【作者】 李鹏；

【导师】 杨世伦；

【作者基本信息】 华东师范大学 ， 河口海岸学， 2012， 博士

【摘要】 河口和邻近海域悬沙浓度的变化和底床冲淤对该区域的环境质量和生态系统具有重要影响,从而在很大程度上涉及到社会经济的发展。而悬沙浓度变化和底床冲淤之间有着内在的联系。长江是我国第一大河,入海水沙分别居世界的第五和第四位。长江三角洲是我国乃至世界上的重要经济区。由于流域高强度人类活动的影响,近期长江入河口泥沙通量呈明显的下降趋势,特别是2003年三峡大坝建成以来,长江平均入河口泥沙通量(2003-2009年)仅为1950s-1980s的30%。在此背景下,河口及邻近海域悬沙浓度的变化和三角洲地貌的冲淤响应是值得深入研究的重要科学命题,它不仅是区域河口海岸管理的重要依据,而且对丰富世界陆海相互作用的理论认识具有重要意义。本文根据长江口和邻近海域10个固定采样点2-12年的每日表层悬沙浓度,结合该区域的动力资料和长江入河口水沙资料,研究悬沙浓度的空间分布格局和时间变化的周期性和趋势性,并探讨其主要影响因子；利用近50年(1958-2007年)不同时段长江口水下三角洲典型区域的地形资料,研究底床冲淤速率的时间变化及其与长江来沙锐减的关系；分析长江口-邻近海域悬沙浓度和海底冲淤对长江来沙减少响应过程中的相互制约关系。主要结果和结论如下：1)河口及其邻近海域悬沙浓度的空间分布格局及其机制。2009年平均表层悬沙浓度从长江口上游节点(徐六泾站)的0.058g/l逐渐向河口口门(九段沙站)增大到0.60g/l(增大一个数量级),继而又向海降低至冲淡水边缘(绿华山站)的0.057g/lo悬沙浓度的这种沿长江口延伸方向变化的空间格局进一步证实了地形和潮动力共同控制下河口最大浑浊带的发育。在沿岸线方向上,悬沙浓度从长江口外海滨(佘山站：0.43g/I)向杭州湾中部(滩浒站：1.28g/1)呈显著增加趋势,其主要原因是潮动力的沿程增大和较低悬沙浓度的长江入河口水体影响的沿程减弱。2)河口及其邻近海域悬沙浓度变化的周期性及其影响因素。长江口及其邻近海域悬沙浓度存在明显的周期性变化。日均悬沙浓度表现出14.9±0.2天(p<0.001)(反映大小潮变化)以及365±6天(P<0.01)(反映季节性变化)的显著周期。代表性站点(大戢山)日均悬沙浓度与日均潮差、波高、长江径流量和输沙率(大通站)的多元回归分析结果是：潮汐、波浪、长江径流量和输沙率对日均悬沙浓度变化的贡献率分别为30%、7%、23%和4%,反映这四个因子对日均悬沙浓度的影响以潮差最大、径流量次之。波浪的影响远低于潮差是因为取样点的水深通常大于5m,在这样的水深条件下,一般的波浪难以扰动底床沉积物从而引起泥沙再悬浮。大通日均输沙率对河口及其邻近海域悬沙浓度变化的贡献远低于大通日均径流量的贡献在理论机制上可能是一种假象,造成这一假象的原因可能是大通径流量和输沙率之间的高度正相关关系(R=0.90)。上述四个因子对日均悬沙浓度的总贡献率为64%,说明它们是主要的影响因子；同时反映日均悬沙浓度的变化机制较为复杂,其它因子的影响也不宜忽视。代表性站点(大戢山)月均悬沙浓度与月均潮差、波高、长江径流量和输沙率(大通站)的多元回归分析结果为：潮汐、波浪、长江径流量和输沙率对月均悬沙浓度变化的贡献率分别是21%,1%,62%和8%。这说明,长江径流量的季节性变化是长江口及其邻近海域悬沙浓度季节性变化的最重要的控制因子,其次是潮差的季节性变化。上述四个影响因子对悬沙浓度季节性变化的总贡献率达到92%,说明其它因素的影响很小。3)河口及其邻近海域悬沙浓度变化的长期趋势及其主要原因。悬沙浓度本文可供长期对比各个测站的年均悬沙浓度在过去的10-20年中均呈现不同程度的下降趋势。这种下降趋势主要归因于长江入海悬沙通量的下降,理由是没有证据表明同期海洋动力条件的变化是主要原因。导致研究区悬沙浓度呈现长期减少趋势的主要因素为长江输沙量的减小。其中,徐六泾站点的悬沙浓度降低幅度最大达56%,与大通站的下降幅度吻合,反映河口上游悬沙浓度对流域来水来沙条件变化的响应最为敏感。而目前遭受侵蚀的长江口门外水下三角洲前缘(佘山站)悬沙浓度下降幅度最小仅5%。长江口及其邻近海域各站悬沙浓度的下降幅度平均值为20%左右。这一方面反映河口及其邻近海域悬沙浓度对流域入海悬沙通量的长期变化趋势有较敏感的响应,另一方面又反映这种响应存在明显的滞后现象。具有连续12年监测资料的代表性站点(小洋山)年均悬沙浓度资料系列与大通站年悬沙通量资料系列的相关统计分析表明,前者的变化相对于后者的变化存在3-4年时间滞后。这些滞后现象归因于长江入海泥沙锐减所诱发的水下三角洲(特别是口门外水下三角洲前缘)的强烈侵蚀和细颗粒泥沙再悬浮。4)三角洲对流域来沙锐减响应的敏感性及其空间差异。近50年来长江口门外水下三角洲(以一个1825km2的典型区域为例)的冲淤演变经历了4个阶段：1958-1977年为快速淤积期[时段-区域平均淤积速率(下同)6.8cm/a],1977-2000年为淤积减慢期(3.2crn/a),2000-2004年为淤-蚀转变期(-3.8cm/a),2004-2007年为侵蚀加强期(-4.5cm/a),侵蚀速率较2000-2004年大。上述四个阶段的水下三角洲冲淤速率与同期的长江入海泥沙通量(大通站)之间存在显著的正相关关系,根据该关系内插得出的水下三角洲冲淤转换临界入河口泥沙通量为270×106t/a,略小于三峡工程运行前的2001-2002年的大通输沙率(275-276x106t/a)。这说明,该淤-蚀转变很可能发生在三峡工程运行后(2003年以来的大通输沙率变化于85×106t/a和216×106t/a之间)。鉴于没有证据表明近几十年海洋动力条件和长江口门外的边界条件发生了明显变化,上述水下三角洲的淤-蚀转变从根本上归因于流域人类活动特别是三峡工程蓄水引起的入海泥沙通量下降。换言之,三峡工程蓄水对长江口门外水下三角洲地形演变产生了重要影响。另一＋方面,三角洲的冲淤及其对长江来沙通量变化的响应存在明显的空间差异。口门外5—10m水深区域对长江来沙减少的响应最为强烈(即近年来侵蚀最为明显),10-20m区域次之,5m以上区域则相对比较稳定。也就是说,长江三角洲对流域来沙减少的响应敏感是限于口门外三角洲前缘的敏感带。5)河口及其邻近海域悬沙浓度和海底冲淤对流域来沙锐减响应的耦合机制。在流域来沙减少的背景下,河口及其邻近海域悬沙浓度的响应和水下三角洲底床的冲淤响应是互为耦合的,即悬沙浓度的变化影响底床冲淤,而底床冲淤又反过来影响悬沙浓度。近10年徐六泾站悬沙浓度的下降幅度(56%)与大通站悬沙浓度的下降幅度(55%)基本一致,反映大通至徐六泾河段底床冲淤不明显。而长江口外水下三角洲前缘的佘山站悬沙浓度的下降幅度(5%)远低于大通站,是因为底床冲淤再悬浮的泥沙补充了悬沙浓度。如果河口及其邻近海域底床是由基岩或砂砾组成,不能补充悬沙,则河口及其邻近海域的悬沙浓度将继承流域入海水体悬沙浓度的下降特征。假定是这种情况,那么从长江口向浙江沿岸和外海输送的泥沙将像流域入海泥沙那样急剧减少。相反,若河口及其邻近海域底床侵蚀再悬浮的泥沙完全补充了流域入海水体悬沙的减少,则河口及其邻近海域悬沙浓度不会下降,从长江口向浙江沿岸和外海输送的泥沙也不会减少。实际情况介于上述两种极端假定情景之间：长江口及其邻近海域悬沙浓度的平均下降幅度(21%)仅为流域入河口水体悬沙浓度下降幅度的38.2%。这说明,流域来沙锐减背景下水下三角洲的侵蚀抵消了约一半以上的悬沙浓度下降。同时也反映,从长江口向浙江沿岸和外海输送的泥沙呈减少趋势,但减少幅度小于流域来沙的减小幅度。6)未来几十年河口及其邻近海域悬沙浓度变化和三角洲冲淤趋势的展望。未来几十年,由于流域新建大坝(特别是金沙江流域梯级水库)、水土保持和南水北调等人类活动的影响可能超过已建水库因历年淤积而减少库容(从而减小入库泥沙的淤积比)的影响。另一方面,在经历了三峡工程运行以来近10年的坝下游侵蚀之后,长江中下游干流河床可供侵蚀的泥沙可能将逐渐减少。因此,未来几十年长江入河口的泥沙可能将进一步减少。在此背景下,长江水下三角洲的侵蚀将可能继续进行。但是,因前期侵蚀导致的地形调整、表层沉积物粗化以及较密实的老淤泥层暴露,总体上侵蚀速率将可能逐渐减小(尽管流域来沙通量将进一步降低)。因此,未来几十年长江口及其邻近海域悬沙浓度很可能进一步降低,从长江口向浙江沿岸和外海输送的泥沙也将进一步减少。更多还原

