Dec 05, 2011

No Space for Detention Ponds

While ponds can be an inexpensive solution to handling post-development storm water, more and more engineers are challenged when it comes to extremely tight sites. Peter Blaisdell Jr. of Hayes Eng. ran into this exact issue when his client approached him.

The client had developed a cul-de-sac with a detention pond, but now wanted to access a back lot. However, in order to access this additional lot, the driveway would have needed to go through the detention basin, which was not possible. To resolve the issue, Hayes Eng. worked with the town of Wakefield, Mass., contractor Blue Chip Corp. and VARI-TECH LLC to design an underground infiltration system using Brentwood Industries ’ StormTank.

“I chose Brentwood StormTank because we had a limited amount of space and we were required to provide the same amount of storage volume in the subsurface system as was provided by the open detention basin,” Blaisdell said.

The Brentwood StormTank has a 97% void space, which is great for areas with limited space, and reduces in the excavated footprint. The town was also concerned about cleanability. This system provides observation ports, which allows for access into the system.

The site was prepared in stages in order to get around the entire system and still have access to the back lot. Once the site was prepared, it took only a couple of days for installation. Several site issues arose during construction; however, Deb Colbert of VARI-TECH LLC was available to discuss and resolve these issues with Blue Chip Corp. and the town inspector. For example, the contractor had exposed a rock ledge in one corner of the proposed system located near a neighboring home. In this instance, the tanks’ modular designs were beneficial. Instead of blasting or chipping away the rock ledge, the contractor was able to install the system around the ledge and relocate the displaced tanks elsewhere in the system.

As seen in the first photo, the driveway has been built, and the system area is currently being used for construction staging for the new home on the back lot. With 2 ft of cover, the StormTank system can handle HS-25 loads, or 40,000 lb per single axle load.

The Brentwood StormTank System is versatile. The standard module is 1.5 ft wide and 3 ft in length, and the height varies from 18 in. to 36 in., in 6 in. increments. In addition, the tanks can be stacked to 9 ft. of height. If a retention or reuse application is desired, the tanks can be wrapped in an impervious liner as seen below in Brooklyn, N.Y. The owner, Brooklyn Bridge Development Corp., wanted useable green space, and Nitsch Eng. of Boston selected the Brentwood StormTank to capture storm water.

“[We] chose the Brentwood StormTank system over a traditional underground piped retention system because of the smaller excavation footprint to install the StormTank system for an equivalent storage volume. The StormTank system has a high storage volume capacity per cubic foot of system,” said Josh Alton of Nitsch Eng.

The tanks were wrapped in a 40-mL PVC liner that was sandwiched between two layers of 8-oz. non-woven filter fabric for protection during construction.

As urban areas develop, engineers will be challenged with how best to handle the post-development runoff. There will be need for versatility, and Brentwood presents a viable option with high void space, cleanability and the ability to handle traffic loads.