Suspended sediment is of importance in river and dam engineering. While, due to its high nonlinearity and stochasticity, sediment prediction by conventional methods is a challenging task. Consequently, this paper establishes a new hybrid model for an improved forecast of suspended sediment concentration (SSC). It is a nonlinear autoregressive network with exogenous inputs (NARX) integrated with a data pre-processing framework (denoted as INARX). In this model, wavelet transformation (WT) is used for time series decomposition and multigene genetic programing (MGGP) for details scaling. The two incorporated modules improve time and frequency domain analysis, allowing the network to unveil the embedded characteristics and capture its non-stationarity. At a hydrological station on the upper reaches of the Yangtze River, the records of daily water stage, flow discharge and suspended sediment are collected and refer to a nine-year period during 2004-2012. The data are used to evaluate the models. Several wavelets are explored, showing that the Coif3 leads to the most accurate prediction. Compared to the sediment rating curve (SRC), the conventional MGGP, multilayer perceptron neural network (MLPNN) and NARX, the INARX demonstrates the best forecast performance. Its mean coefficient of determination (CD) increases by 7.7%-38.6% and the root mean squared error (RMSE) reduces by 15.1%-54.5%. The INARX with the Coif3 wavelet is further evaluated for flood events and multistep forecast. Under flood conditions, the model generates satisfactory results, with CD > 0.83 and 84.7% of the simulated data falling within the ±0.1 kg/m3 error. For the multistep forecast, at a one-week lead time, the network also yields predictions with acceptable accuracy (mean CD = 0.78). The model performance deteriorates if the lead time becomes larger. The established framework is robust and reliable for real-time and multistep SSC forecast and provides reference for time series modeling, e.g. streamflow, river temperature and salinity.

At the multigate sluice structure on a fluvial river, undesired sediment deposition affects the normal operation of the reservoir in question. Physical and numerical models are hybridized to help explore flow and sedimentation patterns. Field and laboratory investigations show that the deposition is attributable to the formation of large recirculation zones at low and medium discharges. As a potential countermeasure, an array of guide vanes is recommended to cope with the concern. Their attack angle with the flow is a dominant parameter that needs to be evaluated. Tests in the fixed-bed model demonstrate that the vanes bend the reservoir flow towards the sluice and suppress the circulation zones along both banks. The favorable range of attack angle is 15–20°. With the examination of sedimentation of both bed and suspended loads, the numerical modeling indicates that the sediment-removal efficiency increases with an increase in attack angle. By weighing the flushing efficiency and the risk of local scouring at the vanes, the 15° vane layout is recommended. This study is expected to provide a reference for guide-vane design in similar situations.

During dredging for subsea tunnels and pipelines, the dredged soil is typically dumped in a designated area. Understanding the settling behaviors of the dumped particles is essential for an accurate prediction of the resulting morphology. This study dealt with the settling processes in the open-water column by means of experimental tests and numerical simulations. Both quiescent and ambient current conditions were taken into account. Particular attention was paid to the induced flow patterns, descent sediment movement features and the resulting topography. Regarding the diffusion width and settling velocity, three key effect factors, i.e., dumped volume, particle size and ambient flow strength, were considered. The results show that the dumped sediment in the water forms a particle cloud, and two vortices with opposite rotations occur on both sides of the cloud. During settlement, three stages corresponding to convective descent, dynamic collapse and passive diffusion are observed. Most of the descending sediment is incorporated in a spherical vortex, resembling an upside-down mushroom cloud, and some sediment is also contained in an irregular trailing stem. The dumped particles exhibit initial acceleration, and then they slow down to converge to a fixed value. Subjected to the ambient currents, the initial acceleration phase is prolonged, and the vortex is somewhat distorted; the particle cloud is also advected downstream with a velocity roughly equal to the ambient current.

Turbulent structures generated by vegetation patches play a dominant role in the dispersion of suspended sediment, which in turn is of great significance for ecosystem cycling and river geomorphology development. High fidelity Large Eddy Simulations (LES) coupled with the Discrete Phase Method (DPM) were used to explore the particle distribution and its variance (the non-uniformity in temporal and spatial space) in a partially vegetated straight channel. The novel findings and conclusions are outlined here. Firstly, the contour of the vertical vorticity component coincides well with particle preferential gatherings in the outer edge of the mixing layer in the near-bed region. Large-scale turbulent structures grow in mixing layer along the side of a vegetation patch (VP), which deplete particles away from the mixing layer into the neighbouring region. Also, higher vegetation densities (Dn) promote this depletion trend. Secondly, the Probability Density Function (PDF) and its variance were defined to quantify these phenomena, illustrating that the VP continuously interrupts the flow condition and promotes higher non-uniformity of particle distribution among the vegetated and non-vegetated regions. The variance of the PDF in the non-vegetated region is significantly higher than that in the neighbouring vegetated region located in the same streamwise location. The particle parcels are highly unevenly located along the periphery of the large eddies and are exchanged by the mixing flow between the non-vegetated and vegetated regions. Finally, the vertical entrainment of particles occurs in the vegetated region of the present cases. This is because the horseshoe structures provide an upwards velocity for the current Dn conditions (Dn<0.1) and an increase of Dn (Dn<0.1) accelerates the upward suspension. These findings complete our understanding of particles’ transportation in both spanwise and vertical directions.

