4 minute read
Rogue River
Grants Pass, Oregon, United States
Building a bioengineered wall to protect a city’s water supply. The Grants Pass Water Treatment Facility sits directly adjacent to and above a high bank of the Rogue River that, following a major flood in 1996, was eroding from beneath the facility’s foundation. This erosion threatened the city’s only water supply for a population of nearly 35,000. Following the 1998 initial site evaluation and then development of a detailed hydraulic model, Inter-Fluve, under contract to the City of Grants Pass, proposed four options: no action, a traditional concrete retaining wall, a bin wall, and a bioengineered wall. Criteria for evaluating the alternatives included cost, aesthetics, constructability, long-term stability, ease of permitting, maintenance requirements, and habitat enhancement potential. The team chose the fourth option, a bioengineered, stacked geocell wall. The U.S. Army Corps of Engineers (USACE)–Portland District performed the actual construction of the 150-meter-long, 15-meter-high structure, adapting the bioengineering technology to a project site that challenges the limits of this emerging technology. The geocell wall reduced the risk of a catastrophic chlorine spill from the treatment facility while also fostering the growth of a diverse vegetative margin along the river.
Article cover: The river 20 years after stabilization. (Photo by Inter-Fluve)
Producing Efficiencies
In addition to mitigating erosion directly below the facility, the design addressed erosion up- and downstream of the site and prevented sand and gravel deposition, which would restrict flows to the plant’s intake structure. The team developed a twodimensional model to better understand how to counteract the complex hydraulics and intense scour caused by the highway bridge and sharp bend just upstream. Further, the bioengineered approach made the structure much more attractive to the regulatory agencies striving to protect endangered Chinook salmon (Oncorhynchus tshawytscha) in the Rogue River.
Using Natural Processes
While traditional approaches to riverbank stabilization—riprap, sheet pile, and concrete—provide structural integrity and long-term protection, they offer no ecological improvements. In contrast, a bioengineered retaining wall using stacked geocells with structural tie backs into the bank provides not only structural protection but also increases internal soil strength through root growth and fosters the development of a diverse vegetative margin that would normally cover a natural riverbank. This provides habitat for a variety of wildlife, such as the federally listed Chinook salmon and the many songbirds and neotropical migrants common to southern Oregon.
Broadening Benefits
Access to clean drinking water is a basic human need, and many freshwater sources are riverine. Combining the delivery of clean drinking water with ecologically beneficial technology represents an important step in providing clean water to a multitude of species. This site demonstrates how engineering with nature can complement a fundamental societal need as nearly 20 years later, the project continues to perform and provide ecological benefits. Further, protecting the water treatment plant, a designated historic landmark, preserves an important piece of Grants Pass history.
Promoting Collaboration
The city began by hiring a consulting engineering firm to perform the initial investigations and design. Then the city partnered with the Portland District, whose two-dimensional hydraulic model, a cuttingedge tool at the time of the project’s construction back in 1998, helped to refine the design. By teaming up with USACE, the city doubled its funding, using federal dollars to match the city’s initial investments, and their many public meetings ensured local input and support from constituents, stakeholders, and regulatory agencies.