【Abstract】 Variations in estuarine and coastal suspended sediment concentration (SSC) and accretion/erosion of sea bed have important influences to the environmental quality and ecosystem in estuarine and coastal waters, thus, effect the the developmemt of social economy to a great extent. There is an internal connection between variations in SSC and sed bed accretion/erosion. As one of the largest rivers in the world, the Yangtze River is the fourth in sediment load (until recently>400Mt/yr), and the fifth in water discharge. The Yangtze Delta is an important economic region to our contury and even in the world. In recent years, the suspended sediment supply from Yangtze has drastically decreased as a result of human activities, especially the operation of Three Gorges Dam in2003. Yangtze sediment discharge during2003to2009was only30%that in1950s-1980s. Under this context, variations in estuarine and coastal SSC and accretion/erosion in the Yangtze subaqueous delta are important scientific topics worth researching, which are not only important basis to estuary management but also have important significance to enrich the land-ocean interaction theory.Researches about spatial variations, periodicity and temporal trend in SSC, and exploring factors controlling SSC variations in the Yangtze (Changjiang) Estuary and nearby coastal waters, based on data of daily SSC in the surface waters, wind, wave tide and the Yangtze water and sediment load to the sea at10fixed stations in the study area ranging from2to12years. Based on the bathymetric maps of the Yangtze subaqueous delta in recent50years (1958-2007), the relationships between variations of the subaqueous delta accretion/erosion rates and the sharp decline of the sediment supply from the Yangtze River to the sea were preliminarily discussed. In addition, we analysed the interaction relationship between response of SSC in Yangtze estuary and adjacent coastal waters and accretion/erosion in the subaqueous delta to the the sharp decline of the sediment supply from the Yangtze River to the sea. The main fruits are as following:1) Spatial pattern of surface SSC and mechanism in estuary and adjacent coastal waters. Quasi-synoptic measurements in2009showed an increase in surface SSC from0.058g/1at the upstream limit of the Yangtze estuary (Xuliujing) to-0.6 g/l at the Yangtze River turbidity maximum at the estuary mouth (Jiuduansha), seaward of which SSC decreased to0.057g/l (Lvhuashan). This cross-shore variation revealed the developemnt of an estuarine turbidity maximum, attributed to combined influence of stronger hydrodynamic forces and shallower water depths at the mouth-bar area. The SSC of longshore transect showed a different pattern:increasing towards the Hangzhou Bay, from0.43g/l in offshore area (Sheshan) to1.28g/l in the middle of Hangzhou bay (Tanhu), which attributed to higher tidal hydrodynamic forces and the reduced influence of discharge from the Yangtze River.2) Temporal variations and factors controlling Periodicity of SSC in estuary and adjacent coastal waters.The SSC in the Yangtze estuary and Hangzhou bay waters have marked periodic variations. SSC