Transport Modes and Grain‐Size Patterns in Fluvial Basins

Abstract

Fluvial transport modes have been defined using arbitrary quantitative limits to separate bedload, mixed-load and suspended-load dominant modes. If estimates of bed grain-size, flood depth and flood slope are available for particular reaches of present-day rivers, this makes it possible to allocate a dominant transport mode to each reach. These fluvial transport modes represent equilibrium flow-transport states, separated by relatively abrupt transitions. Examination of sediment transport relationships using a data set from present-day rivers shows that mixed-load rivers are less efficient than bedload rivers, where efficiency is defined in terms of the flow strength needed to transport sediment load of a given weight. This may be a result of the more important presence in mixed-load rivers of dissipative features, particularly sinuous channel bends and sand bedforms (ripples and dunes). Some further aspects of the transport mode approach are examined in the context of two very different kinds of data sets from fluvial basins. The present-day, intensively studied, Allt Dubhaig river, Highland Scotland, exhibits an abrupt downstream fining from gravel, via a few hundred metres of bimodal gravel and sand, to sand with bedforms. This downstream change corresponds to the transition from bedload to mixed-load dominant transport mode. Within the bedload dominant gravel reaches, Shields parameter and dimensionless width vary around constant values, and the downstream profile decreases in elevation, as predicted. The abrupt downstream transition in mode appears to reflect exhaustion of gravel supply, relative to subsidence and sand accumulation, owing to the raising of downstream base level, in this case produced partly by construction of a local dam. A margin of the Old Red Sandstone (Devonian) basin in East Greenland has been examined using patterns in the fluvial sedimentary structures to identify and characterise the fluvial systems. For these ancient rocks, direct flow measurements are not available, but the transport mode approach still provides important insights. Abrupt downstream transitions from gravel to sand appear to represent changes in fluvial transport modes and made the toes of the basin-margin fans. Exhaustion of gravel downstream appears to have determined the build-out distance of the gravels, as with the Allt Dubhaig, and the decrease of slope produced by general filling of the basin centre by axial rivers may also have been important. Decrease of stream power owing to divergence of channels on the marginal fans, as well as soak-in and evaporation of river water, may also have limited the downstream extent to which the bedload dominant mode existed. In both cases, the transport mode approach provides valuable understanding of the basin-filling systems.