2017-02-21_REVISION - M1988112 (3)

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The final recommendations and decisions as to design parameters and types of structural improvements were discussed and agreed to by the MGC design team and BMRI through a series of technical memoranda and meetings. The general approach that was agreed to and followed for project development was as follows: • Design Basis Flood: PMF based on Rocky Mountain Unit Hydrograph with Constant Loss Method (National Engineering Handbook, Part 630 (Hydrology) using saturated soil hydraulic conductivity rates (Ksat) by hydrologic soil group) assumptions, accommodated via a combination of storage and bypass channel routing, generally as follows: o bypass channel and drop structure will be upgraded to handle a peak discharge of approximately 1500 cfs during the PMF with a minimum freeboard/safety factor; o during a PMF event, a portion of the flood will flow over a 500 -ft long overflow section, which is essentially a low section on the up -gradient (east) end of the inboard (impoundment side) berm of the ditch, and will be stored in the tailing impoundment area; o the upstream overflow weir elevation and length are designed such that the channel and drop structure will fully contain (with no inflows into the impoundment area) on the order of a 500 -year flood event with full recommended freeboard and safety factor. • Channel and Drop Structure Improvements: o regrade channel to remove undulations and enable drainage where practical; o widen the channel bottom width to 15 feet and raise the inside berm as needed for the design basis flood; and o replace the conduit drop structure in left dam abutment with an open, straight concrete chute and stilling -basin type structure; sized for the design event(s) as described above. 1.3 Purpose and Scope The purpose of this report is to document the basis of design, including calculations, criteria, and design references for construction of improvements to the San Luis Project South Diversion Ditch system. 2.0 DATA COLLECTION 2.1 Mapping Lidar data from the Colorado Geological Survey and ground surveys of the tailing impoundment area and existing South Diversion Ditch channel and Drop Structure area were acquired in 2013 as part of the detailed dam safety evaluation. In order to expand the coverage and develop a Civil3D CADD model needed for design beyond the available surveyed areas, MGC subcontracted with Worthington -Miller Environmental (WME) to develop additional topographic contours using the lidar digital elevation model with GIS tools. The base maps thus created in CADD were then made available to the various design team members. ----------------------------------- --- San Luis Project - South Diversion Ditch Drop Structure - Final Design Report 4