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accompanying trench and the other treating the outslope. The west rockfall hazard trench was divided into the segment <br />for A through G and the Segment G through L (Sheet 4, Exhibit 7F). The segment from A through G of the west <br />rockfall hazard trench documents the attainment of water effluent limitations fora 10-year 24-hour storm following <br />• treatment of 56.47 acres, of which 5.42 acres ate distwbed. SEDCAD+ modeling anticipates a 10-year 24-hour peak <br />flow of 3.3 cfs. The trench will be mulched with 2 tons/acre of mulch following seeding, prior to the development ofa <br />vegetated channel, and will flow through a natttral vegetative filter l00' long at the outfall at point G. The segment <br />from G through L of the west rockfall hazard trench documents the attainment of water effluent limitations fora l0- <br />year 24-hour storm following treatment of 25.9 acres, of which 1.85 acres are disturbed. Segment G through [consists <br />exclusively of a berm. SEDCAI?+ modeling anticipates a 10-year 24-hour peak flow of 1.77 cfs. The trench will be <br />mulched with 2 tons/acre of mulch following seeding, anticipating the development of a vegetated channel with <br />discharge at point J. The three west rockfall hazard trench outslope demonstrations (A-G, G-J, and J-L) document the <br />adequate treatment of 4.93 disturbed acres using mulched disturbance areas and 5' long vegetative filters. The <br />demonstrations are filed in Exhibit 8. <br />The water tank hazard trench demonstration consists of the successful treatment of 6.65 acres, of which 0.65 acre is <br />disturbed. SEDCAD+ modeling anticipates a 10-year 24-hour peak flow of 0.61 cfs. The trench will be mulched with <br />2 tons/acre mulch following seeding in anticipation of creation of a vegetated channel. There are two rock check dams <br />in the trench: one at across-section T, shown on Sheet 3 in Exhibit 7F, and another one at the outfall of the trench. A <br />20' vegetative filter below the lower rock check dam treats the remaining water. The outslope of the water tank trench <br />and anticipated associated construction distwbance consists of 0.44 acre resulting in a peak flow of 0.42 cfs. The <br />disturbance will be treated with 2 tons/acre mulch and the underlying 20' vegetative filter will achieve effluent <br />limitations following a 10-year 24-hour storm. The demonstrations are filed in Exhibit 8. <br />The east rockfall hazard trench demonstration consists of the treatment of 18.7 acres, of which 2.22 acres are disturbed. <br />SEDCAD+ modeling predicts a 10-year 24-hour peak flow of 2.12 cfs. The trench will be treated with 2 tons/acre <br />mulch following seeding. A contour furrow will be established approximately halfway (30") up the cut to aid in <br />sediment retention. Rock check dams will be established at cross-sections P, N, and M, as well as at 255' below cross- <br />. section O and 160' below cross-section N, as shown on Sheet 3 in Exhibit 7F. A 175' vegetative filter below the outfall <br />of the east rockfalJ hazard trench will result in achievement of effluent limitations following a 10-year 24-hour storm. <br />The outslope of the east rockfall hazard trench consists of 2.25 acres of fill and associated construction disturbance, <br />whicfi is predicted to result in a peak flow of 2.16 cfs following a 10-year 24-hour storm event. The outslope will be <br />treated with 2 tons/acre mulch and a 20' wide vegetative filter will result in the achievement of effluent limitations <br />following a 10-year 24-hour storm event. The demonstrations are filed in Exhibit 8. <br />TCC intends to attempt collection of a limited number of samples at rock filters during a future precipitation event to <br />further support current field observations. Samples will be compared to applicable stormwater limitations for <br />Settleable Solids. <br />109 Diversion Filter <br />The sediment control for the underground rock filter for the 109 diversion will be provided by an existing rock filter <br />adjacent to Pond F, the vegetation surrounding the site, and the rigid revegetation ofthe site. The site is approximately <br />0.06 acres in size and protected immediately upslope by a collection ditch; this limiu the amount of overland flow that <br />the site will see. <br />Southwest District Ventilation Borehole and Substation <br />The sediment control at the Southwest District Ventilation $orehole will be provided by placement of afilter-fabric <br />fence downslope of the site. In the event the filter-fabric does not function adequately, rock filters may replace the <br />filter-fabric fence at the appropriate drainage points. The ventilation borehole site is approximately 0.2 acres. <br /> <br />APPRO~IE~ ,Jili~l 2 8 2000 <br />PR 99-OS 2.05-83 2/24/99 <br />