Under the provisions of the new Federal Clean Water Act , shopping center developers have to abide by stricter requirements for water retention. Developers can meet these requirements with minimal impact to their land costs by moving the water retention system underground.

At the 203,000 sq. ft. Arlington Heights community center in suburban Chicago, installing an underground water retention system allowed locally based developer M&J Wilkow Ltd. to expand an existing retail center on a tight suburban site.

As part of the total $12.1 million project cost, a $5.1 million contract to build the underground water retention system and a new 53,000 sq. ft. addition, remodel the existing 150,000 square foot center, and connect the two, was awarded to Krusinski Construction of Oak Brook, Ill.

The center's site is without available open space for a grade level retention pond to absorb ground water. Because the limited site acreage could not accommodate a groundwater retention pond, Krusinski Construction suggested an underground retention system with pipes.

Typical problems caused by storm runoff increases include overloads on existing storm sewers, lower ground water tables, soil erosion, downstream flooding, increased chemical pollution and silt build-up of streams and lakes.

When planning studies indicate that existing facilities cannot accommodate projected runoff increases, storm water retention facilities can stabilize runoff rates. These systems collect and temporarily store excess runoff, while discharging the water at rates equivalent to pre-development levels.

While both aboveground and underground retention systems limit storm water runoff from newly developed sites, the high maintenance costs of open ponds and a need to conserve land are key considerations in using underground systems, as was the case in Arlington Heights. Furthermore, underground systems are generally safer than open ground retention systems.

Increasingly, urban communities are imposing strict discharge and runoff regulations. Recent National Pollution Discharge Elimination System (NPDES) storm water regulations limit on site runoff to reduce damage to waterways and municipal sewers. Also, new developments are often required to limit runoff quantities to pre-development levels.

Traditional retention facilities, such as surface ponds and basins, frequently cannot be used for controlling runoff. Underground corrugated metal pipe or pipe-arch storm water retention systems are alternates to surface retention solutions.

Design and Development Factors Underground runoff retention units begin storing runoff water when inflow exceeds the allowable discharge rate of the storm sewer system. As long as inflow exceeds the permissible discharge rate, the retention system accumulates water and discharges it over longer time periods.

These retention systems work as an integral part of the storm sewer system and provide a temporary storage area for excess storm water. Maximum acceptable runoff volume is generally limited by community regulations based on runoff patterns occurring prior to development activities.

To achieve greater operating and installation efficiency, and for maximum long term performance, subsurface retention systems must be site adaptable, durable, economical and with sufficient structural capacity.

Corrugated metal pipe underground retention systems can be sized and shaped to meet most site specific storage needs. Lightweight corrugated metal piping assembles faster, thus lowering installation costs and shortening construction schedules.

These pipe systems are generally fabricated from galvanized steel, aluminum, and aluminized steel Type 2, the most widely used because of its durability and economy. Contech Construction Products, of Middletown, Ohio, provided piping systems for the Arlington Heights shopping center.