Morphological Evolution Of The Pearl River Delta In The Past 165 Years And Its Response To Human Activities

The impact of human activities on rivers and estuaries is the focus of global persistence research hotspot. The explosive urbanization process (? 60 million) and economic growth (> $ 1 trillion) make the Pearl River Delta one of the world's estuarine delta that strongly affected by human activities, therefore this region is the ideal area to carry out studies in the evolution of the estuarine delta under human activities. In this thesis, 165 years of bathymetric chart data (1850-2015) and 60 years of hydrological and meteorological and 40 years of estuarine remote sensing image data, are firstly used to study the basic rules and control factors of the geomorphic changes in the Pearl River Estuary, and to quantitatively analyze the contribution of different human activities to these changes, the main understanding is shown as follow:1. Over the past 165 years, the morphology of the Pearl River Delta and its estuary has undergone tremendous changes. The area of the estuary is reduced by about 35%(1258 km2), of which the area of the Lower LingDing Bay (LLDB) is only reduced by 3% (26 km2), while the area of the Modao-Jitimen Estuary (MJE), the Upper LingDing Bay (ULDB), and the Huangmao Sea Estuary (HSE) is reduced by 62% (5250 km2),36% (405 km2) and 39% (301 km2), respectively. More than 20 islands have been gradually merged into the continent, and almost all of the beaches 165 years ago have been reclaimed into land. The volume of the estuary decreased by about 9 km3 (39%),and the volume of the four study areas is reduced by 26% (LLDB), 38% (ULDB), 50%(HSE) and 58% (MJE), respectively. The average water depth of the whole area decreased by about 0.4 m (from 6.3 m to 5.9 m), and the average depth of each estuarine area varied greatly, with the MJE increased slightly, the ULDB decreased slightly, while the LLDB and the HSE showed a significant downward tendency, indicating that the western estuarine area changes more intensely compared to the eastern estuarine area.2. The volume change calculated from 7 periods of seabed bathymetry shows that in the past 16 years, the Pearl River Delta estuary has received about 10 Gt of sediment,and the actual amount of sediment discharge in the Pearl River is about 10-11 Gt,indicating that sediment discharge of the watershed and the morphology of the delta are basically balanced, and less sediment (- 10%) is transported from the offshore to the sea. Since the 1970s, human activities including dam construction and sand mining in the river basin, reclamation around the outlets and ocean engineering in the estuary,have a significant impact on the bathymetric changes and sediment transport of the Pearl River Delta.3. The results after analyzing the characteristics of human activities in different regions of the Pearl River Delta show that, the human activities in the Pearl River Delta mainly concentrated in the ULDB, MJE and HSE, and the human activity intensity in the LLDB is relatively small. The ULDB has the largest human activity intensity and classes, including reclamation around the outlet that dramatically changes the coastline,while the MJE is mainly in the reclamation around the outlet, and the HSE is mainly in the reclamation and the channel dredging around the outlet.4. According to the rate of shoreline propulsion and the accumulation rate of underwater delta in the past 100 years, the closed time of the four major estuaries of the Pearl River is speculated, that is 170 years (LLDB), 590 years (ULDB), 170 years (MJE)and 150 years (HSE),respectively. In the context of the increasing human activities,the disappearing process of the flared shape of the Pearl River Delta will be greatly accelerated. After the estuary disappear, the southern island chains will be deposited as the front sedimentation core of the future delta, where sediments will accumulate rapidly.Based on the study of the response of the geomorphic changes in the Pearl River estuarine delta to the human activities, the results are further compared with the geomorphic changes of the other eight rivers and estuarine delta in China, which indicate that:1. The quantitative assessment of the contribution of different human activities over the past 60 years to the reduction of sediment in China's 9 rivers shows that, in 1954-2015, about 51 Gt of sediment has been left on the continent due to human activities, among them, about 26 Gt and 49% of the sediment has been intercepted by reservoirs, while 15.4 Gt and 12 Gt of the sediment due to water resource utilization and soil and water conservation, contributing about 29% and 22%, respectively. In the past 60 years, under the impact of human activities in the watershed, the sediment flux of all the rivers and tributaries of the nine rivers in China, except for the upper reaches of the Yellow River (the Tang Naihai River), the upper reaches of the Yangtze River(the Jinsha River), the tributaries of the Pearl River (the Liujiang River) and the upper reaches of the Songhua River (the Nenjiang River) dramatically decrease.2. The main difference between the Pearl River and the other eight rivers in the common background of the decrease of sediment transport in the basin is that, the sudden decrease in sediment flux appears to have little effect on the changes of the shape of estuarine delta, in a period time of the current and the future, the reclamation more than natural factors (such as sea level rise, storm surge, etc.), will still be the most important control forcing the Pearl River Delta continue to advance to the sea. In the future, with the decrease of sediment transport in the Pearl River Basin and the rise of sea level, the decrease of the area and volume of the Pearl River Delta will be mitigated,including humanities activities such as channel dredging and estuarine sand mining, the morphology of the Pearl River Delta will be presented in the polarization characteristics of "the more shallow the shallow shoal, deeper deep trough". The development of estuarine delta requires scientific management.