Experimental Back-sloping with Vegetation Establishment as an Erosion Control Option for Missouri Streambanks
EXECUTIVE SUMMARY: Missouri landowners dealing with streambank erosion problems are searching for affordable and effec-tive techniques they can use to address existing erosion issues and protect their property from further erosion. The search is complicated because the eroding streambank is often a symptom of a larger problem occurring elsewhere within the watershed. Consequently, finding an effective erosion control method can be difficult for a landowner unless they receive appropriate professional assistance. The limitations of currently available methods in terms of high cost, difficult installation, or inapplicability to larger stream systems have caused landowners to try techniques that are ineffective and may lead to increased instability.
As a result, the Missouri Department of Conservation (MDC) decided to evaluate five different streambank stabilization techniques. The back-sloping with vegetation establishment approach was evaluated as a potential alternative to a longitudinal rip rap toe project for controlling excessive streambank erosion. The back-sloping approach is designed to reduce erosive forces acting on the eroding streambank by back-sloping the streambank which gives it a higher width to depth ratio and establishing vegetation that will decrease ve-locities by adding roughness and stabilize the streambank with root systems over the long-term. Back-sloping reduces the slope of the eroding streambank to a 3:1 horizontal to vertical ratio by removing streambank mate-rial. The exposed soil is then covered with erosion control fabric and planted with vegetation to stabilize it over the long-term. Five back-sloping with vegetation establishment projects were distributed among five streams located on four MDC conservation areas across Missouri. The projects were built between September 2006 and January 2009 and all experienced multiple high flow events.
This technique had mixed results. Three of the five projects have either failed or sustained damage and vegetation establishment has been slow to nonexistent on four of the five banks. The California Branch and Fiery Fork projects were the two most successful back-sloping projects, but have also been tested by the few-est number of high flow events. The California Branch project maintained its sloped angle, even though it was built with a steeper slope than was intended. The Fiery Fork project was not a good test of the technique be-cause immediately after project construction the thalweg shifted away from this streambank for reasons that had nothing to do with the project. The Starks Creek project had some toe erosion, but the project has main-tained the bank’s slope. However, the Starks Creek project remains vulnerable to complete failure due to lim-ited vegetation establishment at the site. The two projects built on Union Ridge Conservation Area on un-named tributaries of Spring Creek have both failed. The project on the eastern tributary has suffered significant toe erosion at the apex of the bend downstream and the western project failed due to toe erosion that occurred along the entire length of the project.
Overall just one of the four projects that was actually a good test of the technique has been successful in stabilizing the bank. The projects that failed did so because the vegetation did not become established quick-ly and the erosion control fabric was not strong enough to protect the bank. Given these results, careful consid-eration would have to be given before using this approach. While this approach did save money compared to a traditional approach, its lack of success and the need for repairs could quickly eliminate those savings. The most significant factors limiting the usefulness of this technique are the fact that it is inappropriate for tight bends and its reliance on the quick establishment of vegetation, particularly trees, in determining whether or not the project has any chance at being successful long-term. As previously recommended in the literature, this approach appears to have merit as a supplement to other types of toe protection, but has little or no appli-cation as a stand-alone technique.