Can deciduous tree revetments reduce streambank erosion rates on a sand-bed stream?

Accelerated streambank erosion can threaten infrastructure and land, as well as water quality and aquatic habitats. Streambank stabilization techniques have been developed with the intent to reduce or halt streambank erosion. One such technique is the use of woody revetments. This case study evaluates the effectiveness of deciduous tree revetments on stabilizing streambanks on the Smoky Hill River, a low-gradient, sand-bed stream located in central Kansas in the United States. It was hypothesized that deciduous tree revetments would mimic bank protection processes of permeable-type spurs, capturing sediment and reducing velocities and shear stresses near the toe of the streambank. To test this hypothesis, cross-sectional dimensions of four streambanks were obtained before and after installation of tree revetments and compared to four natural, control streambanks (i.e., not stabilized) over a 5-year period. Rates of bank erosion were calculated and compared. This study found that, in its current design form, deciduous tree revetments were not effective at reducing bank erosion, as all sites had experienced revetment failures by the end of the study period. Furthermore, the installation of tree revetments accelerated bank erosion rates following revetment failure. Increased bank erosion was attributed to both the construction disturbance, as well as improper anchoring of the revetment. The results of this case study show the importance of collecting bank stratigraphic data and incorporating it, as well as expected flow scenarios, in numerical modelling tools to assess designs and adjust accordingly. While conducting these analyses upfront may result in higher design costs, long-term maintenance or replacement costs would be decreased.