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1) U.S. Water Resources Council. Guidelines for Determinina Flood Flow Fre. quency, Bulletin ., 7A. Washington, D.C., 1977. 2) Colorado Water Conservation Board. Manual for Estimatino Flood Charac- teristIcs of Natural-Flow Streams In Colorado, Techmcal Manual +1. Denver. CO 1976. 3) U.S. Department of Agriculture. Soil Conservation Service, procedures fo~ DetermmJna Peak Flows in Colorado. Denver, CO '977. 4) U.S. Department of Commerce, NOAA Atlas #2, Preci itation.Frequency Atlas of the Western United States, Volume III, Colorado. ilve-r Spring. MD 1973. For gauged basins. both urban and rural. with at least 20 years of systematic stream gauging records the hydrologist should perform a probability dlstribu' tlon analysis of the gauge data In order to assign probabilities to different flows. The analysIs should use the log.pearson 1/1 technique. and it should include a regIonal skew coefficIent appropriate to the region being studied. Acceptable procedures are: . Hydrologic Methods Hydrology deals with the water balance in a watershed. Hydroiogic methods are used to determine flood flows and voiumes, SCience has now developed reasonably consistent methods for determining representative flow values.- A hydrologic investigation for a floodplain study will first determine the potential cause of flooding and then determine volumes or rates of flow that would be expected for a given set of probabilities. In a detailed hydrologic analysis these calculations will be performed for the 10..50.. 100.. and 500. year frequency flood discharges. The analysIs will be based on existing drainage baSin conditions except where future development within the basin may be significant. The selected flood discharges may be computed by either an analysIs of available streamflow records or by a synthetIc rainfall, runoff method, During the hydrologic investigations of a floodplain study the engineer will calculate the 1 OO'year flow. A 1 OO'year flood is a flood that would be expec. ted to happen once every 100 years or that has a one percent chance of occurring dUring any given year. Sections 24.65.105. 24-65.1-103. and 24. 65.1'403(3)(b). CRS. 1973. as reVised, established the 1 OO'year flood stan. dard as the baSIS for land'use planning and regulatory activities. The CWCS recommends the follOWing guide In selecting the appropriate method(s) for a hydrologic investigation In a particular community. For developing urban areas, the hydrologist should select a synthetic ralnfall.runoff method and correlate It to available gauge records. Acceptable models are: . 1) U.S. Army Corps of Engineers, Flood Hydrograph Package (HEC.' j (Davis. CA 19691 2) Wright-McLaughlin EngIneering, Urban Storm Drainage Cntena Manual, Volumes I and /I (Denver, CO 1969.) 3) U.S. Army Corps of Engineers. Missouri River District, "Runoff Block," in Storm Water Management Model tSWMM}, U.S Environmental Protection Agency, Omaha. NE: U.S. Army Corps of Engineers, 1973. 4) U.S. Department of Agriculture, Soli Conservation Service. National Engineering Handbook. Section 4 ~ydra/aqy. Washington. D.C.. 197'. For ungauged rural areas. the hydrologist should use a regional analysIs approach. ThiS approach Involves developing representative regressIon curves for the region based on available gauge records for both rainfall and snowmell.Acceptable procedures are: 1) U.S. Water Resources CounCil. GUidelines for Determining Flood Flow Frequency. 2) Beard, Leo R. Statistical Methods In Hydrology. Sacramento, CA, U. S. Army Corps of Engineers. 1962. .