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The sediment carrying capacity is one of the fundamental issues in sediment simulation. It is of great importance both in theory and practice to develop process-based approaches for the sediment carrying capacity for a wider range of silt-sand sediment. The current study focuses on the approach for depth-averaged concentration of silt-sand sediment under non-breaking wave conditions. By integrating process-based suspended sediment concentration (SSC) profiles, new synthetic expressions for depth-averaged SSC for vortex rippled beds and sheet flow conditions were obtained. The proposed expressions involve several basic physical processes, including the effects of bed forms, stratification, hindered settling, mobile bed, etc. A number of experimental datasets were collected for verification and reasonable results were obtained. Discussions were made on the changes of sediment concentration under increasing wave dynamics conditions, which show that the proposed formulas can describe the phenomenon which has been observed in experimental tests that SSC does not always increase when wave dynamics increase due to the effects of bed forms. In short, the current research provides a process-based approach for wave-induced sediment carrying capacity, which is expected to be applicable for numerical modeling and engineering practice.

The evolution and utilization of estuarine and coastal regions are largely restricted by sediment issues. Waves and currents are the main dynamic driving forces of sediment transport in estuary and coastal areas. The study of wave-current movement and sediment transport is of great significance in both academic research and engineering practice, which has received much attention from many scholars and engineers. The northwestern head of Bohai Bay belongs to a transitional area from silty coast to muddy coast, where the sediment movement is complex. Firstly, based on the field observation data measured with a tripod system and the hydrological data in the northwestern head of Bohai Bay, the characteristics of tidal currents, waves, and suspended sediment concentration (SSC) changes under different hydrological conditions were analyzed. Sediment density distribution was detected by a γ-ray densitometer. Results show that the SSC is mainly influenced by wave-induced sediment suspension: Under light wind conditions the SSC was very low, with the peak value generally less than 0.1 kg/m3; the SSC increased continuously under the gales over 6−7 in Beaufort scale, with sustained wind action. The measured peak SSC at 0.4 m above the seabed was 0.15−0.32 kg/m3, with the average value of 0.08−0.18 kg/m3, which is about 3−6 times the value under light wind conditions. The density at the bed bottom was about 1.4− 1.6 g/cm3, and decreased to 1.02−1.03 g/cm3 at 0.1−0.2 m above the bed; there was rarely little fluid mud in the study area. Secondly, the mechanism of wave-induced SSC was studied from the view of flow structure, shear stress in the bottom boundary layer. The friction velocity was calculated by fitting the datasets of the measured logarithmic velocity profile. Comparison shows that the wave shear stresses (calculated from the bottom velocity profile) is higher than the current shear stress (calculated from the upper velocity profile). Following the Reynolds’ decomposition method, expressions of wave-induced and current-induced shear stress were derived by splitting the variables into a fluctuating component, an averaged component and an oscillatory component. Compared with uniform flow, an additional item of wave streaming leads to a higher wave-induced shear stress. Finally, by analytically solving the sediment diffusion equation, a numerical procedure was proposed for sediment concentration under combined action of waves and currents. Some key approaches for fine sediment simulation were presented, including stratification effects, hindered settling, moving bed roughness, reference concentration and critical shear stress. Under combined wave-current conditions, the combined sediment diffusivity is given by the square sum of the wave-related and current-related diffusivities, and a toe-type distribution of wave-related eddy viscosity was employed. The stratification effects are considered by introducing the turbulence damping coefficient. The approaches of hindered settling for silt were employed. The enhanced roughness due to mobile bed effects was included. Comparison shows that, these approaches can properly simulate the measured wave-induced sediment concentration. This study provides basic data for coastal protection and utilization as well as for scientific study of silty-muddy coast, which is of great significance in practice and theory.

Experiments and field observations have revealed that when silt and very fine sand are subject to oscillatory wave motion, a high shear flow layer and a high concentration layer (HCL) exist near the bottom. The behavior of the HCL is still under researched. Firstly, an intra-wave process based 1DV model was established for fine sediment transport under the combined action of waves and currents. Some key processes that were included in the model are represented through approaches for different bed forms (rippled bed and 'flat bed'), hindered settling, stratification, reference concentration and critical shear stress. A number of experimental datasets were collected to verify the model, which shows that the model is able to properly simulate the flow and sediment dynamics. Secondly, sensitivity analyses were carried out on some factors which would impact the suspended sediment concentration (SSC) profile of the HCL by the 1DV model, such as bed forms, flow dynamics, stratification effects, mobile bed effects and hindered settling. Results show that bed forms play a significant role in the HCL and determination of the shape of the concentration profile. When a current is imposed, the SSC profiles become smoother; however, sediment concentration in the lower HCL is still dominated by the wave motions. For finer sediment, the stratification effects and the mobile bed effects strongly impact the HCL. In conclusion, this paper provides a tool for the study of the HCL and an evaluation of several impact factors on the HCL.

