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7 <br />1) an existing watershed runoff interc <br />runoff from a 100 -year, 24 -hour sto <br />and analysis of Ditch D -1 is pan of <br />Stockpile). <br />Post -it® Fax Note <br />7671 <br />Date l <br />► � � <br /># of b, <br />To "1 <br />From <br />fir <br />4 V1,Q, <br />Co. /Dept. <br />Co. <br />Phone # <br />Phone # <br />Fax # <br />Fax # <br />2) a silt fence below the pile. This fence will be constructed prior to any placement of development <br />waste on the pile. <br />3) circumvential ditches (ditches DWP -W and DWP -E) around the upper portions of the pile. Runoff <br />from the upper half of the pile would be divided and flow around the toe -of- slopes in these ditches. <br />These ditches would also capture runoff from small watersheds located between the interceptor ditch <br />(Ditch D -1) and the circumvential ditches. These circumvential ditches will be feathered out inside <br />(uphill) of the perimeter silt fence to distribute discharge along portions of the lower silt fence (and <br />thence into vegetative debris filters). These ditches will be constructed prior to any placement of waste <br />on the pile. During construction, - entrenchment of these-- ditches will be arrested by the exposure and <br />accumulation of the cobble sized fraction of the site soils. After construction (no more development <br />waste to be placed on the pile), these circumvential ditches will be seeded and protected with mulch. <br />These circumvential ditches are designed to accommodate 100 -year, 24 -hour storm events. <br />4) after construction is complete, the pile and disturbed areas about the site will be stabilized with <br />vegetation (seeding and mulched). <br />• <br />.55 natural vegetation and vegetative debris ground litter (equated to a grass filter). Runoff from the <br />downhill slopes of the pile and discharge from the circumvential ditches would flow through the lower <br />silt fence and into the vegetative debris filter. The filter will provide final filtration of all the runoff <br />from the pile. <br />This analysis addresses a "worst case" scenario, i.e., the site is protected by an interceptor ditch, the circumvential <br />ditches have not been vegetated, and the entire pile is considered as constructed with no ground cover. No credit <br />is given to silt fences or straw bales with regard to sediment control or mitigation of affects on the quality of storm <br />runoff. Partial implementation of all the best management practices identified will reduce the potential for erosion <br />and provide a degree of impact somewhat less than the "worst case" scenario. <br />1.3 Criteria <br />To qualify as a Small Area Exemption (SAE), runoff from disturbed ground must not exceed effluent limitations <br />during a 10 -year 24 -hour storm event of 0.5 milliliters /liter (ml/1) of peak settleable concentration (SSp) of sediment <br />(reference Rule 4.05.2, Division of Minerals and Geology). Thus water quality analyses are based on this storm <br />event. <br />The development waste pile is to be protected from 100 -year, 24 -hour, storm events [Rules 4.10.3(2) and <br />4.09.2(7)]. The interceptor and circumvential ditches are designed to provide this level of protection. <br />A 0.3 foot of freeboard for diversion ditches is required by the Division of Minerals and Geology [Rule 4.05.3(7)]. <br />A 0.5 foot of freeboard is used for to design for all the ditches, however, to add an extra measure of protection. <br />(West Elk Lone Pine Gulch Development Waste Pile SAE, 12 October 1995, Page 2 of 7) g- <br />