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- 21 - 0, 1)1);-:8 \,. L,~ lJ A synthetic series of peak flows' were developed by applying rainfall amounts and frequencies to the runoff characteristics of the watershed. A 24-hour rainfall-frequency curve was computed from U. S. Weather Bureau data at Trinidad. In the Rocky Mountain region practically all recorded maximum annual 24-hour amounts of precipitation result from four to six-hour thunderstorms; therefore, the frequency curve was treated as a six-hour duration curve. No reduction was made for areal distribution because of the small size of the watershed. The composite soil-cover complex numbers were gradually increased with increasing depths of rainfall in order to reflect the greater per cent of runoff that occurs in the more intense rainstorms. It was agreed by representatives of the Engineering and Water- shed Planning Unit, Lincoln, Nebraska, the Watershed Planning Party, the Colorado State Office of the Soil Conservation Servic~and the Colorado Water Conservation Board, that the deSign flood for detention storage should control approximately four inches of runoff. Four inches of runoff from the 1.14 square miles above the FPC-I dam site, or 243 acre-feet, and 73 acre-feet from 0.34 square mile on FPC-2 will be provided in the reservoirs for flood detention. Hydraulic characteristics of all channels and structures were determined from field surveys, The channel capacities below the closed conduit on Fishers Peak Arroyo were computed by the Modified Leach's Method for determing water surface profiles. Capacities for the natural stream channels above the Closed conduits on Fishers Peak and Carbon Arroyos were computed by Manning's formula. Routing for floods of various frequencies through damage reaches was limited to adjustments for travel time only. No reduction was made in the flood peaks because of the short reaches, high velocities, and the small amount of channel storage. " The emergency spillways for both structures were designed using criteria ip Section 3.21, hydrology handbook, and Engineering Memorandum No. 27, revised. The emergency spillway hydrograph from 5.0 inches of rainfall will be regulated to principal spillway discharge without the emergency spillway operating. The freeboard hydrograph (12.5 inches of rainfall in 6 hours) determined the minimum emergency spillway dimensions in structures FPC-l and FPC-2. Since both FPC-l and FPC-2 are class "~' structures, the emergency spillway dimensions were increased beyond the determined minimums.