Interplay between suspended particles, hyporheic exchange and bed sediments suppresses bedload transport and produces heterogenous deposits

Abstract:

Rivers transport large quantities of eroded sediments to oceans as both suspended and bed load. Modelling this process is critical to both managing rivers and understanding fluvial biogeochemcial processes. Existing models of suspended and bed load transport do not consider feedback between the two modes of sediment transport, but we show that hyporheic exchange couples bed load and suspended load by delivering suspended particles from the water column into the bed. In laboratory flume experiments, we observed that flow-bed-suspension interactions lead to deposition of clay particles in sand beds. Long-term accumulation of clay reduced bed sediment mobility and permeability, diminished bedform motion, and curtailed further hyporheic exchange. We observed that these coupled dynamics produced two distinct end states. In the locked end state, fine particles stabilized the upper regions of the bed and bedform motion completely ceased. In the segregated case, ongoing bedform scour concentrated fine particle deposition in a layer below the region of active bed transport, transiently reducing bedform motion. These results show that bedload sediment transport rates are reduced by the concurrent presence of fine particles. Further, the history of fine particle loading and the frequency of bedform scour impacts the residence time of both fine and coarse sediments.