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I I I I I I I I I I I I I I I I I I I Hydraulic Analysis Water surface elevations for the 10-, 50-, 100-, and 500-year floods were computed by the U.S. Army Corps of Engineers' step backwater com- puter program HEC-2. (Reference 11). The version used was the HEC-2 release dated November 1976, updated August 1977, with corrections and modifications. Manning's "n" values for the channel were computed using the formula n = 0.222(S + 0.001)0.33 where S is the channel slope. This formula was extracted from a paper by V. V. Golubtstov in "Soviet Hydrology: Selected Papers," published in 1969 by the American Geophysical Union (Reference 12). Resultant "n" values are listed in Table 5. Stream Lefthand Creek James Creek Little James Creek Table 5 Manning's Roughness Coefficients "n" Values Overbanks 0.080 0.080 0.080 Channels 0.060-0.120 0.075 0.080 Corps of Engineers. Water surface elevations for James Creek were started at the flood elevations resulting from coincident discharges on James Creek. Bridges were analyzed using a blockage criteria dependent upon bridge construction and water depth. Concrete and steel bridges were assumed unobstructed until the upstream water surface elevations reached the bridge "low steel" eleva- tion at which time the bridge was assumed fully obstructed. Wooden bridge decks were assumed destroyed due to debris. This type of bridge was assumed unobstructed at all discharges with wingwalls and abutments in place but the deck removed. Head losses at fully obstructed bridges were determined by weir computations. Unobstructed bridge losses were computed by using the normal bridge routine in HEC-2. Velocities throughout this study were sufficiently high (greater than 10 feet per second at some locations) that significant movement or shifting of the stream bed is anticipated. This shifting of the bed may cause some changes in the flood outline or profile which are impossible to predict. The results obtained from the HEC-2 computer model were verified by comparing them to ground photographs of the 1969 flood through Jamestown. No other high water mark data were located which could be used to calibrate the hydraulic model. Starting water surface elevations for Upper Lefthand Creek were obtained from the Lower Lefthand Creek Study conducted in 1979 by the 9