The Responses Of Saltwater Intrusion To Climate Change And Major Projects In The Changjiang River Estuary

The estuary, which is known as a complex zone of land-ocean interaction, is affected by both river basin and ocean. The influences of climate change and major projects on hydrodynamic and biological environments and saltwater intrusion in the estuary have been greatly concerned by government and society. The impacts of climate change on the estuary are mainly reflected in the variations of river discharge that is caused by precipitation and sea-level rise. And the impacts of major projects are reflected in the change of river discharge altered by engineering such as the Three Gorges Project and the South-to-North Water Diversion Project, and the alteration of local river regime. Based on the improved3-D numerical model ECOM-si, the hydrodynamic process and saltwater intrusion in the Changjiang River estuary were simulated and quantitative analyzed in2012as the standard year. And then the situations of saltwater intrusion and water resources in the estuary were predicted under the interactions of climate change and major projects in2030,2050and2100. By maintaining one of the factors, the effects of climate change and major projects on saltwater intrusion were removed quantitatively. Besides, the drought periods like winter in1999, autumn in2006and spring in2011were simulated and analyzed, and the combined effects between lower river discharge and sea-level rise on saltwater intrusion and freshwater resources were discussed. The main conclusions are summarized as follows:1. To attain the boundary conditions affected by climate change and major projects. It is necessary to confirm the sea level and river discharge in different years as boundary conditions for studying the responses of saltwater intrusion to climate change and major projects. The sea-level rise includes the absolute sea-level rise caused by climate change, and the land and crustal subsidences (the relative sea-level rise). And the variation of river discharge that we consider is mainly affected by precipitation caused by climate change, and the Three Gorges Project and the South-to-North Water Diversion Project. In accordance with the contrast between historical analysis and results, the river discharge in2012,2030,2050and2100are attained respectively with the value of13755,13057,12792and13631m3/s, and the sea-level rise in each year relative to that in2012are112.1,281.1and703.6mm respectively. 2. The responses of saltwater intrusion to climate change and major projects in dry season in the Changjiang River estuary were simulated and analyzed. By analyzing the salt flux and its decomposed terms, the dynamic mechanisms of salt transport processes during spring and neap tides were studied. The results showed that the Stokes transport is the main dynamic factor for up-estuary salt transport in the Changjiang River estuary; meanwhile it is an important reason for the saltwater-spill-over (SSO) from the North Branch into the South Branch. Near the river mouth, salt transports upstream along main channel in the North Channel, while it goes downstream on tidal flats during spring tide. However, during neap tide, the directions of salt transport in the channel and tidal flats are contrary due to the strengthened vertical shear. In the South Passage, the Stokes transport is the major mechanism for up-estuary salt transport during spring tide, while during neap tide the salt transports upstream along north side and downstream along south side under Coriolis force, in the meantime, saltwater intrudes upstream continuously under tidal pumping and vertical shear transports near the upper reaches. The salt exchange is distinct between the South and North Passages. Saltwater that transports upstream along the South Channel turns downstream in the North Passage, resulting in the seaward salt Eulerian transport. By the result of mechanism analysis, freshwater resource in the Changjiang River estuary is threatened by the SSO and up-estuary migrated seawater during spring tide, while seawater is the main reason during neap tide. It is most serious in2100as the highest sea level, while in2012the intrusion is comparatively weaker for larger river discharge. During the whole February, the freshwater resources in2030,2050and2100relative to that in2012decrease by18.64%,11.39%and33.43%, respectively.3. The effect of each factor affected by climate change and major projects on saltwater intrusion in the Changjiang River estuary was analyzed and removed quantitatively. The results show that saltwater intrusion can be weakened as the improved tendency of river discharge caused by climate change in the future. Because it increases the freshwater that discharges into the North Branch, the SSO can be suppressed. Near the river