Experimental Farm Rock Weirs 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 ero-sion. 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 farm rock weirs as a potential low cost alternative to a bendway weir project for controlling excessive streambank erosion. The difference between farm rock weirs and traditional bendway weirs are three fold: 1) farm rock weirs are made from shot rock (quarry rock not graded out to a specific size) instead of graded out rip rap, 2) farm rock weirs are not keyed into the bed of the stream, and 3) farm rock weirs are not keyed into the bank. These changes were made to reduce the costs associated with a weir approach while hopefully still stabilizing the bank. Five projects were constructed at four MDC Conservation Areas using farm rock weirs. The projects were built between October 2005 and April 2007 and were all tested by multiple high flow events.
This technique had mixed results. Four of the five projects were repaired, modified, or failed. The two projects built on Jakes Creek were the most successful farm rock weir projects. Both Jakes Creek project’s shifted the channel away from the streambank and got deposition between the weirs, despite having several weirs lose length. Weirs at all projects lost length because they were not keyed into the bed of the stream. The Dry Branch farm rock weir project failed due to inadequately sized rock that washed away. The project was repaired with larger shot rock but is in the process of failing again via the same mechanism. The Weaubleau Creek project failed when weirs three and four lost length and allowed flow to get behind weir five. Weir five was repaired and length was added back to weir four and since the project has moved the thalweg away from the eroding bank. However, little deposition occurred between the weirs and recent flows have resulted in erosion behind weirs three and four that will eventually result in the failure of the project unless it is repaired. The Middle Fork project was a complete failure with flow getting behind weirs two through five and causing them to be washed away.
Overall three of the five projects protected the project streambank during the course of the study, but only one project did so without at least some repair, modification, or added expense. The two complete failures and the repairs needed at two other projects were due to inadequately sized rock, and lack of bed and streambank keys. Given these results, careful consideration needs to be given to whether or not to use the farm rock weir approach at a site because if maintenance is required, the savings over a traditional bendway weir project are lost or greatly reduced. The results from these projects show that while this approach may have limited potential in most cases the traditional approach will be the better choice. Additional modifications to the farm rock weir approach could result in a technique that does have real potential. However, without further study it is uncertain that any modification will reduce the savings over a traditional bendway weir project. The farm rock weir approach should not be attempted by a landowner without the assistance of an experienced professional and at this stage is not an approach we would recommend to landowners.