2011-06-29_PERMIT FILE - C1981019A (26)

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C: EXHIBIT 7, ITEM 20, PART G TERRACE DITCH DESIGN A series of terrace ditches will be constructed across the face of the permanent fills. The locations of these are shown on Map 41A. These ditches will be approximately located along the 100 foot postmining contours. The function of the ditches is to direct overland flow runoff from the face towards the channels of the ditches denoted as the East and West Tributaries of the East Taylor Fill Ditch and the West Taylor Fill Ditch. The design basis for these permanent channels is provided in Exhibit 7, Item 20, Part E. The longest segment of the proposed Terrace Ditches is a westward draining branch of the ditch originating at the 8200 foot elevation and emptying into the West Tributary of the West Taylor Fill Ditch in the NEA of Section 19. This segment is approximately 2,520 feet in length. Using this length and a tributary area of 51 acres as a design basis for all of the terrace ditches, the dimensions of the representative ditch for the terrace ditches was developed using SEDCAD as described below. For a conservative design, the applicable storm was set as a 100 year, 24 hour event. Sensitivity cases were examined using SEDCAD for several smaller sub -areas (3 to 22.6 acres) with shorter times of concentration, but all produced smaller peak runoff flows than the modeled 51 acre sub -area. The 51 acre sub -area that was modeled could therefore be used for all segments. The other terrace ditches will have more capacity than needed to convey their respective 100 -year flows. SEDCAD4 was used to determine the peak flow from this drainage area as detailed on the following SEDCAD output pages. The 100 year, 24 hour design event was modeled using a CN of 77 as adopted in TR -73 for reclaimed land with 1 to 2 years' growth. This would be representative of a short term condition where the upstream area has recently been topsoiled and seeded and has had only a year or two of growth. The long term condition would have a CN of 74 per the same TR -73, and would produce somewhat less runoff. The modeled case produced a peak flow of about 24 cfs under the 100 year event. As shown in the following SEDCAD output, a grass lined asymmetric triangular ditch cross section is assumed, with a channel slope of 0.015 ft/ft and 1.5H:1 V side slope on the uphill side and 3.OH:1 V side slope on the downhill side. It would flow at a depth of 2.4 feet deep when full retardance associated with mature grass lining for the long term is in place. In the short term, with a shorter immature vegetative cover, it would flow slightly shallower and at higher velocity, though still acceptable at 3.7 fps. A nominal freeboard of 1 foot has been added to the calculated flow depth for the long term, resulting in a generic ditch section at least 3.4 feet deep on a 1.5% slope for all terrace ditches on the fill face. This depth has been rounded up to an even 4 feet, with other design parameters as summarized below: Exh. 7 -20G -1 Revision Date: 12/31/10 Revision No.: TR -81