mouth, the up-estuary Stokes transport decreases under increased river discharge during spring tide, indicating the decreased salinity. During neap tide, the salinity gradient between surface and bottom layers in the North Channel are enlarged as the increased river discharge, promoting the strengthened vertical shear. While the Eulerian transport that transports salt along the north side in the South Passage is weakened, resulting in the reduction of saltwater intrusion, and thereby reduces salt transport in the North Passage. During the whole February, the freshwater resources in2030,2050and2100increase by5.46%,30.31%and27.71%respectively as the increased river discharge caused by climate change.The effect of sea-level rise includes the larger tidal amplitude, which can improve tidal strength, and the deeper channel, which means the larger discharge area. Meanwhile, the vertical flow exchange can be strengthened as sea-level rise, and furthermore enhances the vertical stratification.In the North Branch, the SSO is aggravated by the enhanced tidal strength in2030and2100, while in2050it is weakened as the increased freshwater discharging into the channel with a larger discharge area. Near the river mouth, the larger Stokes transport under enhanced tidal strength is the main reason for stronger saltwater intrusion during spring tide. However, due to the deeper topography and weaker tidal strength during neap tide, the vertical shear transports more salt into the estuary in the North Channel, and the Eulerian transport increases along the north side in the South Passage, indicating that saltwater intrusion becomes more serious. During the whole February, the freshwater resources in2030and2100decrease by8.95%and49.45%respectively as sea-level rise, while in2050it increases by4.41%due to the weaker SSO.The Three Gorges Reservoir that releases freshwater persistently in dry season can distinctly increase downstream river discharge, while the South-to-North Water Diversion Project diverts water resources to the north, resulting in the decrease of river discharge. The increased river discharge after the Three Gorges Project suppresses the up-estuary Stokes transport obviously during spring tide, and salinity decreases near the river mouth. The mechanisms in2030and2100are similar, while by the nonlinear effect as sea-level rise in2050, the value of reduced Stokes transport is less in the South Passage, and then the total up-estuary Lagrangian transport increases slightly. The salt transport on the Jiuduan Sandbank is weakened, decreasing salinity in the South Passage. During neap tide, more salt transports into the estuary by strengthened vertical shear, especially in2050that river discharge is smaller. During the whole February, the freshwater resources in2030,2050and2100increase by30.26%,45.46%and32.38%respectively as the influence of the Three Gorges Project. The South-to-North Water Diversion Project decreases river discharge, especially in dry season, and salinity increases obviously in the estuary. During spring tide, the circulation between up-estuary Stokes transport and seaward Eulerian transport enhances. During neap tide, salinity front moves upstream in the North Channel, resulting in a weaker vertical shear. In the South Passage, up-estuary Eulerian transport becomes larger as decreased river discharge, meanwhile the baroclinic force is strengthened in the upper reaches, indicating that saltwater migrates upstream into the South Channel, and the salt transports more to the sea in the North Passage. During the whole February, the freshwater resources in2030,2050and2100decrease by19.21%,42.79%and34.41%respectively as the South-to-North Water Diversion Project.According to the correlation analysis between each factor of climate change and major projects and freshwater resources and the SSO in the Changjiang River estuary, the Three Gorges Project and the South-to-North Water Diversion Project are the main reasons for saltwater intrusion in2030and2050while sea-level rise is the most important factor in2100, and the effects of major projects decrease correspondingly.4. The influences of saltwater intrusion and freshwater resources in drought periods, such as winter in1999, autumn in2006and spring in2011, were simulated and analyzed, and the combined effects together with sea-level rise were discussed. The results showed that compared to that in2012, saltwater intrusion is more serious in1999, and the hazard for Dongfengxisha, Chenhang and Qingcaosha reservoirs sustains a longer time in2006and2011. Under the unfavorable situation that sea level rises, the threat of water intake in all the reservoirs can be extended by saltwater intrusion. Except for the weakened SSO in2006as the increased discharge area by sea-level rise under2050level, salinity decreases slightly.