ARCHIVED REPORTS_1995_2 - Laserfiche WebLink

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ASSUMPTIONS USED AND RATIONAL FOR DRAINAGE CALCULATION VALUES The following rational and assumptions refer to the Storm Design Sheets located in Appendix C of the Preliminary Closure Plan for the Foothill Sanitary Landfill. The first column in moving left to right, labeled "Point of Concentration", describes a specific area which is defined on the Drainage Area Designations figure. These values were obtained by assigning a letter of the alphabet to each respective drainage area. The drainage areas were designated by designing a drainage system based on the final landfill topography, and then dividing that system into logical areas. The second column, labeled "Contrib. Ac (A) 10, shows the corresponding acreage for the respective drainage area. These values were obtained using a routine in AutoCad. The third column, labeled "Runoff Coef. (C) 11 , is assumed by the County 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, and a fair stand of grass. The fourth column, labeled "CA" , is simply the respective product of the second and third columns. • The fifth column,, labeled "Sum CA, is simply the running sum of the fourth column products. The sixth column, labeled "Length L (feet) ", is the distance of the flow path between the respective area's discharge point and the point furthest from the area's discharge point. The seventh column, labeled "Initial Velocity (feet/sec) " , is the assumed initial velocity within a given area. The respective areasf computed actual velocity (pipe or ditch as may apply) is shown in the thirteenth column ("Computed Velocity ft/sec" . The eighth column,, labeled "Inlet Time L/Vel (minutes) " , is the respective area's flow path length (L) divided by the initial velocity; then divided by 60 to convert the time from seconds to minutes. The assumed time to inlet of 20 minutes is only added to the first area's inlet time in a chain for a particular Q. Subsequent areas do not include the initial 20 minute inlet time, however, the respective inlet time for subsequent areas in a chain is a running total. The ninth column, labeled "I in/hr from curve" is the intensity as determined from the 100yr curve. It is determined by taking the respective inlet time for a given area, and plotting that time on the 100yr curve to obtain an intensity respective of that inlet