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• Description of SEDCAD Modeling. <br />The results of three SEDCAD models are presented below. A runoff curve number of 83 <br />representing the "topsoiled and seeded" condition as described in Exhibit 7 introductory <br />text "Methodologies and Assumptions for Sedimentation Pond Design Evaluations, Table <br />1," as revised in TR -73, was assigned to all cases. <br />The first two events modeled were the 10 year, 24 hour storm and the 25 year, 24 hour <br />storm running into the berm treated as a small pond. The analysis assumes the pond <br />starts from empty but ignores any outflow that will occur through the gravel drain section <br />during the storm. This represents the maximum level that the temporary impoundment <br />can surge to. In both cases, the flow depth is very shallow in the 20 foot wide `notch" <br />over the gravel section in the berm, which is modeled as a broad crested weir. Total <br />runoff volume in the 10 year 24 hour storm is 0.13 ac ft, most of which (0.10) ac ft is <br />released as overflow. Peak inflow in the 10 year 24 hour storm is 1.73 cfs. The flow <br />depths in the 10 year 24 hour storm and the 25 year 24 hour storm were, respectively, <br />0.04 feet and 0.06 feet. <br />SEDCAD can be used to model a ditch check only as a porous gravel fill placed in a v- <br />shaped ditch. The user can specify side slopes of the channel but bottom width is <br />assumed to be zero. All flow is assumed to pass through the triangular cross section area <br />• at a velocity determined by the gravel's porosity. For this analysis the porosity of the <br />gravel was assumed to be 50 %. The check dam analysis does not produce a peak pool <br />elevation, but does develop a flow - through rate. The inflow for the 10 year 24 hour <br />storm is the same as that modeled above with the pond, 1.7 cfs. In the model constructed <br />for this application, the gravel check dam is followed by a grass filter, which is the gently <br />sloping (— 4% slope) overland flow pasture below the berm. <br />For the sedimentology evaluation, both as a "pond" and as a "ditch check ", the USLE <br />parameters were selected as per TR -73 for the worst case upstream condition in this <br />analysis, that condition being "Reclaimed, Topsoiled and Seeded." The system treated <br />as a "pond" had a slightly higher trap efficiency (82 %) and the modeled outflow for that <br />approach had 0.00 ml/1 settleable solids. For the system treated as a check dam followed <br />by a grass filter, the analysis indicates that combined effect of the 30.5% trap efficiency <br />of the check dam followed by the 60% trap efficiency of the grass filter will drop <br />settleable solids to less than 0.5 ml/1 in the first 400 feet of overland flow below the check <br />dam. These values bound the expected performance, and both are quite low. It is <br />expected that any solids which escape the sediment trap would quickly be trapped in the <br />gently sloping vegetated surface below the facility. <br />• <br />Exh. 7 -17 -3 Revision No.: TR -90 <br />Revision Date: 04/13/11 <br />