FLOOD11631 (2) - Laserfiche WebLink

4.0 HYDRAULIC ANALYSTS Tn view of the size of Costilla County and the limited amount of time and money available fo~ preparing floodplain delineations for the county, approximate methods of hydraulic analysis were selected. 4.1 Base Maps 7 1/2 minute U.S.G.S. quadrangle maps were used as base topographic maps to show the 100-year floodplain. 21 quadrangle maps were used to provide base mapping for the study area. Of these 21 quadrangles, 6 had a contour interval of 40 feet, 6 had a contour interval of ZO feet (with 1 of those having selected intermediate 10 foot contours and 1 having selected intermediate 5 foot contours), 8 had a contour interval of 10 feet (with 2 of those Yaaving selected intermediate 5 foot contours). and 1 had a contour interval of 5 feet. In the San Luis study area field ~urveyed cross-sections were superimposed on a blown-up version of the San Luis quadrangle. Likewise, the CSU Maps of Floodprone Areas were prepared by superimposing field-surveyed cross-sections ora quadranqle maps. In the case of the CSU maps, however, the cross-sections were far less detailed and much more widely spaced than those used for the San Luis study. 4.2 Flood Depths The basic procedure for floodplain mapping was to determine a representative depth of flooding for the 100-year peak flow based on an elevation-discharge relationship. That depth was assumed to be constant for a given stream reach. Flood contours were drawn on the maps, and the boundary was determined by connecting the end-points of the f~ood contaurs. The key step in this process was to determine appropria~e flood depths. The first sources of information for this selection were the gage analyses perfarmed by the various governmental agencies. These analyses generally included flood staqes for the calculated 100-year flow. The second set of data sources was a set of typical cross-sections that were derived from the base maps (usually 10' contour intervals were used). Hyd.raulic calculations were performed for these cross-sections. In the area around the Town of San Luis, the elevation-discharge relationships determined in the December, 1988 report, Master Drainaqe Studv, Town of San Luis, Colorado, were used. From these analyses draft maps were prepared. Final depths of flooding could not be selected until a field revbew of these draft maps had been conducted. -30-