Stream Barbs and Spurs Application in the Des Moines River

The Des Moines River caused excessive bank erosion at two different sites in Van Buren County, IA USA. At the first site, the observed bank loss occurred along a 600 ft. straight stretch at the left bank and was induced by a recent channel enlargement. In the second site, a 1,800 ft. stretch along the right bank was destabilized by the higher velocity downstream of a river bend. To mitigate this problem, the alternation of stream barbs and spurs was proposed at both sites. The combination of barbs and spurs has been already proven to be an effective approach in controlling streambank erosion at different locations along other large rivers in Iowa with similar geomorphic features. However, their design is site specific and each application needs to be carefully evaluated. A commercially available two-dimensional depth-averaged model (FESWMS) was used to evaluate the efficiency of various scenarios and to determine the optimal number of structures. Field surveys together with laboratory experiments were used to calibrate and validate the numerical model. The effect of the structures on the local flow field was taken in account adjusting the value of the eddy viscosity. Sensitivity analysis of eddy viscosity and Manning's roughness coefficient was performed to establish the uncertainty in the output of the numerical model. The strategy followed in this case study will help county engineers to identify an optimal design, which is at the same time efficient and cost-effective, to reduce bank erosion.