Bedload sediment transport in gravel-bed rivers: Intermittence and continuum

English Abstract:

Understanding bedload sediment transport in gravel-bed rivers remains a challenge for river scientists. Bedload sediment transport is often sporadic and intermittent at various temporal and spatial scales and transport rates do not necessarily increase with increasing hydraulic stresses. Bedload sediment transport occurs mostly during flood events when unsteady flow makes it difficult to obtain simultaneous and continuous records of flow and bedload transport measurements. This paper examines the complex response of bedload transport to turbulent flow. First, we will review the relationships between turbulent flow characteristics and sediment transport rates as observed in a small gravel-bed river in southern Quebec. The measurements were conducted during the rising limb of two flood hydrographs of different magnitudes. Turbulent variables change systematically with discharge through hydraulic geometry relationships. Bedload sediment transport occurred as discrete events in both floods. Among the hydraulic variables, flow acceleration/deceleration at the turbulent scale explained a large proportion of the behaviour of sediment transport rates especially for the less intense flood. This result supports evidence reported in previous studies that flow acceleration/deceleration was important in the mobility of individual gravel particles. Second, this evidence at the turbulent scale will be examined at larger time scales. We have observed that turbulent flow events are clustered in a way that generates larger scale flow pulsations. We have documented such pulsations in several gravel bed rivers at different flow stages. An important question that needs to be addressed is the relationship between these large scale flow pulsations and the dynamics of bedload transport. The impact of flow pulsations on sediment transport may be related to the frequency and temporal spacing of the large magnitude turbulent events that generate high shear stress. These extreme events contribute to the flow acceleration/deceleration patterns observed in flow pulsations and to the emergence of large scale flow properties. This may provide a key to our understanding of the intermittence, intensity and duration of bedload transport events.