2014-04-09_REVISION - M1977493 (2)

Home Browse Search

Ray Lazuk March 19, 2014 Page 7 Table 2. Climax Weather Station Rainfall Depth- Duration for 10 -Year Storm. Duration Depth (inches) 5 -min 0.21 10 -min 0.33 15 -min 0.41 30 -min 0.57 1 -hr 0.73 2 -hr 0.82 3 -hr 0.87 6 -hr 0.99 12 -hr 1.17 24-hr 1.40 3.4 Runoff Hydrographs The runoff hydrographs for the drainage basins in the HEC -HMS model were developed using either the unit hydrograph method or the kinematic wave method. The unit hydrograph method was used for basin configurations where a well- defined main channel is present and basin runoff enters the interceptor at a single point. The Flood Hydrology Manual (USBR, 1989) presents a separate hydrograph lag time relationship for each of the two basic types of storm phenomena apparent in data from numerous Rocky Mountain basins. These two basic storm types are the low- intensity general storm and the high- intensity thunderstorm. Basin -wide roughness coefficients that are representative of general storms, ranging from 0.13 to 0.26, are generally considered more appropriate for developing runoff hydrographs of more common frequency than the 100 -year storm event (USBR, 1989). Basin - wide roughness coefficients of 0.20 were selected for this analysis. The kinematic wave method was used for basin configurations where basin runoff is routed to the interceptor through several small drainages and a well- defined main channel is not present. Using this method, excess precipitation is routed across overland flow planes to collector channels that run more or less perpendicular to the interceptor canal. The parameters used to characterize the overland flow planes and collector channels are flow distance, slope, overland - flow coefficients, and channel roughness coefficients. Overland -flow coefficients of 0.6 for the