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,I I II I I I I I I I I I I I I I I I I Pertinent rainfall data and basin characteristics are shown on the Summary Sheet - Blackmer Gulch Tributary Hydrology, Table 1. Both lOO-Year Flood Hydrographs are shown on Figure 1. HYDRAULIC COMPUTATIONS Hydraulic calculations were don" using the HEC II cOl'lputer program for routing the lOO-year flood flows. A linear increase in flows was assumed from the upstream hydrologic point to 81acknler Lake. 131ackmer Gulch Tributary flows under E1 Camino Drive and Holly Street, cro~;';illlJ at both streets in corrugated "Ietal pipes. A third crossinq, located downstrealll of Holly Street at lIighline Canal, wilS analyzed as though tht' canal were a bridge with an insignificant sized opening. The starting water surface elevation for the lOO-year flood level at Blackmer Lake was taken as ele- vation 5459. Initial computer analysis indicated that flow in the gulch was near or at critical depth for the majority of the length. Flood plain encroachment was calculated for each cross-section based on an egual degree of encroachment on each overbank. The HEC II computer program Method 5 was utilized for the encroachment calculations. This method uses an opti- mization scheme to obtain a target difference in water surface elevation between natural and encroached conditions. The target water surface eleva- tion difference was set at 0.5 feet. Optimization towards the target figure was reguired because during the 100-year flood the majority of the gulch would flow at or near critical depth. Depths near critical can fluctuate greatly as a result of even minor changes in cross-section characteristics. A maximum of twenty-one trials was allowed in attempting to approach the target water surface difference. A summary of the flood hazard information resulting from the computer analysis is shown on Table 2. 600 500 o z 8 400 LlJ III II:: LlJ a.. .... LlJ LlJ u. () aI ::l () Z 300 200 ::: o .J u. 100 ENTIRE BASIN-AT BLACKMER LAKE PEAK::539CFS , VOLUME= 59.2 ACRE-FEET DRAINAGE AREA= 305 ACRES --~i- I I 00 UPPER SUBBASIN-AT EL CAMINO DRIVE PEAK=418 CFS VOLUME=24.1 ACRE-FEET DRAINAGE AREA=122 ACRES \ ~-~-~-\ ~- \ " 2 3 TIME IN HOURS 4 5 BLACKMER GULCH TRIBUTARY IOO-YEAR FLOOD HYDROGRAPHS FIGURE I - 2-