2017-02-21_REVISION - M1988112 (3)

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2.3 Hydrologic Investigations The South Diversion Ditch was originally designed and approved for operational purposes to bypass around the impoundment upland runoff from a 100 -year flood. Larger floods were intended by design to overtop the ditch and flow into the impoundment, in which adequate freeboard would be maintained to contain floods up to the PMF event without overtopping the dam embankment. Engineering analyses provided in the Detailed Inspection Report (MGC, 2014) concluded that the hydraulic conveyance capacity of the existing Drop Structure may be less than was calculated by the original design analysis. The closed -conduit design of the structure makes it susceptible to clogging by debris during a large flood event. The updated hydrology presented in this section and Appendix B of this report indicates that the current 100 - year, 24-hour flood estimate is greater than the estimate for that event which was the original basis of design. These conditions are adverse factors that increase the risk of overtopping of the Drop Structure that could result in scour at the left dam abutment and possible release of tailing and impounded flood waters. This was an identified and highlighted potential failure mode (see Detailed Inspection Report, MGC, 2014). A design objective is that the facility perform within tolerable risk guidelines for dam safety, which were defined in the Detailed Inspection Report (MGC, 2014). An Inflow Design Flood (IDF) has not been established for this facility, because the facility is not under the jurisdiction of the Colorado Division of Water Resources (DWR). However, with the objective being to reduce risk to below tolerable risk guidelines for similar facilities, BMRI has opted to upgrade the South Diversion Ditch and Drop Structure to improve protection for major flood events up to a Probable Maximum Flood (PMF). With this objective in mind, BMRI directed the MGC engineering team to design upgrades to the South Diversion Ditch and Drop Structure with the following specific hydrologic design goals: 1. Intercept and divert upland runoff from events on the order of a 500 -year storm, with full design freeboard on the entire diversion system (ditch and drop structure), and with no release of intercepted flows into the reclaimed tailing impoundment area. 2. Split flows from larger events such that a portion of the inflow is stored behind the dam with adequate freeboard to dam crest, and the remainder of the flood is routed around the dam with minimal freeboard on the diversion structures at the extreme PMF discharge. The original hydrologic basis of design was updated as part of this study because of changes in available hydrologic data and analyses procedures since the original design was completed, as documented in the project records (BMRI, 1989). Hydrologic studies in support of the diversion system upgrade design are described in a separate report prepared by DOWL (2015), which is attached as Appendix B to this Design Report. The chosen Design Basis Flood (DBF) is the probable maximum flood (PMF) based on the Rocky Mountain unit hydrograph with Ksat infiltration assumptions. Table 2.1 summarizes the estimated peak flows from each of the three sub -basins (A, B, and C) of the contributing watershed above the South Diversion Ditch. Maps showing the sub -basin delineations are provided in Appendix B. Table 2.2 lists the contributing flow from each sub -basin, concurrent with the total flow below Basin C. The data summarized on Table 2.2 were used in HEC -RAS routing for sizing and design of the channel improvements and drop structure (described in Section 3 of this report). San Luis Project — South Diversion Ditch Drop Structure - Final Design Report 10