Space can be a limiting factor in effectively managing urban stormwater. The larger impervious surface areas common in urban regions can add to the challenge.
In such cases, underground systems can offer solutions. Added value can be obtained by a stacked approach as is offered by the HydroLogic Solutions’ StormChamber stacked systems.
David Batts, director of system solutions for Construction EcoServices in Houston, TX, says he favors using stacked systems for a tight footprint and even tighter budgets.
Experience underscores his philosophy. His company is a turnkey stormwater consulting and product sales company with three business segments: managing stormwater pollution prevention risks for general contractors; installing stormwater quality, stormwater detention/retention, and green infrastructure-type systems for civil engineers and landscape architects; and maintaining long-term functionality for individual property owners of stormwater best management practices such as pipe treatment systems, underground detention/retention systems, rain gardens, and permeable pavement systems.
“Most of the time when we do underground detention/retention systems, we try to maximize depth,” points out Batts. “Depth is always the mitigating factor of what underground system we use. Generally speaking, the deeper you can go, the less expensive you can make the system, especially if you have water that you don’t have to store.
“Anytime you’re in a high-density area or you’re limited in space and you’ve got depth available, then the stacked system is the way to go.”
Four-Layered System at a Houston Self-Storage Facility
After the success of several two- and three-layer StormChamber installations, Brewer Escalante Engineering of Houston designed a four-layer system for a Houston self-storage facility when it was under construction in April 2013.
The primary challenge in providing stormwater detention for the facility was the small site upon which it was located. The four-story building meant high density, says Batts.
David Brewer, P.E., principal of Brewer Escalante Engineering, says available space is the main factor in a stormwater system selection. He notes that during the past decade, the stormwater product market has become large and vibrant, with many products from which to choose.
Brewer Escalante Engineering and the site’s owner considered concrete vaults and large-diameter pipe, but the costs of those approaches were too high for the budget.
“We know that budget at the end of the day is a really high concern for our clients, and they want an economical solution,” says Brewer.
Every project has a unique set of geometric requirements and a unique solution, he adds. StormChambers fit the bill for both pricing and footprint considerations.
“A lot of time with the StormChambers, it’s the geometry, the shape of the chambers, and how they fit together,” says Brewer. “You need to look at different products in a given volume of space. You might get different storage amounts based on different manufacturers’ products, so that’s an optimization process, and the most storage is going to be good on an economic basis as well.”
With StormChambers, “we had the advantage of a deep enough outlet to allow us to stack that many sets of chambers on top of each other,” he says. “At the end of the day, it’s about geometry. The development was a big storage building on a small site. There was very little extra land left over for anything else.”
Construction EcoServices was able to document from independent testing data and the success of three-layer system installations that the four-layered StormChamber systems would exceed the loading requirements at significantly less expense.
Not only was it less expensive than the concrete vault, but the StormChamber system was able to fit within the small footprint because it could be placed in four layers due to its capability of supporting more than four times the AASHTO required H-20 Wheel Load Rating. The system was able to accommodate the required 25,000 cubic feet of storage within a 126-foot by 33-foot area (4,158 square feet total).
“[The site] has very limited parking area, so being able to stack the chambers deep was quite attractive, because the only other solution that could be used is box culverts or cast-in-place concrete, which can get really expensive,” points out Batts.
In constructing the system, Construction EcoServices’ crews excavated 20 feet deep to install the stacked StormChamber system.
“It was very complicated because it was such a tight site—there was not a lot of room to move around,” notes Batts. “The density was really high with the building pad and everything else going on the site.”
Crews put a 12-inch subgrade at the bottom of the excavation with a perforated pipe running through it to drain the water into a relatively deep storm sewer system. Atop the base material, crews installed the first layer of chambers running the length of the excavation. On top of those chambers, crews backfilled with 12 inches of No. 57 stone.
Construction EcoServices’ crews installed the second row of chambers on top of the rock, perpendicular to the length of the excavation, and backfilled it with 12 inches of rock on top. The same procedure was repeated for the third layer.
