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<br />Coal Creek <br />- 3 - July 13, 1988 <br />At other locations along the channel, the channel bottom will be defined <br />t>y the depth of mining to bedrock. Because the depth to bedrock is not <br />4:nown, the longitudinal profile of the channel bottom is uncertain. <br />E-edload from upstream sources, which should remain unchanged, can be <br />expected to fill in these local depressions where they occur. A number <br />of low-water crossings, or fords, are to be constructed and left in place <br />tit four (4) locations dlong the 4.65 miles of channel proposed for <br />mining. The furthest one of these upstream will act as a headcut <br />control. Sediment can be expected to be deposited behind each one of <br />these fords, and preferentially above the ones located upstream. During <br />mining, sediment deposited in the channel will reduce sediment transport <br />downstream. This effect will continue after mining until the channel <br />achieves an equilibrium grade. <br />l:n the Phase II drainage report received July 7, 1988, the statement is <br />made on page 5 that: "sediment transport will not change from historic <br />conditions, therefore, the bridge piling will not be endangered by <br />channel degradation from the mining." In the conclusion on page 11, the <br />statement is made that: "Natural erosion will continue after the mining <br />process has ended. Some of this material is expected to aggrade with a <br />gradual refilling of the mining area over time." These two statements on <br />scediment transport appear to be inconsistent, and contradict one another. <br />F. comparison of the present channel location (Exhibit C) with the <br />proposed post-mining location (Exhibit F) shows significant changes. The <br />post~nining channel is shown to occur down the center of the area of <br />mining. Where this is narroa~, the channel is located in the pre-mining <br />position. <br />Mlhere the extent of mining is wide, the channel is placed in the center <br />of this area. As all the sand and gravel is proposed to be removed from <br />these areas down to bedrock, and fiine-grained silts and clays are <br />proposed to be re pi aced in the areas of mining, the actual river channel <br />grill vary from 130 feet to 175D feet wide and be from 8 to 22 feet deep. <br />1"hus, it is easy to say the channel has been designed to carry the 100 <br />year storm. What is difficult is to predict where the post-mining Coal <br />Creek stream will actually reside. <br />The bottom material of the post-mining channel will be fine-grained <br />particles, not alluvial -sand. This material will be easily eroded unless <br />it is cohesive_ During high flows, the stream is going to scour into <br />this bottom material, resulting in high suspended sediment concentrations <br />downstream. The stream will also erode laterally into the sandy stream <br />tank and redistribute this material as channel deposits downstream. <br />Trying to establish a stable channel and successfully revegetate the <br />rtiined areas upon reclamation will be difficult within a short amount of <br />tame. <br />1'he plan proposes that, with replacement of the fines in the channel <br />bottom, an a77uvial aquifer can be maintained. Although water will <br />