Downstream fining in large sand-bed rivers

Abstract

Downstream fining of bed sediments is a well-known phenomenon in gravel-bed rivers, but also occurs in (large) sand-bed rivers. The underlying processes, however, are not necessarily the same, due to the difference in sediment mobility, grain-size distribution width and geographical setting. Downstream fining studies focusing on large sand-bed rivers are scarce, nevertheless. In this paper, the processes that affect downstream fining in gravel-bed rivers are reviewed, and it is evaluated to what extent they are relevant for large sand-bed rivers. Furthermore, several ‘new’ downstream fining processes are discussed, ending in a conceptual model of downstream fining in large sand-bed rivers. In deriving the conceptual model, downstream fining processes are divided into three categories: abrasion, selective transport and sediment addition–extraction. Abrasion rates in large sand-bed rivers are small due to the durable lithologies, the small grain size, the high degree of rounding, the dominance of grinding as abrasion process and the dominant suspended transport mode of the bed sediment. Only in the upstream part of large sand-bed rivers, abrasion rates may be somewhat higher. Selective transport rates in large sand-bed rivers everywhere overrun abrasion rates, but are small compared to selective transport rates in gravel-bed rivers, because the unimodal sediments in sand-bed rivers exhibit stronger hiding-exposure effects during threshold-of-motion conditions, but especially because many sand-bed rivers (though not all) are in state of fully mobilised transport, making differences in threshold of motion between coarse and fine grains relatively unimportant. Selective transport in these rivers is probably mainly the result of the presence of suspended load transport in combination with the effects of dune and perhaps bend sorting, which are all processes that act on a relatively large scale. Within sand-bed rivers, the degree of selective transport probably decreases downstream due to the change in mixture bimodality and sorting efficiency. There are indications that coarse grains may sometimes even be more mobile than fine grains in the downstream part of sand-bed rivers. Note that in contrast to abrasion, selective transport only produces a ‘stable’ downstream fining pattern if the river profile is concave. Generally, a stronger profile concavity leads to a higher aggradation rate and a stronger downstream fining trend. Sediment addition and extraction processes can obscure the effects of abrasion and selective transport in sand-bed rivers. Overbank sedimentation, for instance, may significantly decrease the downstream fining rate in large sand-bed rivers, because especially fine grains are removed from the bed. Dredging can also strongly affect the downstream fining trend, depending on the volume and the grain size of the sediment involved. Meander migration may cause a nett loss of coarse grains from the channel in aggrading circumstances, increasing the downstream fining rate. River bifurcations can cause a discontinuity in the downstream fining pattern, because bend sorting upstream of the bifurcation causes the river branch that originates in the outer bend of the main channel to be much coarser than the other branch. All these addition and extraction processes not only have a direct effect on the grain-size composition of the river bed, but also indirectly influence downstream fining by their effect on the longitudinal bed profile. Other sediment addition and extraction processes, such as tributary confluences, non-alluvial sediment sources and dumping are less common in large sand-bed rivers, but can locally have a great effect. A full determination of the relative importance of all downstream fining processes in large sand-bed rivers requires extensive field measurements, the construction of detailed, fractionwise sediment balances and the development of numerical downstream fining models that incorporate all processes described above.

Introduction

Rivers commonly show a downstream fining of bed material (Morris and Williams, 1999). Although downstream fining is more obvious in gravel-bed rivers, it also occurs in sand-bed rivers (Fig. 1). The underlying processes, however, are not necessarily the same due to the greater sediment mobility in sand-bed rivers, the narrower size distribution of the bed sediment and the different geographical setting: sand-bed rivers predominating in lowland areas and gravel-bed rivers predominating in upland areas.

Both geomorphologists and engineers have studied the fining phenomenon extensively, but nearly all downstream fining studies have focused on upland gravel-bed rivers. Downstream fining in sand-bed rivers deserves more attention, because a downstream change in grain size from very coarse sand to silt changes the dominant mode of sediment transport, the bedform dimensions and also the grain size of overbank deposits.

In this paper, the processes that affect downstream fining in gravel-bed rivers are reviewed, and it is evaluated to what extent they are relevant for sand-bed rivers. Furthermore, several ‘new’ downstream fining processes are discussed. This will end up in a conceptual model of downstream fining in sand-bed rivers, which may help to provide directions for future research. The focus is on large, alluvial, sand-bed rivers such as the downstream reaches of the Mississippi, the Rhine, the Ganges and the Niger (Fig. 1).

In this paper, downstream fining processes are divided into three categories: abrasion, selective transport, and sediment addition–extraction. Abrasion processes cause a progressive breakdown of particles during their downstream transport, while selective transport processes cause a preferential downstream transport of fine particles. Sediment addition and extraction processes, finally, may obscure the effects of abrasion and selective transport, for instance at river confluences.