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~' . " . .' MROED-HE SUBJECT: (1 Jul 82) Evaluation of Potential Water Supply Storage at Chatfield Reservoir in Colorado TO: Chief, Planning Div FROM: Chief, Engrg Diy DATE: 29 Sep 82 CMT 112 1. We have completed our preliminary reevaluation of the Reservoir Design Floods on Cherry Creek, Bear Creek and Chatfield. These reevaluation studies were based on providing enough storage at each project to control the standard project rainfall flood or the SOO-year snowmelt flood using the original design release criteria. A detailed outline of the studies that were made for all three projects is presented in the following paragraphs. ,a. Cherry Creek. A standard project flood was deyeloped for the Cherry Creek D~ study which was included in our studies of the Sand and Toll Gate Creek basins. The total volume of this flood can be stored below spillway crest. However, the adverse'emergency operating characteristics of this project make it a very unlikely candidate for use as a water supply storage facility. In addition to the freeboard being inadequate during a probable maximum flood event, the flood water is directed into another watershed as soon as the pool level is higher than the spillway crest. b. Chatfield. The Chatfield Reservoir Design Flood was selected by exaoining "three conditions that can cause major flooding on the South Platte River at the damsite. Based on a review of the design criteria used in developing each rainfall flood condition it became apparent that the criteria in each case was more severe than a standard project or a SOO-year snowmelt event. The following paragraphs describe the amjustments that were made under each condition to change the level of protection to the standards discussed at the outset. (1) Condition ,1. A combination of the 1942 snowmelt flood, wbich was the record spring flood in this region. and a May'standard project flood was the basic assumption for this condition in the Chatfield Reservoir Design Flood. In our ree aluation study we used a 10- ear s rin flood as the coincident event. It was determined on a volume pro ability basis from the South Platte River gaging station record at South Platte using April-May flows only in a 60-day analysis. This resulted in a Chatfield coincident inflow volume of about 40% of the 1942 snowmelt flood. The same S-day sbutdown period and stepped release rate used in the 'design studies was used in the new operation of tbe standard project flood inflow. A comparison of the old and new storage requirements is presented in table ~. (2) Condition 2. This condition was based on a 1000-year spring inflow coincident witb the rainfall runoff flood of record (June 1965). The reevaluation study was based on a SOO-year sprinR inflow coincident with a May I-day, lu-year event on Plum Creek at Louviers. Although our SOO-year ratio was only 1.63 times the 1942 flood versus the design ratio of 1.7 for the 1000-year event, our distribu- tion was more critical during the storage inflow period. Because our rainfall runoff inflow was considerably less, the' shutdown period during, the flood operation was reduced from S days to 2 days and the releases were stepped at 1000 cfs incr~ents per day instead of the design SOO cfs, These results are also shown in the table below. 2