Bed Material Coarsening

Bed material coarsening occurs not only in the downstream channel of a reservoir during the clear water release, but also in a natural stream or the variable back water zone of a reservoir when scour takes place. It usually occurs during channel scour, but may also occur when the channel approaches to equilibrium or even during a mild channel accretion. Bed material coarsening impacts sediment transport, sedimentation quantity and channel roughness. The main contents of this chapter consists of: (1) Massive field data for bed material coarsening are used to group the coarsening into three categories (coarsening due to scour of bed load and suspended load, respectively, and sediment exchange) with six types (coarsening on sand bed, pebble bed or pebble bed mixed with sand); (2) a thorough illustration of the mechanisms for bed material coarsening based on the statistical theory of exchange intensity, which is a breakthrough of the general concept that coarsening only depends on entrainment conditions and (3) The calculation method for thickness of bed coarsening layer and the armor layer and their size distribution, and the proportion of finer particles filled in the armor layer are derived. The calculation results are in good agreement with the field data.