The City of Cookeville is situated on the East Highland Rim physiographic province and is underlain by soluble limestone bedrock. Due to its unique geologic setting, Cookeville has one of the highest densities of sinkholes and caves of any city in the region, and perhaps the nation. Cookeville's stormwater drainage system relies on drainage to the subsurface through sinkholes and caves. Sinkhole flooding occurs when the rate of water entering a sinkhole exceeds the rate of water draining from it. In urban areas, impervious surfaces (paved streets, parking lots, etc.) increase runoff rates, elevating the risk of sinkhole flooding.
Study Objective
The objective of this study was to delineate the 100-year floodplains for sinkholes within the Cookeville Urban Growth Boundary (UGB). In order to do this we used detailed topographic data (LiDAR), GIS, and hydrologic modeling software. The results of floodplain prediction were validated by comparing predicted flood heights with those observed during actual storms. Sinkhole floodplain maps are being provided to the city in GIS format (Geodatabase) and in PDF format, for public dissemination or sharing among offices.
Sinkhole Areas
Sinkhole drainage areas were determined for 218 major sinkholes within the Cookeville urban growth boundary. The sinkhole drainage area is the land area that contributes runoff to the selected sinkhole. Results show that sinkhole drainage areas cover 14,610 out of the total 32,622 acres of the Cookeville UGB, or 45%. Thus land use change and urban development that occurs in almost half of the urban growth boundary directly affects sinkhole flooding. Sinkhole drainage areas average 67 acres in size and, on average, consist of 22% impervious surfaces (paved streets, roofs, parking lots, and driveways). These impervious surfaces include over 13,000 structures and 260 miles of roads.
Sinkhole Floodplains
Sinkhole floodplains as defined in this study are those areas around sinkholes that are predicted to flood in the 100-year, 3-hour duration rainfall (4.5 inches for Cookeville, or 1.5 inch per hour). The total area predicted to be inundated is 657 acres, or an average area of approximately 3 acres per sinkhole. Flood height elevations range from 938 feet ams.l. to 1276 feet ams.l., depending on which part of the city the sinkhole is located. GIS analysis shows that 225 structures and 7.8 miles of roads are currently built within sinkhole floodplains and would be subject to damage during the 100-year storm event.
Recommendations
Although not the purpose of this study, we offer three recommendations for sinkhole flood mitigation.
First, the addition of impervious surfaces should limited by zoning, or offset by properly-functioning detention basins. This will ensure that runoff inputs to sinkholes do not increase dramatically in the future.
Second, sinkhole swallets (the point where water enters the ground) should be monitored and periodically cleared of trash and debris. Some swallets may require a filter or trash-rack type structure to prevent large objects from entering the cave/stormwater system.
Finally, sinkhole drainage rates should be monitored, perhaps in a longer-term study, to observe changes in rates. Any such changes may signal possible clogging of the subsurface drainage system.