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<br />M1592 <br /> <br />Alternatives, Including Proposed Act/on <br /> <br />2-22 <br /> <br />2.4.3.2 Site Selection <br /> <br />Sites for bank stabilization were selected based on a set of technical criteria applied <br />equally to all potential sites. At the COE's recommendation, a bank energy index (BEl) <br />was used to sort and categorize potential sites by susceptibility to erosive flows. This <br />index is a measure of the hydraulic energy available to erode the channel banks. The <br />concept was originally developed by Harvey and Mussetter (1993) for application to the <br />American River in California and has been successfully applied in evaluating other rivers <br />since that time. The advantage of the BEl is that it accounts for both the magnitude and <br />duration of the stresses applied to the channel boundary (i.e., the channel bank). The <br />index is calculated by integrating the stream power4 over the time under consideration. <br /> <br />Current BEl values range from a low of 6 to over 400, and average about 57 through the <br />overall study reach, Approximately 20 percent of the BEl values exceeded 85. Most of <br />the sites with a BEl greater than 85 are currently either actively eroding or have existing <br />bank protection indicating a strong erosion tendency at that location. Sites with smaller <br />BEl values are most likely associated with a very low rate of erosion because of the local <br />hydraulic conditions and/or bend geometry. Therefore, a threshold value of 85 was <br />selected below which the erosion potential is not considered significant. This value was <br />used to select sites for consideration in preparing the bank stabilization plan. <br /> <br />A threshold BEl value of 85 indicated an erosion risk, and a pre- to post-project BEl <br />increase of greater than 10 percent established a significant project impact. The intent is <br />to protect infrastructure and active agricultural lands, avoid channel confinement <br />whenever possible, and keep stabilization on terrace margins whenever possible. The site <br />selection process is illustrated in Figure 2-9. In this flow chart, the bracketed numbers <br />refer to the following protection criteria: <br /> <br />[1] Proximity to infrastructure <br />[2] Proximity to agricultural fields <br />[3] Special circumstances (These are areas where the BEl thresholds of criterion 2 <br />should be waived. They are primarily agricultural fields where the BEl is below <br />85, yet field inspection indicates that the bank is nonetheless actively eroding.) <br />[Ml] Monitoring of infrastructure (These are locations where existing protection is <br />probably adequate; however, if significant project-related erosion occurred, it <br />would have to be corrected.) <br />[M2] Monitoring of agricultural areas (These are places where criterion 3 does not <br />apply or a reasonable buffer currently exists between the cultivated bank and the <br />river; however, if significant river movement occurred, the banks would need to <br />be protected.) <br /> <br />Forty-nine areas, with a combined linear extent of 18,325 ft, were identified in the <br />Uncompahgre Bank Stabilization Study as recommended locations for bank stabilization. <br /> <br />4 <br /> <br />Stream power is the product of the average main channel velocity and the shear stress acting on the bed <br />or banks. <br /> <br />AS Lateral Hydropower Project <br /> <br />July 2000 <br /> <br />:1 >~1 <br />