Experimental Log 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 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 log weirs as a potential technique for controlling excessive streambank erosion. The goal of the log weir technique is to save money over the traditional bendway weir approach by using logs as opposed to using large amounts of rip rap to build the traditional rock weirs. This reduces the costs associated with a weir approach while attempting to stabilize the streambank. Five projects were constructed at four separate MDC Conservation Areas using log weirs. The projects were built between July 2005 and May 2007 and all experienced multiple high-flow events.
The log weir technique had mixed results. The initial project on Jakes Creek failed before the other projects were built. Jakes Creek failed because the streambank keys (area where the log is anchored in the streambank to hold it in place) were not strong enough to hold the logs in place during high stream flows. The lessons learned about key strengths from that failure were applied to the four projects built later. Changes included burying at minimum half the log in the key and to protect the surface of the key with shot rock (quarry rock not graded out to a specific size) to prevent erosion. These alterations helped two of the projects succeed, but the other two projects failed for different reasons. Both projects built on California Branch worked as designed. They moved the thalweg away from the streambank and got enough deposition at the toe and between the weirs to bury the majority of the weirs. The Dry Branch project failed due to improper layout of the weirs during construction. Incorrect spacing and construction angles for weirs four and five resulted in the complete failure of weirs five and six at the lower end of this project. The Mill Creek project failed for multiple reasons: the instability of the reach caused by a head-cut that had recently moved through the system, a change in how the key material was protected with shot rock, and erosion that occurred downstream of the last weir. All four of the projects built after the Jakes Creek project had erosion downstream of the last weir, but the project on Mill creek was the only one where erosion caused the failure of the most downstream weir.
Overall only two of the five projects stabilized the eroding streambank during the course of the study. The failures seen at the other three projects appear to be the result of inadequate key strength, incorrect weir placement, and erosion downstream of the last weir. The information gained from all five projects indicates that this approach has only limited potential as a stabilization technique. Modifications could be made to add additional strength to the key, but they have not been tested so their limitations are unknown and they would not address the need for a professional design in laying out the weirs. The log weir technique is also affected by the limitations associated with the size of logs that will be needed depending on the size of the streambank. If streambank height exceeds 10 feet tall or the channel needs to be moved more than 15 feet, this approach becomes impractical due to the size of the logs required. The most important factors in using the log weir technique are the strength of the key, the distance the thalweg needs to be moved by the weirs, the height of the streambank, proper placement and spacing of the weirs, and a stable stopping point on the downstream end of the project. The log weir approach should not be attempted by a landowner without the assistance of experienced professionals and currently is not an approach we would recommend to landowners.