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<br />- 2- <br /> <br />attendees that 7,000 CFS releases was the least favorable option for downstream political and flood <br />mitigation reasons. <br /> <br />. Outcomes of that discussion were that the storage space allocation committee (Water Users <br />Sub-Committee) recommendation would need to be completed by September 30, 2004. <br />. We would want to elect a representative for the water users to chair the meetings and Larry <br />Lang will be act as a co-chair. <br />. The Water Users Committee meetings will base their discussion on the data from the Brown <br />and Caldwell study. The CWCB will Brown and Caldwell under contract for technical <br />assistance to this group. <br /> <br />Corps Reservoir Modeling Presentation <br />At the May 5th Chatfield Project Review meeting the Corps reconfirmed that the 20,600 AF of <br />proposed additional waster supply storage space would be available under certain changes in the <br />reservoir operations criteria. Bill Doan - Hydraulic Engineer with the Corps gave their presentation. <br />It was designed to give an overview and understanding of the Corps design and operation criteria for <br />Chatfield. Mr. Doan explained the difference between the reservoir design flood and the inflow <br />design flood. The Corps has completed the HEC-5 reservoir routing modeling analysis. <br /> <br />In 1991 there was new US ACE Dam Safety Criteria for New and Existing Dams: ER 1110-8-2(FR) <br />Inflow Design Floods for Dams and Reservoirs that requires five feet of freeboard for all Corps <br />dams. <br /> <br />Reservoir Design Flood: (or standard project flood) in combination with the starting pool level and <br />outlet works operation establishes the storage zones required to provide a level of protection for the <br />project. Refer to slide three (Condition 2) for this write up. This is the capture of floodwaters and <br />then the release pattern. This is a very large flood of the 500- or 1000-year flood magnitude. They <br />looked at the upper basin to come up with a large snowmelt inflow and used the 1942 snowmelt <br />flood and then factored it up. The blue line is the reservoir operations and releases under that <br />scenario. Then the Corps chose the 1965 rainfall event and put it on top of the snowmelt flood. <br />Operations include shut down of releases for five days then increase or ramp up about 500 CFS a <br />day. This is a flood storage pool of 107,000 AF of snowmelt waters and 28,000 AF from rainfall <br />waters. <br /> <br />New Flood Routing Criteria: In 1991 new criteria from the Corps Dam Safety required a <br />(antecedent flood) or pre-flood of 50% of the Probable Maximum Flood (PMF). This gives an error <br />margin and safety factor and assumes you have a big rainfall event before you have the PMF. The <br />process IS: <br />. Run a "Antecedent Flood" of Y2 PMF into the reservoir with no releases <br />. Release flows for 5 days at maximum downstream channel capacity (5,000 CFS) <br />. After five days route the full PMF through reservoir (determines the full spillway capacity). <br />. Need five feet of freeboard for dam safety and wave action <br /> <br />This analysis provided 2200 AF available for reallocation without modifications to Chatfield <br />Reservoir or a 1.5 foot rise in the current operating pool elevation. This allows for the required five <br />feet of freeboard in a the maximum flood event with out any dam safety improvements or changes in <br />operations. <br /> <br />Flood Protection - Water Project Planning and Finance - Stream and Lake Protection <br />Water Supply Protection - Conservation Planning <br />