Quantifying the contribution of surface runoff to karst-related flooding between Le Roy and Caledonia, NY

Karst-related flooding occurs in several depressions between Le Roy and Caledonia, NY, USA. The timing of these flood events suggests that a regional groundwater phenomenon contributes to the flood volumes, in addition to contributions by surface water and subsurface quickflow. To gain better understanding of the proportions of these components, two watersheds near Le Roy (Britt Road and Quinlan Road) were studied in detail. A rainfall-runoff model was constructed in the PCRaster language to quantify the portion of flood volume originating from surface runoff for two different flood events. For an event in October 2006, runoff explained the entire flood volume in the Quinlan Road watershed. For an event at the end of November 2006, the contribution of surface runoff to the flood volume was about 49% and 46% for the Britt Road and Quinlan Road watershed, respectively. The remaining part of the flood volume must have been derived from subsurface quick flow or groundwater. In these model simulations swampy areas and the flood zone were assumed to be impermeable, simulating the presence of a high water table. Without this assumption all the surface water entering the flood zone would infiltrate into the subsurface. From a sensitivity analysis it was found that microtopography storage is an influential parameter in runoff generation in these watersheds. These findings suggest that high groundwater levels are the main driving force behind the floodings. Without a groundwater level close to the land surface in the Britt Road watershed or groundwater mounding at the sinkholes in the Quinlan Road watershed, no flooding would occur. However, to accurately quantify the quickflow and groundwater components contributing to the floodings, additional research needs to be performed at the actual time of flooding