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I I I I : II I II I I II I II I I I I I I I -16- 1. Information provided by the U.S. Forest Service indicated that approximately 10 different soil types comprise the Sheriff Reservoir basin. Upon evaluating the information provided by the Forest Service regarding soils types, we assumed that hydrologic soil class C applies to the entire basin. This assumption was confirmed by field inspection. 2. Initial losses will be significant. Detailed inspection of USGS topography indicates that between 5 and 10 percent of the basin drains into depressions without outlets. The forest is filled with many depressions that will pond water to a depth of over 1 to 3 feet before any surface water runoff occurs. Much of the runoff from talus slopes will be absorbed by the talus and will re-emerge downgradient only after an extended time span. For these reasons and others, high initial losses are appropriate for this basin. 3. The Trout Creek, Sand Creek, and downstream) of Sheriff Reservoir are laden with massive rocks and of 0.08 to 0.10 are appropriate. percent are common. tributary channels upstream (and are steep and rugged. The channels large trees and channel Un" factors Channel slopes of greater than 5 4. The hydrologic characteristics of the basin are best reflected by subdividing the basin into 10 subbasins. In this way, routing pheno- mena can be reasonably accounted for. Based on these considerations, the PMF for the catchment above Sheriff Reservoir was established using the Kinematic Wave subroutine within the model HEC-1. These results were verified via the Snyder's Unit Hydrograph Procedure within HEC-l. When determining subbasin runoff with the Kinematic Wave method, three conceptual elements were used: flow plains, collector channels, and a main channel. The Kinematic Wave technique transforms rainfall excess into sub- basin outflow. An overland flow element is described by four parameters: 1) a typical overland flow length, 2) slope, 3) roughness factor, and 4) the percent of the drainage basin that is represented by the particular overland flow element being modeled. Flow from the overland flow elements travels to the subbasin outlet through one or two successive channel elements. Channel elements are defined by length, slope, roughness, shape, width or diameter, and side slope. Snyder's unit hydrograph procedure is one of the more widely used unit hydrograph techniques. A unit hydrograph is defined as the response of the basin to one unit (usually an inch) of effective rainfall. The unit storm is a rainfall of such duration that the period of surface runoff is not appre- ciably less for any rain of shorter duration. The unit hydrograph represents the integrated effects of factors such as tributary area, shape, drainage pat- tern, channel capacities, and land slopes. The runoff response (design hydrograph) of a design storm is determined by multiplying the ordinates of the unit hydrograph by incremental rainfall excess depths of the design storm and summing each of the resulting incremental runoff hydrographs sequentially