Modulating the lateral migration of a gravel bed channel using the coarse tail of the bed material distribution

Abstract

SummaryMost stream channels around the world have been, to some extent, laterally constrained by non‐erodible banks constructed of concrete or large angular boulders (called riprap). These lateral constraints generally have a negative impact on aquatic ecosystems by reducing stream channel complexity, and limiting the natural disturbance processes that are critical for renewing the physical habitat. In this paper, we test a novel approach to limiting stream channel instability by exploiting the stabilizing effects of the coarsest grains in the bed material distribution, which we call geomorphic river engineering. This approach modulates the rate of bank erosion rather than halting it altogether. It also exploits the geomorphic feedback between bank erosion, channel width, and mean boundary shear stress during a flood event to limit the potential for vertical incision into the stream bed. The basic premise of the geomorphic river engineering approach is to place the coarse sediment on the floodplain adjacent to the channel margins, and rely on bank erosion by the river to recruit the sediment, allowing the river to remain the architect of its own configuration. Our results suggest that the recruitment of a modest volume of coarse sediment to a coarse, gravel bed stream during a flood could potentially dramatically reduce the amount of lateral migration that would otherwise occur, and that the degree of stabilization could be modified by changing the volume of coarse sediment per unit area placed on the floodplain. More generally, our results confirm the fundamental role that the coarse tail of the bed material distribution plays in dictating the channel dynamics of gravel bed streams with cohesionless banks, and indicate the potential for future river stabilization projects to focus on the engineering that can be done by rivers, rather than to them.