Field Measurements and Predictions of River Flow, Sediment Transport and Morphological Changes

Sammanfattning: In an alluvial lowland river, sediment is transported in the form of suspended and bed loads. The fluvial process is the macroscopic view and long-term consequence of sediment movement. The river frequently adjusts its cross-section, longitudinal profile, course of flow and pattern through the natural process of sediment transport, scour and deposition. Anthropogenic factors, e.g., river damming, channelization and other wading projects, also modify the natural processes. With long-term alluvial changes, the river often exhibits patterns such as meandering, braiding and wandering. If the river course has a free connection with open sea, its flow is often bi-directional. The river is typically influenced by the interplay between the runoff and tides, which makes the behaviours of flow and sediment transport extremely complicated. By combining field measurements, numerical simulations, physical model tests and machine learning techniques, this research investigates the fluvial river dynamics and processes, paying attention to the flow patterns, bed shear stresses, steady and unsteady sediment transport and morphological changes. Measurements of flow and sediment, and mapping of bathymetry in both tidal and non-tidal river systems, are presented and discussed. Based on field data, 2D and 3D numerical simulations are performed with the open source code Delft3D, allowing a couped modelling between complex river geometry, bathymetry, and flow and sediment boundaries. A hybrid approach of physical and numerical simulations is adopted for examination of reservoir sedimentation issues, in which both suspended and bed load transport are taken into account. A machine learning method is also applied for predications of suspended load in a river. In a tidal river including a confluence and meander reach, the research elucidates the interplay between freshwater flows and tidal currents. This discloses the circulatory patterns of suspended load transport during the tidal rising and falling. From the interplay also the bed scour changes of hole at the confluence and asymmetric cross-sectional changes at the bends are illustrated. In addition, it is shown that the shifting tidal directions result in a migration of erosion and deposition in both directions, which does not exist in unidirectional runoff flows. The flood tides govern sediment transport and deposition, while the ebb tides with run-offs lead to erosion. Based on the perturbation theory, an improved sediment carrying capacity formula is also derived, suitable for calculations in a tidal environment. At a diffluence-confluence unit, the flow and sediment characteristics and the resulting bed changes are examined. The results indicate that incoming flow variations have a bearing on the diffluence flow partition. Secondary flow structures are found to be more influenced by the thalweg curvature than the flow division. The ‘inlet step’ or differential topography contributes to the unequal flow division. In the confluence, a two-cell flow structure coexists, which may diminish along with the dynamical adjustment of the two waters. The classical bed discordance is also observed.  Based on extensive recorded hydrologic data and surveyed bathymetries, the sedimentation of the 500-km Lower Yangtze River reach is elucidated before and after the commissioning of the Three Gorges dam. The analyses demonstrate that the impoundment modulates the seasonal flow discharges and traps an appreciable amount of sediment, resulting in enhanced erosion potential and coarsening of sediment. The reach has not yet achieved a hydro-morphological equilibrium; the riverbed down-cutting is supposed to continue for some years and the noticeable sediment reduction from upstream is the extrinsic cause for the bed erosion.Some river training measures, e.g., training wall at the diffluence and guide vanes in the reservoir, are employed to modify flow patterns and sediment transport. With proper training wall layout, acceptable flow patterns are achieved in the diffluence and the branch flow efficiently is increased. With respect to the layout of the vanes, 15º‒20º is suitable under typical operating conditions. The vanes modify effectively the flow patterns and suppress the flow circulations, leading to less sedimentation and enhancing the sediment flushing efficiency.In overall, this research provides support a decision-making process when considering the integrated river management and it also provides reference for other similar situations.