figure 24. The main difference in the estimated flood plains is due to the dynamic nature of the DHM model, which accounts for the storage effects resulting from flooding, and the attenuation of a flood wave because of 2-D routing effects. From this study, the estimated flood plain is judged to be reasonable.
Application 4: Small-Scale Flows Onto a Flat Plain
A common civil engineering problem is the use of temporary detention basins to offset the effects of urbanization on watershed runoff. A problem, however, is the analysis of the basin failure; especially, when the floodflows enter a wide expanse of land surface with several small channels. This application is to present study conclusions in estimating the flood plain which may result from a hypothetical dam-failure of the LO2P3O Temporary Retarding Basin. The results of this study are to be used to estimate the potential impacts of the area if the retention basin berm were to fail.
The study site includes the area south of Plano Trabuco, Phase I. It is bounded on the north of LO2P3O Retarding Basin Berm, on the east and south of Portola Parkway and on the west by the Arroyo Trabuco bluffs (see figure 25).
Using a 1" = 300' topographic map, a 200-foot grid control volume discretization was constructed as shown in figure 26. In each grid, an area-averaged ground elevation was estimated based on the topographic map. A Manning's roughness coefficient of n = 0.030 was used throughout the study.