Sinkhole flooding above a shallow bedrock aquitard in an urbanizing community, central Tennessee, USA

Abstract

In the aftermath of a storm on 6 February 2020, central Tennessee, USA flooded sinkholes were concentrated in places where two Late Ordovician aquitards are probably at or within ~10 m of the surface. The ~278 km2 study area is located ~35 km SE of central Nashville along the edge of Murfreesboro, a city of >153,000 that grew in population by ~40 % between 2010 and 2020. Within the study area, the largest amount of flooding happened within the ~12 km2 Shores Road focus area which includes the city boundary and is mostly within the city's urban growth boundary. Analysis of published geologic maps and stratigraphic observations reveals areas where bedrock aquitards are probably at or within ~10 m of the surface, and inundation was observed on 3 m cell size Dove multispectral satellite imagery on 8 February. The largest inundated areas were mapped with ~88 % accuracy through supervised classification of the 8 February scene. Of 1247 inundated features in the study area, 233 (~19 %) are in areas where the Pierce Limestone aquitard is at or within ~10 m of the surface, and this result is statistically significant at the p < 0.01 level because these areas comprise only 8 % of the study area. These 233 features account for ~27 % of the inundated area. If areas underlain by a second aquitard, the Lower Ridley Confining Layer, are included in the analysis, this approach explains ~69 % of the features and ~86 % of the inundated area. This result is also significant at the p < 0.01 level. Although many factors affect sinkhole formation and flooding in general, these results underscore the significance of shallow bedrock aquitards.