GAO Shu and JIA Jianjun
Ministry of Education Key Laboratory for Coast and Island Development, Nanjing University,
Nanjing 210093, China
Progress in marine sediment dynamics and related researches in mainland China that has been made over the past four years is summarized under the headings of numerical modeling techniques, sediment dynamic processes and mechanisms, and instrument analyses. Hydrodynamics-based numerical models are now applied extensively to the studies of sedimentary processes or to the simulation of sediment transport and its environmental influences. Sediment dynamics of estuaries and tidal inlets have been a major concern, with advances being achieved in improvements to the existing transport formulae, studies on fine-grain sediment movement in estuaries/inlets and the resultant geomorphological consequences, and the use of grain size trends as an indication of net sediment transport. Recently, new and high technology has been introduced to mainland China in sediment dynamics, including the laser particle sizer and acoustic instruments (e.g. ABS and ADCP). A lot of work has been done for the calibration and utilization of these instruments; pretreatment procedures for grain size analysis have been evaluated, and the potential of deriving suspended sediment information using ADCP echo signals has been explored. Furthermore, new methods of obtaining suspended sediment concentration data from satellite images have been developed.
Key words: Marine sediment dynamics, numerical models, transport processes, estuaries and tidal inlets, suspended sediment concentration, instrumentation
Marine sediment dynamics and related disciplines are considered as an important research field by the coastal engineers, physical oceanographers and marine geologists in mainland China. A number of research groups, belonging to government institutions and universities, are working actively in this field. Such a situation can be related more or less to the natural characteristics of China: there are large rivers discharging into the sea with extremely heavy sediment load, wide continental shelf seas with strong tidal currents and complex ocean current systems, and numerous estuaries and coastal embayments where sediment movement is active. Solution to the engineering problems associated with harbour siltation and maintenance of navigational channels were once of priority. Recently, new problems caused by modifications to the physical environment, pollution of coastal waters and degradation of marine ecosystems have become the focus for research. In terms of basic research, marine sediment dynamics is relevant to the science of granular material physics and global change studies represented by IGBP. The unique environment of the Chinese coastal waters provides a large potential for these studies. In the present short report, we intend to summarize the progress in this field made by the scientists of mainland China over the past four years, on the basis of a literature review of the refereed academic journals. The research topics of numerical modeling, sediment dynamic processes and mechanisms, as well as instrument analysis, are treated in three separate sections.
II. NUMERICAL MODELING TECHNIQUES
1. Process Models
Numerical models have been used extensively to investigate into sediment dynamic processes. Recently, the method of adaptive mesh has been introduced and tested in the Changjiang estuary (Liu et al., 1999); this will overcome some difficulties caused by complex bathymetry and topography of the coastal zone and has a large potential of applications.
2D tidal models have been used to study sediment movement and resultant deposition on the continental shelf. In order to identify the areas where the tidal current is sufficiently weak to allow mud accumulation, the movement of eight sediment types (with different median sizes) are modeled (Zhu and Chang, 2000); the results show that several areas over the Yellow Sea and East China Sea region are suitable for the formation of mud deposits, and the tidal current alone can explain the accumulation of mud. This preliminary research implies that the presence of a cold eddy is not a necessary condition for the formation of mud deposits. The same authors have applied the model also to the formation of the northern Jiangsu radial tidal ridge system (Zhu and Chang, 2001). By numerical experiments to generate the flow field without the presence of the ridges, they argue that it is the flow field that determines the existence of the ridge system.
Bedform morphology has been investigated using numerical modeling methods. In the offshore areas of the Yellow River delta, there are underwater erosional gullies, 40-80 m in width, 220-400 m in length and 2.4-5.9 m in height. Such a feature has been related to secondary flows by a 3D model (Chang et al., 1999). The ratio of the secondary flow to the main flow is around 0.05; thus, if the main flow has a velocity of 0.5 m/s, then the near-bed velocity of the secondary flow is around 3 cm/s. The secondary flow provides a favorable condition for the development of the gullies.
For the characteristics of the tides and sediment distribution patterns of the Bohai Strait, a 2D horizontal model has been designed to study the long-term transport of sediment over the Bohai Strait region (Jiang et al., 2002). The output of sediment transport model indicates that in the study area net sediment transport is directed towards the northwest over the western Bohai Strait, the southwest along the Liaodong Peninsula, and the east on the southern side of the Strait. In the east of the central Bohai Strait, the net sediments transport forms an anti-clockwise eddy and it weakens toward the center of the eddy. The result of seabed accretion/erosion calculation agrees to field observations.
