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I I I I I I I I I I I I I I I I I I I 001021 Thirdly, a detailed analysis of the currently defined lOO-year floodplain (Greiner, 1984) revealed that the flow conditions are really two separate, hydraul ically disconnected flood profiles for Reach A only. When modelled using HEC-2 (a quaisi-two dimensional analysis) to more closely match the actual flow conditions. the lOO-year water surface profile in the main channel was found to be generally hi9her than previously predicted. Also considerably less flow was found to be in the right (east) overbank area. The analysis was performed using both the high and low n-values to determine the worst conditions. Us i ng the above fi ndi ngs, the pre 1 imi nary des i gn approach for overbank channelization with improvements to the main channel was revised from that previously discussed at the progress meetings for the project. Channel improvements were considered which shaped the main channel to more closely match the average active bed width for Cherry Creek or let the main channel in essentially the present condition, and excavated the overbank area to provide additional capacity. Both configurations were evaluated in detail and recommended for portions of the study area. The proposed channel for Reach A more closely matches the existing channel and therefore the sedimentation and erosion characteristics are similar to the present conditions and will have minimal impact on downstream property owners. Since the unstable flow conditions are in the main channel. the channel banks must be protected at the critical location. Whereas an unprotected artificial channel would not be as stable as the natural channel, a program of regular maintenance combined with bank protection at the critical areas will address this concern. The channelization recommended for portions of Reach B included excavation for the overbank area and revetments for the existing bends in the main channel. By leaving the main channel in its historic configuration. the changes in the sedimentation characteristics are minimal for low to inter- mediate flows (up to the lO-year flood). For larger floods, the increased overbank depths and removal of existing vegetation increases the potential for erosion. To minimize this potential, the overbank areas must be revegetated in accordance with the program outlined in Section V-F. -29-