When it comes to challenging stormwater management projects, Bowling Green, KY, is a city that is overflowing with them. The city and the surrounding region are entirely situated on karst, a terrain composed of soluble bedrock, such as limestone and dolomite. With its unique system of caves, underground streams, and minimal soil cover, karst presents engineers and contractors with numerous topographical difficulties, including sinkholes, flash flooding, and unusual hydraulic grades.

Under the city spans an expansive maze of subsurface waterways flowing through large cave passages in the underlying Mississippian aged limestone. Similar to most urbanized karst areas, the caves serve as Bowling Green's natural storm sewer, so there are more than 700 drainage wells that distribute unfiltered stormwater directly into the underground aquifer. In times of severe rain, the caves, wells, and sinkholes can surcharge, increasing hydrostatic pressure and causing runoff to flow back out of the caves often to flood-stage levels. Karst regions such as Bowling Green are particularly vulnerable to pollution and groundwater contamination because of the ease of water flow. Because the region's clay soils do not retain or infiltrate stormwater easily, natural filtration of urban runoff is nearly nonexistent. The many sinkholes in the region serve as funnels for runoff, creating a direct surface connection to the cave's aquifer. Therefore, any pollutant capable of being transported by stormwater runoff into sinkholes, dry wells, or streams can directly contaminate the community's groundwater basin.

With its extensive expertise in karst, Western Kentucky University (WKU) knew it faced a huge challenge when it decided to launch a $2 million parking-lot expansion project. Located within 1 mi. of where Bowling Green's groundwater basin discharges and situated at the base of the campus hill, the Creason Street parking lot has a history of severe flooding. Just one 25-year storm in the 627.5-ac. watershed area can cause more than 1,000 ft.³/sec. (cfs) of water to flow toward the lot.

Vortechs Systems installed at WKU will help protect Bowling Green's cave system from stormwater pollution.

Increasing the impervious surface area certainly would create an even greater amount of stormwater runoff on the campus, thereby compounding the preexisting flooding problem on the lot, which is a natural collection spot for almost all runoff flowing through the campus. The lot's tendency to flood was illustrated clearly in 1998 when a 100-year storm event caused stormwater levels to rise 8 ft., destroying more than 80 cars. However, the campus needed additional student parking spaces, and this was the only area available for expansion. To further complicate matters, project engineers had to create a solution that would protect the underlying caves from structural damage and pollution from the parking lot, such as contaminated sediments, trash, oil, and grease.

Ed West, director of construction for WKU, says the expansion of the Creason lot is critical for the university. "The number of spaces in the new lot will increase from 349 to 727, which will provide much-needed additional parking for students and sporting events." The project is a part of a $32 million renovation of WKU's Diddle Arena, home to the men's basketball program, which ranks among the top 10 winningest programs in NCAA history.

To help tackle the project, WKU turned to local engineering firm DDS Engineering PLLC along with its own karst and engineering experts, including Nicholas Crawford, the founder of WKU's Center for Cave and Karst Studies. The evaluation of the site began more than two years before new construction even broke ground. With the help of a grant from the city, Crawford's team first conducted a microgravity subsurface investigation of the Creason lot to determine as precisely as possible where and in what direction the cave channels run. The technique works particularly well in extensively developed karst regions, such as Bowling Green, and is conducted by measuring the gravitational pull of bedrock from a portable device on the surface. Once a cave is discovered and confirmed with an exploratory boring, the cave then is mapped by conducting a series of readings, or traverses, about 10 ft. apart. The traverses, along with previous mapping efforts, reveal the general route of the cave passage.

Recognizing the long, problematic history of the site and its impact on the entire region, the City of Bowling Green also got involved in the research phase by asking DDS Engineering to conduct a feasibility study to determine a way to dispose of the stormwater while preventing any hazardous materials from entering the subterranean system. An analysis by the team from DDS showed that previous flood-control efforts, such as dry wells and drains discharging into a local rock quarry, were highly 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," states Jason Blakeman, a DDS engineer on the project. "Two US highways, a state highway, the university, the state department of transportation, the city, and a rail line all have converging interests in this project."

Blakeman explains that while developing plans for the project, there was a fine line between what was economically feasible and what was best for the environment, so DDS performed a number of tests and consulted with a number of contractors, drillers, and stormwater treatment providers to determine the level of construction work needed to implement an effective stormwater management plan. With the presence of transportation facilities on the property, such as the rail line, the list of pollutants that might travel through the site was extensive, but DDS decided to target the ones that were likely to be the most common in the runoff, including trash, contaminated sediments, and hydrocarbons.

High-impact compaction of the clay backfill around the Vortechs units.

Armed with all the research and planning, DDS was able to design a comprehensive stormwater management plan that included a network of stormwater treatment units and flood controls that effectively could manage the extreme water flows. To mathematically analyze the watershed for the parking lot, DDS divided the watershed into two sections. The northern section contains 304.4 ac., including the main campus of WKU. The southern section contains 323.1 ac., predominantly composed of residential properties. The time of concentration for the northern portion is 20.4 minutes, while the time of concentration for the southern portion is 50.4 minutes. When DDS analyzed the two hydrographs in conjunction with each other, assuming that a rainfall event occurs over the entire watershed, they discovered that the peak stormwater runoff arrives at the outfall point (the parking lot) in just 10 minutes.

All of these factors influenced the review process and analysis of potential stormwater best management practices (BMPs) for the site. Although a wetlands proposal had been submitted earlier in the process, it 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 BMP that would meet three overriding requirements: designed to remove the targeted pollutants, sized large enough to handle the volume of runoff, and affordable in terms of both unit costs and system installation requirements. After researching several options, DDS recommended the Vortechs System from Vortechnics Inc. With a documented 80% total suspended solids (TSS) removal on a net annual basis, the system also had a comparatively shallow sump depth, which meant it additionally would require less drilling and coring on the karst terrain.

DDS designed a storm drain plan that channels the urban runoff into one of four stormwater systems, each containing a Vortechs stormwater treatment unit, a bypass structure for flood events, and an 8-ft. drainage caisson-like structure positioned over a 4-ft.-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-ft. trench drains located on adjusted grades throughout the lot. Each Vortechs unit, 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 enter directly into the cave system, the flooding problems should be virtually eliminated, provided that the cave under the parking lot indeed is as big and connected as the studies indicate.

"We did quite an extensive search for an effective stormwater treatment unit that could handle the flow requirements, and Vortechnics 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."

When the students return this fall and park on the new lot, they might not see the stormwater management system working beneath them, but they will definitely see the difference it makes.

SW September/October 2003

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