Variations In Estuarine And Coastal Suspended Sediment Concentration And Delta Accretion/erosion In Response To Decline In Sediment Supply From The Yangtze River

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.