linking 3D particle-scale structures to the evolution of gravel bed morphology

---

Flow strength and bed resistance

The movement of material on a river bed is a balance between the flow strength and the sediments’ ability to resist this force. Typically the stability of a river bed has been related to its grain size. Recent research has demonstrated that the structure of bed material exerts an equally important role in determining stability. Characterising bed structure has been difficult but this project will employ new 3D techniques to quantify measures both in the field and laboratory.

Channel change begins with bedload transport. In order for any sediment movement to occur the force of the fluid, the shear stress, must exceed the force of resistance, the critical shear stress. Traditionally grain size was the main factor used in determining the critical shear stress. However, sediment transport formulae using critical shear stress are typically only accurate within an order of magnitude. Further research has shown that critical shear stress is poorly predicted because it does not take into account the structural arrangement of the gravel-bed, nor the role of finer sediment in cementing larger grains in place.

Project importance

Understanding how particle scale structures impact river bed stability and the initiation of sediment transport is critical. These structures alter flow resistance both spatially and temporally and can dramatically impact sediment transport.

Sediment movement is the first step in the process of channel change allowing for the development of channel forms such as riffle-pool sequences and bars. Through investigation of the fundamental importance of particle scale structure on sediment transport we will therefore also help better understand channel evolution as a whole.