Bed material and bedload dynamics in three small alluvial channels

Abstract

In a river channel the prevailing bedload transport rate is not solely independent upon hydraulic parameters, but also upon the inter-relationship of bed material characteristics and flow properties through the transfer of sediment between the bed material and the bedload. Segregation of the surficial bed material in gravel-bed channels, as expressed by the presence of an armoured or paved surface, inhibits the transfer of sediment between the bed material and the bedload by controlling the availability of transportable material. A knowledge of the way in which segregation of the surficial bed material in gravel-bed channels proceeds, and the way in which it affects the transfer of sediment between the bed material and the bedload, is affirmed to be necessary for the understanding and description of the bedload transport process in gravel-bed channels.The relationship between the particle size distribution of the bed material and the prevailing flow conditions determines the amount of sorting and selective erosion that is associated with the development of a segregated surface, the celerity of the segregation process, and the particle size distribution of both the resultant surface and the bedload. When they are viewed in terms of the exchange of particles between the bed material and the bedload, the temporal changes which occur as the mobile particles are gradually removed from the surface of the bed are manifest in the particle size distribution of the bed material, the particle size distribution of the bedload, and the bedload transport rate respectively. The patterns of temporal change that might be observed in these three parameters, are initially elucidated with reference to experiments which were undertaken in a laboratory flume. These results form the basis for the interpretation and explanation of temporal variations in the composition of the bed material and the bedload which were subsequently observed in two field areas. First, short-term (intra-event) variations in the particle size distribution of the bed material and the bedload are examined and related to changes in the flow conditions in the Sheepstor Beck, a small Dartmoor stream. Secondly, the progressive effect which segregation of the surficial bed material has upon bedload transport rates is considered with reference to the pattern of bedload movement that was observed in the Borgne d'Arolla, a small Swiss, alpine stream. The bedload transport rate - stream power relationship does not adequately describe conditions in gravel-bed channels, since it fails to take into account the effect which segregation of the surficial bed material may have upon the supply of transportable material. More detailed field sampling of the surficial bed material and measurements of bedload transport rates are required if the effects of bed material heterogeneity upon the bedload transport process are to be elucidated further.

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