Laserfiche WebLink
rap dispersion basins to prevent erosion and re-silting . <br />before the water reports to the river. The ponds are <br />designed to hold a sediment load of one-tenth (1/10) acre- <br />foot of sediment per acre of drainage area. The ponds can <br />retain more than the runoff from a tOYr-24Hr storm event <br />with a full sediment load. The runoff will be retained <br />until the water has cleared, at which time the manually <br />operated discharge gates will be opened. After draining, <br />the gates will be closed to catch further precipitation <br />runoff. <br />Water quality i:; also maintained through measures other than <br />sedimentation ponds and diversions. Sedimentation ponds are <br />expensive both in terms of mining costs and in cost to the <br />environment as they distroy much grazing and foraging ground <br />that would otherwise be unaffected. Light-activity areas <br />west of the main access road have been surfaced with gravel <br />in order to prfavent silting of runoff waters. The areas <br />around the west portal facilities have been surfaced with <br />six inches of cleaned gravel as an alternative to pond <br />settlement of silt. All waters treated by alternative means <br />report directly to the Purgatcrie River. <br />En¢ineerinQ Design Calculations <br />For estimating :surface runoff the Soil Conservation Service • <br />has identified the topsoil in and around the Allen Mine area <br />6 <br />