| Sediment transport is the key scientific problem in estuarine dynamics. Study on the sediment transport in estuary can enrich the estuarine science, as well as help to administrate ports & navigation channels, ecological environment, fishery resources and so on. In recent years, with more and more human activities exerting on estuary, the sediment transport in estuary has been impacted greatly. Different estuaries are impacted by different human activities in various ways and levels. As we know, the dams in the drainage of the Changjiang river (especially the Three Gorges Dam) have been intercepting a large amount of sediment from upper reaches, leading to a sharp reduction of river sediment discharged into the estuary, which has caused the change of sediment transport in the estuary, resulting in the erosion of wetlands and siltation of navigation channels. In addition, the Chesapeake Bay in United States has been excavated with large navigation channels throughout the whole Bay. The excavation of navigation channels has greatly changed the boundary conditions of the bed form, affecting the sediment transport in the Bay and causing the siltation of local channels. Based on field observations, utilizing spatial information technology and numerical model, the dissertation has investigated the sediment transport and navigation channel siltation in the South Channel of Changjiang estuary in response to the reduction of river sediment due to dams, as well as the sediment transport and navigation channel siltation in upper Chesapeake Bay in response to the bathymetric variations due to the excavation, also revealed the different responding mechanisms of sediment transport in different estuaries to major human activities and discussed the difference between the resulting navigation channel siltation. The main conclusions are as follows.There are both similarities and differences between the natural conditions of the Changjiang estuary and the Chesapeake Bay. In terms of estuary type, the Changjiang estuary is classified as the typical branched delta estuary, while the Chesapeake Bay is the typical drowned valley estuary. The Changjiang discharges abundant water and sediment, with annual mean water and sediment discharge are 8.91×1011 m3 and 3.84×108t, respectively. While the Chesapeake Bay rivers discharge less water and sediment, with annual mean water and sediment discharge are 5.83×1010 m3 and 4.67×106 t, respectively. M2 tidal constituent is the main tide both in the Changjiang estuary and Chesapeake Bay, with semidiurnal cycle. The tide intensity is moderate in the Changjiang estuary and weak in the Chesapeake Bay, the tidal current is reversing both in the two estuaries, varies with flood-dry seasons, spring-neap and flood-ebb tides. The mixing of saline water and fresh water in the Changjiang estuary includes mainly partially mixed type and evenly mixed type, as well as the upper Chesapeake Bay. The turbidity maximum is formed both in the two estuaries, but the suspended sediment concentration (SSC) in the Changjiang estuary is very high, while the SSC in the Chesapeake Bay is much low. The distribution of bedload in the Changjiang estuary varies along the channel, while the distribution of bedload in the Chesapeake Bay is mainly associated with the bathymetry.The human activities in the Changjiang drainage such as dam construction have caused reduction of river sediment, especially the interception of the Three Gorges Dam (TGD) in 2003 has resulted in a reduction of 60%, and the sediment transport in the South Channel of Changjiang estuary has been responding to this with spatial and temporal variability. The SSC in upper South Channel showed little change from 2002 to 2009, since the SSC has been compensated by bed sediment resuspension during that time, with the coarsening of bed sediment, the sediment carrying capacity of flow decreased, till around 2009 the SSC began decrease obviously, showing the response of the SSC to the reduction of river sediment is of hysteretic. While the SSC in lower South Channel just has decreased slightly and showed no obvious response to the reduction of river sediment as of 2011, since the lower South Channel is just located at the upper end of the turbidity maximum, the SSC is being compensated with both the bed sediment resuspension and the ocean sediment supply. Sediment resuspension contributed more than half of the SSC during maximum tidal currents with the mean resuspension rate is 57.6%. Compared with that before the interception of the TGD, the resuspension rate in the South Channel increased in recent years, the bulk settling velocity increased, the suspended sediment and bedload coarsened, fine fraction of the sediment decreased. At the same time, the bedload transport was intensified and the velocity of sand wave was increased, which has caused the enlargement of sand wave region in the South Channel, extending from the upper part to the lower part.The sediment transport particularly the bedload transport in the South Channel has caused erosion and accretion change of the bed, as well as the navigation channel. Generally, the upper South Channel has been eroding while the lower South Channel has been accreting slightly in recent years. The middle part of the Ruifeng Shoal has been eroding, till 2006 the Ruifeng Shoal was divided into two separated parts, since then the upper part has been retreating and the lower part has been shrinking. The erosion of the upper navigation channel in the South Channel made the bedload move downstream, resulting in the increase of the siltation volume in the lower navigation channel. It is shown that the sediment transport and bed erosion-accretion are still in adjustment to the reduction of river sediment supply and the interaction between river and sea.The channel excavation of the upper Chesapeake Bay has leaded to the change of the width of the channel. In addition, the channel was deepened from 8.2 m to 13 m in recent years; the resulting change of bed morphology has influenced the salt intrusion and sediment transport greatly. Under the condition of low-to-moderate discharge, the location of the turbidity maximum can be controlled by the bathymetry, forming an important mechanism of sediment trapping. The bathymetry can influence the salt intrusion evidently under the same discharge. The salt intrusion has been enhanced obviously after the channel was deepened. Under the low-to-moderate discharge of the Susquehanna river, the change of channel width due to the excavation has altered the boundary of the channel and hence the hydrodynamic condition of the sediment transport, so there has always been an along-channel maximum salinity gradient and a turbidity maximum near bottom at the same time, and they are always located at km 30～36 from the head of the Chesapeake Bay where the channel is wide relatively.The change of the bed morphology due to the excavation in upper Chesapeake Bay has controlled the sediment transport, consequently the erosion and siltation of the navigation channel has been influenced. Generally, the maximum siltation region has always been consistent with the turbidity maximum and the erosion region has always agreed with the region where the channel is narrow relatively. Consequently the maximum siltation region is always located at km 30～36 where the channel is wide relatively, which is being dredged regularly to keep the depth for shipping.At present, the sediment transport and channel siltation in the South Channel of Changjiang estuary is mainly influenced by the reduction of river sediment, so the SSC decreased, the bedload transport intensified, and leading to the channel siltation with bedload. While the sediment transport and channel siltation in upper Chesapeake Bay is mainly affected by the excavation, the location of the turbidity maximum is always controlled, resulting to the channel siltation with suspended sediment. To sum up, the sediment transport and channel siltation in different estuaries in response to human activities are always discrepant. |
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