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BOARD01480
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BOARD01480
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Last modified
8/16/2009 3:02:20 PM
Creation date
10/4/2006 6:56:19 AM
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Board Meetings
Board Meeting Date
3/10/1998
Description
WSP Section - Colorado River Basin Issues - Adaptive Management Work Group
Board Meetings - Doc Type
Memo
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<br />. <br /> <br />- <br /> <br />. <br /> <br />-' <br /> <br />static with time. Additional thought was also being given to the location of stored sediment in <br />the canyon and the mechanisms for moving sediment from the channel bottom to eddy areas. , <br />Extensive modeling by the sediment researchers changed to a great degree the way in which <br />transport mechanisms were viewed. The long term balance of sediment in the Grand Canyon <br />continued to be an important issue in these discussions. <br /> <br />Sediment researchers now believe that flood flows counteract the possible adverse impacts that <br />fluctuations have on beach erosion, thus rebuilding the deposits that would eventually slough <br />back into the eddies, regardless of the nature of the powerplant operations. Some suggested that <br />more frequent floods could allow higher levels of fluctuations. <br /> <br />The Agreement Contained in the 1996 AOP <br /> <br />With this evolving positive view towards spills, a desire for a test of the GCDEIS Beach Habitat <br />Building Flow was expressed by the Transition Work Group beginning in 1994. This request for <br />a purposeful powerplant bypass was strongly opposed by the Basin States because of the <br />GCDEIS language triggering such bypasses, claiming a violation of the 1968 Colorado River <br />Basin Project Act provision of avoiding anticipated spills, interpreted as powerplant bypasses. <br />This opposition created an impasse that blocked such a test. <br /> <br />Additional discussions between members of the Transition Work Group and the Basin States <br />resulted in a proposal for a modification of the GCD EIS preferred alternative, that of moving <br />Beach Habitat Building Flows (BHBF) from years oflow reservoir conditions (when spills <br />would not be required for hydrologic reasons) to years of high reservoir conditions and high <br />inflows. Thus a BHBF would occur in years when there was an expectation of having a <br />hydrologically induced spill. This agreement was institutionalized in the 1996 Annual Operating <br />Plan for the Colorado River, signed by the Secretary of the Interior in December 1995. A <br />subsequent BHBF test was conducting in April 1996, confirming the hypothesis that high flows <br />could rebuild sandbar deposits. In December 1996, the GCDEIS Record of Decision was signed <br />by the Secretary of the Interior and included this modification of the preferred alternative. <br /> <br />Options and Impacts of Using Spillway Gate Extensions <br /> <br />This section addresses key questions raised earlier in this report which combined raise the issue, <br />"Should the extensions be installed?". <br /> <br />The need to reduce the frequency of powerplant bypasses <br /> <br />Current thinking among sediment experts is that, given high flow conditions resulting from large <br />runoff years, releases above 25,000 cfs should be preceded by BHBF's. The BHBF should be <br />greater in magnitude than the highest expected future release. This not only moves sediment <br />higher on beaches away from future releases, but also coarsens the main channel bed which <br />reduces future sediment transport. <br /> <br />The occurrence of high release years is fundamentally tied to the statutory operation of Lakes <br />
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