Study On Sediment Resuspension And Dynamics In The Lianyungang Nearshore Area

Sediment resuspension is a universal physical phenomenon in coastal areas."resuspended by wave, transported by current"is the main mechanisms of sediment transportation in muddy coastal seabed. Sediment resuspension is directly related to local ecosystem alteration, geomorphological evolution, material recycling, sustainable development of the human society and living space. Revealing the processes and mechanism of sediment resuspension is the key problem in the research of sediment dynamics, where many problems need to be further study.This thesis focused on the sediment resuspension process and its dynamics in a typical muddy area near the Lianyungang coast, where the sediment resuspension occurred alternately and the resuspension changed apparently. Hydrological observations at several stations were conducted synchronically under different climate conditions during the spring and half tide in winter and summer respectively. Based on these data, several methods (including mechanism decompositions of suspended sediment flux, wavelet analysis, sediment grain-size distribution analysis and the numerical simulations) were used to discuss the resuspension process and sediment dynamics. The main conclusions are obtained as follows:1. The temporal and spatial variation of suspended sediment concentration (SSC) in the Lianyungang nearshore area is distinct. SSC is characterized by concentration peaks over times, which are usually limited to special depths. The peaks from the bottom are also different seasonally. For the summer, the peaks of SSC are always constrained near the seabed (2m above the bottom). With the water depths increased, the peaks of SSC are higher, and the stratification is also obvious. In winter, the peaks of SSC are higher than that in summer, and sometimes the concentration is uniform. Spatially, the SSC is high in nearshore areas, and is decreased far from the coast. Vertically, the SSC is increased with the water depth, and its distribution patterns have four modes (i.e. linear, diagonal linear, parabolic curve and mixture), which vary in different seasons and tidal periods. The sediment resuspension is studied by using the variation of bottom SSC as an indicator. The sediment suspension process, together with its dynamic mechanism, variation period and influencing factors are also analyzed. Wavelet analysis indicated that the period of the SSC is mainly related to tidal current.2. The method of mechanism decomposition was used to decompose the flux of suspended sediment into several factors. Intratidal variation of the flux of suspended sediment and its mechanism decomposition are analyzed under different seasons and climates. Quantitative study has been carried out for dynamic mechanism of the suspended sediment. Local and allochthonous sediment resuspension are also discussed. Results show that the advection and the tidal pumping effect are the main dynamic decomposition for the flux of suspended sediment. The advection effect plays an important role during the whole observations, with the direction same to the flood current. The tidal pumping effect plays an important role in the nearshore area, which is the main factor for the periodical variation of SSC. The direction of the tidal pumping mainly toward the land, which indicates that the periodic component of SSC is transport to the land with the form of peaks in the bottom layer.3. With the resuspension and settlement, some sediment either enter into the water and keep in suspension or settle on the seabed, which is inevitably lead to the variation of the grain-size distributions. Therefore, the temporal-spatial variation of the grain-size distributions of suspended sediments contains important information of sediment resuspension and settlement. Based on the characteristics of grain size, the inversion of sediment resuspension and settlement processes is conducted. The characteristics of grain-size distributions and its response to the hydrodynamic are analyzed, and the flux of resuspension of coarse sediment in the bottom is also calculated. Results indicated that the variation of intratidal grain-size distributions of suspended sediments mainly exhibit the fluctuation of major peak for fine grain size and the appear or disappear of subpeak for coarse grain size. According to the statistics, the major peaks of fine grain size at most stations are 7phi and fluctuated between the finer and coarser grain size. While the small peaks of most coarse grain size are less than 4phi. The variation of the grain-size distributions curve is the result of sediment resuspension and settlement caused by hydrodynamic forces. The small peak of coarse grain size is the indicator of resuspension from the bottom, and the amount of its resuspension can be estimated accordingly.4. A simplified expression for calculating shear stress under the joint action of wave and current is given. Using this method, the flux of resuspension caused by current, wave, and the joint effect of wave and current are calculated respectively. Results show that the resuspension flux variation range caused by wave-current combined effect is larger than that caused by current or wave solely, but its value is not the simple superposition of other two values.5. The relationship between the wave and the suspended sediment concentration indicates that the trends of their variation are almost the same, but the variation of SSC is usually lags behind the wave. The period of the SSC is mostly affected by the tide, but the wave weakens the periodic relationship between the tide and the SSC.6. Based on the three-dimensional numerical calculations, the relationship between the dynamic factor and the SSC is discussed. The resuspension process and its mechanism on"resuspended by wave, transported by current"are also studied. Results show that both wave and current can make the sediment resuspension in the Lianyungang nearshore area, but wave is the main force to suspending sediment.These results reveal the process of sediment resuspension in the Lianyungang nearshore area. The theoretical results got by this paper not only enriched the knowledge on the processes of land-sea interaction, but also of great significant for ocean exploitation.