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Restoration effort starts for canyons
By Mike Lee
STAFF WRITER
San Diego Union-Tribune
August 13, 2005
Community and environmental groups yesterday christened an unprecedented effort to restore San Diego's canyons and reduce pollution in San Diego Bay.
By reshaping canyon lands and installing underground dams to slow the flow of groundwater, the coalition aims to curb contaminated urban runoff, which ranks as one of the city's worst environmental problems.
Powered by $183,000 in state and local grants, the city's Golden Hill neighborhood is laying the technical groundwork for underground structures conceived by Wayne Tyson, a retired San Diego consultant on ecosystem restoration. If the structures work, similar designs could be used throughout the city's 60 major canyons.
"The idea is to re-create the 'sponge' capacity that the canyons used to have before they were scoured of native vegetation and good soils," said Tershia d'Elgin of the 32nd Street Canyon Task Force.
The combined grants are the largest injection of money into any of the city's 23 "canyon friends" groups developed by Sierra Club San Diego, said Eric Bowlby, the organization's canyon preservation organizer.
"In terms of investing in our canyons as a natural infrastructure, it's a very significant step for our city and for (the) overall quality of our environment," he said.
San Diego's expanding grid of roads and rooftops shunt massive volumes of runoff into the canyons, which connect to storm drains or creeks and eventually dump into the ocean. Over time, that runoff has scoured many of the canyons, baring channels of native plants and whisking pollutants downstream.
In recent years, controlling urban runoff has been a top priority for city staff, environmental groups and pollution regulators in large part because stormwater is a leading cause of beach closures and public-health warnings. The groups have established public education campaigns, started native-plant restoration programs, created neighborhood canyon protection groups and stepped up pollution enforcement.
Still, urban waterways such as Chollas Creek carry large loads of heavy metals and toxic pesticides.
The next step is to restore the canyons' natural functions as biological filters and "sponges" for stormwater, backers of the new project said.
"If we help nature along, it can help clean up the mess we have made," said Jimmy Smith, a scientist with the San Diego Regional Water Quality Control Board. "This project will go a long way toward restoring the (watershed)."
Initially, recent grants will be used to reshape the 32nd Street Canyon and remove some of the fill dirt that has altered the natural flow of water.
The part of the project that may have citywide application is assessing the technical feasibility of installing underground barriers to slow the flow of groundwater. Tyson, the restoration specialist, said he got the idea from a natural underground dike in Santa Barbara County that held back sediments and fostered lush plant growth.
Tyson figured he could replicate the concept in San Diego's canyons.
The manmade structures would be virtually invisible, though some may extend slightly aboveground. The base, attached to bedrock, would be made of an impermeable material such as concrete or clay. The section closest to the surface would be perforated to allow water to pass.
If the design works, groundwater flowing down the canyon would pool underground at the barrier. As the water level rose, it would slowly filter through the permeable top section and keep moving downstream.
Tyson said there are many benefits to holding back the water and spreading it out along the face of the dam. For starters, slow-flowing water doesn't cause as much erosion as torrents do and native vegetation is more likely to take root in what amounts to a shallow aquifer than in a washout gully.
Also, pooling underground water would allow plants to use up some of the nutrients, such as nitrogen, that are common in urban runoff. Tyson said other pollutants would bind to the rocks and soil, while additional contaminants would decompose naturally.
Over time, Tyson said, native plant growth also would help slow surface water by allowing additional sediments to settle out and keeping them from polluting the bay.
"Every canyon in San Diego that is urbanized would benefit from some version" of the underground barriers, Tyson said.
The project's backers acknowledge that the concept remains unproven and they still don't know how much it will cost, though they anticipate needing tens of thousands of additional dollars to complete the work. Initial funding was provided by the California Department of Water Resources and The San Diego Foundation.
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