To detain stormwater runoff at the Metro Fire Station # 21, the Metropolitan Government of Nashville and Davidson County elected to remove an above-ground detention pond and construct an underground stormwater detention system to gain back valuable land for parking during the recent replacement of the facility.

The first design of the new stormwater detention system specified 36-in. corrugated metal pipe but concerns regarding fire truck traffic loading on the system resulted in a change to 36-in. reinforced concrete pipe. This in turn could not be used as it would not fit in the required footprint under the facility’s driveway.

In the final design, a modular concrete stormwater management system was chosen and subsequently constructed under the entrance road, since it reduced the detention system width and overall footprint by over 40%, and easily fit under the fire station roadway.

The modules were manufactured and supplied at a nearby precast concrete plant. The final system contained (16) modules at 3 ft tall, installed on top of (16) base slabs, for a total of 3,700 cu ft of detention storage. In addition, (5) catch basins/storm structures, 15-in. and 18-in. reinforced concrete pipe, and (3) sanitary manholes were provided for the project. The installation of the entire drainage system was completed in (1) one day.

The detention system was a portion of the overall project to construct a new 21,000-sq-ft fire station for Nashville’s Fire Department. Fire Station #21 is expected to achieve LEED Silver certification.

Littlejohn Engineering designed the stormwater system and Summit Constructors performed the installation for Messer Construction, the general contractor on Fire Station # 21.

A new low-impact development (LID) stormwater management system has been developed as an all-inclusive stormwater run-off control system that manages water volume in addition to protecting water quality by providing integrated pretreatment. This system combines the advantages and versatility of structural precast concrete modules (vaults) with the aesthetics and performance of permeable interlocking concrete pavers to provide a stand-alone, low-maintenance, LID green solution for stormwater retention, detention, reuse, groundwater recharge and flood management.

The success of this stormwater management system begins with the permeable paver system. Due to the highly permeable aggregates that surround the specially designed concrete pavers, large volumes of stormwater originating on the permeable paver area, or running on from adjacent impervious surfaces (e.g. roofs), are infiltrated into the underlying aggregate base and ultimately drain to the underground storage modules (vaults). Not only is surface water flow virtually eliminated, but the paver system also provides initial water quality treatment through filtration; total suspended solids and metals are captured in the joint material, which can in time be vacuumed out as part of the regular maintenance program.

A major component of the total stormwater management system is the underground precast concrete module with permeable lid. After infiltration through the permeable pavers, the stormwater flows through the permeable lid and collects in the precast modules for controlled management. This allows maximum runoff collection over the entire system’s footprint without the need for grated inlets. The lid permeations and permeable base materials above are strategically sized to eliminate the need for a geotextile fabric that has the potential to clog the system. The modules store the water and allow for controlled infiltration into the underlying soils.

In addition, cleanout and maintenance of the stormwater system is easy through maintenance access into the below-grade modules.

A stand-alone solution, suitable for a large number of engineered applications including parking lots and roadways, this complete stormwater management system reduces or eliminates runoff, minimizes area disturbance, offers impervious cover and underground storage for filtering, treating, storing, evaporating, detaining and exfiltrating stormwater runoff close to its source and is accepted as best management practices (BMP) to control stormwater runoff.

Oldcastle Precast is the leading manufacturer of precast and polymer concrete products in the U.S., providing engineered product solutions nationwide to a variety of market sectors. Contact: 7921 Southpark Plaza, Suite 200, Littleton, CO 80120. Tel: 888-965-3227. Fax: 303-794-4297. E-mail: Jackson.bishop@oldcastle.com www.oldcastleprecast.com