FLOOD06904 - Laserfiche WebLink

Home Browse Search

28 LAWN LAKE DAM AND CASCADE LAKE DAM FAILURES, COLORADO The Fall River streamflow prior to the flood was estimated to be about 150 ft'/s. The channel slope flat. tened rapidly in Horseshoe Park to 0.7 percent (fig. 2); this decrease in slope dramatically altered the flood-wave characteristics. The combination of the small slope on the prehistoric glaciallakebed, flood-plain widths in ex. cess of 1,000 ft (table 4), and very dense brush (fig. 18) attenuated the flood wave through Horseshoe Park (fig. 11), so that the leading edge was slowed to 2.1 mi/h and was no longer a wall of water (fig. 15). Peak dis- charge was greatly reduced through Horseshoe Park (fig. 11). The water was very tranquil, as noted by the smooth water surface of the flood near its peak in Horse- shoe Park (fig. 19). The flow was subcritical in this seg. ment. Peak discharge decreased from an estimated 18,000 ft'/s (see "Dam-Break Modeling") at Lawn Lake dam, to 7,210 ft'/s (Site 1) at the downstream end of Horseshoe Park. The majority of this attenuation pro- bably occurred in Horseshoe Park. A large amount of organic debris from the Roaring River was deposited or trapped by the dense brush at the upstream end of Horseshoe Park, and the majority of the sediment load was deposited in a large alluvial fan at the base of Horseshoe Falls (fig. 1). Little debris or sediment was transported by the flood wave in Horseshoe Park. It was fortunate for the numerous people in residences and commercial establishments downstream from Horseshoe Park that the park acted as an efficient flood- retarding basin (fig. 19). By slowing the flood wave, the time available for warning and evacuation was increas- ed. Most other Front Range valleys do not have large mountain meadows (that is, former glacial lakebeds) such as Horseshoe Park; without Horseshoe Park, the flood peak and resulting damages would have been much greater downstream. FALL RIVER: CASCADE LAKE DAM TO ESTES PARK POWERPLANT Channel slope increased to 8 percent (fig. 2) through Aspenglen Campground to the Estes Park powerplant (fig. 1), resulting in dramatic channel changes. Inflow floodwater to Cascade Lake overtopped the dam. The tremendous forces of the water caused the entire dam to fail (figs. 9A.D). At the time of failure, flood discharge was 4,500 ft3/s, and water was flowing 4.2 ft over the top of the dam. Failure of the dam released a total of 12.1 acre-ft of stored water in addition to the volume of FIGURE lB.-Surveyed cross section in Horseshoe Park showing topography of the flat glacial-lake floor. Looking downstream at man standing at cross section at river mile 5.36.