Facility Expansion Maximizes Space with Underground Stormwater Detention

By Gina Carolan

When Reinhart, the third largest food service distributor in the United States, received a contract with a national restaurant chain, the company needed additional storage space at its Reinhart FoodService’s NATCO facility (New Bedford, MA) by the beginning of January 2010. It was decided that a 27,600-square-foot addition to Reinhart’s frozen and non-frozen foods warehouse should be built.

Because the expanded warehouse and additional parking lot would result in an increased quantity of stormwater runoff, the engineers from Tibbetts Engineering Corp. came on board to design a new stormwater system. The full-service civil engineering company was familiar with the distribution center’s site, as it had originally designed its surface detention pond.

Figure 1: Reinhart Foodservice’s detention basin.

The existing drainage pattern featured roof and parking lot runoff flowing west to east, following the grade. The stormwater was then collected and conveyed to the detention basin and piped out to a concrete-lined swale in the street layout. While designing a new stormwater system during the current expansion, the engineers kept the original drainage pattern and worked to tie in the existing drainage elements with the new system.

In accordance with the Massachusetts Department of Environmental Protection’s 2008 stormwater management standards, Reinhart’s new stormwater system needed to limit the post-development peak flow rate of runoff from the site to no greater than the pre-development rate. The engineers also needed to provide onsite infiltration to promote groundwater recharge. This requirement was based on the type of soil onsite, which at Reinhart’s facility consisted of glacial till – Type C – soil with a poor infiltration rate. Thus, according to the stormwater requirements, the engineers needed to infiltrate 1/4 inch over the new impervious area.

A pre- and post-development hydrologic analysis for the 2-, 10-, and 100-year storm events was conducted. The engineers calculated the site’s pre-development runoff rate based on an as-built survey of the drainage system and compared it to the potential impact of the additional building and parking area’s runoff. The peak rates of runoff were held at or below the pre-development levels for the 2-, 10- and 100-year Type III 24-hour storm events, which for the project location amounted to a volume of 3.2, 4.8, and 7.2 inches of rainfall, respectively, in 24 hours. The peak flow rate generally occurred at the 12.5-hour mark.

Located on a 9.4-acre plot, Reinhart’s facility already housed an existing 90,000-square-foot building, 117-space parking lot and a surface detention pond, so the building addition presented the engineers with a challenge of space constraints, a common issue in urban areas where space is at a premium.

“We had little choice but to put the addition in place of the existing parking lot,” said Robert J. Rogers, Project Engineer with Tibbetts Engineering Corp. “After that, we had to decide how we were going to fit 72 new parking spaces in the remaining area that was mostly occupied by the detention pond.”

Figure 2: Artist’s rendering of a CULTEC parking lot system.

Sheamus Kelleher, Operations Manager with Emerald Excavating, agreed: “The site did not have enough space to accommodate a detention pond that would be large enough to handle the volume of stormwater runoff from the added impervious areas.”

The engineers’ solution was a subsurface plastic chamber system from CULTEC.

“Underground chambers are a good option when land availability is an issue,” said Rogers.

The chambers can be used as subsurface retention or detention systems and as replacements for ponds, concrete structures or pipe and stone installations. A variety of chamber sizes accommodate almost any site constraint, and the company’s higher profile Recharger® series makes optimal use of large storage capacities in a smaller footprint.

The engineers replaced about three quarters of the detention pond with 442 units of Recharger 330XL, the model that has a capacity of 7.5 cubic feet per linear foot. This chamber model was used in conjunction with the HVLV™ 180 header units, which have an open bottom to allow for the maximum flow volumes at a retarded rate of speed, similar to the design of the plastic chambers.

The site’s existing and proposed drainage piping needed to be connected to the system at each end and in the middle, so the engineers used the HVLV 180 header system to provide equal flow distribution to the Recharger 330XL chambers. The combined storage capacity of the installed header units and chambers provided 58,000 cubic feet of the required storage.

Specific to Reinhart’s site, the system was located beneath the groundwater level at the same depth as the detention basin. While this positioning allowed the connection of the new system with the existing drainage elements, it also posed a question of meeting the infiltration requirement. In the end, the engineers added a shallow infiltration trench to the underground chamber system. The trench was designed to collect roof runoff, with its overflow discharging into the CULTEC system.

Design Strategies

According to the system’s design, the site runoff is piped to the underground system and surface detention basin, then slowly discharged by the outlet control box to the street swale, causing stormwater to collect and rise in the system to the point where one foot of freeboard remains at the surface basin during a 100-year event.

The engineers used HydroCAD® modeling software to analyze the site and design the system. The program already included the Recharger 330XL parameters, so the engineers only needed to specify the six-inch layers of stone above and below the chambers.

Figure 3: Installation of the underground chamber system.

The system’s footprint was determined by the required storage volume, and the engineers tried to make the system as narrow and long as possible to be able to tie it in with the existing roof and parking lot drainage and detention pond locations. The required storage capacity of the system was calculated using the post-development rate of runoff requirement. The storage volume per installed chamber was 11.3 cubic feet per foot, including the storage capacity of the stone.

Before finalizing their plans, the engineers had CULTEC specialists review the designs. They advised the engineers on selecting the right header system and number of access ports.

Installation

The installation of the chamber system began with excavating an 18,000-square-foot bed, laying non-woven polypropylene filter fabric along the sides and the bottom of the bed and then adding a six-inch layer of crushed stone. The chambers were arranged in the bed and fed using the company’s HVLV 180 manifold system.

The entire installation of the system took about two weeks. The HDPE units were light, did not require heavy installation equipment and were made with interlocking connections for a fast and straightforward installation.

Figure 4: Installation of the underground chamber system.

“We installed the system quickly and easily,” said Kelleher. “When properly installed, the units are very strong. That allows the installation crew to drive a skid steer over the top of the units during backfill operations. This saves a lot of time.”

Maintenance of the system is minimal, required only of the preliminary collection system prior to feeding the bed. The maintenance plan for Reinhart’s facility includes:

Sweeping of the parking areas twice a year.
Quarterly inspection of all catch basins, detention basin and outlet control structure.
Cleaning of catch and detention basins and control structure after the last spring snowfall as well as when sediment is within two feet from the outlet.
Regular inspection of catch basins for presence of hydrocarbons, which, if found, should be removed by a licensed contractor.
Removal and proper disposal of all sand and debris from the sumps.
Yearly mowing and cleaning of the surface detention basin.

The installation of the underground chamber system was completed in January 2010. According to Rogers, the system performs as specified. It captures and detains a large amount of runoff onsite without taking up valuable space.