Title: Closure to "Modeling the Evolution of Incised Streams. II: Streambank Erosion" by Eddy J. Langendoen and Andrew Simon Authors
Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 1, 2009
Publication Date: November 13, 2009
Citation: Langendoen, E.J., Simon, A. 2009. Closure to "Modeling the Evolution of Incised Streams. II: Streambank Erosion" by Eddy J. Langendoen and Andrew Simon. Journal of Hydraulic Engineering. 135(12):1107-1108. Interpretive Summary: Application of one-dimensional channel evolution computer models is problematic in meandering streams because of the strong three-dimensional nature of the stream flow. The topography and flow dynamics in meander bends increase the forces exerted by the flowing water on the streambank at the outside of a meander bend compared to those exerted on streambanks in straight channels. A semi-empirical approach was developed to estimate this increase in applied forces using a survey of meander bend topography. Application of this approach to a meander bend on the Goodwin Creek in North-Central Mississippi showed excellent agreement with values derived from observed erosion rates between 1996 and 2001. The presented methodology is important to all users of one-dimensional channel evolution models. The presented methodology improves the accuracy and reduces the uncertainty in model outcome for streams with a meandering planform. One-dimensional channel evolution models are widely used by federal and state agencies, such as the US Geological Survey, the US Bureau of Reclamation, the US Corps of Engineers, the Natural Resources Conservation Service and the US Environmental Protection Agency, to design stream-channel conservation measures and assess their long-term stability and benefits.
Technical Abstract: In 2008 Langendoen and Simon presented a numerical model in the Journal of Hydraulic Engineering (JHE) that predicts streambank erosion within the one-dimensional channel evolution computer model CONCEPTS. The model was validated against observed retreat of the outer bank of the Goodwin Creek Bendway study site in North-Central Mississippi. The rate of streambank erosion is greatly affected by the rate of removal of previously failed streambank material deposited at the bank toe. The resistance to erosion of the failed material and the bank soils can be described by an excess shear stress equation, which is a function of a critical shear stress at which erosion commences and the rate of erosion once the critical shear stress is exceeded. In the validation study the critical shear stress (measured in situ with a jet test device) was reduced by a factor of two near the apex of the bend and by a factor of eight at the apex of the bend to account for the increased shear stresses exerted by the flow on the outer bank of meander bends. This brought about a discussion by readers of JHE on the magnitude of the used reduction in critical shear stress and how one could relate the reduction to meander bend hydraulics and topography. The application of a three-dimensional computer model of open-channel flow by the National Center for Computational Hydroscience and Engineering (Univ. of Mississippi) showed that boundary shear stress on the bank toe for a large flood event on April 6, 2005 was increased by a factor of 4 to 8 along the apex of the bendway. This agrees very well with the decrease in critical shear stress used in the CONCEPTS simulation. A simple semi-empirical approach is presented to determine the reduction factor (that must be applied to the critical shear stress in meander bends) from surveyed meander bend geometry. Application of the approach yielded a reduction factor that varied from 2 upstream and downstream of the apex to 6 at the apex of the bend. The approximated reduction factors are in excellent agreement with those obtained by calibrating CONCEPTS against the observed toe erosion.