FLOOD06904 - Laserfiche WebLink

Home Browse Search

DESCRIPTION OF FLOOD.WAVE CHARACTERISTICS 29 FIGURE 19.-The flood peak in the downstream end of Horseshoe Park. Flow peak is approximately 7,200 cubic feet per second. Photo courtesy of Grace Davis George and Jennifer D. George. floodwater from Lawn Lake. Because of the near- instantaneous failure and relatively small volume of released water, the resulting flood hydrograph from Cascade Lake dam resembled a "spike"-shaped hydro- graph, superimposed on the relatively broad inflow flood hydrograph (fig. 46). The peak discharge of the in- flow hydrograph was 7,210 ft'/s (Site 1), and the dam failed at a discharge of 4,500 ft'/s (Site 2), indicating that the dam failed just prior to the arrival of the in- coming flood peak from Lawn Lake dam. Peak dis- charge of the Cascade Lake dam failure was extremely difficult to determine because of its inherently unsteady nature. However, a peak discharge of 13,100 ft'/s was computed 1.0 mi downstream from the dam after con- siderable attenuation probably had occurred. Dam- break modeling results (discussed later) suggested a peak discharge of 16,000 ft'/s resulting from the Cascade Lake dam failure. Flood depths in this high-gradient (S-percent slope), wide flood plain of the Fall River generally were less than 5 ft, as flood conditions resembled overland flow (fig. 20). Several eyewitnesses described the flow as cascading through Aspenglen Campground. The major- ity of flow probably was subcritical due to shallow depths, the low relative submergence of bed material, energy losses from transporting boulders, and dense vegetation obstructions (fig. 20). Several witnesses at Aspenglen Campground described the flow depths as relatively constant, with a secondary surge that pro- bably corresponded to the "spike" of the flood hydrograph, resulting from the failure of Cascade Lake dam. The two flood deaths in Aspenglen Campground oc- curred near the area shown in figure 20. Two campers, having been warned by other campers of an approach- ing flood, were last seen going into the island campsites. FALL RIVER AND BIG THOMPSON RIVER: ESTES PARK POWERPLANT TO LAKE ESTES Channel slope in this part of the flood path gradual- ly decreases from the upstream end of Fall River to the downstream end on the Big Thompson River (fig. 2), with an average slope of 2.3 percent. The channel was relatively narrow in this reach (figs. 21 and 22). Many buildings were located adjacent to the channel on the flood plain (fig. 21); these structures tended to slow the passage of the flood wave. This was particularly true in Estes Park, as shown in the photographs in figures