In river systems, transported suspended sediment interacts with in-channel and riparian vegetation. The involved physical processes are complex and still poorly reproduced by numerical models. This study compares the performance of background horizontal eddy viscosity models in reproducing flow fields and suspended sediment transport processes inside partially vegetated flows. Particularly, we selected an experiment with a partly-vegetated flume which we numerically replicated with the Delft 3D-2DH model. We applied three existing horizontal eddy viscosity approaches: constant value, Elder model and hybrid model. Besides, we represented the vegetation by using Baptist formulation. The results show that the Elder viscosity model reproduces the explicit development of vortices along the flume, while all three viscosity models show an underestimate of the sediment deposition in the vegetated area and an overestimation of sediment deposition in the non-vegetated area. Further investigation is needed to reproduce the experimental introduction of the sediment and to link transversal suspended sediment dynamics with the resolved dynamics in numerical models.

Baptist’s method, Drag Force and Single-Stem approaches are the commonly used tools implemented in Delft3D to model water and sediment transport processes in vegetated channels. Despite their wide application, the model reliability has seldom been tested against data of controlled flume experiments with solid suspension. Here, we investigate the ability to reproduce suspended sediment transport through emergent vegetation by comparing the results of 2D simulations to existing experimental data. The results show that in low vegetation density, the Baptist and Drag Force approaches are not sensitive enough to density variations. The Single-Stem approach reproduces detailed flow structure and sediment deposition around stems, but its high computational time is a limitation for long-term simulations or dense vegetation. Furthermore, we observed that the simplification of 2D depth-averaged models and the non-equilibrium of sediment transport in both experiments and numerical simulations may also affect the overall performance of the vegetated modelling approaches.

Climate change has a direct influence on both hydrology and floodplain vegetation of water courses, which are key players in river morphodynamics. The river system response to climate change is complex and the effects of non-linear interactions between alterations in water, sediment and vegetation remain incompletely understood. Which of these components becomes dominant in shaping the river channel when climate becomes drier or wetter? To answer this question, we investigate the cross-sectional response of sand-bed rivers to climate change focusing on channel width and depth, which respond directly to changes in boundary stresses. Thanks to the exceptional availability of long time-series of daily discharge and cross-sectional profiles, the Pilcomayo River is an ideal living lab for this investigation. We constructed a two-dimensional model of the river using the open-source state-of-the-art, structured Delft3D code. The highly dynamic behaviour and quick morphological adaptation of the Pilcomayo allowed reducing the period of time covered by the simulations because the river cross-section adapts its morphology to a new value of the water discharge within hours or days, which is crucial for modelling investigations. Calibration and validation were successfully performed by comparison with historical data. We considered several scenarios representing current, dryer and wetter climates. The results show that a dryer climate reduces the river channel depth and enlarges the width. A wetter climate increases the channel depth but produces negligible widening. Vegetation, sparser with a drier climate and denser with a wetter climate, is found to control the channel width. This analysis is unique and shows which alterations can be expected in alluvial sand-bed rivers with natural vegetated banks due to climate change.

The Lower Mara River and Wetland, Tanzania, is an important ecosystem and unique water resource for a vast semi-arid area. The river, an affluent of Lake Victoria, and the wetland are experiencing morphological and vegetation changes resulting in channel avulsions and wetland expansion. This study analyses the changes over the last 100 years and investigates natural and anthropogenic behaviors to explain the increase of the Mara Wetland area. We collated historical topographic maps and satellite images. We conducted two field surveys in low and high flow condition with an unmanned aerial vehicle, a sonar and an ADCP. We mapped selected areas as well as the bed topography in some stretches of the river, measured discharges, and collected river bed and suspended sediment samples. The analysis of the sediments shows that the wetland system, dominated by papyrus sp., is very efficient in trapping sediment, releasing clear water to the Lake Victoria. The historical reconstruction using topographic maps, satellite images and a multivariable analysis including hydrology and land cover, shows that 4 major avulsions occurred in the last 70 years due to a combination of natural behaviors, hydrological fluctuations and anthropogenic factors such as basin deforestation, farming and grazing along the river banks and in the wetland. Each avulsion led to substantial expansion of the wetland. Combined, they increased the wetland area by a factor of 3.6. Describing the Lower Mara River dynamic behavior, this work provides relevant information for sustainable future water and sediment management in order to preserve wetland habitats and natural resources.