C153833 Feasibility Study - Laserfiche WebLink

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I I I I I I I I I I I II II I II I I I I I I I INDIAN CREEK RESERVOIR REHABILITATION An earth fill dam with a maximum height of 34 feet and a crest length of approximately 2000 feet impounds Indian Creek Reservoir. The reservoir stores approximately 1900 acre-feet of water at the decreed gage height of 26 feet. The existing Indian Creek Reservoir spillway is a poorly defined natural low area to the east of the left abutment of the dam as shown in Figure 6. The area surrounding the reservoir has been subdivided into 35-acre parcels with each parcel having lakefront boundaries. The spillway area lies within the property boundaries of three of these parcels. Land owners have planted trees, built structures, and placed underground electrical lines within the spillway, The spillway does not meet State regulations and NPIC has been given until August 9. 2001 to bring the spillway into compliance, Spillway design has been complicated by the fact that NPIC has an operation and maintenance agreement on Flood Retarding Dam B-4 upstream of Indian Creek Reservoir and is therefore liable for handling floodwater from this dam entering Indian Creek Reservoir, Dam B-4 is a class II dam and was not designed to pass 75 % of the Probable Maximum Flood as is required of Indian Creek Reservoir. In a flood greater than 50 % of the PMP, Dam B-4 would most likely breach, sending a large sudden inflow of water into Indian Creek Reservoir, Preliminary surveying conducted for this feasibility study indicate that there is a low portion of the Cheyenne Ditch. adjacent to where Indian Creek enters Indian Creek Reservoir, that is several feet lower in elevation than the crest of the Indian Creek Reservoir Dam, It appears that this low area is actually the historic route of Indian Creek and that at least a portion of the water from the Indian Creek Basin, including water released from dam B-4, would flow through this route and not enter Indian Creek Reservoir as all previous hydrology studies have reported, This low section of the existing ditch bank is at USGS elevation 5345 feet and its location is shown in Figure 6, Further survey data is needed, but preliminary elevations indicate that it would be possible to lower this portion of the ditch embankment further to allow more water to follow the natural drainage rather than enter Indian Creek Reservoir, To assure that water would follow this route, an earthen berm could be built across the Indian Creek drainage immediately upstream of the Reservoir, The location of this berm is shown in Figure 6. The Indian Creek drainage basin covers over 16 square miles and would produce a volume of runoff that is very large compared to the capacity of the reservoir. For this reason, it is possible that an incremental damage study could be the most economical way to deal with spillway adequacy, While providing a spillway compliant with State Regulations is the top priority, seepage at the toe of the dam has also been reported in the latest State Engineers Inspection Report. Making repairs to the dam while construction equipment is mobilized for the spillway project could result in long-term savings. Having two spillway areas for consideration, the possibility of having to make repairs to the dam in the near future. and the large drainage basin being a good candidate for an incremental damage study. results in a multitude of options possible for this project. Three options were analyzed for cost. The first of three options considered for the Indian Creek Spillway would fill the existing spillway to dam crest height and add a new 100-foot long spillway on the existing dam, This new spillway would be designed to pass a minimal amount of floodwater and an incremental damage study would be conducted to determine NPIC's liability for damage caused by the remaining floodwaters. The cost estimate for this option is found below in Table 7. Page 15