Sediment incipient motion is a fundamental issue in sediment transport theory and engineering practice. Silt has transitional behavior between cohesive and non-cohesive sediment and its incipient motion is still poorly understood. This study aims to find an expression for incipient motion from silt to sand from a unified perspective and analysis. From the analysis of forces, using the derivation method for the Shields curve, an expression for sediment incipient motion is proposed for both silt and sand under conditions of combined waves and currents. The differences and similarities in the sediment motion threshold were analyzed under the effects of waves and currents, and fine and coarse sediment. The Shields number was revised by introducing the cohesive force and additional static water pressure, which indicates that this study could be seen as an extension of the Shields curve method for silt. A number of experimental datasets as well as field data were used to verify the formula. The effect of bulk density on fine sediment was discussed and tested using experimental data.

Suspended sediment concentration (SSC) is one of the fundamental topics in sediment study. The parameterization of the SSC profile of silty sediments is still under-researched. This study focuses on the mean SSC profile for silty sediments under non-breaking wave-dominant conditions. First, inspired by a 1DV model, different types of the distribution of mean eddy viscosity were proposed, i.e., a toe-type distribution over flat bed and a constant-toe type distribution over rippled bed. Then, the time-averaged diffusion equation for suspended sediment transport was analytically solved, and expressions for the mean SSC profiles were derived. The expressions involve several basic physical processes, including the effects of bed forms, stratification, hindered settling and mobile bed. Verification using a number of experimental datasets showed that the proposed expressions can properly calculate the mean SSC for silt and are applicable for sand as well. In conclusion, this research provides an approach to estimate the mean SSC for silty sediments under wave-dominant conditions, which is expected to be applicable for engineering practice and numerical modelling.

Understanding the long-term morphodynamic evolution of tidal inlet systems and their control mechanisms has great significance in the management and sustainable development of coastal regions. This study addresses this issue by applying a process-based morphological model (Delft3D) to hindcast morphodynamic evolution of the Laolonggou tidal inlet (LTI) for a duration of 68 years and then analyze the mechanism of morphologic evolution and factors affecting it in tidal inlets. The morphodynamic model is validated against two periods, a natural stage (1938–1983), and a recent erosion stage with reservoir construction (1983–2006). Results show that the evolution pattern, sediment volume changes, and hypsometry curves simulated by the model agree well with field data. A series of scenarios were designed to analyze the mechanism of morphologic evolution and impacting factors. The model results reveal that the asymmetry of the tidal currents in the channels leads to tide residual sediment transport, which causes morphodynamic evolution. The inconsistency of tidal flow path, i.e., ebb dominance occurs in the west channel and flood dominance occurs in the east channel, is an important mechanism for the evolution of the whole system. In addition, the decrease of sediment supply caused by reservoirs in Luanhe River, and extension of the barrier island contribute to the evolution of LTI. Although this study is site-specific, the results are expected to provide valuable information for sustainable management of tidal inlet systems.

The evolution and utilization of estuarine and coastal regions are greatly restricted by sediment problems. This thesis aims to better understand fine sediment transport under combined action of waves and currents, especially in the wave-current bottom boundary layer (BBL). Field observations, experimental data analysis, theoretical analysis and numerical models are employed. Silt-dominated sediments are sensitive to flow dynamics and the suspended sediment concentration (SSC) increase rapidly under strong flow dynamics. This research unveils several fundamental aspects of silty sediment, i.e., the criterion of the incipient motion, the SSC profiles and their time-averaged parameterization in wave-dominated conditions. An expression for sediment incipient motion is proposed for silt-sand sediment under combined wave and current conditions. A process based intra-wave 1DV model for flow-sediment dynamics near the bed is developed in combined wave-current conditions. The high concentration layer (HCL) was simulated and sensitivity analysis was carried out by the 1DV model on factors that impact the SSC in the HCL.Finally, based on the 1DV model, the formulations of the mean SSC profile of silt-sand sediments in wave conditions were proposed. The developed approaches are expected to be applied in engineering practice and further simulation.

The coastal zone continuously changes due to natural processes and human activities. In order to understand and predict these morphological changes, an accurate description of sediment transport, caused by waves and currents (tidal or wave-induced), is important. This paper presents a review of the state-of-the-art knowledge in this field, including sediment incipient motion, bed forms, bed roughness, bed-load transport, suspended-load transport, equilibrium sediment concentration, and sheet flow. Some possible research fields and topics for future study also are proposed.

