The Bolsa Chica Lowlands Restoration Project is in Orange County, California, adjacent to the City of Huntington Beach. The purpose of the project was to restore portions of the wetland ecosystem of the Bolsa Chica Lowlands. The project area covers about 1,247 acres and is the largest, most expensive coastal wetland restoration project in southern California and the second largest construction project ever directed by the U.S. Fish & Wildlife Service. A steering committee comprised of eight Federal and State agencies coordinated the planning and design of the Bolsa Chica restoration project. Bringing the wetlands habitat back to life involved reconnecting the lowland to the invigorating influence of ocean tides, and creating a full tidal basin and managed tidal areas.
Elements of the restoration project included the design, construction and management of a muted tidal basin, pocket muted tidal basin, nesting islands and groundwater barrier. A 4,500 linear-foot impermeable barrier wall was to be anchored into a low-permeability silt/clay layer to prevent groundwater and salinity intrusion into the adjacent neighborhoods. A water-extraction system on the neighborhood side of the tidal basin was added to control water levels in the neighborhood. Following subsurface modeling, a vertical barrier was chosen because it provided a positive cut-off against potential saltwater intrusion beneath the neighborhood and required an overall lower level of long-term maintenance than a linear drain-based system.
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. The Full Tidal Basin was opened to the ocean on August 24, 2006. Since the restoration of the wetlands, the seepage barrier has performed as intended.
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.