The Initiation of Motion and Formation of Armour Layers

Accurate estimation of sediment flux remains a major requirement in assessments of river channel mobility and evolution, solid material supply to reservoirs, identification of potential hazards such as bridge collapse or culvert blocking, and maintenance of instream habitat and environmental flow. In order to estimate sediment flux we first need to understand under what conditions grains begin to move and the factors that influence this threshold of motion. The circumstances necessary to initiate sediment motion are a function of the characteristics of the sediment, the fluid, and the flow conditions. The critical shear stress necessary to initiate motion has typically been given as one value per grain size but significant variation in critical shear stress per grain size has been recorded. The variation is due to (i) definitions of entrainment threshold; (ii) measurement techniques used; (iii) grain and flow characteristics; and (iv) the use of reach averaged rather than actual estimates of τc⁎. Grain-scale processes caused by the variation in sediment sizes present on the bed surface, such as bed structuring and bed armoring, serve to alter entrainment thresholds, and subsequent sediment transport patterns. Over the shortest temporal scales (seconds to minutes) the turbulent parameters that control motion are relatively poorly constrained despite having been shown to be key in controlling grain entrainment. Over the longest temporal scales, the flow regime to which a sediment bed is exposed affects entrainment thresholds whether this is as a result of periods of sustained low flow or during the passage of a flood wave.