The fourth layer was installed perpendicular to the length of the excavation, with 12 inches of rock on top, and the excavation was backfilled with stabilized sand. A 6-inch concrete parking lot was installed on top.
As the building takes up most of the site, most of the stormwater comes from downspouts, says Batts.
“A lot of it is direct-piped into the underground detention system from the rooftop,” he says. “The water coming off of the parking areas goes through standard graded inlets that are piped into the StormChambers.”
Maintenance depends on the amount of sediment that ends up inside the system, but Batts doesn’t anticipate a great deal given that most off the runoff will come from the roof. He anticipates the system will need to be vacuumed every five to 10 years. Inspection ports within the system are situated over the top of HydroLogic Solutions’ SedimenTraps beneath selective chambers to provide vacuum truck access.
Three-Layered System for New York Condo Development
When the Glen Hill Condominiums in Ramapo, NY, were being developed several years ago, engineers were faced with having to shoehorn a stormwater system into one corner of a site constrained by a proposed building property boundary and a utility easement.
That lack of space available to accommodate traditional stormwater facilities presented a challenge for Leonard Jackson Associates of Pomona, NY.
The engineering firm designed a large-diameter perforated pipe system for stormwater detention. Although it fit within the tight footprint, the budget required pricing of alternative systems.
This can be a driving factor in many projects where economics dictate the acquisition of the smallest possible parcel of land and the need to severely constrain building costs, including those of stormwater infrastructure.
For the Glen Hill Condominiums project, a triple-stacked StormChamber system was determined to be the least expensive alternative, allowing the end user to reduce the size of the drainage footprint through stacking. ELQ Industries of New Rochelle, NY, installed the system.
The choice of using a three-stacked approach was possible because StormChambers are capable of supporting more than four times the AASHTO required H-20 Wheel Load Rating (more than 32,000 pounds per square foot) as documented by independent laboratory testing. The StormChamber system was able to accommodate the required 24,265 cubic feet of storage within a 91-foot by 45-foot area (4,095 square feet total).
“There are always options,” notes Leonard Jackson, principal of Leonard Jackson Associates. “You can use an open detention system. You could use a pipe detention system. You could use StormChambers. As long as you have an area to provide a volume of storage that you can temporarily detain water, you use whatever fits in the site and what seems reasonable at the time.”
Space savings is the major benefit of the StormChamber system, notes Jackson. There was a need for a large volume of storage in a limited area of space. “By stacking these units, we managed to achieve the volume that we needed,” he says.
StormChamber is designed as an open bottom, high-density polyethylene infiltration chamber BMP that functions in permeable and non-permeable soils for subsurface retention, detention, reuse, and conveyance of stormwater runoff and as a water-quality BMP.
Its elevated side portals allow for overflow connection to adjacent row(s) of StormChambers, eliminating the need for inflow and outflow header pipe manifold systems, and their associated manholes with weirs and the extra excavation and stone required for their placement. They also provide greater engineering and hydraulic design flexibility.
The chambers’ open bottom provides maximum infiltrative surface area. The open infiltration surface area functions similarly to a septic drain field for nitrogen, phosphorus, hydrocarbons, and other pollutants, with formation of a bio-mat of microorganisms upon the underlying stone and soil.
The infiltration of stormwater also helps replicate preconstruction hydrology as it recharges groundwater supplies, maintains base flow to streams and wetlands, and, in coastal areas, helps to counter saltwater intrusion.
In non-pervious areas, underground storage also can be provided with regulated outflow. Under these conditions, a StormChamber system is less expensive than the typical specified pipe and concrete vault systems. Significantly better water-quality enhancement also is achieved due to the enhanced remediating effect of the bio-mat due to the higher moisture-holding capacity and higher organic content of clay soils.
Maintenance of StormChamber systems also is designed to be quicker, easier, and less expensive than other types of underground stormwater management systems. A vacuum truck tube is inserted down a riser pipe directly above StormChamber SedimenTraps in the first and last chamber of the row(s) receiving the stormwater inflow.
