Laserfiche WebLink
<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />001019 <br /> <br />morphology work presented in Section III-C. The section is comprised of a 230 <br />feet wi de mai n channel area approximately 4 to 5 feet deep and a total <br />floodplain/floodway width of around 700 feet and 9 feet deep. The main channel <br />configuration aproximates the existing shape that nature has preferred in the <br />project area, and has a capacity around the 10-year flood. <br /> <br />A third approach, consisting of a simple trapezoidal section was also <br />investigated. This option was not found to be acceptable due to the <br />potentially unstable hydraulic conditions which resulted in supercritical flow <br />and standing waves. To resolve this instability problem, a flatter channel <br />grade and a check structure was required. However, the flatter grade changed <br />the sediment transport characteristics of the creek, which would deposit large <br />amounts of sediment in the channel area. This condition was considered <br />undesireable due the maintenance, flood control, and cost factors therefore <br />this option was not recommended. <br /> <br />"K" - Levees to Contain Flood: This alternative consists of constructing <br />berms or levees along the channel bank to contain the IOO-year flood. The <br />1 evees, however, create 1 oca 1 flood problems and requi re spec i a 1 approval by <br />the County. the Urban Orainage and Flood Control District (UD&FCD), and FEMA <br />and therefore were not investigated further. <br /> <br />3. Evaluation of Channelization Alternative <br /> <br />Because of the potential benefits of channelization for reclaiming flood- <br />plain property, a detailed analysis of the hydraulic and sedimentation condi- <br />tions was performed. The analysis included an investigation of the effects of <br />channelization on roughness, flow stability, history of creek bed movement, and <br />definition of the floodplain. <br /> <br />To determine the appropriate roughness factor (Mannings n-value) for the <br />HEC-2 analysis, calculations were performed using the methods described in <br />Reference-ll and the grain size data obtained for the sedimentation analysis <br />(Chapter III). The calculated n-values ranged from 0.013 to 0.015. After <br />rev i ew of the assumptions and adj ustment for velocity and depth of flow, the <br />n-va1ue of 0.20 was considered appropriate for the main channel area and 0.025 <br /> <br />-27- <br />