C150313 Feasibility Study - Laserfiche WebLink

Home Browse Search

Cucharas Feasibility Study| Introduction 4 required to safely route the runoff generated by 75% of the probable maximum storm as defined by the National Weather Service (NWS). URS in their 2006 feasibility report estimated that the current spillway has a capacity of 42,000 cfs, but the routed reservoir outflow from the 75% Probable Maximum Precipitation (PMP) event was over 100,000 cfs and would overtop the dam by approximately 8 feet. Since that report, the SEO has promulgated new Rules and Regulations for Dam Safety and Dam Construction that allow reductions in the NWS PMP values for altitude and the use of a site specific weather model developed by the SEO for high altitude precipitation referred to as the Extreme Precipitation Analysis Tool (EPAT). Reductions of 40% of the PMP values have been observed in some basins with the use of this tool. Over 90% of the 640 square mile drainage basin flowing into the reservoir is above 6,000 ft. Mean Sea Level (MSL), the lower limit for use of the EPAT process. GEI has prepared a draft hydrology report which has estimated that a significant increase in the Probable Maximum Flood (PMF) is justified. This large increase and the results of the Incremental Damage Assessment (IDA) may allow for first phase spillway improvements to accommodate a smaller Inflow Design Flood (IDF). This would be based on the fact that a dam failure at the 10,000 AF of storage level would not impact the downstream reach any more than the 100% PMF. The hydrology report that has been prepared by GEI has not been peer reviewed or accepted by the State Engineer. Part of the future work in design studies would have to address this issue. We believe it is feasible to lower the existing spillway to an elevation that could safely route an approved IDF. The goal would be to only lower the spillway to an elevation that could safely pass the required IDF and maintain reservoir storage at the current fill restricted level while Phase 2 plans are being developed. This would be considered an interim fix. While there are significant structural issues with the present dam embankment that make permanent reservoir storage at higher elevations not feasible, the dam has performed adequately for the past twenty years at the current restricted level of gauge height 100 feet. No additional embankment consolidation or displacement has been measured and only minor increases in seepage have been noted at the maximum restricted storage level. Lowering the spillway by removing the ogee crest in the first phase appears to provide adequate protection of public safety while the evaluation and design of the second phase work is finalized. This also conserves funding to be used on a permanent fix in Phase 2. Given the proposed phased approach of dam improvements, the goal would be to minimize phase one work to meet public safety standards with the major expenditures being made in the second phase. For example, although the outlet works gates are currently in need of repair, they do function safely. Rather than spending money on replacing the gates in the first phase, it would seem prudent to defer this cost until a later phase when the decision is made on what type of dam would be constructed in that later phase. D) FIELD INVESTIGATIONS Existing data from previous field investigations was reviewed and determined suitable for this level of study. Further field investigations may be required based upon more detailed engineering analyses. Previous geotechnical investigations include those performed for the