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15 <br /> downstream effect of dam failure is necessary. Several modes of failure are possible; the most <br /> likely are 1) structural failure (blow out) of the dam due to some weakness or extreme <br /> hydrostatic pressure, or 2) an elevated water level which washes over the top of the dam, erodes <br /> it away, thereby causing greater erosion and washing, etc. If failure occurs, it will probably <br /> occur during or shortly after a large rainfall event. Assuming proper spillway improvements are <br /> made, the second failure mode can largely be ruled out. Since the dam is relatively low, and <br /> has been in satisfactory use for many decades, the most probable failure mode will stem from <br /> 0 some sort of erosional seepage through weak areas of the dam. This kind of failure starts slowly <br /> but could culminate in a fairly sudden breakthrough, depending on the horizontal and vertical <br /> location of the breakout area. If such a worst-case event were to occur, the immediate area <br /> below the dam would be quickly inundated, and the water would almost certainly flood part of <br /> Old N.C. 10 and the filled area on the south side of the road. The downstream railroad <br /> embankment will effectively act as a dam, since it is elevated well above the surrounding <br /> landscape entirely across the area which would otherwise be the flowpath. A single box culvert <br /> under the railroad would then be a regulating outlet which should restrict the downstream flow <br /> to a reasonable amount. Depending on the height of water rise in this area, a small amount of <br /> water could flow along the northern ditchline of the railroad, through the back yard areas of <br /> three dwellings, and cross under the railroad at another culvert. Nuisance flooding of these yard <br /> areas could result, but no structure flooding is expected. <br /> The result would be a fairly well contained area of inundation between the existing dam <br /> /1/ and the railroad embankment. The duration of the submergence of Old N.C. 10 could be very <br /> lengthy however, and the road probably would be impassable for several hours or longer. No <br /> flooding of buildings in the area is expected due to their elevation or their protection by <br /> intervening high ground. <br /> 0 Below the railroad, the flowrate would be limited to the culvert capacity, which is on the j' <br /> order of 300 - 400 CFS. This amount should be passed on downstream in a fairly safe manner, <br /> Nsince it is only about one-half of the expected 100-year flow rate for the stieam at this point. <br /> Certainly, flooding of private property would result, most severely at the mobile home lot in <br /> Joppa Oaks and at the small pond downstream. The depth of flooding in the floodplain area <br /> behind the mobile home would only be about 1.0 feet average, based on rough calculations. The <br /> mobile home itself does not appear to be subject to flooding in this case. At the small pond, the <br /> water elevation can be expected to rise several feet, and the driveway embankment and dam <br /> overtopped. The paved driveway serving the Byrdsville Park would also be overtopped and <br /> rendered temporarily unsafe for travel. All these driveways probably would suffer substantial <br /> I erosional damage. No dwellings in this area appear to be subject to flooding under these <br /> conditions. Further downstream, no severe flooding conditions are expected beyond nuisance <br /> flooding of private property and a high water condition at the New Hope Road stream crossing. <br /> uric licaion 1 <br /> Mdam does not appear to meet the requirements for State regulatory jurisdiction, <br /> unless it is declared to be a high-hazard (Class C) dam by the Director of the Division of Land <br /> 1 <br /> Page 3 <br /> !II <br />