2 feet per second. The County decided to use the 0. 6 value since it is more conservative. The contributing acreage is then multiplied by the runoff coefficient: 31. 6 acres X 0. 6 = 18 . 96 which is rounded to 19. 0 acres which is then simply transferred to the next column since it is the area furthest from its respective site discharge point. The next step is to determine the intensity (I) for area M. This is accomplished by scaling the flow path between the point within area M furthest from area M's discharge point into the next area (D) in the chain. This distance as scaled from plan sheet 2 (Site Development Plan) is approximately 3 , 000 feet. This distance is then divided by the assumed initial velocity of 2 feet per second, as shown in column six. 3 , 000 feet / 2 feet per second = 1,500 seconds = 25 minutes This figure (25 minutes) is then added to the Assumed Initial Time To Inlet (shown near the top of the Storm Design Sheet) of 20 minutes to get 45 minutes for the area's inlet time. The next step is to refer to the graph of the 100yr storm curve, and plot the 45 minute inlet time against the curve in order to obtain an intensity (I) . This yields an intensity of approximately 1. 2 inches for area M. The f inal step is to multiply the CA product of 19 acres by the intensity value of 1.2 inches per hour to get a flow rate Q of 22 . 8 cubic feet per second.