S. L. Yang

Influence of the Three Gorges Dam on downstream delivery of sediment and its environmental implications, Yangtze River

[1] Water and sediment supplies from ungauged areas were calculated and combined with datasets from gauging stations to establish sediment budgets. Using sediment budgets and regression relationships, the influence of the Three Gorges Dam (TGD) on downstream delivery of sediment was quantified. We found that 151 mt/yr (1 mt = 10 6 tons) of sediment has been retained by TGD since it began operation (2003–2005). In response to this, significant erosion has occurred in the downstream riverbed. This erosion did not offset the sediment lost in the reservoir, and the sediment flux into the estuary decreased by 85 mt/yr (31%). This decrease has lead to conversion from progradation to recession in the delta front. In combination with other anthropogenic impacts, TGD was expected to decrease the sediment flux into the estuary for centuries, which is of great importance for delta ecosystem and human development. Citation: Yang, S. L., J. Zhang, and X. J. Xu (2007), Influence of the Three Gorges Dam on downstream delivery of sediment and its environmental implications, Yangtze River, Geophys. Res. Lett., 34, L10401,

Decline of Yangtze River water and sediment discharge: Impact from natural and anthropogenic changes.

The increasing impact of both climatic change and human activities on global river systems necessitates an increasing need to identify and quantify the various drivers and their impacts on fluvial water and sediment discharge. Here we show that mean Yangtze River water discharge of the first decade after the closing of the Three Gorges Dam (TGD) (2003–2012) was 67 km3/yr (7%) lower than that of the previous 50 years (1950–2002), and 126 km3/yr less compared to the relatively wet period of pre-TGD decade (1993–2002). Most (60–70%) of the decline can be attributed to decreased precipitation, the remainder resulting from construction of reservoirs, improved water-soil conservation and increased water consumption. Mean sediment flux decreased by 71% between 1950–1968 and the post-TGD decade, about half of which occurred prior to the pre-TGD decade. Approximately 30% of the total decline and 65% of the decline since 2003 can be attributed to the TGD, 5% and 14% of these declines to precipitation change, and the remaining to other dams and soil conservation within the drainage basin. These findings highlight the degree to which changes in riverine water and sediment discharge can be related with multiple environmental and anthropogenic factors.

Morphological response of tidal marshes, flats and channels of the outer Yangtze River mouth to a major storm

Systematic morphological changes of the coastline of the outer Yangtze River mouth in response to storms versus calm weather were documented by daily surveys of tidal marshes and flats between April 1999 and May 2001 and by boat surveys offshore during this and earlier periods. The largest single event during 1999 to 2001 was Typhoon Paibaian, which eroded the unvegetated tidal flat and lower marsh and led to accretion on the middle-to-upper marsh and in the subtidal channel. The greatest erosion of 21 cm occurred at the border between the marsh and the unvegetated flat due to the landward retreat of the marsh edge during the storm. Strong waves on the flats increased suspended sediment concentration by 10–20 times. On the upper marsh, where the frequency of submergence by astronomical tides is only 3%, Typhoon Paibian led to 4 cm of accretion, accounting for 57% of the net accretion observed over the 2-yr study. Typhoon Paibian led to 4 cm of accretion, accounting for 57% of the net accretion observed over the 2-yr study. Typhoon Paibian and other large storms in the 1990s caused over 50 cm of accretion along the deep axis of the river mouth outlet channel. During calm weather, when hydrodynamic energy was dominated by tides, deposition was centered on the unvegetated flats and lower, marsh with little deposition on the high marsh and erosion in the subtidal channel. Depositional recovery of the tidal flat from typhoon-induced erosion took only several days, whereas recovery of the subtidal channel by erosion took several weeks. A conceptual model for the morphological responses of tidal marshes, flats, and subtidal channels to storms and calm weather is proposed such that sediment continually moves from regions of highest near-bed energy towards areas of lower energy.

The sharp decrease in suspended sediment supply from China's rivers to the sea : anthropogenic and natural causes

Abstract Based on data from river gauging stations, the multi-year variations in suspended sediment flux (SSF) from Chinas nine major rivers to the sea were examined. The decadal SSF decreased by 70.2%: from 1.81 Gt/year for 1954–1963 to 0.54 Gt/year for 1996–2005. The decrease in SSF was more dramatic in the arid northern region than in the wet southern region; from north to south, the SSF decreased by 84% in the Yellow River, 42% in the Yangtze River, and 22% in the Pearl River. Dam construction was the principal cause for the decrease in SSF. At present, approximately 2 Gt/year of sediment is trapped in the reservoirs within the nine river basins. Reduced precipitation and increased water extraction and sand mining have also played a role in the decrease in SSF. Although water and sediment conservation programmes have not counteracted the influence of deforestation, they have enhanced the decrease in SSF in recent years. It is concluded that human activity has become a governing factor on riverine sediment delivery to the sea in China.

