CHARACTERIZATION OF COVER-COLLAPSE SINKHOLE MORPHOLOGY ON A GROUNDWATER BASIN-WIDE SCALE USING LIDAR ELEVATION DATA: A NEW CONCEPTUAL MODEL FOR SINKHOLE EVOLUTION

Airborne lidar data were collected in April 2012 over highly karstified areas of southwestern Illinois. Two missions were conducted using differing flight line orientations to optimize penetration of the dense woodland and vegetative cover, one at a nominal point spacing (NPS) of < 1.0 meters and a second collection at NPS < 0.5 meters, resulting in point densities (PD) of 1.0 pt./meter² and 4.0 pts/meter², respectively. Hybrid point cloud stacking approaches were employed to merge the lidar datasets to maximize the ground returns and better characterize the 1,500+ sinkhole features existing within the 30 km² Antler-Annbriar Spring and Dual Spring groundwater basins. With these data, we were able to map and complete an inventory of all sinkhole features greater than 30 cm in depth for the study area.

Lidar data revealed an evolutionary model guiding long-term cover-collapse sinkhole development within groundwater basins in highly karstified landscapes. Relatively large dendritic drainage-shaped sinkholes form along the margins of the groundwater basins of southwestern Illinois, and initially feed a single drain to the aquifer. The focused recharge of these sinkholes accelerates the chemical and mechanical erosion of underlying bedrock crevice and conduit systems, resulting in their enlargement. This, in turn, lowers the water table near the edge of the growing basin resulting in the loss of hydrostatic pressure within the soil zone, the formation of new drainage-shaped sinkholes, and the outward migration of the groundwater basin boundaries. As the basin margins migrate outward, the older drainage-shaped sinkholes become isolated and develop numerous drains. These sinkholes join other sinkholes via erosion and sinkhole formation along sinkhole boundaries and become amoeboid-shaped, compound sinkholes. Continued erosion of sediments overlying bedrock within these compound sinkholes reduces the elevation of the ground surface within and adjacent to the compound sinkholes until their multiple drains become smaller individual sinkholes that follow crevice patterns. With increased erosion of sediments overlying bedrock near the discharge point(s) of a groundwater basin, the individual sinkholes form a pattern that can be defined by Theissen polygons due to competition for surface runoff among adjacent sinkholes.