2014-04-09_REVISION - M1977493 (2)

Home Browse Search

Ray Lazuk March 19, 2014 Page 8 flow planes and channel roughness coefficients of 0.1 for the collector channels were selected based on ground surface characteristics. 3.5 Base Flow The NRCS Fremont Pass SNOTEL site is located on Chalk Mountain near Fremont Pass, at the southwest side of the Climax Mine site. The snowpack typically peaks in May and snowmelt runoff typically occurs over about a 45 -day period in June and July. To account for the scenarios where the 10 -year rainfall event could overlap the tail end of the snowmelt runoff period, the drainage basins were modeled both with and without a snowmelt base flow. A snowmelt base flow rate of 7.76 cfs per square mile of drainage basin area was used for this analysis. This rate coincides with the 30 -day average snowmelt runoff from a 2 -year snowmelt, based on a previous analysis of the Black Gore Creek basin near Vail. The Clinton Canal portion of the East Interceptor was modeled with a base flow of 100 cfs to account for possible releases from Clinton Reservoir. This is a conservative assumption, because the Clinton Canal is typically operated at a much lower flow rate. A 1996 report by Resource Engineering indicates that the Clinton Canal channel would begin to erode at a flow rate of about 100 cfs even though the channel volumetric capacity is much greater. A base flow of 100 cfs in the Clinton Canal was selected for this analysis because it is unlikely that the Clinton Canal would intentionally be operated at a flow rate exceeding this limit. Note that releases from Clinton Reservoir into the Clinton Canal are controlled by the Clinton Reservoir Company. 4.0 FLOOD ROUTING CAPACITY HEC -RAS geometry, including the cross - sectional dimension, alignment, and profile data, were developed from the surveyed cross sections for each of the interceptors. A Manning's roughness coefficient of 0.030 was used for the main channel and a Manning's roughness coefficient of 0.035 was used for the left and right overbank areas in the HEC -RAS model. These Manning's roughness values were selected to be somewhat conservative to produce a deeper water depth. Lower Manning's roughness values would produce a slightly higher flow velocity and lower water depth. Steady state and unsteady state simulations were used to route the runoff hydrographs generated from the HEC -HMS model through each of the interceptors