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I. I I I I I I I I I I I I I I I I I I . . -10- established according to procedures outlined in HMR-49. Initial abstractions and infiltration losses will be computed using two or three different methods, possibly including the use of infiltrometers in the field to accurate establish on- site infiltration characteristics. We will make every effort to consider all of these factors in order that the basin's capability to store water and infiltrate rainfall will be accurately accounted for so that reasonable PMF values will be derived. Ranges of parameters will be utilized for initial modeling runs until we have selected the combination that we believe most accurately reflecta conditions of the study area. These parameters will be used to develop flood flows for the basin. Incremental Damage Assessment The process of evaluating incremental impacts of dam failure for purposes of spillway design is a relatively new concept. Colorado is one of only a few states in the nation which will review and possibly approve the design of spillways based upon an "Incremental DamalJe Assessment" (IDA). The principal emphasis of an IDA is to define flood con- ditions for both base flood conditions and for the base flood plus dam break flows. The incremental difference between these two cases is evaluated according to two criteria: 1. Incremental property damage potential: In this case, if the incremental increase in flow depth is less than 1 foot, the SEO considers the potential of incremental property damage to be negligible. 2. Incremental loss of life potential: In this case, if the mathematical product of velocity times the flow depth in the incremental zone is less than 7 (square feet per second), the SgO considers the potential for additional loss of life to be negligible. Base flood flows are generally those which pass through the spillway and over the top of the dam to the point where dam failure begins. In Colorado, the SEO will permit the eva- luation to consider tributary flows from adjoining catchments above the point in question. In the case of Sheriff Reservoir, the first significant personally owned property is located approximately 6 miles downstream of the dam, off of the National Forest property. We believe that the SEO would accept a spillway size based on an IDA which meets the criteria for negligible impacts at the 6-mile point downstream of the reservoir because the National Forest property provides a buffer against any permanent pro- perty developments which would increase the magnitude of pro-