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Cool Stuff at Cooley; Warm Springs Settles In

Two South Bay projects could act as prototypes for restoring some of the Cargill ponds (see opposite). At the 115-acre Cooley Landing site, south of the western end of the Dumbarton Bridge, a not-very-subsided salt pond was restored to tidal salt marsh in December 2000. As with most salt ponds, when Cooley Landing was created, sediment was dug up from the Bay bottom and used to build levees around the pond. Because these "borrow ditches" tend to capture tidal flows when the levees are breached, Philip Williams & Associates designed "cut-off" and "training" berms to prevent capture and direct tidal flows into the silted-in-but still visible-original channels. Those channels then re-established themselves quickly, says PWA's Michelle Orr. PWA also decided not to excavate inlet channels through the mudflat, as has been done in past projects. Instead, those channels scoured out on their own within a few months. Nor was dredged material necessary because the site hadn't sunk too low, explains Orr.

"Cooley indicates that we can restore shallowly subsided ponds cheaply and efficiently provided we understand where to put the breaches," explains Williams. Sizing the breaches to match the predicted long-term channel depth and width was another critical factor. Within eight months, the site had good tidal exchange, and monitoring data indicates that sedimentation is occurring rapidly.

At the other end of the spectrum, Warm Springs, a 200-acre site on Coyote Slough, had sunk almost 15' below sea level after it was excavated as a borrow pit for a nearby development. Warm Springs was opened up to tidal flows in 1986 and has filled in fairly rapidly with sediment, partly because suspended sediment concentrations in the Bay nearby are very high. "We don't expect those same rates of sedimentation for the Cargill ponds, but we are getting better at predicting suspended sediment concentrations," says Williams. "One of the issues we have to come to grips with quickly on such a large-scale restoration is how it will affect the sediment dynamics of the entire South Bay."

Contact: Phil Williams (415)262-2300 LOV

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