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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 Div DATE: 29 Sep 82 GMT H2 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 500-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 developed for the Cherry Creek Dam 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 adyerse'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 examining 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 500-year snowmelt event. The following paragraphs describe the adjustments 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, which 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 reevaluation study we used a lO-year spring flood as the coincident event. It was determined on a volume probability 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 5-day shutdown period and stepped release rate used in the design studies was used in the new operation of the 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 alOOO-year spring inflow coincident with the rainfall runoff flood of record (June 1965). The reevaluation study was based on a 500-year spring inflow coincident with a May I-day, 10-year event on Plum Creek at Louviers. Although our 500-year ratio was only 1.63 times the 1942 flood versus the design ratio of 1.7 for the lOOO-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 5 days to 2 days and the releases were stepped at 1000 cfs increments per day instead of the design 500 cfs. These results are also shown in the table below. 2