The influence of stress history on the stability of graded sediment beds.

European and national legislation is increasingly being imposed on river management practice in the UK. Successful implementation requires detailed understanding of the interaction between flow and sediment transfer within river systems. Environmental organisations are calling for research aimed at accurately predicting the instant at which sediment motion begins and improving our understanding of the mechanics of sediment transport. This is because predictive sediment transport models are essential tools for engineers and scientists whose responsibility it is to plan and manage river regulation, restoration and re-alignment. However, existing models do not perform well. Improving the accuracy of these models is of immediate importance in predicting the sensitivity of sediment transport to changing flow patterns resulting from climate change and catchment modification. To date, it has been widely assumed that the stability of sediment comprising a river bed is only affected by high flows capable of sediment transport. Yet, there is clear evidence that the period of low flow between two flood events is actually a very important control on the point at which sediment is mobilised during a subsequent high flow event. Whilst, sediment transport occurs at lower shear stresses following short inter-flood periods, it is delayed until a higher shear stress when floods are separated by a longer inter-flood period. Accepting, quantifying and understanding the influence of antecedent low flows (termed the stress history ) on river bed stability are crucial if sediment transport predictions are to be improved. As such, detailed, systematic investigations aim to quantify the effect that stress history has on the stability of graded sediment beds, determine the mechanisms responsible and develop a practical method for integrating a stress history variable into sediment transport relationships. Whilst this new area of experimental research will significantly benefit the active international research community at a fundamental level, the motivation for this research is firmly rooted in its application to improving sediment transport models imperative to civil engineers, councils, consultants, water companies, reservoir managers, fisheries and the environment agencies directly involved in implementing the EC Water Framework Directive in areas such as river regulation, river restoration and flood risk.