Bed-load transport and resistance to flow in alluvial meandering streams

Andrew Binns, Ana Maria Ferreira da Silva

Monday 29 june 2015

16:00 - 16:15h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 3B. Sediment - River

The effect of various types of bed forms (dunes, ripples and alternate bars) on resistance to flow and sediment transport in alluvial streams has been the focus of considerable research. Conversely, few studies have investigated changes in sediment transport rate and flow resistance due to the large-scale, laterally adjacent erosion pools and deposition bars invariably occurring in alluvial meandering streams. These pool-bar complexes, induced by stream curvature, occur once per half meander length and grow in magnitude until equilibrium conditions are reached. Here, the laboratory runs presented by Binns and da Silva (2009) are revisited, with the goal of investigating the relationship between sediment transport, flow resistance and deforming and equilibrium meandering bed topography. Five runs were conducted in a 0.80 m-wide 70° sine-generated laboratory channel. In all runs, the flow was turbulent and sub-critical, the flow rate was constant, and sediment was transported as bed-load only. All runs started from a flat initial bed with a given longitudinal slope. The bed deformed under the action of the flow until a time sufficiently beyond equilibrium conditions was reached. The flow was stopped at prescribed times during the bed development process to accommodate measurements of bed surface elevation and volume of sediment exiting the channel. At the beginning of the runs, sediment eroded from a pool was transported downstream parallel to the channel centreline and deposited to form the bar directly downstream. As the bed became more deformed the bars deflected the flow towards the pools, resulting in sediment being transported primarily along the thalweg of the stream. Measured sediment transport rates are analyzed in conjunction with bed morphological development, enabling distinction between the two components of the sediment transport rates: (1) the local transport rates associated with the development of the pool-bar complexes; and (2) the overall transport rate of sediment in transit in the channel. It is found that the latter remained almost invariant throughout the runs and that the resistance to flow increased only minimally throughout the development of the pool-bar complexes – which is in sharp contrast to the case of a bed covered by dunes or ripples.