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 merging flows. 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. 

I en alluvial låglandsflod transporteras sediment suspenderat i flödet eller längs flodbädden. Den fluviala processen är det makroskopiska förloppet och den långsiktiga konsekvensen av denna sedimentrörelse. Floden justerar ofta sitt tvärsnitt, längsgående profil, flödesförlopp och mönster genom naturliga processer som sedimenttransport, erosion och deponering. Antropogena faktorer, till exempel dämning, kanalisering och andra hydrauliska projekt, modifierar också de naturliga processerna. Vid långsiktiga alluviala förändringar visar floden ofta mönster som t ex. slingrande-, flätade- och vandrande sträckor. När flodsträckan har en fri anslutning till öppet hav är flödet ofta dubbelriktat. Floden påverkas vanligtvis av samspelet mellan avrinning och tidvatten, vilket leder till extremt komplicerade flödes- och sedimenttransportbeteenden.

I den här avhandlingen kombineras fältmätningar, numeriska simuleringar, modellförsök och maskininlärningsteknik för att klarlägga flödesdynamiken och de fluviala processerna, med fokus på flödesmönstren, bäddskjuvspänning, sedimenttransport och morfologiska förändringar. Vidare presenteras och diskuteras mätningar av flöde och sedimentation och förändringar i batymetri i flodsystem med och utan tidvatten kartläggs. Baserat på fältdata utförs numeriska simuleringar i 2D och 3D med Delft3D, vilket möjliggör en koppling mellan komplex flodgeometri, batymetri och flöden samt sedimentvillkor i en och samma modul. En hybridmetod för modellförsök och numeriska simuleringar används för undersökning av sedimentationsfrågor, där både det suspenderade materialet och det som transporteras längs bädden beaktas. Utöver fältmätningar används också maskininlärning som ett alternativ för att förutsäga sedimentation.

I en tidvattenflod innefattande ett sammanflöde och en slingrande sträcka belyser forskningen samspelet mellan sötvattensflöden och tidvattensströmmar. Bland annat  klarlägger forskningen cirkulationsmönster för suspenderad sedimenttransport under ebb och flod och uppkomsten av ett erosionshål vid sammanflödet. Vidare illustreras asymmetriska tvärsnittsförändringar vid flodkrökarna. Den skiftande flödesriktningen resulterar i migrering av erosion och avlagring i båda riktningarna, vilket inte finns i enriktade avrinningsflöden. Flodvatten styr sedimenttransport och deponering, medan tidvatten (ebb) leder till erosion. Baserat på störningsteorin härleds också en förbättrad formel för bärighet av sediment som är lämplig för beräkningar i en tidvattenmiljö.

Vid en bifurkation-sammanflödessenhet undersöks flödes- och sedimentegenskaperna och de resulterande bäddförändringarna. Resultaten indikerar att variationer i inflöden har betydelse för bifurkationsdelen. Sekundära flödesstrukturer har visat sig vara mer påverkade av thalwegkurvaturen än flödesuppdelningen. ‘Inloppssteget’ eller skillnaden i topografi bidrar till en ojämn flödesfördelning. I sammanflödet samexisterar en tvåcells flödestruktur, vilken kan minska beroende av den dynamiska förändringen av de två vattendragen. Den klassiska diskordansen för flodbädden kan även observeras.

Vissa hydrauliska flodåtgärder, t.ex. en ledvägg vid bifurkationen och ledskenor i reservoaren, används för att modifiera flödesmönster och sedimenttransport. Med rätt ledväggsgeometri uppnås ett acceptabelt flödesmönster i bifurkationen och grenflödet ökas. När det gäller ledskenorna är 15º‒20º vinkel till huvudflödet att föredra under normala driftförhållanden. Då modifierar ledskenorna flödesmönstren effektivt och undertrycker flödescirkulationer, vilket leder till att sedimentationen mildras.

Baserat på omfattande uppmätta hydrologiska data och undersökta batymetrier studeras sedimention i en 500 km lång sträcka av nedre Yangtze-floden, före och efter idrifttagningen av Three Gorges-dammen. Analyserna visar att uppdämningen reglerar säsongsvariationerna och fångar en avsevärd mängd sediment, vilket resulterar i ökad erosionspotential och grövre sediment. Sträckan som studeras har ännu inte uppnått en hydromorfologisk jämvikt. Det innebär att nednötningen av flodbädden kommer att fortgå en tid framöver och den märkbara sedimentminskningen från uppströms sträckor av floden är den yttre orsaken till bäddens erosion. 

