Compact subsurface system manages stormwater quality and site discharge for a new commercial development.
Park Place Tower stormwater detention, Jeffersonville, Ind.
Stormwater regulations established in most communities limit the runoff volume or rate and pollutant loads from a developed site. Owners are required to manage runoff actively to minimize the effect it may have on the environment and existing sewers. Rarely is a commercial or industrial site exempted from these regulations.
There are a variety of new products—and new applications of existing products—that have been introduced to help engineers meet stormwater regulations. Surface storage systems (ponds or basins) are commonly used to hold runoff; an overflow pipe is used to control the runoff discharged to a sewer or other outlet. While ponds can be made relatively attractive, they sometimes pose safety concerns and consume expensive real estate that could be used for building or other applications. Subsurface storage systems, typically comprised of an underground pipe network, eliminate many of the negatives associated with surface systems.
Storage systems, whether a surface or subsurface design, are generally not depended on to remove pollutants. That is accomplished by other products, including stormwater quality units (SWQUs), which have been introduced during the last several years as regulations have become more restrictive. SWQUs operate by capturing a portion of the initial runoff which generally contains the most concentrated pollutants. Pollutants are settled or floated out of the runoff, while the relatively clean portion flows to a storage system or another discharge point.
Park Place Towers, a Sprigler Development project, included a two-story office building and a few retail stores on a parcel that had previously been an open farm field. It is located in Jeffersonville, Ind., across the Ohio River from Louisville, Ky.
The city of Jeffersonville limits runoff from new development to that which existed prior to development for 2-, 10-, and 100-year storm events. In addition, the city requires the use of an SWQU to improve the quality of the runoff. The Park Place Towers development was only about an acre; a parking lot covers about half the area and the building encompasses about a quarter of the site. The remaining area is open space. The addition of the impervious surfaces to what was originally an open area meant that stormwater runoff would definitely increase. The engineer on the project, Heritage Engineering, also of Jeffersonville, determined that 0.08 acre-feet would need to be stored to meet the stormwater regulations for post-development runoff.
The engineer wanted to make the most of the small site and one of the best ways of doing that was by placing a subsurface stormwater storage system beneath the parking area. A LandMax detention system using Hancor, Inc., Sure-Lok ST HDPE pipe was designed for the site. The LandMax system for the Park Place Tower project required about 740 feet of 30-inch-diameter pipe. The entire system is contained beneath just 13 parking spaces. A Hancor SWQU was installed to capture pollutants prior to the runoff entering the LandMax system and to meet the city’s requirements. Additional Hancor HDPE pipe in 12-, 15-, and 18-inch diameters was used elsewhere on the site for storm sewers.
The LandMax system for the Park Place Tower project required about 740 feet of 30-inch-diameter pipe. The entire system is contained beneath just 13 parking spaces.
Design and layout of a Hancor LandMax and similar systems require input from both the design engineer and the manufacturer. Manufacturers have a standard product offering for subsurface storage systems and can provide custom-fabricated components if required. Special fabrications may include headers, tees, wyes, or elbows with certain dimensions, or pipe of a certain length. Once delivered to the site, the components can be assembled easily and accurately using the plan drawings.
The LandMax system at the Park Place Tower site was installed in December 2005. The system went together quickly and easily, even though it was the first subsurface pipe system the project manager, Leo Messmer of Dan Cristiani Excavating, Clarksville, Ind., had installed. An excavator was used to remove the native soil and position the LandMax system. LandMax components are lightweight and can easily be handled by a few laborers and ordinary construction equipment.
Specifications required a #57 stone for the backfill. The contractor used the excavator bucket to pour the backfill on the pipe crown so that it flowed uniformly on either side of the HDPE pipe laterals. A laborer shoveled the backfill into the pipe haunch area to minimize any voids that might have been created. One pass was made with a plate compactor when the backfill was about at the springline. The pipe did not misalign during backfilling or compaction. Backfilling continued until there was about 12 inches of gravel over the pipe crown; the contractor then used a dozer to level the backfill.
The total cover over the system varied from 2 to 3 feet. The #57 stone backfill extended 12 inches to 24 inches over the pipe crown. The asphalt base, binder, and surface layers added nearly 12 inches more.
The Hancor SWQU was installed near the LandMax system. The SWQU was 42-inches in diameter and 20 feet long. These units are generally backfilled similar to pipe on the project. The units themselves are sized to capture the "first flush" from a storm event so that most of the pollutants can be captured. Flow in excess of the first flush is less contaminated and flows through a bypass pipe to the subsurface system.
LandMax subsurface systems are designed to withstand standard AASHTO HS-25 highway loads with 1 to 2 feet of compacted cover. Some vehicles, including some construction equipment, are heavier than highway loads and should not be allowed to travel over the system without some additional cover.
Subsurface systems, regardless of pipe material, may require a geotextile (filter fabric) to separate the backfill from the native soil to prevent soil migration. Conditions at the Park Place Tower site did not require use of a geotextile since the native soils and stone backfill were unlikely to create migration concerns.
The subsurface system in this project used non-perforated pipe and functioned as detention. The system holds the runoff temporarily and then discharges it at a controlled rate. Perforated or non-perforated pipe can function as a detention system; however, a detention system will always have a controlled discharge. Pipe is used in detention applications with either soil-tight or watertight joints, depending on the project specifications. Like detention systems, retention systems temporarily store runoff; however, retention systems allow the stored stormwater to recharge to the surrounding soil. Perforated pipe with soil-tight joints are used in retention applications. Whether a system should be a detention system or a retention system is often determined by local regulations.
The LandMax detention system at the Park Place Tower project has been performing well for more than 18 months. The contractor was pleased with how easy it was to install the system and looked forward to installing more of them. As real estate costs continue to rise, demand for subsurface systems will be increased. These systems allow efficient management of stormwater without creating an eyesore or a safety issue.
Tori Durliat is corporate manager of marketing and communications for Hancor, Inc. She can be contacted at firstname.lastname@example.org.