On the Longitudinal and Transverse Advection and Dispersion of Bed Load Pulses Induced by a Local Sediment Surplus

Local sediment surplus can originate from various endogenous and exogenous sources and generally forms sediment pulses that propagate downstream and spread in transverse and longitudinal direction. Despite the enormous relevance in natural and anthropogenically influenced river systems, only a few studies deal with the advection and dispersion of local surplus. By the use of physical scale model tests and an image-based, non-invasive measurement technique, the two-dimensional advection and dispersion characteristics of installed sediment deposits on pulse-scale were investigated. In addition to the qualitative analysis of the erosion process, quantitative transport and spreading parameters are determined. These parameters are further used to assess the experimental results and to model the dispersion based on the two-dimensional advection-dispersion equation (2D-ADE). The present results show that the advection and dispersion properties of a local sediment surplus are strongly influenced by the local stream constriction and the associated 3D flow phenomena. Therefore, unlike transport at dynamic equilibrium, advective pulse slowdown is most likely associated with a continuous decrease of the blocking ratio rather than with the burial and trapping of bed load particles. The presented 2D-ADE based model is capable to reproduce shape, magnitude and extent of the propagating sediment pulses. In closing, the insights gained from this laboratory study help to better understand the underlying processes and the introduced modeling approach provides a suitable tool for assessing the transport and spreading of a local sediment surplus.