showed statistically significant bi-weekly (14.9±0.2days, P<0.001)(i.e., spring-neap tides) and annual periodicities (365±6days, P<0.001) linked the seasonal variations in the discharge from Yangtze River. Based on a multiple regression analysis between daily SSC and influencing factors at Dajishan, tidal range, wave height, riverine discharge, and suspended sediment flux contributed around30%,7%,23%, and4%, respectively, to daily variations in SSCs. The analysis results suggest that tidal range and water discharge from Yangtze were the most and second important factor among the four factors to the daily SSC. The contribution of tidal range to variability in daily SSC was significantly higher than the contribution of wave height was attributed to the water depth less than5m normally. So in the study area, normal waves do not feel the bottom that results in strong sediment resuspension. The contribution of daily suspended sediment flux at Datong to variability in daily SSC was much lower than the contribution of daily water flux at Datong. The result is likely a feint which is related to the strong positive correlation between discharge and suspended sediment load at Datong (R0.90).Tides, waves, riverine discharge, and suspended sediment flux together contributed around64%to daily variations in SSC, which indicate that the four factors are dominated influencing factors to daily variations in SSC. At the same time, the results indicate that the mechanism of daily variations in SSC was very sophisticated, and the other influencing factors can not be ignored. Based on a multiple regression analysis between monthly SSC and influencing factors at Dajishan, tidal range, wave height, riverine discharge, and suspended sediment flux, the contribution of them are around21%,1%,62%, and8%to the monthly variation in SSC, respectively. The results of multiple regression for monthly SSC and major influencing factors suggest that the seasonal changes of water discharge of Yangtze was the most important controlling factors to the variations in estuarine and adjacent coastal waters, and the seasonal variation of tidal range was the second important controlling factors. The four influencing factors above-mentioned together contributed around92%to the monthly variations in SSC, which suggest the contribution of other influencing factors were minor.3) Long-term variation trend of SSC in the Yangtze estuary and adjacent coastal waters and the major causes. Over the past10-20years, annual SSC at all gauging stations has shown decreasing trend in different degrees. The major cause of the dcreasing trend is mainly the drastic decline in suspended sediment load from the Yangtze, because no evidence can be found that changes of ocean hydrodynamics was main reason for the changes in SSC. The decrease of SSC at Xuliujing was maximum, the amount of decrease even reached56%. Over this period, at Xuliujing has decreased almost by the same amountas at Datong (55%). The similarity between Datong and Xuliujing suggests that SSC in upper estuary was more sensitive to variation of water and sediment discharge from the basin. While at subaqueous delta front (Sheshan station), the smallest and statistically insignificant decrease (5%) in SSC occurred, where severe erosion was observed. SSC in the Yangtze estuary and adjacent coastal