2. Simulation Models
2D numerical models are now widely used to solve the various coastal engineering problems such as coastal erosion, shoreline protection, and tidal land reclamation. For instance, Wang et al. (2000) have used a model to predict the deposition rate over a reclaimed estuarine shoal in Hongzhou Bay. The calculated deposition rate by this model agrees well with the measured data. Chen (1999) constructed a 2D wave propagation model from which longshore drift can be derived. Compared with previous models that provide single-point estimation of longshore drift, this study represents a progress in that longshore transport rate can be estimated at each grid location within the surf zone. Cao et al. (2001a) derived an expression of suspended sediment concentration distribution in the surf zone adjacent to the Yellow River estuary. Further, a 2D non-uniform model on the basis of this expression was established (Cao et al., 2001b). According to this model, suspended sediment is selectively transported by currents in the coastal waters adjacent to the Yellow River estuary; fine particles can move towards the open sea, whilst relatively coarse suspended material is mostly deposited over the Yellow River delta. Finally, suspended sediment transport has been modeled for the situation of the combined waves and tides and the results are applicable to seabed erosion/accretion evaluation (Bai et al., 2000) and navigational channel maintenance and harbour management (Zhu et al., 2002).
The Changjiang River estuary is another focus of study for coastal engineers. Tidally-affected horizontal and vertical distribution patterns of suspended sediment concentrations (SSCs) in the estuary have been simulated (Cao et al., 2002; Shi and Zhou, 2000). These studies show that the vertical profile of SSCs is controlled mainly by vertical diffusion and flocculation processes. To a certain extent, these SSC models generally agree with measured data.
Hydrodynamic characteristics under the combined action of waves and tidal currents and morphodynamic feedback processes have been a topic for modeling. A comprehensive 2D numerical model, taking into account a number of factors such as estuarine flow, tidal currents, waves, sediment movement and bed topographic changes, has been developed (Ma and Li, 1999). Wu et al. (1999) have studied turbulent current structure within the boundary layer affected by both waves and currents. They found that the current structure within the boundary layer shows non-linear characteristics: wave action evidently affects the current profile, whereas currents have little influence on the structure of wave – induced currents.
For applications to harbour design and post-engineering evaluation of coastal engineering projects, Guo et al. (1998) developed a model, which is based on tidal water level change to compute cross-sectional sediment transport rates. The result was successfully employed to predict the volume of sediment deposition in the Dadong Harbour, Liaoning Province.
In terms of the application of 3D models, the Hamburg Shelf Ocean Model, HAMSOM, has been adopted and modified to simulate the suspended particle movement in the Bohai Sea (Jiang and Sun, 2000, 2001). In the simulation, the baroclinic effects of tide, wind and atmospheric pressure were considered; for the sediment aspects, settling, resuspension and diffusion processes under the combined action of tidal currents and waves were taken into account. The model output indicates that most sedimentary materials from the Yellow River are deposited within the Bohai Sea, with a small proportion being transported towards the Yellow Sea through the southern Bohai Strait. Further, the sediment movement is strongly influenced by winds.
3. Modeling in Combination with Tracer Information
The use of tracers is an important approach to marine sediment dynamics. In this aspect, on the basis of an analysis of the mass conservation of tracers contained in the bulk sediment, Gao (2000) has suggested that sediment transport modeling can be undertaken by combining the tracer information and the flow field data. The result obtained can provide independent signals with regard to the model output of hydrodynamically-based models. Like in any existing sediment transport models, the determination of sediment diffusion coefficient, for different temporal scales, remains an important issue.
III. SEDIMENT DYNAMIC PROCESSES AND MECHANISMS
1. Sediment Transport in Coastal Waters
In examining the empirical formulae for sediment transport rate, Huang and Xi (2000) have emphasized that it is the effective shear stress, rather than the total shear stress (including particle stress and form drag), that determines the sediment transport rate, for both suspended load and bedload. Wu and Ma (2002) have suggested that the particle size to be used in the calculation of sediment transport rate is a crucial issue, and the median grain size of moving sediment rather than that of the bed material should be used.