Project Case Study: Groundwater walls

October 2006 » Feature Articles
Bolsa Chica wetlands restoration in Huntington Beach, Calif.
Eric Stein

Advanced construction materials support Bolsa Chica tidal wetlands restoration and groundwater protection project.

Project
Bolsa Chica wetlands restoration, Huntington Beach, Calif.

Civil engineer
Moffat & Nichol, Long Beach, Calif.

Product application
A vertical, barrier wall constructed from vinyl sheet piles stops salt water intrusion into a residential development.




The Bolsa Chica wetlands in Orange County, Calif., is a place full of life. Flocks of sanderlings skim the water. Peregrine Falcons soar over the mudflats. Peeking through the marsh grass are scores of other endangered birds, native plants, and animals. Yet, not long ago, this unique wildlife paradise was almost wiped out.

"We came close to losing Bolsa Chica many times over the years," said Dave Carlberg of the Amigos de Bolsa Chica, an environmental preservation group. "Bolsa Chica is part of the Pacific Flyway, where migratory birds travel from the Arctic to South America. They need to be able to stop here in order to rest and feed."

After more than a hundred years of human encroachment and environmental degradation, birds are now returning to this strip of coast in Huntington Beach. It is a rare triumph in a state that has lost more than 95 percent of its wetlands during the last century. Like many of the state's coastal areas, Bolsa Chica became the victim of urban growth. In the 1920s, oil field operations began in the lowland areas. As Huntington Beach grew, more development started to invade Bolsa Chica from all sides. Over the decades, the Pacific Coast Highway, beach facilities, and a housing community were all built on the formerly pristine wetlands.

In 1980, Signal Oil Company unveiled plans to build 5,700 homes, marinas, and retail establishments on Bolsa Chica. That would have been the final blow to the dwindling salt marsh, which was already suffering from pollution and construction. Two powerful grassroots organizations comprised of local residents—the Amigos de Bolsa Chica and the Bolsa Chica Land Trust—fought back to reclaim land and block further development. It took several hard-won legal battles, but today, 1,200 acres of Bolsa Chica are undergoing the largest restoration project ever carried out in Southern California. Launched in 2004, the project will cost an estimated $65 million, which has been funded partially by the ports of Los Angeles and Long Beach, Calif., to compensate for marine habitat destroyed during their expansion. Voter-approved bonds provided additional funding.

On-site crews have been busy dismantling 64 oil wells and 98,000 feet of pipeline, restoring habitat, and constructing two bridges. The most crucial part of the project involved reconnecting the lowland areas to the ocean to allow tides to flow into the marsh twice each day. Without the tides, the wetlands cannot support native and endangered plants and wildlife. In late August, a 367-acre section of the salt marsh was opened to tidal flows for the first time in 107 years when bulldozers removed the last portion of an earthen dam that separated the wetlands from the ocean.

Since homes border this part of the wetlands, the new tidal patterns must be contained within the confines of the marsh. Therefore, the U.S. Fish and Wildlife Service, along with outside engineers Moffat & Nichol and prime contractor Kiewit Pacific, decided to construct a groundwater barrier to prevent saltwater from seeping into the adjacent neighborhoods.

"You can't have a marsh without tides. But when they re-flood the marsh area, they have to make sure the saltwater doesn't flow into private property because that can cause all kinds of problems, including killing lawns, trees, and frequently, valuable inland vegetation," said Mike Wallace from Crane Materials International (CMI), the company that produced the material for the groundwater barrier.

Instead of using traditional construction material such as steel, crews constructed the barrier with vinyl sheet piling. Many studies, including tests by the U.S. Army Corps of Engineers, have concluded that vinyl sheet piling surpasses steel and wood in long-term durability because it is far less likely to corrode and succumb to parasitic infestations. Millions of feet of vinyl have been successfully used around the globe in residential, public, and commercial projects.

