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The infiltration rate is the rate at which runoff passes through the soil surface. This rate decreases as tne soil becomes saturated and generally reaches a steady rate. The value used in the Lena Gulch Study varied frOIil 0.72 to 1.6 inches/hour "hich "as determined from field measureinent, The value used in this report '\;vas 0.5 :lnches/hour due to 1:he greater affect that South Table Mountain has upon this smaller basin. The Im'ler infiltra- tion rate used '\;vould theoretically proctice a higher runoff rate and volume. C. FLOOD PLAIN BOUGDARY DETEP~lINAtION 1. Criteria The flood plain evaluation "as completed by using the Corps of Engineers HEC-2 Water Surface Profiles Computer Program. This program Has developed by the Corps of Engineers at Davis, California and Hill compllte the Hater surface elevation ir~ natural or improvE?d channel conditions. The data used in computing this "ater surface "as provided by the design engineer for a particular project. This project Has defined by use of 35 channel cross-sections. The exist:Lng culve.rts "ere considered to be plugged by the floatable debris which is r<eadily available along the drainage-\my. Nannings roughness coefficient useand 0.045 to 0.08 ill the overbank Ureas. The flo" rates used varied along the drainage-"ay since thE,re are several lateral drainage-Ha:rs Hhich contribute.. The tabulation of these floH rates is given in the HydJeaulics Section ill Table IV-I. 2. Analysis The lOa-year Flood ,"imits Here determined for the drainage-"ay since the current Jefferson County Subdivision Regulations use this as the standard flood from which subdivisions are to be protected. The lOa-year Flood Limits are shm-m in Figure III-3.as provided in the envelope at the end of this report. //1 T