Filtration and infiltration of urban runoff keeps California creek clean

Craig Wenzlick
Area Manager
CONTECH Stormwater Solutions, Inc.
Escondido, California

A mere 25 miles from Los Angeles, the city of Calabasas, Calif. is tucked into the southwestern edge of the Santa Monica Mountains. Despite its frequent use as a set for films and television programs, Calabasas residents boast about the town's rustic smalltown feel. So, when construction along the median of Lost Hills Road began impeding traffic flow, the town's residents were quick to inquire about the digging.

Those asking discovered a project called the "Bioremediation and Infiltration of Urban Runoff to Las Virgenes Creek." Its purpose was to improve water quality of the local creek by diverting polluted urban runoff into a new stormwater biofiltration system. According to Alex Farasati, environmental services manager for Calabasas, the city was concerned about the dry-weather flows from the 670-acre watershed which contains a mixture of open space and urban areas, including a landfill and freeways.

Dry-weather flows typically occur April through October in California, and are a contributing factor to adverse water quality in the area. The term refers to flows through a storm drain system that are not from a storm event. Dry-weather flows can include lawn irrigation runoff, washwater from cars and industrial sites, or pretreated industrial wastewater, to name a few. Just like stormwater flows, dry-weather flows can mobilize pollutants on impervious surfaces, carrying everything from automobile oil and grease to trash and debris to lakes, streams, rivers and oceans.

City and conservation groups had been able to identify Las Virgenes Creek as a contributor of trash, debris, nutrients and bacteria found at Surfrider Beach, one of the most famous surfing spots in the world. Dry-weather flows were being discharged untreated into the creek, then flowing into Malibu Creek and ultimately into the Pacific Ocean. Therefore, improving the water quality of the creek would provide positive effects throughout the watershed.

Two funding sources
Calabasas tapped into two sources of funding for the bioremediation and infiltration project to protect Las Virgenes Creek—and ultimately the ocean—from pollutants. The first was $440,000 from California's Proposition 13 Clean Beach Initiative (CBI). CBI provides grants to help local agencies, nonprofit organizations, and public agencies carry out projects that protect and restore California's coastal water quality. Proposition 40, the California Clean Water, Clean Air, Safe Neighborhood Parks, and Coastal Protection Act of 2002 and the California State Coastal Conservancy, supplied another $300,000.

This funding allowed the city of Calabasas to install a two-step treatment solution from CONTECH Stormwater Solutions. First a precast StormScreen treatment system screens the water to remove trash and debris, then water is directed into a series of StormChamber infiltration structures.

Installing the system
The location of the new equipment is deep below the Lost Hills Road median in Calabasas where an existing 102-inch diameter storm drain conveys runoff at a depth of over 20 feet. The large storm drain accumulates water not only from residential and commercial land uses, but also from open space, landfills, and freeways. All of these land uses contribute to polluting the Las Virgenes Creek.

The depth of the storm drain created a unique installation challenge. In order to connect the system, Terry Fleming, president of Fleming Environmental, said that his company had to dig the 25-foot hole in the street's median. Working in the center of the street, Fleming said that because of the tight area, traffic considerations and the depth of the hole, it took about five days to prepare for the installation. Installing the system was easy and only took two days, he said.

Typically, this screening system is installed at subsurface levels, but the Calabasas installation is the deepest to date.

Preparation work for installing the screening system in the median of Lost Hills Road narrows the traffic flow.

Two-stage process
Now, a 12-inch pipe diverts runoff from the 102-inch storm drain to the screening system. Comprised of a 12-foot diameter concrete manhole, the unit houses six cartridges that have a total screening capacity of three cubic-feet per second (cfs). It captures gross solids, screening one hundred percent of the average dry-weather flows through the storm drain and removing all solid pollutants larger than the 2.4-millimeter screen holes, even at high flow rates.

As flows subside, a patented siphon-actuated surface cleaning mechanism in the cartridge pushes energetic air bubbles across the surface of the screen to help prevent screen clogging and maintain continuous flow through the system.

After installing the screening system, the contractor backfilled the hole with rock.

In order to avoid extra digging and installation costs, the city decided to install the infiltration units at subsurface level. Therefore, the system designers integrated a pump inside the unit near its outlet. After flows are treated, the pump forces the water out of the system upwards about 15 feet to infiltration system.

The perforated, domed infiltration systems cover 2,400 square feet. An integrated pump forces treated runoff into the infiltration units, and a flow meter placed at the pump discharge line monitors the flow. Once in the infiltration chambers, the water percolates into the ground.

A manhole provides easy access for cleaning debris and trash out of the chamber. The frequency of cleaning depends upon the debris and dry season rainfall.

Except for its manhole covers, the screening system is almost hidden from view in the median.

By using special funds from Calif. Propositions 13 and 40, Calabasas was able to install a two-stage screening and infiltration system to improve the water quality of Las Virgenes Creek. This environmentally sound project not only works to decrease pollutants in the local creek, but it also protects a popular ocean area Californians, surfers and tourists share and enjoy.

Craig Wenzlick can be reached at (760) 743-3284 or