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Mr. Rex Burns <br />Page Five <br />May 1, 1998 <br />When mining activities come within 440 feet of the river channel at the upstream end of the <br />Timnath Gravel Pit, the mining applicant has two options: (1) quit mining and leave the last 340 <br />feet of potential pit area undisturbed (there is a required setback of 100 feet in any case), or (2) <br />build agrade-control structure in the cut-off flow path to prevent channel cut-off. <br />Grade Control Structure <br />A grade control structure extending the full width of the proposed gravel pit and located at the <br />north end of the proposed pit would prevent erosion of the strip of land separating the river and <br />the pit and would prevent channel cut-off from occurring. <br />There are several options available for construction of the grade control structure. The options <br />include riprap, concrete blocks, formed concrete, soil cement, and~con ete le. The <br />applicant desires that the type of grade control structure be left unde ermined at this time, <br />provided that it is designed by a registered professional engineer before construction. Of the <br />alternatives examined, the applicant favors a structure composed of concrete rubble. This is a <br />material the applicant can accumulate over time in sufficient quantities for construction of such a <br />structure. We anticipate that the structure would extend from the undisturbed ground surface <br />down to bedrock. The width of the structure at bedrock level would be on the order of ~D feet; <br />final design will determine the specific dimensions. After construction of the grade control <br />structure, the area would be backfilled to final grade, and mining up to the-structure would be <br />possible without increasing the chances of a .cut-off. The grade-control structure would be left in <br />place, but would be buried and therefore not visible. <br />2. Impact on Flood Hydraulics and FEMA Aspects <br />Assuming that a channel cut-off does not occur, and that FEMA regulations are followed, there <br />should not be unacceptable impacts on the 100-year flood surface eCevations because the river <br />channel itself will not be affected. There is potential for some increase in 100-year water <br />'surface elevations due to activity in the overbank (primarily stockpiling of material). However, if <br />the applicant follows FEMA rules and does not stockpile soil or otherwise effectively raise the <br />ground surface within the designated floodway, any rise in the flood elevation will be within that <br />amount allowed Larimer County (1/2-foot, FEMA is less restrictive, permitting a 1-foot increase <br />in water surface elevations). The applicant is aware of the presence of and location of the <br />floodway, and has agreed that such "filling" activities will not occur within it. The only activities to <br />take place within the designated floodway will be excavation, which will not raise the 100-year <br />water surface elevation. <br />Conclusions <br />This letter summarizes our qualitative and quantitative analyses addressing the impact of the <br />Timnath Gravel Pit on flood patterns. We have shown that mining can take place without <br />adversely impacting the flooding patterns provided appropriate measures are taken. Such <br />measures include limiting the extents of mining or construction of a grade control structure in the <br />potential channel cut-off path and adhering to FEMA regulations regarding activity in a <br />designated floodway. <br />BURNS-4A.LTR <br />34-0648.00 <br />