Simulating Sediment Sorting of Streambed Surfaces – It’s the Supply, Stupid

Abstract:

The grain size of the streambed surface is an integral part of the transport system because it represents the grains immediately available for transport. If the rate and size of grains entrained from the bed surface differ from that delivered to the bed surface, the bed surface grain size will change. Although this balance is intuitively clear, its implications can surprise. The relative mobility of different sizes in a mixture change as transport rates increase. At small transport rates, smaller sizes are more mobile. As transport rate increases, the transport grain size approaches that of the bed. This presents a dilemma when using flumes to simulate surface sorting and transport. When sediment is fed into a flume, the same sediment is typically used regardless of feed rate. The transport grain size remains constant at all rates, which does not match the pattern observed in the field. This operational constraint means that sediment supply is coarser than transport capacity in feed flumes, increasingly so as transport rates diminish. This imbalance drives a coarsening of the stream bed as less mobile coarse grains concentrate on the surface as the system approaches steady-state. If sediment is recirculated in a flume, sediment supply and entrainment are perfectly matched. Surface coarsening is not imposed, but does occur via kinematic sieving. The coarsening of the transport (and supply) accommodates the rate-dependent change in mobility such that the bed surface grain size does not change with transport rate. Streambed armoring depends on both the rate and grain size of sediment supply – their implications do not seem to be fully appreciated. A coarsened bed surface does not indicate sorting of the bed surface during waning flows – it can persist with active sediment supply and transport. Neither sediment feed nor sediment recirculating flumes accurately mimic natural conditions but instead represent end members that bracket the dynamics of natural streams. Transient conditions of flow, bed sorting, and transport can be simulated if a surface-based transport formula is used. Simple conclusions connecting bed sediment sorting and flow are subject to unresolved detailed interactions and have the capacity to surprise.