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, '. 5 1\ " Hydraul i cs Information from several sources was used to determine the 100- year flood limits on the flood-prone-area maps. Channel cross sec- tions were field-surveyed at points 2 to 3 miles apart on the streams. This provided detailed information on the actual shape of the stream- bed in 1979-80 and its abilitY'to convey floodwaters. Some channels had been significantly altered since the topographic maps were prepared, either by floods or by man's construction activities, and occasionally by both. For this study, U.S. Geological Survey computer program E431, given in "User's Manual, Computer Applications for Step-Backwater and Floodway Analyses" (Shearman, 1976), was used to calculate hydraulic properties of the stream at each surveyed cross section. By use of the mapped stream slope and 100-year flood discharge, flood height was computed and transferred to the enlarged base maps. Flood width, depth, and velocity also were plotted adjacent to the numbered cross sections shown on the maps. ECONOMIC CONSIDERATIONS ! A flood having a recurrence interval of 100 years has been selected as the design flood by 'the Colorado Water Conservation Board for the administration of flood-plain management programs. Many parts of Elbert County had floods in 1935 and 1965 that equaled or exceeded the 100-year flood. Knowledge of areas which will be under water from a 100-year flood is helpful for those persons who must consider risks of land use within the flood plain. Generally, livestock and farming operations are among the acceptable uses of low-lying areas. Permanent buildings should be placed in locations not likely to be flooded-- usually at sites away from the streambed and above the expected 100- year flood levels. Building sites higher than and at some distance from water courses are not always safe from flooding, although elevating the structures to allow for proper drainage in all directions can minimize the possibility of damage. Intense thunderstorms can send sheets of water down sloping hillsides. Such an overland flow, termed by one resident as a "lake tipped sideways," also can destroy bui ldings in its path. To lessen this natural hazard, contoured furrows (ditches) have been cut into numerous hillsides in the county. They intercept overland runoff and divert it to existing waterways at either end of the ditches. These furrows are visible in some of the aerial phot~- graphs mentioned earlier. State and county officials have been given the responsibility for regulating development along streams. The foregoing discussion, along with appropriate flood-prone-area maps, will help them to meet this responsibil ity.