Propagation of sediment waves associated with coarse gravel nourishments

Abstract

Sediment nourishments have become an increasingly attractive alternative to deal with continuous bed degradation problems in the river system. When supplied to the river, sediment nourishments induce a sediment wave that propagates through the system causing changes in bed level and bed surface texture. In the present study we use a one-dimensional numerical model to analyse how the propagation of these sediment waves varies under different conditions. We find that the propagation of the sediment wave tends to be predominantly dispersive for large spatial variations of the flow, which is the dominant case when the height of the wave is large in comparison to the flow depth, or when the sediment is dispersed over a large enough reach to lead to a backwater effect. In artificial sediment nourishments the height of the sediment wave is usually limited to prevent creating an additional obstacle to navigation, and it is then that the temporal variations in the flow, due to high discharge events for example, determine the propagation of the wave. We study the morphodynamic response in a mixed-size sediment river system to a sediment nourishment, and identify the physical mechanisms that impact the propagation of mixed-sized sediment waves. In order to prevent it from being easily flushed downstream, the grain size distribution of the nourished sediment is typically selected to be coarser than the river bed. This reduces sediment mobility and enhances sediment deposition over the nourished reach, which in turn causes a deficit in sediment supply to the downstream reach and leads to the formation of a degradational wave. The propagation celerity of the degradational wave is found to be significantly faster than that of the sediment wave, which means that the effects of the nourishments in a river system can be observed significantly further downstream than from where the front of the sediment wave is found. We find that the celerities of both the sediment wave and the degradational wave are predominantly affected by the adjustments in surface grain size in the system. These adjustments migrate in the river system as a fining wave, which both accelerates the sediment wave, and decelerates the degradational wave. We learn that higher content of the finer fractions in the nourished sediment accelerate the evolution of the wave and lead to a predominantly translational behaviour. It also affects the magnitude of the incision depth, the closer the composition of the nourished sediment is to that of the bed surface, the less severe the scour. And even though it does not affect the propagation celerity of the degradational wave, it does impact how far downstream the wave travels, affecting a larger river reach the coarser the grain size composition of the sediment wave is. Finally, we analyse the results from a nourishment pilot project carried out in the Bovenrijn from 2016-2019. We find that the propagation of the sediment wave associated with the nourishment is only observed during high discharge events. The wave is predominantly dispersive, and only during a prolonged extreme discharge event did the wave show slight translational behaviour. We observe that lateral sorting mechanisms caused the tracer sediment to have a different trajectory from the sediment wave, which highlights the limitations of using a one-dimensional model. We also observe that the changes in bed level are related to the changes in surface grain size, with sediment deposition over the nourished reach and bed degradation just downstream.