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<br />GLENWOOD SPRINGS FIS <br /> <br />C. Surveys <br />The digital terrain used in the hydraulic and hydrodynamic models was based on <br />aerial surveys conducted for the City in November 1990. AutoCAO files with vectored <br />contours at 2 ft intervals were then imported into a Microstation formatted design file. <br />These aerial surveys were flown at relatively low flows but, no new bathymetric data was <br />collected as part of this study. The channel bottom along each of the reaches was <br />estimated from bridge surveys. Cross-sections and terrain information required for the <br />models were then generated from this digital terrain model. <br /> <br />4. River Floodplains <br /> <br />A. HEC-RAS Analysis, Flood Hazard Estimates for the Colorado and Roaring Fork <br />Rivers, March 1998 <br />HEC,RAS (River Analysis System Version 2.0, April 1997, developed by the <br />U.S. Army Corps of Engineers-HE C) was utilized to model the Colorado and Roaring <br />Fork Rivers near their junction at Glenwood Springs. HEC,RAS is a computer program <br />used to calculate water surface profiles for steady, gradually varied flow in channels. The <br />system can handle a network of channels, as well as a single river reach. Subcritical, <br />supercritical or mixed flow regime profiles can be calculated. Computations are based on <br />the solution of the one-dimensional energy equation (standard step method). Energy <br />losses are evaluated by friction (Manning equation) and contraction/expansion (a <br />coefficient is multiplied by the change in velocity head). The momentum equation is <br />utilized in situations where the water surface profiles rapidly varies, such as around <br />bridges and at stream junctions. The CADD software MicroStation and the Civil Design <br />software INROADS were used to generate the cross sections in HEC format from a <br />terrain model of an aerial survey performed in 1990. Profiles and river stage infonnation <br />for the 10,,50-, 100- and 500-year return period flows were determined from the <br />subsequent HEC,RAS runs. <br /> <br />No bathymetry was performed at the time of the survey, the channel cross section <br />below the water surface on the day the topography was flown is not available. The cross <br />section below the water surface was estimated using a few past surveyed cross sections, <br />and the underwater portion of the cross section was increased or decreased in size until <br />the low flow water surface determined from HEC,RAS matched the water surface <br />indicated by the topography. The estimated portions of the each cross section typically <br />represented 5 to IS percent of the 100'year flood conveyance area thus having a minor <br />impact on the I OO-year water surface profiles. <br /> <br />3 <br /> <br /> <br />Some of the available bridge plans provided were old and hard to read. A field <br />visit verified some of the bridge data, but no field cross section survey of the channel <br />geometry and bed or water surface slopes was conducted. <br />