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Horsecreek Reservoir Dam <br />1. No Action Alternative <br />One course of action considered was to not do no major repair or rehabilitation of the dam and outlet <br />works. The results of this alternative would not necessarily result in no cost to HID. The result of <br />not repairing the outlet and controlling the seepage in the dam will ultimately result in major <br />restrictions on the reservoir storage. We would expect restrictions due to the high phreatic surface <br />in the dam embankment to be required in the near future if seepage control improvements are not <br />initiated. Restrictions due to the outlet conditions would likely not be placed on the reservoir within <br />the next five years. However, both conditions can cause failure of the dam and the risk and cost of <br />the results of a failure would financially be catastrophic. <br />While this alternative has no immediate capital expenditures, the risk and potential costs can be very <br />great. A problem with the outlet or a slope failure in the dam embankment during the height of the <br />irrigation season coulcl result in the loss of the reservoir for the season. Thus the cost of not <br />providing for the dam repair can potentially very high even without failure of the dam. A problem <br />developing during the irrigation season can result in restrictions that could either require the reservoir <br />be emptied or alternatively only allow limited discharges. <br />2. Construction Alternative 1- Repair <br />This alternative considered repairing the outlet conduit by lining the existing steel pipes using Insitu <br />Form technology to insert a chemically stabilized liner in outlet. The Insitu Form liner will add to <br />the structural integrity and will seal the outlet from infiltration. <br />The concrete inlet and outlet structures will be patched and repaired as needed to maintain their <br />integrity. The control gate, stem, and operator will be checked for any problems and repaired to <br />reduce the potential for future problems or failure of the gate and control system. <br />To control the seepage in the dam embankment, a toe drain will be installed between stations 31+00 <br />to station 41 +00 along the downstream berm or downstream toe. The toe drain will consist of a 12 <br />inch PVC pipe surrounded by a minimum of 12 inches of sand as a filter and 12 inches of gravel. <br />The existing drain channel at the toe of the dam extending from station 25+00 to station 38+00 will <br />be filled. The channel will stripped of vegetation and topsoil, the bottom covered with C33 concrete <br />sand, and an 8 inch PVC drain pipe with 0.01 inch slots installed. The drain pipe will be covered <br />with C33 sand and the existing ditch filled to the surrounding ground surface level. The existing <br />drains that flow into the ditch will be tied to the drain line. <br />Included in this alternative is the installation of a Parshall flume in the outlet channel downstream <br />of the dam. This is needed to allow for accurate measurement of discharges from the reservoir. <br />Horsecreek & Prospect Reservoirs - 06.014 <br />Henrylyn Irrigation District Page 8 <br />