2016-06-08_PERMIT FILE - C1981008A (2)

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Runoff Calculation Results The results of the runoff calculations for 10 & 100 year - 24 hour storms follow and were generated in the attached SEDCAD model printout. Event Rainfall Peak Runoff Volume Pond Capacity 10 year -24 hour 2.0 inches 13.0 cfs 1.03 ac -ft 3.55 ac -ft 100 year -24 hour 3.0 inches 22.8 cfs 2.25 ac -ft 3.55 ac -ft Pond 018 capacity is 3.55 ac -ft and is sufficient to hold the 100 year -24 hour volume of 2.25 ac - ft. The peak 100 year -24 hour storm flow of 22.8 cfs was used to check the emergency spillway capacity. The emergency spillway handles 22.8 cfs at a depth of 0.58 feet with over 3 feet of free board. Thus, the emergency spillway is adequately sized. SEDCAD model allows for an estimate of the annual sediment collected in the pond. The methodology is described on page 64 of the SEDCAD User's Manual, and requires an Annual R factor (rainfall -erosivity factor). The value of R at 27 can be interpolated from Figure 5.3 presented in Barfield, Warner and Haan (1981). In addition the total tonnage of sediment from the modeled storm, in this case 10 year, 24 hour storm, is required. This is obtained from the SEDCAD output by subtracting the sediment tonnage into the null below the pond (1.7 tons) from the tons flowing into the pond (54.4). The net 52.7 tons is then converted to volume, using a density of 78 lb/cu ft, to arrive at the retained sediment volume of 0.031 ac -ft from a single 10 year, 24 hour storm. Finally, this sediment volume is converted to an annual value by multiplying by the ratio of the annual rainfall erosive factor (R of 27, determined as described above) to the 10 year, 24 hour storm erosive factor (18.85, calculated by SEDCAD based on the 2.0 inch 10 year, 24 hour rainfall input) to arrive at an estimated annual sediment accumulation of 0.044 ac -ft. Thus, over a period of 10 years approximately 0.44 ac -ft of sediment would be accumulated in the pond bottom. Even with 10 years of sediment, the 100 -year event will be contained. Pond Construction Pond 018 has been designed as a partly subgrade impoundment with a small embankment along the southern and western perimeter. It is known that a hard layer of caprock exists beneath most of the floor of the proposed impoundment, and this layer will form the base of the impoundment. A series of backhoe pits have been excavated previously to identify the location and extent of the rock layer. The embankment is along the west and south edges of the impoundment. The details can be observed on Map 2. Along the south edge, where the embankment backfills over the no longer needed portions of Ditch C 18, its maximum height above the ditch invert is about 10 feet at the westernmost corner of the pond. The remaining portion of Ditch C18 west of this point then drains to the west. The emergency spillway will discharge into the C4 Ditch which will be routed to Calamity Draw upon completion of the Pond 018 construction, as shown on Map 2. Attachment 2.05.3(3)-28 Revision Date: 04/21/16 Page 3 Revision No.: TR -72