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® Table 1 FEMA Flood Flows l0 year 25-Year 100-year 500-year 730 cfs 940 cfs 1,025 cfs 1,110 cfs Looking only at peak flow results in an incomplete hydrologic analysis.Because large magnitude flows are controlled by upstream dam releases, the duration of flows can be extended over the period of days rather than hours that would be expected in natural system.Typical winter release flows can be maintained at 100 cfs for manyeonsecutive days.Long durations of moderate flows can be more erosive than short duration, higher velocity flow events. 3.2 Hydraulic Considerations When designing the reconstructed channel,numerous hydraulic factors need to be taken into account. The channel must function as a natural riparian corridor, require little or no maintenance once the vegetation is established, and should provide 100-year flood protection accounting for a heavily vegetated floodplain.The vegetation planting and subsequent management is very important to the hydraulic function of the channel. The physical constraints of the site, including limitations on channel slope and depth,must be balanced with the riparian functions of the system.The discussion below explains some of the physical factors and vegetative concepts were used to the develop the recommended channel configuration. 3.3 Channel Slope and Depth The channel slope and depth of the new channel was driven by the invert elevations of the existing channel at the start and end of the new channel.The invert elevation at the beginning of the channel realignment is 24.25 feet The upstream channel invert was 31.25 feet creating a seven-foot drop in elevation.The length of the new channel is approximately 7,600 feet. The slope between these two points along this alignment is generally 0.001 ft/ft(very flat gradient)When this slope is imposed on ground surface profile along the alignment a maximum average channel depth of the six to seven feet is obtained. 3.4 Vegetation Class Concepts and Management The intent of the project is to create a vegetated channel that meets flood control and habitat restoration objectives. The benefits of riparian (creek side)vegetation are numerous. Riparian areas provide significant ecological, water quality, flood management, and aesthetic functions. Riparian vegetation provides shade which helps keep creek water cool for invertebrates and fish,and is a food source for some of these species. Herbaceous vegetation and shrubs provide soil cover, and help prevent erosion. Further, the roots of riparian trees help stabilize the soil and protect against bank failure and deeper-seated erosion. • Questa Engineering Corporation+ 3 99279CDR/Ap)-ii 6, 2000