Hydraulic roughness is a fundamental property in river research, as it directly affects water levels, flow strength and the associated sediment transport rates. However quantification of roughness is challenging, as it is not directly measurable in the field. In lowland rivers, bedforms are a major source of hydraulic roughness. Decades of research has focused on dunes to allow parameterisation of roughness. This study aims to establish the predictive capacity of current roughness predictors, and to identify reasons for the unexplained part of the variance in roughness. We quantify hydraulic roughness based on the Darcy-Weisbach friction factor calculated from hydraulic field data of a 78 km long trajectory of the Lower Rhine and River Waal in the Netherlands. This is compared to predicted roughness values based on dune geometry, and to the spatial distribution of the local topographic leeside angle, both inferred from bathymetric field data. Results from both approaches show the same general trend and magnitude of roughness values (friction factor f=0.019-0.069, mean 0.035). Roughness inferred from dune geometry explains 42% of the variance, for the best performing predictor. Efforts to explain the remaining variance from statistics of the local topographic leeside angles, which supposedly control flow separation, were unsuccessful. Unexpectedly, multi-kilometer depth oscillations explain 34% of the total roughness variations. We suggest that flow divergence associated with depth increase causes energy loss, which is reflected in an elevated hydraulic roughness. Depth variations occur in many rivers worldwide, which may imply a cause of flow resistance that needs further study.

Regarding transport of macroplastic (>5 mm) in rivers, its division over bifurcations is still an understudied topic. However, this is a critical knowledge to estimate plastic emission to oceans from rivers. To quantify the spatiotemporal variability of plastics in the Hong-Duong bifurcation, we executed a field campaign on a weekly basis and applied visual counting method where we count the floating plastics flowing through three cross sections to determine cross-sectional distribution of floating plastic and classify polymer categories of plastics from bridges located in the Hong-Duong bifurcation. These bridges include Nhat Tan located in northern Hanoi in the Red River, Long Bien is about 8km to the south from Nhat Tan in the Red River, and Dong Tru located in the tributary Duong River (~7km from Nhat Tan). We aim to determine the spatiotemporal changes of macroplastics across the Hong-Duong bifurcation over the period from May 2021 to November 2021. Until July 2021, we found that the total average macroplastic fluxes at the cross sections in Nhat Tan, Long Bien, and Dong Tru were 698, 159, and 113 items/hour, respectively. Notably, these values do not follow the expected plastic balance between total plastic flux in the parent river and its tributaries, which is likely explained by the accumulation, transport below the surface, or sedimentation of plastics in the space between measurement locations. Additionally, over three months of May, June, and July, the total average plastic fluxes in all cross sections showed an increasing trend (~10%). Furthermore, we also found that most plastics were distributed in the right side (downstream perspective) of Nhat Tan and Long Bien, while Dong Tru saw the opposite. Regarding the plastic classification, based on the River-OSPAR category, we found that food wrappers, polystyrene fragments, low density polyethylene (LDPE) bags, and polyethylene terephthalate (PET) bottles were the top 4 items. These findings together with the distribution of macroplastics along the cross sections are expected to apply in correlation analyses with hydrodynamic components in the bifurcation to determine their connectivity. This information is crucial for improving the efforts on quantifying macroplastic emissions from the Red River to the ocean which is still unknown up to now.

Tides influence the allocation of river discharge to distributary channels interconnected by tidal junctions. Geometrical properties of distributary channels and nearshore coastal morphology might also play a significant role in tidal propagation, and hence control river discharge division and tidal energy transport at tidal junctions. Model simulations reveal how under different river discharge regimes, the discharge division is controlled by the tidal behavior at the junctions. Furthermore, analysis of tidal energy at the junctions indicates that the side distributaries drain or add tidal energy to the main river channel depending on the upstream discharge condition.