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Pond Construction <br />Pond 018 has been designed as a largely subgrade impoundment with a small embankment along <br />the southern and western perimeter. It is known that a hard layer of caprock exists beneath most <br />of the floor of the proposed impoundment, and this layer will form the base of the impoundment. <br />A series of backhoe pits have been excavated previously to identify the location and extent of the <br />rock layer. <br />The embankment is a low, wide berm along the northwest and southeast edges of the <br />impoundment. The details can be observed on Map 3. Except where it fills over Ditch C18 on <br />the southeast edge of the pond, the embankment is only 1 to 2 feet in height. Along the southeast <br />edge, where the embankment backfills over the no longer needed portions of Ditch C18, its <br />maximum height above the ditch invert is about 8 feet at the westernmost corner of the pond. <br />The remaining portion of Ditch C18 west of this point then drains to the west. Flows out of the <br />emergency spillway of Pond will flow southward into Calamity Draw, as shown on Map 3. <br />The spillway consists of a shallow 3 -foot deep vegetated channel with a 20 foot wide bottom and <br />2H:1 V side slopes. It traverses the through the crest of the embankment for about 70 feet before <br />dropping onto native ground and towards the channel of Calamity Draw. <br />The volume of the below grade pond was input into the SEDCAD model, along with its spillway <br />details. The model watersheds were input as a series of sub -watersheds, each with its own <br />acreage, its own flow response parameters (slope, distance, time of concentration), and the <br />specific runoff curve number from the above table. The 10 year, 24 hour storm was then applied <br />to the composite watershed, and routed down to the sediment pond. The pond was assumed to <br />be full to the spillway elevation at the start of the storm. It can be observed that the pool rises <br />0.41 feet to an elevation of 5678.41, thus flowing 0.41 feet (5 inches) deep in the spillway. <br />SEDCAD model allows for an estimate of the annual sediment collected in the pond. The <br />methodology is described on page 64 of the SEDCAD User's Manual, and requires an Annual R <br />factor (rainfall -erosivity factor). The value of R at 27 can be interpolated from Figure 5.3 <br />presented in Barfield, Warner and Haan (1981). In addition the total tonnage of sediment from <br />the modeled storm, in this case 10 year, 24 hour storm, is required. This is obtained form the <br />SEDCAD output by subtracting the sediment tonnage into the null below the pond (1.7 tons) <br />from the tons flowing into the pond (46.5). The net 44.8 tons is then converted to volume, using <br />a density of 78 lb/cu ft, to arrive at the retained sediment volume of 0.0264 ac -ft from a single 10 <br />year, 24 hour storm. Finally, this sediment volume is converted to an annual value by <br />multiplying by the ratio of the annual rainfall erosive factor (R of 27, determined as described <br />above) to the 10 year, 24 hour storm erosive factor (18.85, calculated by SEDCAD based on the <br />2.0 inch 10 year, 24 hour rainfall input) to arrive at an estimated annual sediment accumulation <br />of 0.0378 ac -ft. Thus, over a period of 10 years approximately 0.38 ac -ft of sediment would be <br />accumulated in the pond bottom, which is small enough to not significantly affect the trap <br />efficiency of this pond. <br />Upon construction, an as -built drawing and construction report will be completed by a <br />professional engineer. <br />Attachment 2.05.3(3)-28 Revision Date: 11/24/15 <br />Page 3 Revision No.: TR -69 <br />