Sammantaget kan forskning som presenteras i avhandlingen stödja en beslutsprocess för den övergripande förvaltningen av floder genom att använda de studerade fallen som referens.

The impoundment of the Three Gorges Dam on the Yangtze River begins in 2003 and a full pool level is first attained in 2010. This process leads to reciprocal adjustments in flow discharge, sediment transport and morphology downstream of the dam. Based on 26-year recorded hydrologic data 1990–2015 and surveyed bathymetries 1998, 2010 and 2015, this study elucidates, before and after the commissioning of the dam, the alterations along the 500-km reach of the river. Two-dimensional numerical simulations are performed to predict future morphological changes by 2025. 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. On a multi-year basis, the maximum discharge varies by a factor of 1.3 and the corresponding suspended load concentration and transport rate differ by a factor of 3.0 and 3.8, respectively. Combinations of surveyed and simulated bathymetries reveal its morphological responses to the changes. A general pattern of erosion is observed along the reach. In its upper 120 km, the process slows down towards 2025. In the middle 200 km, the erosion shifts, following the gradual impounding, to slight deposition, which then shifts back to erosion around September 2018. In the final 180 km, erosion continues without any sign of de-escalation, which is presumedly ascribed to tidal actions. The reach has not yet achieved a hydro-morphological equilibrium; the riverbed down-cutting is supposed to continue for a while. The combination of the field and numerical investigations provides, with the elapse of time, insight into the morpho-dynamics in the 500 km river reach.

A stop-log gate, installed in water intake of hydropower project, has become an effective facility in achieving selective withdrawal and temperature control for the sake of benefiting downstream ecosystems. Hence, it is of great importance to comprehensively explore the water intake hydraulics with the gate, not limited to some specific case studies. This study deals, through laboratory experiments and numerical simulations, with flow features of such a gate-functioned intake. The physical model test is used to validate the numerical simulation. Subsequently, a series of numerical cases considering different hydraulic and geometric conditions are performed to help look into the behaviors. Particular attention is paid to the flow regimes, head loss and flow velocity distributions. The results showcase the effect of the gate on the intake flow regime, and in terms of head loss and flow velocity distribution, the influences of the upstream water head, intake chamber width and withdrawal depth are revealed in detail. An empirical expression, with regard to the coefficient of head loss, is derived and validated by data from the available literature. Moreover, it is found that the maximum velocity at trash rack section is dependent exclusively on the relative withdrawal depth and always occurs at a certain height range above the gate. These results may provide a meaningful reference for the research of water intake with similar situations.

In a coastal environment, this paper investigated the formation process and the cumulative shape of subaqueous mounds formed by hopper dredged discharges. Hydrological observations and field tests were performed to examine the flow features and ultimately generated morphology characteristics. A high-precision digital elevation model (DEM) was established by multi-beam depth sweeping (MBDS) in the experiment. Particular attention was paid to the formation of the mounds, the three-dimensional shape and the influence factors. The field measurements showed that the mounds were roughly symmetrical in space, and the tidal current, though of weak strength, played a certain role in shaping the profiles. Cone and volcanic cone mound tops were observed, featuring the main top shapes. The height and covered area of the mounds were proportional to the amount of dumped sediment, and they were also affected a lot by the water depth. The results of superimposed tests showed that the second placement over the existing mound resulted in a similar overall shape, but there was pronounced movement around the mound; additional discharged volumes at the same location mainly increased the mound height. The field tests provided a reference for understanding the sediment dumping in other similar coastal areas.

A diffluence‐confluence unit is an elementary component within a river system and presents a complex yet linked pattern of both flow and sediment transport in between. This study deals, by means of field investigations and numerical modelling, with morpho‐dynamics of such a unit on the lower Yangtze River reaches. The unit comprises, looking downstream, a secondary (left) course and a main (right) course. Field surveys are performed for measurements of flow discharge, sediment loads at selected locations and river bathymetry at certain intervals. The field data show that the reach is mainly composed of suspended load, whose amount exhibits a declining trend with the elapse of time. Simulations in 3D are made to complement the field data and clarify the basic features of the unit, especially the partitioning of flow and suspended sediment in the diffluence and their subsequent reciprocal adjustment in the confluence. The results indicate that approach flow variations have a bearing on the diffluence flow partition. To augment flow discharge in the left branch, a training wall is devised in the diffluence to modify the intake flow. 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. With the typical flow and sediment features, the main course is prone to slight erosion, while the secondary branch faces up with gradual siltation. These findings contribute to the understanding of the alluvial behaviours of such units, and provide reference for studies in similar situations and river management.