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II -11 Maple Grove Reservoir. Maple Grove Reservoir is a water storage faci 1- ity owned and operated by the Consolidated Mutual \/ater Company. The dam, an earthen structure, is 34 feet high, has an approximate crest length of 1,000 feet and contains near 800 acre-feet of storage to its crest elevation of 5,529.5. Normally, the reservoi r operates at a level of 5524, storing approximately 580 acre-feet. The spillway is basically a conduit 84 inches in diameter, the hydraulics of which are primari Iy controlled by the inlet. We have estimated the spillway capacity to be 850 cfs with a water surface elevation at 5529.5 feet, taking into consideration the bevel shaped ent"ance and side chan- nel spillway entrance controls. This estimate is 300 cfs lower than estimates prepared by consultant to the Consolidated Mutual Water Company. It is apparent that over-topping would occur on a frequency simi lar to once-in-a-hundred-years. This situation will worsen as the Lena Gulch watershed is further developed. It would seem likely that the dam could not withstand the overtopping caused oy the once-in-a-hundred-year flood under future conditions. The low capacity spi llway situation_~usl~-E~9Irecte(~ so that there is adequate emergency capacity to safely meet the State Engineer's Dam Re- qui rements. Since this dam is above a heavi ly developed residential area, it poses a hazard which must be corrected. The Canso I i dated Mutua 1 Wate r Company's eng i nee r has proposed to remedy this situation by using the enti re darn crest for wei ,. flow and pro- tecting the face of the dam with a rock slope mattress. The entire downstream face of the dam and 5 to 10 feet up the abutment on both sides of the dam would be protected from the high velocity water as it would run down the slope during a design flood. The estimated cost of the rock slope mattress is $600,000. This amount cloes not include any major cost related to modification of th,e existing spillway, should such be requi red. The reader can refer to Section IX of this report for an evaluation of Maple Grove Reservoir. Bas i ns 9 and 11. The lower reach of Lena Gu 1 ch has been res t r i cted, real igned and abused by development to such an extent that almost the entire lower area of the watershed is subject to flooding. Three major items cause this flooding: I. The channel has been restricted to a minimum capacity by development. 2. The Red Barn Store was built over Gena Gulch, and the thalweg was replaced with a small conduit, resulting in the backup and diversion of the major portion of the flood flows to the north. 3. The Blevins property fill near Kipling Street diverts overflows fu rther to the north and causes hi 9he r damage to p r i vate homes as evidenced by the spring 1973 flooding. This situation must be recitified bY...Erc~tidi~~La clear outfa1...l.....!:..~ major flood.