Pollutants will be removed from stormwater by a series of separation and treatment actions that are illustrated and described below:
Temescal Canyon Storm Drain and Low Flow Diversion
Currently, urban runoff/rainwater from a 1,600-acre area in Pacific Palisades flows into the Temescal Canyon Storm Drain, a concrete box culvert. A low flow diversion was built near the storm drain in 2003 to intercept dry weather runoff of less than 1.3 cubic feet per second (cfs) and direct it into a sewer pipe that connects to the Hyperion Treatment Plant in Playa del Rey during the summer months. Flows greater than 1.3 cfs exceed the capacity of the diversion structure and pipeline and must empty directly, without treatment, into Santa Monica Bay at the surf zone at Will Rogers State Beach.
Stormwater flows will be piped to a device that captures litter and other pollutants. Under low flow conditions (dry weather and less than 2.6 cfs), this “pretreated” stormwater will flow through the upgraded low flow diversion and to the Hyperion Treatment Plant for further treatment.
During higher flows (typically during storms) the stormwater will be pretreated, then placed in an underground detention tank. The detained water will be pumped from the tank into the downstream sewer as pipeline capacity allows and will flow to the Hyperion Treatment Plant.
Low Flow Diversion Upgrade
In a separate project, the existing low flow diversion will be upgraded: the capacity will be doubled from 1.3 cfs to 2.6 cfs and it will be in operation year-round.
This key project element will provide stormwater pretreatment. The unit will have a peak capacity of 35 cfs and will effectively capture trash, debris, coarse sediment, oil, and grease prior to downstream treatment at the Hyperion Treatment Plant. The separator will be located underground at the edge of the park or within the road right-of-way to give vacuum trucks easy access to remove the debris and other pollutants.
Underground Detention Tank
The detention tank will serve to temporarily hold runoff during storms and higher flows. The detained water will be pumped from the tank into the downstream sewer as capacity allows and will flow to the Hyperion Treatment Plant. The 1.2 million gallon tank will be located in the park in an area where its location and any construction activities will not encroach into the drip zone of the surrounding trees.
Discharge Force Main
The pumps from the detention tank will discharge the pretreated runoff into approximately 1,000 feet of a new force sewer main (pipeline under pressure). It is estimated that the diameter of the new force main will be 16″. The pipeline will connect to an existing unused 16Ó force main near the intersection of Pacific Coast Highway and Temescal Canyon Road. The existing 16Ó force main runs under and adjacent to Pacific Coast Highway for approximately 3,000 feet at which point it connects to the sewer that flows to the Hyperion Treatment Plant.
If you have questions or comments, please call the outreach coordinator at (213) 978-0333. The project description, concepts and design are preliminary and subject to change.