Progradation and erosion of a fine-grained tidally dominated delta

Abstract

A large proportion of the world population lives in the coastal zone, which is facing a variety of natural and anthropogenic impacts on its evolution. A better understanding of coastal evolution can be obtained when it is based on an accurate knowledge of coastal dynamics. This thesis aims to study the long-term (hundreds of years) coastal evolution of the Jiangsu coast, China, under both natural and anthropogenic impacts. The recent (hundreds of years) evolution of the Jiangsu coast is closely related to the shifting course of the Yellow River. The coastline of Jiangsu experienced a rapid progradation stage (1128~1855), when the Yellow River discharged into the Yellow Sea, and a following modification stage (after 1855), when the Yellow River abandoned its piror course. Simultaneously, the sedimentary environment of the Jiangsu coast changed from sandy to silty. In contrast, the tide, which is a dominant driving force for the sediment transport and morphological changes along the Jiangsu coast, has been stable for at least thousands of years. As one of the fundamental factors controlling the coastal evolution, local hydrodynamics has at the outset drawn our attention. A large tidal wave model containing the Bohai Sea, the Yellow Sea and the East China Sea is established to investigate the near-field hydrodynamic conditions of the Jiangsu coast. For instance, existing knowledge always suggested that the geographical position of the Shandong Peninsula is crucial for the local tidal wave system off the Jiangsu coast. However, this is a hypothesis without any verification. Moreover, the role of the local bathymetry on the formation of the radial tidal current off the Jiangsu coast is debatable. These two factors of influence are assessed based on the tidal wave system (Chapter 2). The results demonstrate that the Shandong Peninsula plays a secondary role, rather than a crucial role on the Jiangsu local tidal wave system. The radial tidal current is independent of the geographic position of Shandong Peninsula and local ridge-channel morphologies. Besides the existence of the special tidal wave system, another characteristic of the Jiangsu coast is the existence of the silt-dominated sedimentary environment. To gain insight into the characteristics of silt-dominant sediment, flume experiments with various wave and current conditions have been conducted with two sediment samples from the field (Chapter 3). A high concentration layer is observed near the bottom together with ripples under wave-only conditions. Moreover, a significant vertical sediment sorting phenomenon has been found near-bottom for both sediment samples. As a widely used instrument for measuring suspended sediment concentration in both field and laboratory conditions, the Optical Backscatter Sensor (OBS), has been introducted, requiring additional caution due to its sensitivity to sediment grain size. As observed in the flume experiments, vertical sorting leads to vertical grain size differences in a water column. Thus, traditional approaches converting OBS signals based on one fixed calibration curve (i.e. against the bottom sediment) may lead to large deviations over silt-sand mixed suspensions. To extend the application of OBS for fine mixed sediment, we take the grain size effect into account and propose a new approach (Chapter 4). The new approach introduces an accompanying sediment sample besides the original bed samples for calibrating the OBS instrument. Besides, a multi-fraction sediment model is developed to predict the vertical distribution of sediment grain sizes. The reliability of this approach has been validated by the flume experiments. Regarding the aforementioned coastal evolution of the Jiangsu coast since 1128, previous studies using a geological approach failed to reach a satisfactory agreement. To investigate the long-term evolution of the Jiangsu coast, a large-scale morphodynamic model is established (Chapter 5, 6). Due to the scarcity of historical data, we have simplified several data, such as the bathymetric data and the hydrodynamic conditions (e.g. river discharge). Historical maps, records and geological measurements have been utilized for determining model settings and validating the model results. We first focus on the formation and the development of the deltaic system (Chapter 5). In spite of the simplifications on model setting, the model shows a good capability to reproduce the development of the AYD. Sensitivity analyses of uncertainties (e.g. sediment discharge, accommodation space) on the performance of long-term morphodynamic model are further conducted. Subsequently, we focus on the modification stage of the AYD, when the Yellow River shifted its course after 1855. In this stage, the fluvial impact on the coastal evolution disappeared, while the impact of wave and human activity rose. Therefore, we take tides, waves and artificial revetments (sea dikes) into account in the model to understand the ongoing coastal behavior (Chapter 6). Based on modelling results, the Yellow River has been proven as one of the sediment sources for the RSRs, and the relevant contribution is neither restricted to the northern RSRs nor to a certain period (even at present). Moreover, the results show that the sea dikes play a significant role on the evolution of the nearshore regions, and that wind waves play a remarkable role on the erosion of offshore shoals. In the perspective of time and hydrodyanmic forces, tide is the most dominant force governing the overall evolution of the Jiangsu coast. This thesis addresses the long-term morphological evolution of the Jiangsu coast through a process-based morphodynamic modelling approach. The corresponding insights and findings improve our understanding on the long-term evolution of the Jiangsu coast.