| Sediment from the continental land mass delivered by the river to the world’s oceans is the fudamental feature of geology and biogeochemistry. Recent years, The sediment flux to the ocean show sharply decrease because the climate and human activities impacted on the river systerm, moreover, resulted the significant changes of esturaine morpology and biochemistry. As a result, the delta responsed to the river material flux has become an international hot research field, and become an important part ofL and-Ocean Interactions in the Coastal Zone (LOICZ).Southern China, as one of the seven partition of china, Is located in the subtropical and tropical area. Many rivers distributed in the area. There is a rapid development in social and economic activities, these river systerm is suffering huge impact by the human, and these resulted the material flux to the ocean is decreased. The thesis mainly selected Pearl River, which is an most important river in China. The Pearl River, as an most complex delta river network system in the world, has important significance. The Nandujiang river and Wanquanhe River, located in Hainan province, is two representative river controlled by the wave. So Based on the Pearl River, Nandujiang River and Wanquan River studies, the main results and conclusions are as follows:(1) Based on water discharge and sediment load data, the trend and the changes of three phases in precipitation, water discharge and sediment load in the Pear Rvier were analysed, in addition, preliminaryly analysed the trend of Nandujiang River and Wanquanhe River based on the limited data. The result indicated that the precipitation in Pearl River basin show insignificant increasing trend from1954to2010(P=0.24). Water discharge showed insignificant decreasing tend(.P=0.24) and sediment load showed significant decreasing trend (P<0.001). Compared to1954-1983, the sediment load in Pearl River droped86%in2006-2010, and compared to1956-1969, the sediment load in Nandujiang River droped78%in1998-2007, and compared to1962-1965, the sediment load in Wanquanhe River droped76%in2001-2007. The water discharge and sediment load in Pearl River can be divided into three phases:(1) Increasing phase (1954-1983),(2) undulating phase (1984-1993) and (3) decreasing phase (1994-2010). Between1954and1983, water discharge and sediment load increased by18%and32%, respectively. During an undulating phase between1984and1993, a marked up in water discharge and sediment load was followed by suddenly rebounded discharge. From1994to2009, water and sediment decreased by32%and83%, respectively.(2) The contribution rate of Human activates on water discharge and sediment load in Pearl River. These trends were generally in agreement with changes in precipitation, suggesting climatic influences on a decadal timescale, although the changes in sediment load were also related to human activities. Human impact on sediment load can also be identified as three major phases. In the1950s to1970s, deforestation in the basin was balanced by dam construction, resulting in no significant net change in sediment load. In the1980s, however, the influence of the deforestation outweighed dam construction, resulting in an increase in sediment load. Since the1990s, dam construction and soil preservation have decreased sediment load quickly, and the monthly sediment loads were lower in post-dams period than in the pre-dams period. Since the closure of the Longtan and Baise Dams in2006, the sediment load in the Pearl River has decreased by～70%relative to the level of the1950s-1980s. Of this change,～90% was caused by dam construction and～10%was due to by climate change. In the coming decades, the sediment load in the Pearl River will probably continue to decrease as the new dams are built within the watershed.(2) The thesis analysed main control factors of the long-term trend and phase change of water discharge and sediment load in Pearl River basin, and prelimilary attempted to quantitatively separated impact of human activities from climate change. The result showed that:before1960s, the impact of human on Pearl River was very little, so water discharge and sediment load were near nature state. The relationship between precipitation and runoff and between runoff and sediment load were significant positive correlations. The correlation coefficient (R2) were0.78and0.92, respectively. These reflected the control impacts of climate change on water discharge and sediment load. Since1960s, Human activities on sediment load influenced more and more. The influence of human activity was also divided into three stages:Between1950s and1970s, the impact of deforestation was balanced to the impact of reservoir construction in the basin, therefore, sediment load had no significant change. To1980s, the soil and water loss caused by deforestation was much greated than that of intercepting sediment of reservoirs. The influence of human activities on sediment transport rate could be fully refected. Sediment load caused by deforesation increased about20%in Pearl River basin.1990s, many large reserviors were began to construct, and at the same time soil and water preservation was used in China. Both of them were far exceed to the effects of deforestation. These leaded sediment load to rapid drop. After2006. the contribution rate of the reservoir for the reduction of sediment occupied about90%, and the climate change’s contribution rate was about10%.(3) Based on the topographic data of differernt years in Pearl River and Nandujiang River, the thesis built the3D terrain model and caculated the erosion rate in defferent stages, the changes of coastline of Wanquanhe River were caculated according to satellite remote sensing image of different years. The thesis analysed changes of the subaqueous delta responeding to sediment flux of the rivers. The result showed that:The accretion rate of subaqueous delta in Modaomen estuary(area:509509km2) decreased1.3cm/a in1964-1977to0.15cm/a (down88%). The accretion rate of subaqueous delta in Lingdingbay decreased4.5cm/a in1955-1964to2.01cm/a to1998-2008(down56%). These were greater than the descrent rate of sediment flux in the same period (the decreased rate was19%from1964-1977to1977-2005and38%from1955-1964to1998-2008, respectively). In addition, the accretion rate in Nandujiang River from4.17cm/a in1963-1980into the erosion rate in1996-2003, the erosion rate reaches2.21cm/a. The sediment flux reduced from55.04×104t (1956-1959) to11.9×104t (1998-2007) at the same period in Nandujiang River. The droped rate was78%. The area of Yudai beach in Wanquan he River reduced from52.6×104m2m2(1988) to34.3×104m2(2005), the decreased rate was35%. The sediment flux reduced from31.7×104t (1988) to11.2×104t (2001-2007) at the same period. The decreased rate of sediment load was64%. These indicated that subaqueous delta was sensitively in response to the reduced sediment load in rivers.(4) Combined with the development trend of global climate changes and human activities in the future, the thesis forecased the trend of water discharge and sediment flux and delta evolution in Pearl River in the coming decades. In the future, Global climate change in the Pearl River basin might still mainly for the interannual and time scale fluctuation in precipitation, and there was no significant long-term trend in precipitation. On the other hand, Many new reservoirs would be constructed to meet power generation, flood control and irrigation needs, The sediment trapping efficiency of the new reservoirs may be over the built reservoirs capacity. Further, water and soil conservation measures would strengthen. Therefore, water discharge in Pearl River maybe not change, but sedimant rate is likely to further reduce in the next decades. The progradation rate of delta will continue to decline. As for the Pearl River would like the Yellow River and Yangtze River delta as a whole transformation of silt-corrosion. It needs to be further studied. |
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