Stabilizing Soils in a Challenging Proving Ground: The Mississippi Delta
Clambering along the eroding banks of a stream in north-central Mississippi’s Yazoo Basin, a team of installation specialists and scientists from the Montana-based company Trout Headwaters Inc. (THI) carefully plants willow and sycamore cuttings. Farther downstream, the team positions reinforced fiber mats over the fine, crumbling soil and then anchors the mats with more live stakes.
The half-mile project is an effort to demonstrate the effectiveness of new ecofriendly techniques for bank stabilization and river restoration across highly varied river systems. Working through a cooperative research agreement with the United States Army Corps of Engineers Coastal and Hydraulics Laboratory (CHL) in Vicksburg, MS, the THI team is enjoying great initial success in the challenging proving ground of the Yazoo Basin.
Since the early 1800s, people have manipulated the Mississippi Delta to make use of its rich soils and abundant natural resources. Channelization of streams and rivers and construction of dams and levees to contain water have restricted sediment and freshwater supply to natural floodplains. Combined with the removal of streamside vegetation, manipulation of stream courses has severely impacted water quality.
In 1984, Congress directed the Army Corps of Engineers and the Soil Conservation Service to establish demonstration watersheds to study erosion and sedimentation in areas heavily affected by agriculture. The resulting Demonstration Erosion Control (DEC) Project in the Yazoo Basin is a multiagency systems approach to sediment, erosion, and flood control. When complete, the DEC Project will install more than 2,300 grade control structures, 72 floodwater-retarding structures, 200 debris basins, and 250 mi. of streambank stabilization.
The erosional history of the streams in north-central Mississippi has been called a “tragedy of neglect, ill-planned land use, and channelization.” With average annual precipitation reaching 50 in., the frequent, intense rainstorm events wreak havoc upon the highly erodible sand, silt, and clay soils of the region. Channelization has resulted in significant degradation and widening of stream channels throughout the 17 watersheds under study within the DEC Project. Ongoing channel degradation has presented consistent challenges for bank stabilization, as channel incision historically has undercut attempted stabilization measures.
Bank stabilization methods currently in use within the DEC Project are primarily hard-armor structures, such as longitudinal toe riprap, transverse dikes, and bendway weirs. Biostabilization work previously attempted in the project has underperformed. As a result, the use of vegetative biostabilization techniques has been extremely limited throughout the region.
“For decades, hard-armor techniques have been used to stabilize streambanks despite growing evidence that these approaches are causing more problems than they’re solving,” states Michael Sprague, president of THI, a leader in biostabilization technologies for streambanks. “We needed environmentally superior strategies that were predictable. We looked around and discovered that there weren’t any. Basically the lack of predictability for natural solutions had driven engineers, hydrologists, and wetland specialists to fall back on the well-known attributes and structural predictability of hard armor. Our firm has been working for seven years to generate that same predictability for soft materials and complex systems.”
In 2001, THI and CHL entered into a two-year agreement to facilitate the cooperative 0.5-mi. demonstration restoration project in the Yazoo Basin using the innovative bioengineering bank stabilization treatments developed and refined by THI.
The middle fork of Worsham Creek, near the town of Winona, MS, is typical of many hill country streams in the Yazoo Basin. Historic channelization and denuded riparian areas have occurred as a result of agricultural practices and associated channel alteration. There also is some evidence of relatively recent channel incision. Using Worsham Creek as an initial demonstration site, these new technologies for bank stabilization could then be deployed as vegetative alternatives to hard-armor riprap throughout the DEC Project’s 17 watersheds.
Along with dormant plant material (live stakes and fascines), THI specified North American Green’s C350, an erosion control/turf reinforcement mat (TRM) comprising of a permanent, super-high-strength, three-dimensional matting structure incorporated with a 100% coconut fiber matrix. According to hydraulic computer channel models used for river hydraulic analysis, the combination of vegetation and TRM would provide the soil stability needed to allow for recovery of the riparian corridor.
Field tests were installed during January 2002 on Worsham Creek and neighboring Harland Creek. Results from the initial test sites were then used to design and implement restoration on 2,700 lin. ft. of Worsham Creek.
Initial tests indicated very good growth and survival percentages for all types of plantings. The C350 is assisting recruitment of native vegetation and trapping sediment while retaining general bank soil stability. As an added benefit, short-term plant viability appears to be increased with the use of C350 on the highly porous substrate due to increased soil moisture retention.
Since January 2002, the tests and the demonstration project site have been routinely monitored by THI and CHL. The demonstration restoration project site on Worsham Creek has become a project used by corps officials for its bank stabilization workshops. Long term, the demonstration restoration will be part of many demonstrations in the corps’ DEC Project watersheds and will be monitored at least twice per year by visiting professionals attending corps-sponsored workshops.
THI has several research and development agreements in place with the corps’ Environmental Laboratory aimed at assisting development and widespread deployment of soft-armor stabilization and restoration technologies. Short-term project findings from the Worsham site have been widely circulated by corps personnel in both field and classroom instruction. A field day held on the site in January 2003 was attended by industry representatives, corps personnel from two districts, and The Nature Conservancy. Ongoing use of the site for corps classes and instruction will continue to disseminate this information to interested audiences.
“We’re excited to be working with the corps and other leaders in the industry to forge a brighter future for our nation’s waterways. I believe the work underway here will set a new standard for both stream restoration and bank stabilization,” concludes Sprague.