Morphological Evolution At The Mouth Area Of The Yangtze Estuary, During The Recent Half A Century, And Its Development Tendency In The Future Decades

In recent decades, many deltas in the world have challenged by erosion result from decreasing sediment flux to the sea caused by human activities, such as riverine damming. The delta evolution has been one of the hottest issues in the coastal and estuarine research, because it greatly impact on delta ecological environment and social economy. Focus has been paid to the Yangtze subaqueous delta’s evolution, as the Yangtze River is one of the greatest rivers in the world. Recently due to the impacts of the Three Gorges Project and other high intensive human activities, the sediment flux from the Yangtze River towards sea has been noticeably decreased. Under this circumstance, it requires attention being paid on how the Yangtze subaqueous delta’s develops in the future. Although numerous studies have been carried out to study subaqueous delta geomorphic evolution problems after the reduction of sediment flux, there is no agreement yet because of data limitation and the differences of research methods. The influence of the Three Gorges Dam on estuarine delta is in a long time scale. It is necessary to add new data based on the previous studies to further understand the evolution of subaqueous delta. In the present study, bathymetric maps of the Yangtze subaqueous delta in recent 55 years (1958-2013), including the mouth area of the north branch and south branch of Yangtze River, was collected and digitalized. Using GIS software, rate of isobaths changes of the Yangtze River estuary isobaths and subaqueous accretion and erosion rate near the north branch and south branch entrance through were calculated. Moreover, we elaborate the causes of the change in the Yangtze subaqueous delta with the sediment load data from Datong, wind data from Sheshan, and engineering works in the estuary. Furthermore, we discuss the evolution trend of the subaqueous delta in the future combining with the reasons of accretion/erosion change. The main results are:1) Subaqueous delta’s isobaths changes have spatial and temporal differences. In periods of 1958-1978,1978-1986,1986-1997,1997-2004 and 2004-2013, the area growth rates of the four tidal flats of Yangtze estuary (Eastern Chongming, Eastern Hengsha, Jiuduansha and Eastern Nanhui) above 0 m isobaths (most isobaths are envelope structure) were 0.53,5.1,9.2,16 and 18 km2/yr respectively; the area growth rates of the 5 m isobaths (part of isobaths are envelope structure) are 13,4.8, 3.8,3.7 and 13 km2/yr, respectively; the progradational rates of 10 m isobaths (most isobaths were smooth and straight) outside the mouth were 165, -28,3.2,5.4 and -35 m/yr; respectively; and the progradational rates of 20 m isobaths outside the mouth were 179,28,-14 and -26 m/yr, respectively (lack of data in the period of 2004-2013). The evolution rates of different parts are significantly different in the same period. For instance, the four tidal flats area were -0.35 (Nanhui Coast),-0.37(Jiuduansha), 1.1(Hengsha Coast) and 4.8 km2/yr, respectively, in 1978-1986. The change of the average vertical accretion/erosion rate near the south branch entrance:in periods of 1958-1978,1978-1986,1986-1997,1997-2004 and 2004-2013, the average vertical accretion/erosion rate near the south branch entrance were 4.9, 1.9,0.51,3.7 and 0.17 cm/yr, respectively. The net accretion/erosion volume was 268, 105,28,198 and 9.0 Mm3/yr (Mm3=106 m3), respectively. The evolution near the mouth channel changed stronger than the subaqueous delta temporally and spatially. The evolution near the north branch entrance:in periods of 1959-1981,1981-1997 and 1997-2012, the average vertical accretion/erosion rate were-1.4,1.0 and -7.7 cm/yr, respectively, and the net accretion/erosion volume was -13,9.0 and -56 Mm3/yr respectively. Negative value represents erosion.2) Causes of evolution in study area. Although, the evolution of the study area is complicated in time and space, the total net accretion/erosion volume (the north branch area and the south branch entrance) clearly shows a decreasing speed in accretion rate, even shows transition from accretion to erosion on the whole:in periods of 1958-1978,1978-1986,1986-1997,1997-2004 and 2004-2013, the net accretion/erosion volume were 255,101,37,142 and -47 Mm3/yr (amounted to 332,131,48,185 and -61 Mt/yr) (Mt=106 t; 1 cm3=1.3 g), respectively. The wind condition of the Yangtze estuary hasn’t shown a significant trend in the past five peri- -ds, and the sediment load from Datong were 469,445,347,282 and 136 Mt/yr, respectively. So the decrease of sediment flux from the Yangtze River might cause the decline of accretion rate in the study area in recent years. Associated with decrease in sediment load from river, lots of estuarine engineering (even the ones promoting sedimentation and reclamation engineering) might lead to a increase of the accretion rate of the tidal flat above 0 m isobaths in the present study area. On the other hand, the lost sediment (the net accretion/erosion sediment removed from the sediment load from Datong) was 137,301,299,97 and 197 Mt/yr, respectively, from the Yangtze river in the five periods, and the estuarine engineering might cause the lowest lost sediment. So the Yangtze River sediment flux and the estuarine engineering were the two main annual control factors of the evolution of the present study area.3) The evolution trend of the Yangtze subaqueous delta front in the coming decades. The critical threshold of river sediment discharge that separated delta progradation from recession was 1.9×10 t/yr. The sediment flux from Datong was presently below this threshold. In view of the facts that the sediment flux would continue to fall with the comprehensive influence of the Three Gorges Dam project and Jinshajiang cascade reservoirs in the coming decades, as well as the sea level would rise further, sea wind speed level would be greater than or equal to the last few decades, the corrosion resistance of the seabed sediment (within the depth of several meters) changed a little, the Yangtze subaqueous delta front might continue to erode, unless the effective engineering measures were carried out. The conclusion had important reference value for the Yangtze river estuary tidal flats resources use, coastal protection, underwater facilities (such as oil and gas pipeline and cable protection).