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AYRES <br />ASSi1C~~~T- <br />May 1, 1998 <br />Mr. Rex Burns <br />Latimer County Engineering <br />218 West Mountain Avenue <br />Fort Collins, Colorado 80521 <br />Re: Timnath Gravel Pit <br />Dear Rex, <br />At the request of Dan Kehn, representing Don Kehn Construction, Inc., we have investigated the <br />potential hydraulic impacts of their proposed mining operations at the Timnath Gravel Pit. The <br />Timnath Gravel Pit is located in the SW'/4 of Section 11, T6N, R68W of 6th P.M. and the NE%4 of <br />the NW'/4 and the NW'/4 of the NE'/< of Section 14, T6N, R68W of 6th P.M., in Latimer County. <br />Our investigation is in support of their application for a mining permit through Latimer County. In <br />our investigation, we examined two specific issues related to potential mining activities: <br />1. Potential for Channel Cut-off <br />2. Impact on Flood Hydraulics, and FEMA Aspects <br />These issues are addressed individually in the following paragraphs. Supporting computations <br />and figures are included as attachments to this letter. <br />1. Potential for Channel Cut-off <br />The potential for the river to shortcut through the Timnath Gravel Pit and abandon the current <br />river channel is of significant concern. As shown in Figure 1, the proposed mining area is <br />located in the neck of a meander on the Poudre River. Meander necks are prime candidates for <br />river cut-off because the potential channel gradient across the neck is much steeper than it is <br />along the existing ^ver channel. Construction of a large grave! pit in the neck area, separated <br />from the river only by narrow strips of channel bank, provides essentially a pilot channel for a <br />channel cut-off, making scut-off much more likely. During the flooding that occurred along the <br />Poudre River in 1983, two such cut-offs occurred (near Putt-Putt Golf and Batting Cages on <br />West Mulberry Street, and near Fort Collins Wastewater Treatment Plant #2 [WVVfP2]. The cut- <br />off near WWTP2 led to abandonment of the main channel near the treatment plant outfall, <br />resulting in water quality problems downstream of the outfall. Ultimately, a large earthen plug <br />was constructed, forcing river flows back into the old channel. <br />There is potential for a similar adverse impact should a river cut-off occur through the Timnath <br />Gravel Pit, for the diversion structure for Greeley Canal No. 2 is located on the meander bend in <br />question. If a cut-off were to occur, it is possible that the river channel carrying water to the <br />diversion structure would be abandoned and much agricultural land to the east would be without <br />irrigation water until some means of returning flow to the original channel is constructed. <br />- .- . BURNS-4A.LTR <br />Engineers/Scientists/Surveyors 34-0648.00 <br />