Drainage Design Manual - the City of San Luis Obispo

4.1.4 Example ProblemA sample basin has been selected to illustrate the application of the Rational Method in the SanLuis Obispo Watershed. The sample basin is located west of Broad Street near Orcutt Road,south of Downtown San Luis Obispo. The City of San Luis Obispo general plan shows a mix ofresidential, open space, and industrial land use at general plan build out. Figure 4-3 illustratesthe basin and the soil, general plan land use, and slope characteristics of the basin. Thefollowing example computes flow at the outlet of the basin assuming general plan build-out landuse conditions and existing channel conditions.The composite runoff coefficient for the basin is computed using the area of each uniquecombination of land use, soil hydrologic group, and slope range in the basin. The total area ofeach combination has been tabulated in Table 4-7. The composite runoff coefficient iscomputed by assigning a runoff coefficient to each landuse/soil/slope combination, multiplyingeach respective coefficient by the total area represented by each combination, summing thisproduct for all combinations, and dividing the sum by the total basin area.The Time of Concentration is computed by defining the flow path from the most remote locationin the watershed to the basin outlet, splitting this path into several segments, and computing thetravel time in each segment according to the appropriate method (i.e. overland flow, shallowconcentrated flow, or channel flow). This computation is illustrated in Table 4-8.Table 4-9 illustrates the drainage calculations for the basin for the 2-year, 10-year, and 100-yearrecurrence interval events. C and T c are as computed in Table 4-7 and Table 4-8, respectively.Annual precipitation is taken from Figure 4-2, and the rainfall intensities are taken from theappropriate table (Table 4-4, 4-5, or 4-6 depending on annual precipitation). The rainfallintensity for the time period T c , i Tc , is linearly interpolated between the nearest tabulated values(in this case the 15-minute and 30-minute values). C a is taken from Table 4-2, and Q iscomputed according to Equation 1.SLO Creek Drainage Design Manual 31 February 2003