The vast karst cave landscape that extends from southern Indiana into northern Alabama exhibits the unique ability to transport stormwater directly into the underground rivers throughout the region.
At the center of this region lies Bowling Green, Ky., a community entirely situated on karst. Karst is a terrain based on soluble bedrock such as limestone or dolomite. Karst regions present challenges such as sinkholes, flash flooding, and unusual hydraulic grades. Because of this, city planners are constantly facing the environmental, hydrologic and structural issues associated with this landscape.
The underground caves serve as the Bowling Green’s natural storm sewer, with more than 700 drainage wells distributing unfiltered stormwater directly into the aquifer. During heavy rain events flooding is common. “In general caves are very stable because the ceilings are similar to arches, which are very sturdy. But when we create things like highways and parking lots, we create runoff in amounts and pressures not found in nature — and this can cause geologic problems,” explains Annie Croft, a research hydrologist with the Center for Cave and Karst Studies at Western Kentucky University (WKU).
Typical of karst regions, Bowling Green is particularly vulnerable to pollution and groundwater contamination due to ease of water flow and lack of natural filtration.
The uncontrolled and contaminated stormwater flows are not only capable of eroding the caves’ structural integrity and polluting the aquifer, but also seriously damaging the delicate underground karst ecosystem. WKU knew it faced a huge challenge when it launched a $2 million parking lot expansion project to increase parking capacity from 349 to 727. Located within one mile of where the region’s 75-square-mile groundwater basin discharges, the Creason Street parking lot has a history of severe flooding. Just one 25-year storm in the 627.5-acre watershed can cause more than 1,000 cfs of water to flow toward the lot. Increasing the impervious surface area would create an even greater runoff volume and stormwater pollution.
To help tackle the project, WKU turned to local engineering firm DDS Engineering, PLLC and its own experts, including Dr. Nicholas Crawford, the founder of WKU’s Center for Cave and Karst Studies. The evaluation of the site began more than two years before construction started. Crawford’s team determined that the cave measures about 18 feet deep and between 80 and 200 feet wide. It is comprised of three feet of air, two feet of water and 12 feet of silt and sediment.
An analysis by DDS showed that previous flood control efforts such as dry wells and drains discharging into a local rock quarry were ineffective due to their insufficient size and compounded by the buildup of silt and sediment on the site. “This site was known for having a lot of issues, and it was critical that we conduct a feasibility study to look at all of the environmental impacts,” explains Jason Blakeman, a DDS engineer on the project.
DDS performed a number of tests and consulted with various contractors, drillers and stormwater treatment providers to determine the level of construction that would be needed to implement an effective stormwater management plan. The list of pollutants at the site were narrowed down to common urban nonpoint source pollutants including trash, contaminated sediments and hydrocarbons. A wetlands proposal was eliminated because it would not have been able to handle the runoff volume in the amount of land space available. Instead DDS opted to go with a manufactured best management practice that would meet three overriding requirements: designed to remove the targeted pollutants; sized large enough to handle the volume of runoff; and offered affordably, both in terms of unit costs and system installation requirements.
“We did quite an extensive search for an effective stormwater treatment unit that could handle the flow requirements, and Contech Construction Products Inc. offered us something that was the best fit and most economical for the job,” says Blakeman. “Because it was a state project, the bidding for the job was open, but based on the extreme flows and the various site parameters the Vortechs unit was ultimately chosen. We feel confident that we are going to get the maximum amount of targeted pollutant removal possible considering the volume of runoff on the site.”
DDS designed a storm drain plan that channels the urban runoff into four separate stormwater systems, each containing a Vortechs, a bypass structure for flood events, and an eight-foot drainage caisson-like structure that is positioned over a four-foot diameter hole in the roof of the cave. Each of the four systems is designed to handle up to a 25-year storm, where runoff is collected through a series of 22-foot trench drains located on adjusted grades throughout the lot. Each Vortechs, which utilizes a combination of vortex motion and flow controls to remove the targeted pollutants, is designed to treat a one-year storm, or about 100.5 cfs. Flows exceeding that treatment capacity are channeled through each of the stormwater systems’ bypass structures to prevent surcharge. When stormwater flows on the site exceed the 25-year storm level, the caisson structures, which are strategically placed within traffic islands, will provide additional flood control. By creating an effective “overflow” process for water to directly enter into the cave system, the flooding problems should be virtually eliminated, provided that the cave under the parking lot is indeed as big and connected as the studies indicate.
“This project is a great stormwater management example for others to look at,” said Ed West, director of construction for WKU. West relocated to Bowling Green several years ago and now serves as an advisor for the city on stormwater-related issues in addition to his WKU position. “It was strange moving here and discovering that the whole city is on rock – and as long as people dump water directly into the caves without pretreatment there will continue to be problems. Hopefully a lot of people will learn from this job.”