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and post -flood conditions. The BEMA is restoring the dams back to original condition with no <br />enlargement of the dams or reservoirs. As such, it was logical to assume a datum relative to <br />elevations used for reconstruction of the dams. For this investigation, elevations of the spillway <br />crests for each dam were set to the respective elevations for the proposed reconstruction; as <br />presented in Big Elk Meadows Association Dam Reconstruction Project Hydrology Report by G E I <br />Consultants, Inc., February 2014. The datum for the reconstruction project is based on the <br />National American Vertical Datum of 1988 (NAVD88). All units for the models and results <br />presented in this report are U.S. Customary. <br />10.5 Basin Delineation <br />Drainage basin delineation was performed using a combination of the United States Geological <br />Survey (USGS) StreamStats Web -based GI interactive mapping tool, USGS Hydrologic Unit Code <br />(HUC) GIS shapefiles, and various other GIS georef ere nced mapping sources. Basin delineation <br />was included for basins tributary to the BEMA dams and drainages tributary to the X Bar 7 cross <br />section. Figure 17 shows the basin delineation used in the rainfall and runoff models. <br />J t MMUGGrNS GULCR <br />MEADOW <br />yea ' <br />"4 , .. ; J <br />.L©WER WEST FORM <br />M1 '' <br />y ®F <br />71y i �N • e.IXla Fi <br />CORK <br />- CCpffL41Ei� iiiFFF��' <br />� a,� Lsysnd <br />Figure 17 - Map of basin delineation for rainfall- runoff modeling showing the locations of interest for model results. <br />Limits of drainage basin delineation were chosen to enable modeling of the runoff contributions <br />from selected drainages at several locations of interest. The runoff into each reservoir is critical to <br />understanding the magnitude of overtopping that led to the failure of those dams. The extent of <br />damage to US Highway 36 in the vicinity of the confluence of the main stem of the Little <br />Thompson River with the West Fork of the Little Thompson River is also of interest. The study <br />needed to be able to determine with some certainty the timing of the peak discharges from each <br />Report of the September 2013 Little Thompson River Flooding COLORADO <br />and Big Elk Meadows Dam Failures, June 2014 (Revised, Dec 2014) Division of Water Resources <br />Page 20 of 48 Department or r.alup.M 4„5o rc�, <br />DAM SAFETY BRANCH <br />