waters over this period decreased was20%on average and was less than that at Datong. The differences reflect that SSC in the Yangtze estuary and adjacent coastal waters was more sensitive to long-term vaiation of sediment discharge from the basin, and on the other hand the responses between them had a significant time lag. At Xiaoyangshan, where observations of SSC have been conducted since1998, a linear regression between the annual average of SSC at Xiaoyangshan and the annual SSD at Datong revealed that there was a3-4year time lag between the decrease in coastal SSC and the recent decrease in SSD at Datong. This de-coupling is attributed mainly to severe erosion of the subaqueous delta, compensating for the decrease in the supply of riverine suspended sediment. 4) Sensitive and spatial difference of response of the subaqueous delta to drastical sediment decrease from the basin. In recent50years, the changes of the Yangtze subaqueous delta (area:1825km2) experienced four periods. Between1958and1977, the subaqueous delta accumulated rapidly with the average shoaling rate of6.8cm/yr. Between1977and2000, most of the Yangtze subaqueous delta continued to prograde, but the net shoaling reduced to3.2cm/yr. Between2000and2004, conversion from accretion to erosion, with the average vertical erosion rate of3.8cm/yr. During2004to2007, the erosion rate speed up in comparison with in2000-2004, with the net erosion of4.5cm/yr. During the four study periods bathymetric changes within the study area have shown a strikingly linear correlation with the Yangtze’s sediment load (at Datong). The fitting result suggested that the Yangtze delta will continue to erode as long as the river annually discharges less than-270Mt/yr of sediment, which was less than275-276Mt/yr as measured at Datong before the impoundment of TGD from2001to2002. The results indicated the conversion of the delta front from accretion to erosion occurred after the impoundment of TGD propably. Because of no evidences were found that ocean hydrodynamics changed obviously in recent decades in the study area. The mainly reason of the Yangtze subaqueous delta reverted from mainly accretion to erosion was decrease in suspended sediment discharge to the estuary (east sea) due to catchment human activities, especially the impoundment of TGD. That is to say, the impoundment of TGD has brought forth incisive impact to the evolution of the Yangtze subaqueous delta. On the other hand, the accretion/erosion of the subaqueous delta response to the changes of the sediment supply from the Yangtze has significant special difference. In the study,5-10m water depth area was the most sensitive to the change (decrease) of the sediment supply from the Yangtze,10-20m water depth area was more sensitive, while shallower (<5m) areas remained relatively stable. In other words, the response of Yangtze delta to the decline of sediment supply from the basin was only limited the sensitive zone of delta front.5) Interconnection mechanism between response of the SSC in estuary and adjacent coastal waters and accretion/erosion of sea bed to the drastic decline of sediment supply from the catchment. Under the background of decrease of sediment supply from the catchment, the response of the SSC in estuary and adjacent coastal waters and accretion/erosion of sea bed are interconnection, that