Ao Chu

Has Suspended Sediment Concentration Near the Mouth Bar of the Yangtze (Changjiang) Estuary Been Declining in Recent Years

ABSTRACT Dai, Z.-J.; Chu, A.; Li, W.-H.; Li, J.-F., and Wu, H.L., 2013. Has suspended sediment concentration near the mouth bar of the Yangtze (Changjiang) Estuary been declining in recent years? There are considerable concerns about the decrease in suspended sediment discharge (SSD) into the large estuaries of the world as a result of extensive anthropogenic activities in their catchment areas. With the operation of Three Gorges Dam (TGD) in 2003, the riverine loads into the Yangtze (Changjiang) Estuary have been greatly changed with the sharp decrease of SSD and suspended sediment concentration (SSC). However, according to our analysis on the SSC in the surfacial water measured at different stations in the Yangtze Estuary, we conclude that the spatial characteristics of the annual mean SSC around the mouth bar area show no apparent change yet, even though the TGD was constructed with an ascending trend at the upper part of the estuary. The spring–neap periodicity of the daily mean SSC after the TGD was constructed remained the same as before. Moreover, the seasonal and annual mean SSC at the inner side of the mouth bar was relatively low due to the large reduction of upstream sediment supply after the operation of TGD began in 2003. But the seasonal and yearly mean SSC at the outer side of the mouth bar during 2007–2009 is comparable with those before the TGD operated, even though there is a decreasing trend of SSC into the Yangtze Estuary in corresponding years.

Is the Three Gorges Dam the cause behind the extremely low suspended sediment discharge into the Yangtze (Changjiang) Estuary of 2006

Abstract In 2006, the suspended sediment discharge (SSD) into the Yangtze (Changjiang) Estuary, China, reached the historical low value of 85 × 106 t. One hypothesis is that this was caused by the second impoundment, i.e. the second stage of the water-level increase behind the Three Gorges Dam (TGD). However, coincidentally, a significant drought occurred in the same year. From our analysis of long-term data on discharge and SSD, we conclude that the SSD decrease in the upstream catchment area resulting from the extreme drought is primarily responsible for the historical low SSD into the Yangtze Estuary. We quantified the contributions of the extreme drought and the second impoundment to the reduction of SSD into the Yangtze Estuary in 2006 as 82% and 18%, respectively. Even though the TGD is the largest dam in the world, the results indicate that the extreme drought conditions had a greater impact than such a manmade river regulation. Citation Dai, Z. J., Chu, A., Stive, M, Du, J. Z. & Li, J. F. (2011) Is the Three Gorges Dam the cause behind the extremely low suspended sediment discharge into the Yangtze (Changjiang) Estuary of 2006? Hydrol. Sci. J. 56(7), 1280–1288.