Discussion of “Modeling the Evolution of Incised Streams. II: Streambank Erosion” by Eddy J. Langendoen and Andrew Simon

Abstract

The authors are to be congratulated on having successfully implemented a conceptual model for two bank failure mechanisms into their numerical model CONservational Channel Evolution and Pollutant Transport System CONCEPTS and on having validated it against long-term field data of streambank erosion. A comparison of measured and simulated cross sections at the study site indicates reasonable behavior of their numerical model of bank stability for streambank failure. As noted by the authors, CONCEPTS does not predict the increased shear stress along an outer riverbank caused by the helical flow pattern within a bend. Curved channel flow is associated with a helical motion—often termed “secondary flow”— generating a transverse velocity component in addition to the main streamwise velocity. Yalin and da Silva 2001 mention the prominence of the secondary flow that progressively decreases as the width-to-depth ratio B / h of a curve increases. The authors’ treatment of the missing helical motion within their field validation is, however, questionable. The increased shear stresses at the bend apex due to secondary flow are accounted for by a reduction of the critical shear stress. However, the range of critical shear stresses used, between 1 Pa up- and downstream of the bend and 8 Pa at the bend apex, is surprising. This is nearly a one-order-of-magnitude difference, while the magnitude of the secondary flow velocity is typically some 10% of the streamwise velocity magnitude Blanckaert and de Vriend 2004. The range of the selected critical shear stresses appears therefore to be extremely large. This is also reflected by the results presented. Cross sections with a small critical shear stress of 1 Pa underlie stronger and temporally increased streambank retreat than those considered using a higher critical shear stress of 4 Pa. The discusser therefore questions the sensitivity of the authors’ approach relative to the selected model to account for secondary flow. Were simulations also made with a larger or smaller ratio of critical shear stresses at cross sections along a typical bend? Finally, the discusser would like to ask the authors how the critical shear stresses can be selected in terms of bend hydraulics, topography and geotechnical data?