2008-11-10_ENFORCEMENT - C1981019 (2)

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�h I� 0 "'VAr SWS6 drainage basin. Using the aerial footprint of the containment area and assuming a depth of about 3.5 feet, this trap has a capacity of about 2 acre -feet. As shown in Figure 1, the August 9, 2008 thunderstorm produced 0.80 inch in a 30 minute period and 1.23 inches during the first hour. Figures 4 and 5 present the cumulative and increment rainfall amounts for the 10 year, 24 hour rainfall under the NR II distribution. The 10 year, 24 hour design storm with this distribution produces abo 0.46 in hes during the most intense hour of the storm. Given the intensity of the thunderstor event and the return �Z period in excess of the 50 year event, it is not surprising that some damage occurred. Attached to this document are a series of SEDCAD model runs. These were prepared to model current hydrologic conditions under a variety of rainfall events. The first 15 pages of the attachment are the 10 year, 24 hour design storm. Pages 16 through 28 consist of the actual rainfall experienced during the August 9, 2008 thunderstorm. The final SEDCAD run on pages 29 through 40 shows the results of the second run when the outlet drain holes and the drop inlet spillway are blocked to prevent flow. During the actual thunderstorm, it is known that the drop inlet spillway was quickly blocked as a result of sediment deposition and other debris. With the exceptions noted above, all three SEDCAD runs are identical in other respects. The 10 year, 24 hour design storm generates a peak flow of 58 cfs and runoff volume of 8.2 acre -feet as shown on page 4 of the attachment. The August 9, 2008 thunderstorm generates a higher peak flow of about 66 cfs due to the intensity of the event. The runoff volume from the storm is slightly less than the design storm at 6.6 acre -feet (page 19). The third model run produced identical results to the second run, except the plugged outlet results in the peak flow through the spillway reaching an elevation of 6752.42, about 2.58 feet below the dam crest. None of the models showed the spillway overtopping the dam crest. While this was known to have occurred, this model was the closest to demonstrating that. The storm also moved a considerable amount of sediment into the impoundment. The pond had most recently been cleaned last fall. Plans are underway to clean out the recent sediment. Several photographs of the conditions at Prospect Pond following the event are also attached. On September 16, 2008, 1 inspected the conditions at the Prospect Pond as documented in a separate engineering report provided as an abatement action. One of the findings of that report was that the open channel emergency spillway became partially blocked at some point during the event. This may have become a contributory factor to the embankment overtopping but it is difficult to model these impacts. Direct evidence of the debris marks as the overtopping flow crossed Highway 13 were found during this inspection. It was determined that at least 310 feet of the pavement were inundated. Rounded cobbles up to about 6 inch nominal diameter were moved across the highway and were found downstream. Assuming these rocks were submerged, and applying the broad - crested weir formula of Q = 1.5 b (H ^1.5) results in an estimated outflow across the highway of about 210 cfs. In this case, the estimated depth of flow was assumed to be 8 inches. This 2