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Table 1 FEMA Flood Flows 10 year 50-Year 100 year 500 year 1,095 cfs 1,480 cfs 1,550 cfs 1,700 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 many consecutive days.Long durations of moderate flows can be more erosive than short duration, higher velocity flow events. Sizing an appropriate sized low flow channel is difficult. There is no maintain flow gage for the creek and thus determining channel forming discharges is quite difficult. Field parameters are non- existent due the constant maintenance activities on the channel. On the north branch we erred on the side of a larger, wider channel knowing deposition would occur. Small gravel/silt point bars developed within the graded low flow channel. With this channel we believe a different and simpler design approach should be used. We believe creating a channel that has a bottom contour which pivots upon the center point of the channel to the left and right to create a thalweg. The representative sections show this concept. Thus the low flow channel will adjust to its appropriate ® dimensions and more of a transitional vegetation zone will develop based inundation regime and geometry instead of sole on channel geometry. 4.2 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 develop the recommended channel configuration and ultimate design channel roughness parameters. 4.3 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 25.83 feet. The upstream channel invert was 27.66 feet creating a 2-foot drop in elevation. The length of the new channel is approximately 3,200 feet. The slope between these two points along this alignment is generally 0.0006 ft/ft(very flat gradient). When this slope is imposed on ground surface profile along the alignment a channel depth between 10 and 12 feet is obtained. Questa Engineering Corporation 6 270124CDR/November 16, 2007