Long-Term Morphological Evolution of a Tidal Island as Affected by Natural Factors and Human Activities, the Yangtze Estuary

Abstract Bathymetric data based on sounding surveys and intertidal elevations measured using Real-Time Kinematic Global Positioning System methodology were analyzed using Geographical Information System techniques to examine the morphological evolution of the tidal island Jiu Duan Sha (JDS) in the Yangtze estuary. It was found that, from 1958 to 2005, the area and sediment volume of JDS increased by 96% and 156% above the 5 m isobath and by 331% and 504% above the Lowest Astronomic Tide (LAT). Specifically, the maximum elevation of the island increased from 0.3 to 4.9 m above the LAT, and the salt marsh increased from 0% to 40% of the intertidal area. The dominant factor controlling the growth of this wetland was found to have varied with time. The growth was mainly due to the deposition of riverine sediment before the 1970s but was primarily the result of the merging of a shoal into JDS between 1971 and 1994 and, later, the local engineering projects between 1994 and 2005. It was concluded that human activities are playing an increasingly important role in the morphological evolution of tidal wetlands in the Yangtze estuary.

Seasonal changes in coastal dynamics and morphological behavior of the central and southern Changjiang River delta

Seasonal changes in sea level, tidal range, wind, riverine discharges, nearshore SSC (suspended sediment concentration) and bed-level of intertidal flat at 4 different sites were shown. In addition, the statistical relationships between the dynamics and the behavior of the sediment surface were examined. The average intertidal elevation seems negatively correlated to sea level while positively correlated to nearshore SSC. The effect of wind on seasonal cycle of average intertidal elevation is not evident although wind is an important factor governing short-term erosion/accretion events. The influence of riverine discharges on seasonal cycle of deltaic intertidal flats is masked by other factors. It is concluded that seasonality on mudflats is more complicated than on beaches.

Erosion potential of the Yangtze Delta under sediment starvation and climate change

Deltas are widely threatened by sediment starvation and climate change. Erosion potential is an important indicator of delta vulnerability. Here, we investigate the erosion potential of the Yangtze Delta. We found that over the past half century the Yangtze’s sediment discharge has decreased by 80% due to the construction of >50,000 dams and soil conservation, whereas the wind speed and wave height in the delta region have increased by 5–7%, and the sea level has risen at a rate of 3 mm/yr. According to hydrodynamic measurements and analyses of seabed sediments, the period when bed shear stress due to combined current-wave action under normal weather conditions exceeds the critical bed shear stress for erosion (τcr) accounts for 63% of the total observed period on average and can reach 100% during peak storms. This explains why net erosion has occurred in some areas of the subaqueous delta. We also found that the increase with depth of τcr is very gradual in the uppermost several metres of the depositional sequence. We therefore expect that the Yangtze subaqueous delta will experience continuous erosion under sediment starvation and climate change in the next decades of this century or even a few centuries.

Temporal Variations in Water and Sediment Discharge from the Changjiang (Yangtze River) and Downstream Sedimentary Responses

This chapter focuses on anthropogenic impacts on the water and sediment discharge of the Changjiang over the past 150 years and the sedimentary response of its delta and adjacent waters. Increased water consumption and dam construction have reduced the annual water discharge by ~10 % in the past 150 years. This decrease is mainly attributable to the reduction between August and November each year when the reservoirs store water. In contrast, the water discharge in January and February each year has shown a significant increasing trend, presumably due to water release from reservoirs in the dry season. Human impacts on the riverine sediment load are even more pronounced than impacts on river discharge. The annual sediment load has decreased by more than 70 % since the 1970s and by more than 50 % since the 1990s, due to dam construction and soil conservation in the drainage basin. The decline in sediment discharge has caused a drastic decrease in the progradation of intertidal wetlands on the delta front and has generated erosion on the subaqueous delta. Sediment resuspension from delta erosion partly offsets the decrease in riverine sediment supply, so that the suspended sediment concentration (SSC) in the estuary and adjacent waters only decreased by an average of 26 % over the period 1992–2009. It is predicted that these temporal trends will continue to occur in response to anthropogenic impacts for the remainder of this century.