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I. I I I I I I I I I I I I I I I I I I . . -9- through the soil, and finally re-emerges downgradient as surface flow available to contribute to flood flows, is highly comple~ and poorly defined in high mountain basins. Depending upon assumptions made for the process of interflow, flood discharges can be either relatively high or relatively low. o We would expect debris ~lockage of the principal streams feeding Sheriff Reservoir by trees and other debris from the basin d~ring a PMF. This phenomenon would create a series ot "pools" throughout the basin (below timberline), and the net effect of these pools would be to attenuate peak discharges and lead to a more manageable PMF. These and other considerations will be studied carefully ~y our project team. We have f~und that the SEO is flexible 1n its interpretation of conditions that will exist in high mountain basins during PMFs; cO~sequently, manageable PMF flows can be derived using soun~ engineering assumptions. Should the design engineer adopt: a regimented, "textbook" approach for the Sheriff Reservoir basin, we expect that flood flows on the order to 3,000 to 4,000 cfs per square mile, or a total of 43,000 cfs for the bas~n, would be derived. By contrast, knowledge of the phenomena described above would lead to PMF flows of more on the order of 1,500 to 2,000 cfs per square mile, or a total of 18,500 cfs for the basin. The consequences of these two PMF v~lues are obvious with respect to the incremental damage assessment, spillway design, and dam overtopping considerations. Using the state-of-the-art rainfall document for Colorado, the National Weather Service publication HMR-49, our team will develop a range of flood flows beginning with the 100-year flood and extending through the PMF for Sheriff Reservoir. The HEC-l model will be utilized for this purpose. HEC-l is the most commonly used and widely accepted hydrologic model for flood studies in the U.S. today. Through review of aerial photographs, on-site inspection, and experience with similar basins, we will formulate appropriate parameters for modeling studies. By providing a range of flood flows from the 100-year to the PMF, subsequent incremental damage assessment studies and spillway enlargement studies can be carried out on a rational basis. Our modeling studies will subdivide the drainage area into approximately five subbasins with hydraulic routing of flows from one subbasin to the next. This discretization of the basin is preferable to an approach that would treat the basin as a homogeneous area that can be modeled as one unit. We believe that subdividing the drainage basin will help to decrease the size of the PMF. Rainfall distribution will be