Experimental Farm Rock Toe Protection as an Erosion Control Option for Missouri Streambanks
EXECUTIVE SUMMARY: Missouri landowners dealing with streambank erosion problems are searching for affordable and effective techniques that they can use to address existing erosion issues and protect their property from further erosion. This 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. Farm rock toe was evaluated as a potential low cost alternative to a traditional longitudinal rip rap toe project for controlling excessive streambank erosion. The differences between farm rock toe and a traditional longitudinal rip rap toe protection approach are four fold: 1) farm rock toe is made from shot rock (quarry rock not graded out to a specific size) instead of graded out rip rap, 2) farm rock toe is not keyed into the bed of the stream, 3) farm rock toe is not keyed into the streambank at the upper and lower end of the project, and 4) instead of placing each rock, the rock is dumped from the top of the streambank and then adjusted as necessary to fill in gaps. These changes were made to reduce the cost of a rock toe protection approach while hopefully still stabilizing the bank. Six farm rock toe projects were distributed among five streams located on five MDC conservation areas across the state. The projects were built between September 2006 and February 2008 and all experienced multiple high-flow events.
The farm rock toe technique had mixed results, but four of the projects have remained stable during the length of the study. The Starks Creek project experienced numerous high flow events and protected the toe while the slope of the streambank was reduced and vegetation became established. The Weaubleau Creek project experienced the most frequent and largest flow events of any of the projects. The toe rock has stayed in place, there has been a slight reduction in streambank slope and vegetation has established on the bank. The Sulphur Branch project was only tested by a small number of flow events that resulted in no actual changes to the bank, but there has been a rapid and extensive establishment of vegetation at this site that will help protect the streambank long-term. The second of two farm rock toe projects constructed on an unnamed tributary of Fiery Fork was not tested at all because the stream shifted away from the project immediately following construction and the resulting deposition has filled in the old channel and buried the project and bank.
Two projects were considered failures. The first of two farm rock toe projects built on an unnamed tributary of Fiery Fork failed completely when all the rock from the apex of the bend downstream was washed away during a high flow event. The California Branch project is considered a failure because all the rock located at the downstream ¼ of the project has washed away. Upstream of this the rock is in place and vegetation has begun to establish on the bank, however the lower end does not have any vegetation establishment occurring and there is almost no rock still in place. Although currently there has not been any erosion of the streambank it is considered a failure because there is nothing to prevent it from occurring going forward. The two failures both appear to be due to inadequate rock size and not a flaw in the stabilization technique.
Overall four of the five projects that were tested by flow events protected the streambank during the course of the study, although one of these is now starting to fail. The sixth project built was never tested because a channel shift occurred immediately following construction taking all the pressure off the project and should not be considered a successful use of the technique. The results indicate this approach does have potential for use as a streambank stabilization technique and the information gained at five of the six projects will be applicable to improving this stabilization technique. The most important factors in the success of a traditional rip rap toe protection project are the stability of your starting point and the size of the rock used. At these six projects we did not have any issues related to not using bed or streambank keys. The lack of keys did not affect project performance because our projects started and stopped at stable points. Two of the projects were negatively impacted by using shot rock instead of rip rap as the undersized rock we received resulted in the failure and partial failure of those projects. Additional modifications to the farm rock toe approach, such as using rip rap instead of shot rock could result in a technique that has potential, and may be worthy of further investigation in the future.