For the Bolsa Chica project, engineers at Moffat & Nichol first conducted a 3-D, finite-difference model of the site using Groundwater Vistas software with MODFLOW96. Input to MODFLOW96 included the geology of the site, the hydraulic conductivities measured in the field, the piezometric levels measured at the piezometers installed in the wetland and the neighborhood, recharge information, and assumed anisotropy in hydraulic conductivity.

In the early phase of design, two options were considered. The first consisted of a linear drain trench penetrating into or through a saturated sand layer, acting as a water-extraction system to control water levels adjacent to the neighborhood. The second option consisted of an impermeable, vertical barrier anchored into a low-permeability silt/clay layer, accompanied by a water-extraction system on the neighborhood side of the tidal basin to control water levels in the neighborhood. Following the subsurface modeling, the vertical barrier was chosen because it provides a positive cut-off against potential saltwater intrusion beneath the neighborhood and requires an overall lower level of long-term maintenance than a linear drain-based system.

In designing the barrier solution, several methods for constructing the vertical wall barrier were considered, including a slurry wall, steel sheet piles, vinyl sheet piles, and a thin membrane system. The vinyl sheet piles were selected based on future maintenance considerations. Strength was not a factor because the sheets are completely buried in the ground.

The vinyl wall design called for 20- to 35-foot-long sheet sections with a total wall length of nearly a mile. The product chosen—ShoreGuard 325 from CMI—has a section modulus of 11.1 and is manufactured from co-extruded polyvinyl chloride. The sheets are box-shaped, offer smooth driving characteristics, and require little in the way of environmental consideration.

Once the site modeling was completed and all design parameters established, the rest of the project went smoothly and quickly. The installation contractor, Hughes Construction (a subcontractor for Kiewit Pacific), began installation of the 4,500 feet of CMI vinyl sheet piling in September 2005. Installation was accomplished using a mandrel attached to a hydraulic, ABI pile-driving machine. Given the sandy nature of the subsurface soils, no problems were encountered during installation. The sheets were, in many cases, driven at a rate of 200 feet per day, with the entire wall being constructed in just over a month.

The owner of the installation subcontracting company, Bob Hughes, said he was impressed by the vinyl design profile. "I like working with vinyl sheet products because they are very cost effective," Hughes said. "The box sheet profile means there are fewer seams [compared with traditional Z-shaped profiles], so you don't have to use as much sealant. That reduces cost and the chance of seepage, which can cause environmental damage."

It will still take years of labor to complete the entire Bolsa Chica restoration. Yet, every time Carlberg looks at the endangered birds that have returned to the area, he said he is reminded of why the monumental effort continues to be worth it. He also predicted that fishing will improve along the coast as ocean tides once again swamp the land. "Tidewaters flush in a lot of fish that depend on wetlands for spawning. This is going to have a great impact on fishing. It will also give people a tremendous place to hike and take part in other outdoor activities."

Members of the Amigos de Bolsa Chica say they have seen a huge shift in support for the restoration project since they founded the group. "Wetlands were viewed as good places for marinas," said Shirley Dettloff, a founding member. "The greatest change has been the public's growing appreciation of these places."

However, the public will only embrace restoration that also protects private property. State-of-the-art products such as vinyl and fiber-reinforced polymer make both the preservation groups and nearby homeowners happy. "We don't want more homes built here, but we still have to restore the wetlands in a way that acknowledges and respects the existing houses," said Carlberg. "The wetlands extend right up to private homes. Without the barrier, we could not have pulled this off."

Eric Stein is manager, marketing and communications, for Crane Materials International in Atlanta. He can be contacted at estein@cmilc.com .


More information about the Bolsa Chica project is available from the following organizations:
• U.S. Environmental Protection Agency (www.epa.gov/region09/features/bolsa-chica );
• U.S. Fish and Wildlife Service (http://bolsachica.fws.gov );
• Amigos de Bolsa Chica (www.amigosdebolsachica.org ); and
• Bolsa Chica Land Trust (www.bolsachicalandtrust.org ).


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