INVESTIGATION OF RAPID AND UNPREDICTIBLE FLOODING FROM A WESTERN NY SINKHOLE

A depressional feature in western NY is the source of rapid and unexplained flooding. The feature has many of the characteristics of a sinkhole formed in a karst environment, except that it is located in an area that occasionally receives considerable groundwater discharge. Water originating from the feature rise rapidly over several 10's of hours and can inundate an area of over 20 acres in size. What makes this flooding so unique is that there is no direct contact between the fields that flood and a stream or river as a source of water. In this paper we describe the geology of the feature and present a series of hydrologic modeling simulations for an observed November 2005 flood event to elucidate the source of the floodwater.

The depression is located on Quinlan Rd in the Village of Leroy NY. Bedrock making up the walls of the feature is the Onondaga Formation, a fractured limestone unit that has been observed to have solution-widened voids in other locations. Using topographic information and stream locations, the drainage area associated with the feature was delineated. Survey data and field observations were used to estimate the volume of floodwater from this event. A lumped-parameter runoff model utilizing landcover mapped from 2002 aerial photography was developed to estimate the surface runoff from the November 5 flood event. The model uses the SCS curve number approach to estimate runoff from pervious areas and assumes that the surface area of all roads within the watershed contributed to the flood. By all accounts this model should probably overestimate the amount of surface runoff. The simulations suggest that the watershed could only produce 30% of the volume of the observed floodwater. It is proposed that the source of this additional water is from an open fracture system that is capable of moving groundwater quickly from source areas outside of the watershed.

A monitoring program is underway to evaluate the hydrogeology of the site and to determine the underlying hydro-meterologic causes of the flooding. Preliminary results indicate that precipitation events at the sinkhole site are only weakly correlated to groundwater rises in the feature. Work is underway to identify the source of this water and to better understand the reasons for its flashy and unpredictable response to weather events.