Identifying Areas of Higher Potential Vulnerability to Sinkhole or Karst-Related Flooding in the Inner Bluegrass Region#Lower Kentucky River Basin

Identifying Areas of Higher Potential Vulnerability to Sinkhole or Karst-Related Flooding in the Inner Bluegrass Region—Lower Kentucky River Basin Karst areas present a unique set of scientific, engineering, and decision-support challenges to hazard mitigation managers, planners, and community stakeholders. Natural karst drainage systems are susceptible to flooding caused or exacerbated by surface runoff exceeding the drainage capacities of sinkholes, and by dynamic changes in groundwater levels, storage, and flow in underlying karst aquifers during storm events. In many karst aquifers, water levels rise dramatically during storms and surface- flood events. When subsurface water storage and karst conduit-aquifer drainage capacities are exceeded, the rising water table may reach the surface, resulting in backflooding of sinkhole depressions and sinking streams, and reactivation of surface flow routes in former stream valleys. Monitoring and characterization of conditions under which sinkhole or groundwater-induced karst flooding occurs are technically and logistically difficult as surface manifestations of flooding typically vary greatly from place-to-place and flooding may occur only during extreme storms and at relatively long recurrence intervals. Hence, hazard risks, such as sinkhole flood inundation levels and frequency, usually cannot be defined, and information needed to improve prevention and remediation of flooding in karst areas is often unavailable. With approximately 55-60% of the Commonwealth underlain by karstic limestones, and in light of anticipated changes in climate which are expected to increase storm frequency and intensity, Kentucky faces increasing challenges and costs in mitigating sinkhole and karst-related flood hazards. In the Inner Bluegrass Lower Kentucky River Basin, flooding of sinkholes and karstic watersheds is a relatively common occurrence that negatively impacts urban-suburban and rural residents and communities. In a study of storm-water improvement projects implemented in Fayette County over a 6-year period (1985- 1990), over $1.5 million in expenditures were attributed directly or indirectly to problems caused by urban development and stormwater disposal in sinkhole areas. Repair of flood and sinkhole property damages has been a recurrent problem in Lexington’s Cindy Blair, Stonewall, and Joyland subdivisions, all of which are built in karst sinkhole valleys underlain by active cave streams. Major karst-related flooding damages have also occurred elsewhere in the region; for example, in 1989 in the Tashamingo subdivision in Jessamine County, and in 2018, in the City of Burgin in Mercer County. In each of these cases, flooding severity and resulting damages were, at least in part, intensified by housing development in karst valleys and inadequate or improperly-designed stormwater disposal, especially in or near the surface catchments of sinkholes and sinking streams. Flooding in the City of Burgin is apparently groundwater induced, due to limitations in the stormwater discharge capacity of an underlying cave, and is thought to have apparently intensified due to complications from a recent installation of a county sanitary sewer pump station. In spite of need, there is a lack of detailed local and regional information about sinkhole and karst- related flood hazards in the Inner Bluegrass Lower Kentucky River Basin. Among the hazard mitigation plans written for counties and municipalities within the region’s Bluegrass Area Development District (BGADD), only one, Lexington-Fayette, specifically identifies and discusses sinkhole flooding. The areas having highest potential vulnerability to sinkhole and karst-related flooding are unidentified, and all BGADD hazard mitigation plans lack detailed information needed to assess the hazard risks, or plan and implement effective sinkhole or karst flood control and mitigation measures. This project is intended as a first step in remedying the critical lack of information by conducting research and compiling data needed to identify and map areas with higher potential vulnerability to karst sinkhole and groundwater- induced flooding. The project will be conducted using advanced GIS techniques, high-resolution LiDAR topographic and sinkhole location data, and a combination of selected other new and existing karst hydrogeologic geospatial data. Data will be processed and visualized using ArcGIS Pro terrain-modeling and watershed delineation tools, and digital data compiled and evaluated will include, at minimum, hillshade topography and geography, NHD designated stream reaches and subwatershed boundaries delineated to 10-digit hydrologic unit scale (40-250 K Acres, 62-390 mi²), elevations and boundaries of FEMA mapped flood zones, and karst drainage features, which include springs, sinking streams, traced subsurface karst flow paths and groundwater basin boundaries, LiDAR-mapped sinkhole locations, and delineated surface catchment boundaries for sinkholes and sinking streams. The dataset of catchment boundaries for individual sinkholes and sinking streams provides critical information needed to improve urban-suburban development and stormwater disposal planning, and improve effectiveness of sinkhole and karst-related flood mitigation efforts. Because groundwater-level data are unavailable for most parts of the Inner Bluegrass region, potential vulnerability of sinkhole and sinking stream catchments to groundwater-induced flooding will be assessed and mapped using a schema based on factors such as proximity of the catchments to FEMA designated surface flood zones, and relative differences between the lowest elevations within individual catchments and mapped elevations of flood zone boundaries. Where available, other data, such as elevations of peak flood or high-flow events measured by USGS stream gages, and reported permeability measurements of mapped soils, will also be incorporated into the evaluation of potential vulnerability. Anticipated products for the project include a published digital map of sinkhole-or-karst flooding vulnerability which will be publicly accessible from the KGS website (http://www.uky.edu/KGS/), and can be accessed by or linked to the Kentucky Division of Water Water Maps Portal (https://watermaps.ky.gov/), and a KGS published report written to summarize project data compilation methods and discuss project outcomes. It is also anticipated that presentations on the issue of sinkhole- or-karst flooding hazards will be given at the Kentucky Association of Mitigation Managers annual conference, and similar other professional gatherings.