FLOOD04744 - Laserfiche WebLink

Home Browse Search

FLOOD MAGNITUDE A study of the hydrology of the Lost Creek Basin was made to determine the relative magnitude of the flood of February 10, 1980 (see Figure 11). The results of the study showed that the magni- tude of the flood decreased discontinuously, as it progressed downstream, due to reservoir storage effects and tributary drainage areas. Directly downstream of Prospect Reservoir, the flood was far in excess of a 500-year event. It was greater than a 100-year, or one percent chance flood, over most of the length of the unnamed tributary. A 100-year flood has a peak flow magnitude with a one percent chance of being equalled or exceeded in any given year, and a frequency of occurrence of about once in a hundred years on the long-term average. At the confluence with Lost Creek, the relative magnitude of the flood dropped sharply to correspond to approximately a 25-year event. It decreased gradually to about a 15-year event at Lord Reservoir. Directly downstream of Lord Reservoir, the flood was slightly greater than a 3D-year event and decreased steadily. COMPARISON TO THE "ZONE A" FLOODPLAIN The Federal Insurance Administration (FIA) has prepared a Flood Insurance Study for the unincorporated areas of Weld County. In that study, the 100-year floodplain was delineated for the reach of Lost Creek below Lord Reservoir using approximate methods. The term "approximate methods" means hydraulic routing of flood discharges using surveyed channel cross sections was not performed. Approximate floodplain areas are identified by the FIA as "Zone A". A comparison of the dam break flood (3D-year maximum return interval) with the Zone A (IOO-year flood) outline on the Flood Insurance Rate Map shows poor agreement for about 1 mile below Lord Reservoir and at the northern terminus of the flood (see Figure 12). The Lost Creek flood outline was within the Zone A boundary for approximately 60 percent of the flood length. 12