FLOOD08228 - Laserfiche WebLink

Home Browse Search

1 1 1 1 1 I: 1 1 I 1 1 1 I 1 1 1 I I 1 The values of the loss rate parameters for various model runs are shown in Table 2. The Green and Ampt equation is selected as the best practical method, and the selected parameter values are: hydraulic conductivity (XKSAT) = 0.25 inch/hr capillary suction (PSIF) = 4.3 inches soil moisture deficit (DTHETA) = 0.10 to 0.35. The surface retention loss is estimated as 0.35 inch. The Horton method for estimating infiltration losses was not used because data are not available to estimate the Horton equation parameters for this watershed. The Horton equation is not as amenable to adjustments for anteced- ent soil moisture as the Green and Ampt equation. A unit hydrograph was used to route the rainfall excess from the watershed. Two types of unit hydrographs were used; S-graphs and the Snyder unit hydrograph. The S-graph that was selected is the one that was developed for Buckhorn Creek near Masonville, Colorado. That S-graph was developed by the USBR in the reconstitution of a severe storm flood. The Buckhorn Creek S-graph is recommended for use for thunderstorms in the Rocky Mountains (Table 3-12, Desion of Small Dams, Third Edition, USBR, 1987), and the use of S-graphs is described in that reference and in the USBR Flood Hvdroloov Manual (1989). The S-graph is a unit hydrograph with one parameter, lag. The watershed characteristics for each subbasin that were used to calculate lag are shown in Table 3. HEC-1 does not have an option to use S-graphs directly. However, the Hydrologic Engineering Center has written a HEC-1 preprocessor program that converts S-graph input into a unit hydrograph. That program, LAPREl, was used with the S-graph runs. The Snyder unit hydrograph was used with the parameters calculated according to the procedure in the UDFCD Urban Storm Drainage Criteria Manual. The parameters for each subbasin are listed in Table 3. 3