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FLOOD10929
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Last modified
1/26/2010 10:13:24 AM
Creation date
10/29/2007 3:08:11 PM
Metadata
Fields
Template:
Floodplain Documents
County
Mineral
Community
Creede
Stream Name
Upper Willow Creek
Basin
Rio Grande
Title
Upper Willow Creek Watershed - FLood Control and Stream Stability Study
Date
10/1/2002
Prepared For
Willow Creek Reclaimation Committee
Prepared By
Agro Engineering
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />Section 2 - Analysis of Problems <br /> <br />2. 1 Development of Topographic Information <br /> <br />Good topographic information was necessary to meet the project objectives. The requirements <br />for topographic resolution and accuracy varied throughout the study area depending on the <br />required analysis. <br /> <br />2.1.1 Low-Resolution Topography <br />The watershed condition analysis required general topographic elevation and slope data. To <br />meet this requirement, large scale but lower resolution topographic data were derived from <br />United States Geological Survey (USGS) Digital Elevation Model (DEM) coverages of the area. <br />USGS DEMs are digital files that document the elevation of an area on a regularly spaced grid. <br />The elevations are derived from available USGS topographic quadrangle (quad) maps. For the <br />Creede area, DEMs are available with coverages equal to the 7.5-minute quad map and a 30- <br />meter square grid. Although each DEM grid cell records the average elevation to the nearest <br />meter, the actual resolution is similar to the original quads maps contoured at 40-foot intervals. <br /> <br />DEM grids of four quadrangles (Creede, San Luis Peak, Half-moon Pass, and Wagon Wheel <br />Gap) were acquired and processed for use in ArcView Geographical Information System (GIS) <br />software. The DEM data is represented in Figure 2.1.1 with elevation represented by color and <br />shaded topographic relief. The Willow Creek watershed boundary is shown with a thick line. <br />The DEMs were then used to generate topographic contours for use within the GIS. 40-foot <br />elevation contours of the watershed area and the watershed boundary are shown in Figure 2.1.2. <br /> <br />2.1.2 High-Resolution Topography <br />The 40-foot topographic resolution from the DEMs was insufficient for the flood elevation <br />analysis. Therefore, traditional survey data and color aerial photographs were used to develop <br />high-resolution topography in and near the floodplain of Willow Creek within the study area. <br /> <br />A grid of survey points was measured in the floodplain upstream of the flume entrance, on the <br />Commodore Mine tailings piles, and at a few additional locations in order to create topography <br />in the wider floodplain areas. At these locations, a differentially corrected Global Positioning <br />System (GPS) unit was used to record the horizontal position, while traditional optical surveying <br />was used to measure an accurate elevation. 661 points were measured using this method. The <br />horizontal accuracy of differentially corrected GPS readings is typically about 1 meter, but the <br />steep canyon walls of Willow Creek degraded the GPS signal and accuracy at several locations. <br /> <br />Although the sub-meter horizontal GPS accuracy was adequate in the floodplain, higher <br />horizontal resolution was desired in the main channel of Willow Creek for hydraulic modeling. <br />Channel cross-sections were measured using a higher resolution method at regular intervals at <br />representative and critical locations. These cross-sections comprised the majority of the cross- <br />sections used in the hydraulic model. A survey tape was fixed across each cross-section, and <br />points across the tape were surveyed for elevation. The horizontal location of the cross-section <br />end points was established using GPS. Fifty-five cross-sections were surveyed from the flume <br />entrance to the Commodore Mine on West Willow Creek and to about 2000 feet upstream of <br /> <br />2-1 <br />
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