2013-09-19_PERMIT FILE - M2012052 (4)

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For Disturbed Area: Area 0.12 acres Runoff Curve Number (CN) 60 Surface Storage (S) 6.67 inches Initial Abstraction (la) 1.33 inches RunOfflooyearswrll (Q) 0.17 inches Volume,00yearA.. (V) 74 cubic ft (24 hrs) Average Flow 24hr5 8 X 10 -4 cfs These calculated results produce reasonable numbers as far down the list as the total estimated volumes. The average flows do us little good, so additional calculations are needed to produce useful instantaneous flow numbers. For this, we go back to the predicted rainfalls from NOAA Atlas 14. Even though we do not agree with the NOAA data as discussed in the responses to the last set of comments, it does include a sufficient population of predictive rainfall values to allow a solution. The approach then is as follows. The la for the grass area is 2.8 inches, thus, using the SCS method, no runoff will be generated until the 100 -year event for approximately 2.5 hours has been reached. At that point we can average the number of inches of rainfall that the NOAA model shows that will occur between the 2 -hour and 3 -hour 100 -year events. That average will be 0.22 inches (2.58 in — 2.36 in) over a 60 minute interval. Assuming 100 percent runoff, the total average flow from the grassy area during that interval would be 0.24 cfs. The S for the area is shown to be 13.8 inches, so at this precipitation rate (assumed to be rain), the runoff would be slightly less than 10 percent, based on a comparison of the total V100 produced in the SCS calculations as compared to the runoff generated from the hypothetical asphalt surface producing 100 percent. Thus, the number shown on the curve on the drawing reaches a maximum of about 0.13 cfs at about 10 hours after the beginning of the storm event. This same calculation can also be made for the disturbed area, where A equals .12 acres, la equals 1.33 inches, and S equals 6.67 inches. The la value for this area is achieved during the half -hour event, so using the average precipitation rate from the NOAA Atlas (as rain) for the 15- minute to half -hour interval of 0.42 inches (1.58 in —1.16 in), we get a total hypothetical flow, assuming 100 percent runoff rate on asphalt, of 0.1 cfs. Using the same ratio as was used for the grassy area and comparing the flows predicted by the SCS model versus the 100 percent runoff value, we obtain a maximum flow of .017 cfs at 1 hour after the start of the storm event. The curve shown on the engineering drawing shows the 5