Laserfiche WebLink
<br />GLENWOOD SPRINGS FIS <br /> <br />B. HEC RAS Analysis, March 2003 <br />HEC,RAS 3.1, the current version ofHEC-RAS, was used to add additional cross <br />sections at the upstream end of the Roaring Fork River. There is a FEMA requirement <br />that the water surface elevation at the limits of the study for the 100-year flood match that <br />of previous studies, to ensure continuity. This has been accomplished by adding <br />additional cross sections upstream of the original study limits of the study reach on the <br />Roaring Fork River. New cross sections were added upstream of section 339+96 to <br />ensure that the water surface calculated from the new study (sec 200+67 to 376+61) <br />merged with the one from the older HEC,2 study (sec 348+46 to 376+61) (1977). The <br />additional cross sections came from a HEC2 model study, completed in November 1977, <br />of the Roaring Fork and Colorado River around Glenwood Springs, CO. The other <br />objective of the re-study is to eliminate negative surcharges that were produced in the <br />previous modeling efforts (1998). A negative surcharge is defined when the calculated <br />floodway water surface is lower than the floodplain water surface at the same flow. <br /> <br />C. Results <br />The plates show the profiles, floodplains and floodways for the Colorado River <br />and Roaring Fork River. A summary of the plates can be found in the 'List of Plates'. <br /> <br />The areas with the greatest potential for flood damage on the Colorado River are <br />near the bridges. Specifically, these are the areas downstream of the West Glenwood <br />Bridge (north bank), downstream of the Devereux Road Bridge (north bank) and <br />upstream, under and downstream of the dual Grand Avenue Bridge and Grand Avenue <br />Pedestrian Bridge (north bank). These flood prone areas include the Glenwood Hot <br />Springs facilities area and Interstate Highway 70. All these bridges will convey the 100- <br />year flood, except the Two Rivers Pedestrian Bridge, which will be inundated during <br />large events. <br /> <br />The Roaring Fork River also has potential for causing flood damage near two of <br />the bridges and at the mobile home park on the west bank upstream of the 7th Street <br />Bridge. Some flooding is predicted upstream of the railroad bridge on both sides of the <br />river. The wastewater treatment plant is within the 1 OO'year floodplain. During a 500- <br />year event, predicted river stages indicate 7th Street will be inundated and flows may <br />even pass under a viaduct and cause some shallow flooding to the northeast of the <br />viaduct. <br /> <br />4 <br /> <br /> <br />The predicted floodplain inundations are based on river stages for a steady state <br />condition at peak flows. <br /> <br />FEMA defines a floodway ".. .as the channel of a river or other watercourse and <br />the adjacent land area that must be reserved in order to discharge the base flood without <br />cumulatively increasing the water-surface elevation by more than a designated height." <br />