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- 2 - CorDS of En2:ineers ReDort Study Overview . Marty Timmerwilke presented the current Project Performance Schedule and Estimated Project Costs (attachment). There is $276,000 in the President's budget for the Chatfield Project. The Corps will not likely get all ofthat, but they now have adequate funds to keep this project going. . Contract amendment # 3 allows the funding to transfer from the CWCB to the Corps. The change order and the FCSA are mechanisms to bring the water users into the agreement. Antecedent Flood Study (PowerPoint Attachment) . Doug Clementson reported that the Corps has not yet completed the Antecedent Flood Study (AFS), but results are forthcoming. . The existing flood storage allows for 210,000 AF. The proposed flood pool would reduce that amount to around 190,000 AF and therefore allow for water supply storage. . The Corps Engineering and Design Regulation for "Inflow Design Floods for Dams and Reservoirs" is 1110-8-2 (FR). . One of the factors that dictate the amount of needed flood storage space is the antecedent flood. The antecedent flood was originally estimated at one half (50%) of the Probable Maximum Flood (PMF). However, Corps analyses have indicated that the 50% assumption is very conservative. An antecedent flood value of 40% ofPMF (or lower) is more realistic. . One year ago three alternatives were proposed to get to the target pool volume of20,600 AF. They were (1) widen the spillway 100 feet at an estimated cost of$18 million, (2) build a three-foot (parapet or wave action wall) at an estimated cost of $2 million, or (3) take a closer look at the Corps Inflow Design Flood Criteria and do an Antecedent Flood Study. Option 3 was selected and is nearly completed. . The current operating criteria require that the Chatfield Reservoir outflow would be shut down for five days following a major event, based on the assumption that areas in the lower basin are saturated and receiving tributary inflow as well. After the 5 days, the Corps would then ramp up outflows on a graduated basis to 5,000 cfs to evacuate the flood storage pool. . It is likely that a 3-day shut down period is realistic and could be achieved through a revision of the operating manual. A 3-day vs. 5-day shut down would increase the evacuation time of the flood pool. . Water Users mentioned that the water yield from this new storage space is based mostly on free river conditions on the South Platte and may only come a few years in ten. The Corps acknowledged but that, but reaffirmed that the analysis must be based on a worst-case scenario. For public safety, the full water supply storage pool needs to be assumed. . Mr. Clementson has provided the preliminary results of the study for peer review by the Corps Hydrology Committee. There do not seem to be any major obstacles regarding the AFS at this point. . The current schedule is estimated to be: 1) Refine Monte Carlo simulations by September 1st, 2) Complete draft report by September 15th, 3) Complete Internal Technical Review by October 15th, 4) Obtain approval by Northwest Division by November 15th, and 5) Obtain approval by Headquarters by December 15th. This represents a slippage from the initial deadline of September 30th. . Key summary items: o Corps staff will recommend that a waiver be approved to use an antecedent flood assumption of 40% of the PMF, rather than the original 50%. o Corps staff will recommend that the operating criteria be revised in order to allow for the flood pool to begin evacuation in three days instead of five days. Releases will gradually increased up to the 5,000 cfs maximum release rate. o The Corps has determined that dam safety would not be compromised if the above recommendations are accepted. Flood Protection . Water Project Planning and Finance. Stream and Lake Protection Water Supply Protection. Conservation Planning