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, I I I 3.0 ENGINEERING ME~HODS I I For flooding sburces studied in detail in the county, standard hydrologic and hydraulic Istudy methods were used to determine the flood hazard data , required for this study. Floods having recurrence intervals of 10, 50, 100, and 500 y~ars have been selected as having special significance for flood plain management and for flood insurance premium rates. The analyses I reported here Ireflect current conditions in the watersheds of the streams. I 3.l Hydrologi!c Analyses , Hydrolog~c analyses were carried out to establish the peak discharge- , frequenc~ relationships for floods of the selected recurrence inter- vals for leach stream studied in detail in the county. Annual m~ximum peak flows for the 10-, 50-, lOO-, and SOO-year floods on the s~reams studied by detailed methods were based on records from the U. S. IGeological Survey stream gaging st.ations tabulated on the followinq page, and on the results of a regional peak flow frequency analysis.i Statistical data, calculated by the log-Pearson type III method (Reference 9), for stations with more than l2 years of record were used in the regional analysis. Using logarithmic-probability I graphs, ~he mean discharge for each station was correlated to various basin pa~ameters (i.e., shape and size of the drainage area, exposure I and permeability of soil, prevailing winds, and vegetative cover). A I log-log Blot of mean discharge-versus-drainage area provided the I highest correlation. i Peak flo~s in areas studied by approximate methods were deterlnined from looiyear peak flow envelope curves. These curves were developed by plotting the largest recorded peak flows in the region for both rain and!snowmelt flood events against drainage area. , , The drai~age areas that were studied by detailed methods required the use of t~e rainfall-runoff computer program HEC-l (Reference 10). This program utilized input data, such as unit hydrograph coefficients, soil los~ rates, rainfall depths and distribution, and, when necessary, routing 60efficients, to compute a flood hydrograph for a given drainage I area and area of storm coverage. Peak flows from the com- puted f100d hydrographs were compared to values obtained from the , regionaljfrequency curves and adjustments were made to the computed flood hydrographs to assure their consistency with the frequency I curves. i The comp~tational methods and techniques are generally accepted for hydrologic analyses and produced results considered reasonable for , ' . Mesa County. Unexpected results were not encountered 1n carrY1ng out the hydrblogic analyses for this Flood Insurance Study. Drainage area-peak discharge relationships for the Colorado River, Horizon , Drive Channel, and West and Leach Creeks are shown in Table 3. ! 15 _______ _ _ ._L_ ~ .......-,.....