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I I I I I I I I I I I I I I I I I I I SECTION I SUMMARY WATERSHED , 1- ("II The South Boulder Creek Watershed encompasses 135.7 square mile~ of area starting from 13,294 feet elevation at the Continental Divide to its co~nce with Boulder Creek at 5,160 feet elevation. As depicted on Drawings 1, 2, and 3 (see separate full size drawings), the watershed is 27.2 miles long on a generally east west axis, and up to 6.3 miles wide. There are about 93.2@5) above Gross Reservoir and 42.5 €i:P below. The watershed is heavily forested between elevation range 6,400 to 11,000 feet, particularly on the north facing slopes. There are significant areas of rock outcrop, and steeply sloped and narrow streams, and incised stream banks which can encourage flash by runoff response. The foothills are steeply sloped, have far less ground cover, and higher peak rainfall then the mountains; which would often lead to high runoff response. The lower valley and floodplain generally has higher soil infiltration characteristics, but substantial rainfall runoff response because of development impervious areas and street drainage conveyance. ~'-'e. - ~..J- Gross Reservoir, completed in 1954 as a water rese oir for the Denver System, inadvertently provides a substantial flood control f nc~on for the Boulder community. Its purpose is to store up to 43,06 AcFt\ Of water for domestic water supply. It is also authorized to generate power. Under current regulations, and due to the lack of any formal agreement to provide flood storage, this study can only assume storage above the crest of the spillway. The modeling herein estimates that during a 1 OO-year flood, assuming the reservoir was full to the spillway crest, that 1.780(;icr~ Rf dynamic storage would occur. The total 100-year rainfall volume tributary tOA~servoir would be approximately 20,400 AcFt., while the total Basin Rainfall Volume would be 29,000 AcFt. Actual runoff would be roughly 5600 AcFt. to the reservoir and 8000 AcFt. for the total watershed. Therefore, Gross Reservoir, even if full to the spillway crest, has a significant effect on hydrology. ~<!S, In the 1969 Event, all runoff (5485 AcFt.) from the m.~untains above Gross.l\ was captured with no discharge over the spillway. Grosd)ias a significant effect on lessening flood levels downstream even if full to the spillway, and greater under historic operating conditions. The reservoir statistics indicate that the average storage available for the months of May through August is 20% of the reservoir volume (8362 acre feet) with an I - 1