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C. Appurtenant Structures <br />In order to size the spillway the inflow design flood for the spillway was determined. The flood <br />hydrology was performed in accordance with the SEO guidelines [SEO, 2007]. Both dams would be <br />considered High Hazard according to SEO guidelines, due to the development downstream of the <br />alignments. According to SEO guidelines a high-hazard dam must be able to pass the probable <br />maximum flood (PMF) resulting from the probable maximum precipitation (PMP). The PMF was then <br />routed through the basin and reservoir to determine required the spillway size. <br />The spillway chute, chute walls, and stilling basin were assumed to be constructed of structural concrete. <br />The final location and configuration of the spillway will be studied during the design process. <br />1. Flood Hydrology <br />The Grand Valley Lake dam alignments all lie entirely in the Colorado River Drainage. In 1984, <br />NOAA published HMR 49, Probable Maximum Precipitation Estimates — Colorado River and <br />Great Basin Drainages [NOAA, 1984], which were used for calculating the PMP values for the <br />study. By definition, the PMP is the largest rainfall, which s theoretically possible given terrain, <br />elevation, and location of the basin. Two types of PMP storms can occur. The general PMP <br />storm generally would be associated with the passage of a weather frontal system. The local <br />PMP storm is a result of a shorter duration, intense thunderstorm or cloud burst type storm. <br />The basin was delineated and hydrology performed for dam Alignment 1 only. Because the <br />alignments are so close in proximity, there should be very little difference between the <br />hydrology for Alignment 1 and Alignment 2. The basin area was delineated in ArcGIS and <br />determined to be 18.3 mi <br />The development of the unit hydrographs was based on the USBR dimensionless unit <br />hydrograph technique presented in the Flood Hydrology Manual [USBR, 1992]. A Kn value of <br />0.235 was calculated on the Uncompahgre River at Delta by the USBR [1992]. This value is <br />within the boundaries of Kn values based on the basin data, and was used to calculate the lag <br />time applied to the unit hydrograph. <br />The hydrologic soil groups were determined by visual field inspection and referring to the NRCS <br />soils map available in GIS shapefile format for download from the NRCS website <br />(http://www.soils.usda.gov/surve�geo�raphy/ssur�o�. An infiltration rate of 0.10 was used <br />based on the hydrologic soil type in the basin. <br />AECOM <br />34 <br />2/22/2010 <br />