is to say:the changes of SSC influence the accretion/erosion of sea bed, and conversely, the accretion/erosion of sea bed influence the changes of SSC. In recent10years, the similarity decrease rates in SSC between Datong (55%) and Xuliujing (56%) suggests that no significant erosion or deposition has occurred along the reaches between these two sites. While the SSC at Sheshan in the delta front decreased far lower than at Datong (only5%), because re-suspension of sediment eroded from the subaqueous delta has partly offset the decline in suspended sediment from the Yangtze River. If the sea bed was made up of rock or gravel in estuary and adjacent coastal waters, the sea bed can not compensate the suspended sediment, thus the SSC in estuary and adjacent coastal waters would follow the declining character of SSC of water from the catchment to the sea. It is assumed that the sediment discharge transfer from Yangtze estuary to Zhejiang coast and outer sea would drastic decline following the drastic decline of sediment from the catchment to the sea. The other way round, if the re-suspension of sediment eroded from the subaqueous delta had offset the decline in suspended sediment from the Yangtze entirely, the SSC in the estuary and adjacent coastal waters would not decrease and the sediment discharge transfer from Yangtze estuary to Zhejiang coast and outer sea would not decline. The reality condition is in between two assumed extremes mentioned above:the decrease amount of SSC in Yangtze estuary and adjacent coastal waters was21%on average, only38.2%that decreased in SSC of water from the catchment to the sea. This illustrated that the severe erosion of the subaqueous delta had compensated more than half of the decrease in SSC. Moreover, this reflected the sediment discharge transfer from Yangtze estuary to Zhejiang coast and outer sea had shown decreasing trend, but the decrease amount was less than that decreased in sediment form the catchment.6) Expectation of variations in estuary and adjacent coastal waters SSC and accretion/erosion of sea bed for the coming decades. In the future decades, the influences of new dams are in construction or will be constructed in the Yangtze basin (especially cascaded in the Jinshajiang basin), the Water and Soil Conservation Project and the South to North Water Diversion Project et al.(catchment human activities) would propably exceed loss of capacity of reservoir of the built dams (decrease the settlement rates of SSC) because of sediment deposition. On the other side, the river sediment supply for erosion in the middle and lower reaches of the Yangtze would decrease gradually, when experienced10years of erosion in the lower reaches after the operation of TGD. Therefore, the Yangtze sediment flux to the sea is expected to continue to decline in approaching decades. It is no doubt that the subaqueous delta would probably suffer further erosion in future. But the topography adjustment, coarsening of bottom sediments and the exposure of older and firmed silt stratification resulting from bed erosion in earlier stage would slow the erosion rate gradually. So, SSC in the estuarine and adjacent coastal waters would experience further decrease, and the sediment discharge transfer from Yangtze estuary to Zhejiang coast and outer sea would decline for the coming decades.更多还原