2014-03-07_REVISION - M1988112 (3)

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of the flip bucket discharge. According to the Drawings, the plunge pool has a bottom width of 5 ft, a minimum established depth of 2 ft and a design depth of 6 ft, and 2H:1 V side slopes. Calculations for the PMF included with the documentation identified in paragraph 4.1 were based on a rainfall event of 355.6 mm (14.0 inches). Using a Soil Conservation Service (SCS) curve number of 70.0, for the total combined area contributing to the basin of 830 acres, the calculations yielded an estimated runoff volume of 680 acre -feet, and a peak runoff discharge of 9,223 cfs. For events larger than the 100 -year storm up to a PMF event, the original operational concept was that the South Diversion Ditch would overflow, allowing much of the PMF flows to enter and be retained in the impoundment without overtopping the dam. The original hydrologic analyses were completed with the computer program WASHED using an SCS type II rainfall intensity curve. 4.2 INITIAL ASSESSMENT OF ADEQUACY OF FACILITY FOR FLOOD EVENTS 4.2.1 Flood Events Evaluated An Inflow Design Flood (IDF) has not been established for this facility, as the facility is not under the jurisdiction of the Colorado Division of Water Resources (DWR). For purposes of this dam safety evaluation, hydrologic and hydraulic check analyses and potential failure modes analysis were completed for two different flood events as follows: • 100 -year event, which was the basis for the operational storm water diversion facilities design; and • Probable Maximum Flood (PMF) event, which was the basis for establishing dam freeboard (non- overtopping) for the impoundment capacity. 4.2.2 South Diversion Ditch Through field observations it was apparent that the South Diversion Ditch is not configured in accordance with the generalized dimensions, geometry and slopes indicated in the design calculations as summarized in the documentation cited in Section 4.1. Ample depth appears to be present, but the slope of the channel invert varies and even reverses in some locations along the ditch. To further evaluate the channel geometry, topographic mapping was developed based on new field surveys performed in October 2013. The field survey data were augmented by use of USGS LIDAR data in areas outside the limits of the field surveys. The data were then utilized to develop CADD files from which cross - sections were cut for use in HEC -RAS modeling that was used to determine actual channel capacity, flow velocities and depths. These analyses are described in Section 4.3 of this report. The channel was found to have an irregular cross section and slope with localized depressions, constrictions, high points and locations with lowered embankments. These conditions were considered in the HEC -RAS modeling. San Luis Project Miller Geotechnical Consultants Tailing Dam Data Report 26 February 2014