| Morphological change in estuaries is an important factor of salinity intrusion.The hydrodynamics in estuaries changes with morphological change,which makes a difference to salinity intrusion.In this paper,a 3-D numerical model ’Ecom-si’ was used to simulate the influence caused by morphological changes in decades and millennia.The differences of hydrodynamics and salinity intrusion within different estuary regimes were compared and analyzed to reveal the effects caused by morphological changes.For the morphological changes in decades,charts of the Changjiang estuary in 1950s and 1970s were collected and digitalized.Together with the observational data in 2012,the effects of morphological changes in the three decades on hydrodynamics and salinity intrusion were simulated and analyzed.To quantify the morphological changes,the Changjiang estuary was divided into 12 regions and the area and volumeof each region were calculated.The analysis showed that,from 1950s to 1970s the volumes of upper reaches,middle reaches and lower reaches of the North Branch had changed by-64.13,-306.60 and-639.27× 106 m3 and the rates were-16.30%,-22.74%and-25.69%.From 1970s to 2012,they had changed by-199.06,-504.61 and-654.12×106 m3 and the rates were-60.45%,-48.44%and-35.38%.The South Branch remained stable after the formation of Xuliujing node.From 1950s to 1970s,the volumes of upper reaches,middle reaches and lower reaches of the South Branch had changed by-28.61,-35.69 and 126.43 ×106 m3 and the rates were-1,30%,-2.12%and 4.36%.From 1970s to 2012,they had changed by 92.34,193.01 and-163.62X 106 m3 and the rates were 4.24%,11.73%and-5.40%.The North Channel was affected by the siltation of Chongming Shoal and Hengsha Shoal.The volumes of upper reaches and lower reaches of the North Channel had changed by 109.21 and-797.14× 106 m3 and the rates were 5.01%and-15.25%from 1950s to 1970s.AfterⅣthat they had changed-154.64 and-511.79 × 106 m3 and the rates were-6.75%and-11.55%from 1970s to 2012 because of the reclamation of Qingcaosha Reservoir and Hengsha eastern tidal flat.From 1950s to 1970s the volumes of upper reaches、north lower reaches and south lower reaches of the South Channel had changed by-238.95,203.58 and 153.34×106 m3 and the rates were-8.96%,6.85%and 3.26%.The volume of upper reaches decreased because of the siltation and the volume of northern lower reaches had increased because the cutting off of Tongsha Shoal increased the depth of water.The volumes of upper reaches,north lower reaches and south lower reaches of the South Channel had changed by 136.39,-658.28 and-1266.11 ×106 m3 and the rates were 5.62%,-20.73%and-26.06%from 1970s to 2010.The volume of upper reaches had increased because the channel in the upper reaches became deeper and the volume of lower reaches had decreased because of the siltation and reclamation of Jiuduansha and Nanhui tidal flat.The results by the numerical model showed that,the change of tidal range in the three decades mainly occurred in the North Branch.From 1950s to 1970s then to 2012,thetidal range of the North Branch decreased first then increased.In 1950s,the residual water transported toward the upper reaches in the lower reaches of the North Branchand its value was low in the upper reaches of the North Branch,and it was greater in the North Channel than in the South Channel.In 1970s,the residual water transported toward the lower reaches in the upper reaches of the North Branch and its value was low in the lower reaches of the North Branch,and it was greater in the South Channel than in the North Channel.In 2012,the residual water transported toward the upper reaches in the North Branch,and its value was equivalent in the North Channel and the South Channel.The net water split ratios of the North Branch in three decades were 2.2%,6.8%,-2.5%during spring tide,and 3.0%,5.0%,0.6%during neap tide.The net water split ratio in 2012 during spr:ing tide was less than zero,which indicated the water spilled over from the North Branch to the South Branch.The net water split ratios of the North Channel in three decades were 60.8%,44.8%,55.2%during spring tide,and 61.2%,48.9%,56.0%during neap tide,which was agreed with the change of the main channel in the South Branch.The distribution of salinity showed that,in 1950s saltwater intrusion was severe in the North Branch and it decreased greatly in 1970s,and in 2012 it was very severe.In 1950s and 2012,saltwater spilled over from the North Branch to the South Branch and this phenomenon didn’t appear in 1970s.In the South Branch,fresh water occupied the most area in 1950s and 1970s,and in 2012 saltwater with salinity greater than 0.45 appeared in the upper reaches because saltwater spilled over from the North Branch.In the South and North Channel,saltwater intrusion in the 1950s was the most severe because the net water diversion ratio in the North Channel was higher than that in the South Channel by 21.6%,which caused the saltwater from the open sea intruded mainly through the South Channel,and returned over from the South Channel to the North Channel.In 1970s,the net water diversion ratio in the South Channel increased,causing the salinity in the South Channel decreased and the saltwater intrusion became weak in the South and NorthChannel.In 2012,the net water diversion ratio in the South Channel decreased again,resulting in the saltwater intrusion in the South Channel greater than that in the North Channel.For the morphological changes of in millennia,the coastlines and the bathymetries under former sea level were used to simulate the paleotidal and paleosalinity regimes of the Changjiang estuary 10000,9000 and 8000 years ago.The results showed that the amplitudes and phases of M2 tide were similar in the continental shelf and became different near the estuary in the three millennia because of the change of coastlines.There was an amphidxomic point near the south coastline 10000 and 9000 years ago,and the amphidromic point disappeared 8000 years ago because of the smaller scale of coastline.The amplitude of M2 tide varied along the estuary 10000 and 9000 years ago,and it did not change a lot 8000 year,because the Changjiang Estuary was single and widened that time.There was an amphidromic point in the Yellow Sea in each millennium and the locations of the amphidromic point in the three millennia were similar.10000 and 9000 years ago,there were two islands inside the estuary,north channel of ’outer island’ is deeper than south channel 10000 years ago,which caused the tidal current in north channel greater than that in south channel,and the flux mainly transported along north channel.The condition was reverse in 9000 years ago;8000 years ago,the Changjiang estuary was single and widened,so the tidal current flowed along the direction of coastline,and the flux transported divergently after getting out of the estuary.Salt water intruded mainly from north channel of ’outer island’ 10000 and 9000 years ago.10000 years ago,the north channel was deeper than south channel,so the runoff of Changjiang transported from north channel,and obstructed the intrusion of saltwater.But 9000 years ago,since the south channel was deeper than north channel,the runoff of Changjiang transported from south channel which caused the salinity intrusion was very severe in north channel and a great salinity front was formed,which caused salinity had a great difference along vertical.8000 years ago,the distribution of salinity was similar with the distribution of depth. |
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