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stream Channel, Rocky stream Banks, Brushy Grassy Channels stream Basin, Composite 0.03 to 0.05 0.10 to 0.15 0.04 to 0.07 0.08 to 0.10 I I I I I I I I I I I I I I I I I I I I B-7 III. FLOOD LEVEL DETERMINATION A. FACTORS AFFECTING FLOOD LEVEL Factors affecting the flood level produced by the selected flow include characteristics of the stream bed, channel sides and basin, and stream bed slope. Quanti- fication of the stream bed and channel characteristics involves the most judgement. Our calculations are made using Manning'S formula for open channel flow (V= 1.486 R 2/3 sl/2 ). n We feel that for this relatively short stretch of river, that this approach is sufficient, and that step- backwater type calculations are not needed. The hydraulic radius (R) and stream bed slope(s) are readily determined for any stream section. The rough- ness factor (n) is subject to judgement. Normal ranges of values for (n) are as follows: B. COMPARISON WITH OBSERVED CONDITIONS In this case we compared our theoretical flood level calculations with actual conditions observed in 1983 at a location just upstream of the island. Actual condi- tions in 1983 indicated a water level of approximately 7978 at a flow of 2500cfs. Using (n) values of 0.03 for the stream bed and 0.07 for the composite basin, we cal- culated a water level of 7978.4 for the same flow. Al- though these values are somewhat lower than we would normally select, the calculation results compare favor- ably, and on the conservative side, with the observed conditions. C. SUMMARY OF RESULTS Locations of stream sections are shown on the attached Figure 4. Results of calculations for a flow of 4600cfs are summarized as follows: SECTION FLOW DEPTH WATER SURFACE ELEV 1 4.45 FT 7976.15 2 5.45 FT 7978.70 3 4.35 FT 7979.30 4 6.15 FT 7982.35 5 7.45 FT 7984.65