Hydrologic analysis of a 100-year flood event in a small rural watershed in Clinton County, PA

The US EPA reports precipitation from extremely heavy storms has increased 70 percent in the Northeast since 1958 (2016). NASA warns that Northeast will experience more heavy downpours and changes in patterns of precipitation will pose growing challenges to many aspects of life (2017). As per the IPCC (2007), evidence indicates that the net damage costs of climate change are likely to be significant and to increase over time. The residents of Sugar Run sub-watershed (1.25 mi2) within the Beech Creek watershed in Clinton County, PA experienced a total of 6.5 inches of rain in two hours on June 27, 2013. This event resulted in a flash flood that caused $17 million damage to property, one casualty, and 32 miles of road damage in the area. The amount and the intensity of rainfall corresponded to a 100-year event for the region. However, the rain event was localized in nature and did not have much impact on nearby USGS gaging stations. Field data on personal property and infrastructure damage caused by the flood were documented during and after the flood. The extent and the nature of the flood were analyzed using several hydrologic theories to determine frequency of the rain event, velocity and discharge of the Sugar Run at bankfull stage and flood stage, peak discharge during the flood, time of concentration of surface run-off following the rain event, and the duration of storm flow. The velocity of the flow (0.8 ft sec -1) and discharge (\u3c1 ft3 sec -1) in Sugar Run during normal flow conditions were determined to compare with values calculated for the flood event. The data show that the calculated flow velocity at bankfull stage (6 ft sec-1) and the peak discharge during the flood (690-780 ft sec-1) were an order of magnitude higher as compared to those values during normal flow conditions. Based on hydrologic analysis of the data, it is concluded that the flood water must have been flowing at 0.6 ft sec-1, which corresponded to a peak discharge rate of 690 ft3 sec -1 over the floodplain (with a cross sectional area of 1180 ft2) in Sugar Run during the flood peak discharge. The calculated values are in agreement with eyewitness accounts. The hydrologic methods applied to verify the field evidence and reported observations about the flood damage proved to be valuable tools, which can be used to predict the extent of future floods caused by similar intense precipitation events in small watersheds