Numerical modelling of suspended sediment transport in pre-alpine river basins

Abstract

Rivers can transport large amounts of suspended sediment, which is the cause of problems such as reservoir siltation, increased risk of flooding and reduced water quality. Therefore, it is of paramount importance to be able to identify the sources and to make predictions of suspended sediment load in river basins. However, this understanding is hindered by the very strong spatial and temporal variability of the mechanisms of sediment production and transport. In this thesis, I developed a new hydrology-sediment model for soil erosion and sediment transport in pre-Alpine basins and carried out a numerical investigation of the sources of such variability. In the first part of the thesis, I investigated the impact of spatially variable climatic and landscape controls for diffuse hillslope erosion and suspended sediment transport. To this aim, I compared simulations on a pre-Alpine basin with uniform and spatially variable precipitation and erodibility. I found that spatial variability of such drivers affects the suspended sediment dynamics, because it determines the location, productivity, and connectivity to the basin outlet of the sediment sources. In particular, our results highlighted the key role of clusters of high soil moisture produced by spatially variable precipitation, and the effect of specific landscape elements acting as obstacles to the sediment flux by a variable surface erodibility. In the second part of the thesis, the role of localized sources of sediment on the dynamics and provenance of sediment is investigated. In this step, the modelling of hillslope erosion, landslides and incised areas was combined with geomorphic mapping, terrain analysis and sediment tracing with 10Be. The results highlighted that hotspots of soil erosion may dominate the sediment budget of pre-Alpine basins, and the control exerted by their morphological properties on the hillslope-channel connectivity and the sourcing of sediment. In the third part of the thesis, I explored how the presence of sediment available for transport on the hillslopes and in the channels of a river basin affects the variability and temporal dynamics of suspended sediment. To do so, I simulated conditions of transport and supply limitation, and compared them with observations in several Alpine and pre-Alpine basins. Both simulations and observations indicated that supply limitation reduces the variability of the suspended sediment concentration at the outlet, while the alternation between low and high sediment availability favours it. The observed seasonality of sediment load in the case study is also compatible with the seasonality of the sediment reservoir predicted by the model. To conclude, in this thesis we present a new model for diffuse and localized soil erosion on the hillslopes and in the channels, and for the routing of such eroded sediment to the basin outlet. The model is used to gain insights into the suspended sediment dynamics of pre-Alpine basins.