Experimental Gravel-rolls with Back-sloping and Vegetation Establishment as an Erosion Control Option for Missouri Streams
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 the use of a gravel-roll with back-sloping and vegetation establishment as a potential technique for controlling excessive stream-bank erosion. The gravel-roll 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 velocities by adding roughness and stabilize the streambank with root systems over the long-term. In addition, a gravel-roll is built at the toe, which is intended to protect the streambank toe in the short-term, allowing vegetation to stabilize the streambank over the long-term. Back-sloping reduced the eroding streambank to a 3:1 horizontal to vertical slope. The exposed soil was then covered with erosion control fabric and planted with vegetation. A gravel-roll was constructed at the toe of the streambank to provide additional protection. The gravel-roll consisted of gravel wrapped in C2 erosion control fabric sewn together with nylon twine. Two projects were constructed at two separate MDC Conservation Areas in Missouri using gravel-rolls with back-sloping and vegetation establishment. The projects Mill Creek and Starks Creek were built between January 2007 and July 2007.
The technique failed at both sites during the first high flow event that occurred following construction. Reasons for failure were the same at both sites, the inability of the gravel-roll to adjust to areas of scour and because it was not large enough to protect a high enough percentage of the streambank (typically 1/3 of the streambank height is covered with other toe stabilizing techniques). The gravel-roll functioned as a large unit, and it could not adjust to fill in small areas of streambank scour as they occurred. Water scoured areas above and behind the gravel-roll, and streambank erosion continued while the roll itself stayed in place. The size (i.e., diameter) of a roll is controlled by the width of the erosion control fabric so it cannot be adjusted. To al-leviate this problem multiple rolls could be stacked at the toe of the bank; however, this adds considerable time and expense to an already expensive technique and does not address the other reasons the project failed. The reasons for discontinuing this technique after just two projects focused on the inability to adapt the technique to address reasons for its failure. Thus, the gravel-roll with back-sloping and vegetation establishment tech-nique will not be recommended because it has no utility to landowners.