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Page 5 <br />July 22, 2008 <br />existence of such a dike would result in a maximum increase in river stage (comparing conditions prior <br />to pit excavation and no dike with conditions after pit excavation and construction of the proposed dike) <br />of 0.25 ft. In fact, placement of the dike actually results in reduced river stages at river station 2.3 of as <br />much as 0.4 ft during the 100-year flow event because of dynamic critical flow characteristics <br />associated with river channel morphology during this event. The impact of the proposed dike on final <br />river stage at various flows is shown on Figure 2. The dike would prevent more direct, higher velocity <br />upstream flows from entering the pit, but would not prevent the pit from flooding as increasing flows <br />would still enter the pit from the outlet, or downstream area of the pit. This pit flooding from the <br />downstream end would continue to provide an opportunity for the river to breach the barrier between <br />the river and the pit and, possibly, eventually scour enough of the barrier that most, or all of the river <br />flow would be captured by the pit. <br />It follows that if such a dike were constructed and large flows occurred, a greater percentage of flow <br />would be realized in the area of Interstate 70 and the island during the peak flow time. The impact of <br />this to river morphology can not be easily distinguished from such an event without the dike in place. <br />Proposed River Capture Mitigation <br />In order to address the concerns regarding the possibility of the pit capturing the river flows and <br />becoming the main river channel, two provisions are proposed and discussed as follows: <br />A levee dike is proposed to be constructed along a portion of the pit, from the upper end <br />(approximately river station 2.7) to just downstream from the upper erid of the island (about <br />river station 1.95). The dike will be constructed of overburden material which has previously <br />been removed from the pit and will be compacted. It will have a top width of 12 ft, river-side <br />slopes of 3-ft horizontal to 1-ft vertical and pit-side slopes of 2-ft horizontal to 1-ft vertical. It <br />will not be armored on the river side but will be revegetated with native vegetation to minimize <br />scour potential. The top elevation of the dike will be at least 2 ft above the 100-year river <br />stage. It must be recognized that, because the proposed dike does provide a complete barrier <br />between the river and the pit, and it is not armored, it only provides partial protection against <br />river capture by the pit.' <br />2. The most likely sequence of events which would result in river capture by the pit are as <br />follows: <br />a. The part of the,natural (unmined) barrier between the river and the pit which will be <br />overtopped initially is just downstream from the split in the river at the upstream end of the <br />island, at approximately river station 1.95. Because the water level in the pit would be lower <br />than the river stage under those conditions, the water from the river will cascade into the pit, <br />creating a scour point. <br />b. The pit outlet would be overwhelmed by the amount of flow entering the pit and <br />scouring in the vicinity of the outlet would occur, further lowering the water level in the pit and <br />increasing the scour in the area of the original barrier breach. <br />C. Scour in the vicinity of the original barrier breach would cut head-ward as well as <br />laterally and eventually reach a point in the river near the upper end of the island. At that <br />point, most, if not all, of the river would be diverted into the pit and, subsequently, into the <br />south channel.