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Page 3 July 22, 2008 <br />Tipping of GJ Pipe, Mr. Glenn Miller, Mr. Travis Marshall of DRMS, and Mr. Ken Jacobson of the <br />ACE. <br />Discussions with Mr. Glenn Miller involved a review of his primary comments and concerns that were <br />initially incorporated into the permit. WWL and WE acknowledged Mr. Miller's concerns and <br />addressed <br /> <br />Small floods may build-Lip sediment in the pit lake and later large floods may scour and re- <br />entrain this fine sediment. A given flood event could cause a "slug" of suspended sediment to <br />enter the downstream channel of the Colorado River. <br />The possibility of such an event occurring is real, however, HEC-RAS simulations indicate that <br />water velocities in the area of the pit during events with return periods that would inundate the pit <br />area) are relatively small. If,vegetation has been established in the reclaimed pit area, this will <br />further reduce scour of previously deposited sediment. In addition, the large discharge events will <br />be carrying relatively greater quantities of sediment in any case, the potential for a transient <br />"slug"of suspended sediment to be released may not create a perceptible change in overall river <br />behavior and morphology. With the originally proposed "Texas Crossing", flows through the pit <br />would be sufficiently large that the sediment scour potential would be great during larger river flow <br />events. However, as will be !,discussed in subsequent paragraphs, the proposed inlet structure has <br />been modified from the originally proposed Texas Crossing to consist of a pipe inlet. One of the <br />reasons for making this modification was to limit flow through the pit during high river flow events <br />and, thus, reduce the possibility of massive removal of previously deposited sediment due to pit <br />flow-through scour. <br />2. Larger, high velocity flood events will create a "plunge" pool 100 ft wide and 10 to 15 ft deep <br />on the downstream side of the inlet structure. <br />WWL and WE agree that such a pool could exist with a Texas Crossing type inlet design. WWL <br />and WE are proposing a culvert inlet design that will contain such a pool to an area specifically <br />designed to allow energy dissipation while minimizing the potential for creation of a large plunge <br />pool. <br />A moderate-to-large flood would discharge several 1,000 cubic ft per second (cfs) over the <br />inlet into the pit. "Such flows would likely destroy the structure, allowing rapid down-cutting <br />to the level of the riverbed, or lower." Such flows would divert most of the river flow through <br />the pit, and result in very, low flow or no-flow in the "island" area. <br />While given a proper design, it could be possible to design a Texas-Crossing that would be <br />sufficiently hardened against scour that it would not be destroyed during a large event, as <br />mentioned in following paragraphs, the flows and velocities through the inlet would be very high <br />and it would be very costly to sufficiently harden such a structure. A culvert design inlet would be <br />much simpler to construct, significantly less costly, less susceptible to damage during large flow <br />events and will help in controlling overall flow to the pit. <br />4. Head cutting in the main channel would progress upstream from inlet which could result in <br />drying-up of some sloughs or backwaters in the Walter Walker Wildlife Area. <br />See response to concerns 2 and 3 above. <br />Past experiments with similar inflow structures at the Walter Walker Wildlife Area and an old <br />pit near 30 Road failed, in ways similar to those described above.