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<br />Memo to Wallace Erickson I I Ntay 3 2001 <br />the proposed operation. To better withstand scour and down cutting during a flood, riverside berms <br />must have a stand of vegetation that closely resembles native vegetation along the stream in question. <br />Based on observations at the Line Camp Pit location, this requirement is met. In fact, the land between <br />the river and the proposed north pit is well vegetated and wooded. <br />To determine if the riverside bean width of 200 feet along the south reach of the proposed operation is <br />adequate, a comparison of the hydrographs of the South Platte in Adams County and the Dolores River <br />in Dolores is illustrative, remembering that a 400 foot crest width is considered protective in all cases on <br />the South Platte. Information provided for United States Geological Survey Gauging Stations 06720500 <br />at Henderson in Adams County, Colorado and 09166500 at Dolores indicates that the watershed above <br />Henderson is 4713 square miles and the watershed above Dolores is 504 square miles. This is a 9-fold <br />difference in the size of the basins contributing to flooding at these two locations. The historic <br />hydrographs for the two gauging stations, copies of which are enclosed, indicate that both streams are <br />subject to frequent snowmelt runoff flooding and that peak flows on the South Platte are substantially <br />higher volume than on the Dolores. Most importantly, the duration of flooding on the South Platte is <br />appreciably longer than on the Dolores. A hydrograph for 1984, which was a flood year on the Dolores, <br />is enclosed. The graph shows that flows on the Dolores exceeded 3000 cfs for approximately three (3) <br />weeks that year. A hydrograph for the South Platte during 1995, a flood year, is also enclosed. This <br />graph shows that flows exceeded 3000 cfs for more than eight weeks during that flood. Longer duration <br />floods have much greater scour potential, and a greater potential for damage if a river is routed into <br />adjacent gravel pits. <br />By analogy with the standard setback practices on the South Platte, the DMG has determined that the <br />200 foot setback proposed for a small portion of the Line Camp Pit operation is adequate, and that this <br />issue need not deter DMG from recommending approval of the Line Camp Pit application. The well <br />vegetated and 500 foot wide setback at the upstream end of the pit also enters into the DMG's finding <br />on this matter, in that the upstream end of the pits is the most likely location for scour to initiate during a <br />flood. Also entering into the decision is the fact that the Dolores River flow drops below 100 cfs on an <br />annual basis (see attached historical hydrograph), which allows for relatively straight forward diversion <br />and repair procedures in the event the riverbank berms are ever damaged by flooding. <br />attachment(s) <br />cc: Harry Posey, DMG (w/attachments) <br />Carl Mount, DMG (w/o a[tachments) <br />c:hvindmvs\persanal\line camp pit 3.doc <br />