C150138 feas study - Laserfiche WebLink

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I',' , Alternative 1- Rehabilitate dam to current height of Gage Height 20 feet II' We have investigated the feasibility of rehabilitating the two dams and spillway which form Knoth Reservoir. The items of concern to the State Engineer are addressed by the rehabilitation and the dams brought up to current standards. The first alternative is to rehabilitate the dam to approximately its current height and size which will store approximately 220 acre-feet of water with three feet of freeboard. The repair work included in Alternative 1 is shown on Figure 2 and includes the following. 1 2 3 � 5 6 7 8 9 Reshaping the upstream face of both dams to flatten the slopes from approximately 2.3 to 1 to 3 to 1. This will require removing the existing riprap from the upstream face of the dams, stockpiling it, and replacing it after placing fill and reshaping the upstream face. The reshaping is necessary to not only flatten the slope but to place a compacted clay liner over the upstream face to inhibit seepage. A cutoff trench will be excavated at the upstream toe of the north dam and tied into the clay liner indicated above to inhibit the seepage through the dam foundation. The cutoff will extend in depth to the underlying shale which is approximately fifteen feet below existing grade at the dam toe. On the south dam, where no seepage has been noted in the past, a cutoff trench similar to that on the north dam will be excavated to a depth of only five feet to act a cutoff between the dam embankment and the dam foundation to ensure seepage does not occur at this contact. A toe drain will be installed along the north dam to intercept seepage that bypasses the cutoff trench. A spillway approximately 200 feet in length will be constructed at the right abutment of the south dam. The spillway will consist of a concrete cutoff wall and riprap for erosion protection. The dam is currently a Class 3 small dam, however, the State Engineer has indicated the dam will most likely be reclassi�ed as a Class 2 dam which will require the spillway pass an inflow design flood from a 0.5 PMP storm. Both dams will have riprap erosion protection placed on the upstream face of the dam from the toe to the dam crest. The existing riprap will be utilized as well as importing additional riprap to fmish the job. The dam crest on both dams will be raised to gage height 20 feet. Currently the crest varies from gage height 17.5 feet to 20 feet. The two abandoned outlet works will be excavated, removed, and backfilled with compacted clay backfill. The existing outlet will be repaired as needed to placed it into good working condition. The condition of this is acceptable as far as can be determined at this time. Some adjustment of the gate and operator is expected and some patching and repair of the concrete is expected. Install an 8' x 8' box culvert at the inlet end of the reservoir to allow for access. , Knoth Reservoir-Feasibility Study � Page 7