The construction of hydropower projects, such as Xiluodu and Xiangjiaba Reservoirs, in the upper Yangtze River will lead to variations of the incoming water and sediment conditions and in turn changes in the deposition and erosion processes in the fluctuating backwater reach of the Three Gorges Project (TGP). In this paper, based on the water and sediment characteristics, a 2D mathematical model in the boundary-fitting orthogonal curvilinear coordinate system has been employed to predict the space-time changes of sedimentation in the Chongqing reach, part of the fluctuating backwater reach of TGP, in a period of 100 years with repetition of the 1961–1970 hydrological series with and without the construction of upstream reservoirs. The results show that in the case of natural hydrological series (NHS) without upstream reservoirs, severe deposition occurs in bays with concave bank lines, recirculation flow areas and wide-shallow reaches. In the case with the adjustment of upstream reservoirs, only a little deposition exists in bays with concave bank lines, and the amounts of deposition in the Yangtze River and Jialing River reaches are about 10–20% of those in the case of NHS. Therefore, this is helpful to maintain the effective capacity of the TGP Reservoir and utilize the bank lines of the Chongqing reach.

Vast bay-type tidal inlets can be found along the coastal zones of China. They are generally suitable for deep water channels and large harbors because of the presence of large water depth and good mooring conditions. The deep channel, in front of the head of Caofeidian Island in Bohai Bay, China, is a typical bay-type tidal inlet system. The tidal current, a type of reverse flow, makes the key contribution to maintain the deep water depth. The co-action of waves and tidal currents is the main dynamic force for sediment motion. Waves have significant influence on the sediment concentration. Based on the characteristics of waves, tidal currents, sediment and seabed evolution in Caofeidian sea area, a 2D mathematical model for sediment transport under influence of waves and tidal currents is developed to study the development schemes of the Caofeidian Harbor. The model has been verified for spring and neap tides, in winter as well as in summer of 2006. The calculated tidal stages, flow velocities, flow directions and sediment concentrations at 15 stations are in good agreement with the observations. Furthermore, the calculated data on pattern and magnitude of sedimentation and erosion in the related area agree well with the observations. This model has been used to study the effects of the reclamation scheme for Caofeidian Harbor on the hydrodynamic environment, sediment transport and morphological changes. Attentions are paid to the project inducing changes of flow velocities and morphology in the deep channel at the south side of Caofeidian foreland, in the Laolonggou channel and in various harbor basins. The conclusions can provide the important foundation for the protection and use of bay-type tidal inlets and the development of harbor industry.

The surrounding area of the Bohai Bay has one of the highest economic development rates of China. Rapid industrialization and urbanization has resulted in extensive changes in land use, including the construction of harbors and embankments. For the analysis of the changes of the coastlines, multi-temporal Landsat images have been used, in combination with topographical and nautical data. From the change analysis, it can be concluded that the largest variations in the position of the coastline over time occurred in the Tianjing Port Zone and Caofeidian Port Zone, mainly caused by extensive harbor construction. Based on the research of the coastline change in recent decades, it is suggested that the human activities have become one of the major factors affecting the morphological processes.

In last 20 years, the hydrodynamic and fluvial processes of the Boluo reach have been greatly changed by the human activities such as the large-scale sand excavation from riverbed, the construction of reservoirs. On the one hand, the working of the reservoirs decreases the mean sediment concentration and the mean sediment discharge and increases the runoff in the dry season. On the other hand, the large-scale sand excavation has the great effects on the fluvial processes include increasing of the channel volume, lowering of the riverbed altitude and deepening of the water depth, which causes severe drop of water level and obvious strengthen of tidal action. Therefore, human activities have become the dominant factor controlling the changes of the hydrodynamic and fluvial processes instead of natural processes.

Caofeidian sea area is the only natural site suitable for building a large-sized berth of 300 000 tons without excavation of waterways and harbor basins in Bohai Bay. In Caofeidian harbor area, the stability of the shoals-inlets coastal system under natural conditions is a key point. The harbor could be developed only if the coastal system is stable. In this paper the characteristics of waves, tidal currents, sedimentation and seabed evolution in the Caofeidian shoal area is briefly described, and the trend of stability of shoals and channels is predicted based on research on the coastal system formation. Engineering effect of some projects, such as the dam linking the Caofeidian Island and the coastline, the shoal reclamation and ore terminal construction, is also studied. The analysis show that deposition and erosion of the seabed in front of Caofeidian foreland will reach its equilibrium in 2～3 years after the implementation of the proposed scheme.