The physical processes of bed armouring in mixed grain sediment transport
A major difficulty encountered by river engineers is the unavailability of a reliable relationship between the flow discharge and the sediment transport rate. Most of the published transport rate prediction methods have been developed using measurements of uniformly sized sediment. It has been found that non-uniformly sized sediment exhibits very different behaviour from that of single sized sediment. If no sediment is introduced into a reach then as the bed degrades coarse material progressively accumulates on the surface, significantly reducing the transport rate. This can be effectively reduced to zero, the bed is then said to be armoured. Four experiments were conducted, in which there was no feed or recirculation of sediment and the flow rate was held constant. These experiments indicated a two phase development during the formation of an armour layer. The initial phase was dominated by the rapid coarsening of the bed surface with a large drop in the sediment transport rate, followed by a second longer phase characterised by the re-arrangement of the surface into a stable grain layout. Numerical simulations were carried out using a mixing layer model. Comparison of the simulated and observed development of transport rate bedload and bed surface composition with time indicated that processes other than grain size sorting were important. Two further experiments were carried out in which the re-arrangement of the bed surface and the adjustment of the near bed flow environment was investigated. This work presents experimental evidence that mixed grain size transport is controlled by a series of inter-related physical processes. All the important parameters such as bed surface composition and grain arrangement, the near bed flow field and the amount and composition of the bedload need to be considered if a mixed grain transport system is to be simulated successfully.