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In reference to the boreholes and control building area, collection berms will be established downslope of the area
<br />• to direct an estimated 0.12 acre-feet of runoff from the 10-year, 24-hour event to either the adequately-sized natural
<br />depression west of the area, or to a pair of rock filters on the north side.
<br />EMD Dewatering Treatment Facility -The SAE demonstrations for the EMD dewatering ponds located in the
<br />NW1/4SW1/4 Sec. 21, TSN; R86W, 6th P.M. on Binco silty clay (a D soil) cover approximately 3.12 acres of
<br />disturbance and 4.4 acres total. SAE demonstrations have characterized three areas; the west side of the topsoil pile, the
<br />east side of the topsoil pile, and the cut-slope west of the dewatering ponds. Sediment control will be achieved by
<br />seeding and mulching the topsoil pile and cut-slope with 2 tons/acre straw. Vegetative filters of unmanaged cropland
<br />vegetation will provide adequate sediment detention to achieve effluent limitations following a 10-year, 24-hour storm
<br />event. Peak flows from this storm are 1.14 cfs, 0.64 cfs, and 0.38 cfs for the upland and west side of the topsoil pile, the
<br />east side of the topsoil pile and the area downgradient, and the cut-slope.
<br />Fish Creek Ventilation Borehole and Dewatering Site (Revised - MR07-216) -Another SAE exists at the Fish
<br />Creek Ventilation Borehole/Dewatering Site. A vegetative filter will provide sediment control for the 0.6 acre area
<br />of disturbance. Until the vegetation becomes reestablished, however, the 0.06 acre-feet of runoff from the 10-year,
<br />24-hour event will flow to the collection area around the Fish Creek Borehole ponds, where it will be controlled
<br />and treated by the gravel filters in the ditch. This SAE is shown on Map 26, Fish Creek Borehole Area Facilities
<br />and Access Pond.
<br />Rockfall Hazard Trenches -There are three rockfall hazard mitigation zones. The zones consist of various
<br />combinations of trenches and berms established between the Twentymile Road and the Twentymile sandstone cliffs
<br />north of the facilities area. Two to three SEDCAD+ demonstrations were developed for each trench; one addressing
<br />the overlying watershed with the accompanying trench, and the other addressing the outslope. The west rockfall hazard
<br />trench was divided into segment A through G, and segment G through L (Sheet 4, Exhibit 7F). For Segment A through
<br />. G of the West Rockfall Hazard Trench, the SEDCAD+ demonstrations document attainment of water effluent
<br />limitations fora 10-year, 24-hour storm following treatment of 56.47 acres, of which 5.42 acres are disturbed.
<br />SEDCAD+ modeling anticipates a 10-year, 24-hour peak flow of 3.3 cfs. The trench will be mulched with 2 tons/acre
<br />of mulch, following seeding and prior to the development of a vegetated channel, and runoff will flow through a natural
<br />vegetative filter 100 feet long at the outfall at point G. For Segment G through L of the West Rockfall Hazard Trench,
<br />the SEDCAD+ demonstrations document the attainment of water effluent limitations fora 10-year, 24-hour storm
<br />following treatment of 25.9 acres, of which 1.85 acres are disturbed. Segment G through I consists exclusively of a
<br />berm. SEDCAD+ modeling estimates a 10-year, 24-hour peak flow of 1.77 cfs. The trench will be mulched with 2
<br />tons/acre of mulch, following seeding and prior to the development of a vegetated channel, with discharge at point J.
<br />The three West Rockfall Hazard Trench outslope demonstrations (A-G, G-J, and J-L) document the adequate treatment
<br />of 4.93 disturbed acres using mulched disturbance areas and 5-foot long vegetative filters. The SEDCAD+
<br />demonstrations are provided in Exhibit 8.'
<br />The Water Tank Hazard Trench demonstration consists of the successful treatment of 6.65 acres, of which 0.65 acres
<br />are disturbed. SEDCAD+ modeling indicates a 10-year, 24-hour peak flow of 0.61 cfs. The trench will be mulched
<br />with 2 tons/acre mulch, following seeding and prior to establishment of a vegetated channel. There are two rock check
<br />dams in the trench; one at across-section T, shown on Sheet 3 in Exhibit 7F, and the other at the outfall of the trench.
<br />A 20-foot vegetative filter below the lower rock check dam treats the remaining water. The outslope of the Water Tank
<br />Trench and anticipated associated construction disturbance consists of 0.44 acres, resulting in a peak flow of 0.42 cfs.
<br />The disturbance will be treated with 2 tons/acre mulch and the downgradient 20-foot vegetative filter will achieve
<br />effluent limitations fora 10-year, 24-hour storm. The SEDCAD+ demonstrations are included in Exhibit 8.
<br />The East Rockfall Hazard Trench demonstration consists of the treatment of 18.7 acres, of which 2.22 acres are
<br />disturbed. SEDCAD+ modeling predicts a 10-year, 24-hour peak flow of 2.12 cfs. The trench will be treated with 2
<br />. tons/acre mulch following seeding. A contour furrow will be established approximately halfway (30 inches) up the cut
<br />to aid in sediment retention. Rock check dams will be established at cross-sections P, N, and M, as well as at 255 feet
<br />below cross-section O and 160 feet below cross-section N, as shown on Sheet 3 in Exhibit 7F. A 175-foot vegetative
<br />filter below the outfall of the East Rockfall Hazard Trench will result in achievement of effluent limitations following a
<br />TR08-65 2.05-79.2 08/20/08
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