Case Study: The Future Is In The Balance

By Rex Hansen, P.E.
Stormwater Management, Inc.

A project in California's Simi Valley demonstrates how the need for new multi-use construction can successfully be balanced with regard for environmental care.

When Selleck Properties presented its plan for a new project to the Simi Valley City Council in June 2002, the major southern California development company took the first step in what would be a long but successful venture demonstrating a thoughtful balance of commerce, community and stewardship of the area's surroundings.

The envisioned project – a 35-acre phased site developed to include commercial space, 167 affordable apartments for seniors and 200 townhomes – promised to provide an economic boost to the community, both during and after construction, as well as much-needed new housing. Moving ahead, however, meant facing a barrage of city, county, and state regulations and restrictions.

One critical issue: urban stormwater runoff, the number one source of coastal water pollution in the region. During rainstorms, runoff can cause a cascade of pollutants – oil, grease, trash and sediments – to be flushed into drains that feed into rivers and oceans, jeopardizing aquatic habitats, the area's important tourism industry, and livability in general.

At El Paseo SimiTowne Centre, the first part of the phased project to be completed, runoff drains into Arroyo Simi Creek, which flows into the Calleguas watershed and river and then into Magu Lagoon. The Lagoon is a highly sensitive water body, already impaired by upstream contaminants. For Selleck Properties, the fact that potential runoff from the proposed project would eventually drain into these waterways meant the developer would need to select a treatment system that could effectively remove pollutants to the necessary levels.

"There's been a continual evolution in regulations regarding stormwater treatment," said Selleck Properties principal David Ghirardelli. "Just a few years ago, all you needed was a clarifier system or screen at the inlets to get rid of fossil fuels. Now you need a total filtration system for the entire project."

Weighing the Options
Selleck Properties enlisted the civil engineering and land planning expertise of The Haaland Group, who explored a variety of different stormwater treatment systems in order to balance both the aesthetic and environmental requirements of the project.

Use of a swale was considered. However, continuous maintenance of such a system is required to keep up with surface pollutants, such as trash and debris, and swales require sizeable chunks of valuable land that can eat into parking opportunities.

"That could cause problems," said Ghirardelli, "since a certain amount of parking space is required for projects like ours."

Another stormwater treatment option was a large, concrete cistern that relies on a swirling-type system to settle out pollutants. But that option also has its downside. For example, the system must be buried quite deep in the ground, which makes installation costly.

Maintenance too is an issue, since regular removal of pollutants every three to six months is required to keep the system operating properly. Even buried deeply, cistern-type systems can present a problem with stagnant water, not just in terms of odor, but vector control. Systems that collect water can attract mosquitoes – a major problem, given the concern about West Nile virus.

The Solution
The Haaland Group had on an earlier project become familiar with another system, the StormScreen®, that was designed to meet the current regulations. For the Simi Valley project the system seemed ideal, addressing the operational, aesthetic, and economic needs of the developer, and the environmental demands of regulatory agencies.

The StormScreen, a widely approved BMP system manufactured by Stormwater Management, Inc. in Portland, Ore., is a passive, high-flow screening system designed to remove trash and debris and sediment from stormwater runoff. Its siphon-actuated screen cartridge with patented surface-cleaning mechanism provides removal of solids as small as 2.4 mm in diameter, regardless of flow rate. Its modular construction allows for removal and replacement of the stainless screens. The StormScreen at the El Paseo Towne Centre treats a water quality flow rate of 7.5 cfs.

An enhancement used at the El Paseo project – a drain-down system – ensures complete dewatering of the StormScreen vault within 24 hours of a rain event, decreasing the digestion of trapped pollutants and minimizing vector problems. Unlike other systems, what is left for removal are dry pollutants.

"There's no stagnant water, no odor, no vector problems," said Ghirardelli, "Plus, once-a-year vacuuming is all that's needed – a real cost savings in maintenance."

The device's small footprint – the interior dimensions of the vault are 8' x 16', which includes 14 StormScreen cartridges – and underground installation met the aesthetic and practical needs of the project. The developer was able to put parking over it.

Other needs were addressed as well. One of the peculiarities of the El Paseo project is the fact that three different zones use one drainage system. Storm drain pipelines from the residential components all feed into the lines running to the commercial portion of the project where the water is treated. From there it flows into detention where it is slowly released at the proper rate.

Historically, residential units in Simi Valley haven't been allowed to rely on underground stormwater storage since homeowner associations typically aren't equipped to maintain such systems. But regulators allowed the El Paseo configuration without requiring separate residential filtration systems, because the commercial component will manage the stormwater management and maintenance function for all three entities.

How it's Working
Since the StormScreen system was put on-line during the construction phase, Selleck Properties was able to witness effectiveness even before the project was complete.

"You get a lot more debris – dirt, mud, you name it – going into the system during the construction process than would normally happen," said Ghirardelli.

Engineers and officials from various agencies who were present during the system's first maintenance were clearly impressed by how well the filtration system was working. Ghirardelli expects to get an even better idea of the system's performance during the upcoming winter. Selleck Properties routinely sweeps the parking lot at the Towne Centre, but contracts StormScreen system maintenance to Stormwater Management. However, even with larger than usual amounts of debris, Ghirardelli anticipates cleaning will be needed at most only twice a year.

Source: Stormwater Management, Inc.