May 11, 2010

Old School to New School

A new $28-million elementary school in Brunswick, Maine, relied on engineered storm water products to complement low-impact development (LID) best management practices (BMPs). Before the new Harriet Beecher Stowe Elementary School could be built, the town’s old high school had to be demolished. This required designers to upgrade the existing drainage system to meet the state’s most recent storm water BMPs.

Roll Call

In line with the most current regulatory trends in the U.S., Maine’s regulations promote the use of storm water design practices that are intended to reduce and provide treatment for the annual runoff volume at the source. Due to site constraints and building footprint, however, reducing the site’s impervious cover and providing storage volumes dispersed uniformly throughout the site proved to be a challenge for the designers.

Of the site’s existing 13 acres, drainage was dispersed and entered several surrounding municipal storm drains without treatment or control. The proposed drainage system included pretreatment, storage, infiltration and biofiltration prior to discharging into the municipal storm drain system.

Dispersed pretreatment and storage could not have been accomplished without the use of engineered products provided by Hydro Intl. The First Defense vortex separator and Stormbloc storage media were used to maximize the efficiency of the proposed infiltration and biofiltration systems.

Answer Key

A vortex separator captures runoff and removes pollutants.

The vortex separator was chosen because it has features that are well suited for source control in LID designs. The designers placed a 4-ft and 6-ft-diameter separator prior to independent subsurface storage and biofiltration systems.

The separators capture surface runoff and use its energy to create the vortex motion needed to promote the removal and capture of fine sediment, trash, debris and hydrocarbons. The vortex motion and internal components minimize turbulence within the device so that the area needed to remove pollutants is much less than that of other gravity-based traps. Overall site work costs also are reduced because its smaller footprint means lower installation expenses and it qualified for a 25% reduction in biofiltration storage requirements.

In order to provide the required discrete storage needed for the biofiltration systems, the Stormbloc media was positioned in two locations where surface detention was not an option. One of the locations was directly beneath parking spaces, where minimal cover was available and existing utilities prevented spreading out the storage. The columnar design of the block supported HS-20 traffic loads with less than 24 in. of cover. With each block having 95% open volume, a large storage volume was provided in a much smaller area compared to arch chamber, pipe or stone storage solutions.

Once the blocks were connected together, a maintenance channel was formed that ran the entire length of the system. Specially designed inspection chambers allow access for inspection and periodic flushing. They also allowed the 18-in. storm drain pipes to be connected easily and in a way that did not restrict incoming flow.

Special chambers allow for easy inspection and maintenance.

All of the components connect together without needing heavy lifting equipment for installation. The two-man crew was able to unload and install about 26,300 cu ft in three and a half days.

Progress Report

Prior to the redevelopment project, “old school” storm water drainage design conveyed untreated runoff off site into the municipal storm drain as quickly as possible. Although redevelopment of the site did not significantly change the land use, an innovative “new school” storm water management approach was able to take advantage of engineered products. The intent of LID could still be accomplished, and runoff from the new school will be reduced significantly and treated.

About the author

Dave Scott is storm water product specialist for Hydro Intl. Scott can be reached at 207.756.6200.

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