Numerical Simulation Of The Stagnation Points In The Changjiang Estuary

In this paper, an improved3D numerical model ECOM-si, which can simulate the interaction of runoff, wind stress, tide, baroclinic gradient force and turbulence, is employed to explore the stagnation points in the Changjiang Estuary. The model validated the water level, current, salinity and the water transport, with the in situ data in the Changjiang Estuary. The result indicates that this numerical model could simulate the temporal and spatial variations of the currents, salinity and the water transport well in both dry and flood seasons. The stagnation points in the North Channel, the North Passage (NP) and the South Passage (SP) around the sand bars are simulated. Furthermore, the impacts of the Deep Waterway Project (DWP) and the variations of the runoff and wind speed on the stagnation points are analyzed in both dry and flood seasons. The main conclusions are listed as follow:1) Under the runoff (11000m3/s) and wind condition (northerly wind of5m/s) in the dry season, during the spring tide, the stagnation point is not present in the reaches of the North Channel around the sand bars (NCS), and the near bottom water converges at the middle and lower reaches, which has the same effect as the water convergence near the stagnation point. The stagnation point is not present in the NP, whereas the near bottom water converges at the middle reaches. The stagnation point in the SP is present in the upper reaches, where the near bottom water convergence is also detected. During the neap tide, the stagnation point in the NCS is present at the upper reaches, and massive near bottom water converges at the middle reaches. The stagnation point in the NP is present at the upper reaches, and there are still massive near bottom water converges at the middle reaches. The stagnation point is not present in the SP, and massive near bottom water converges at the upper reaches.Under the runoff (45000m3/s) and wind condition (southerly wind of4m/s) in the flood season, during the spring season, the stagnation point is not present in the NCS, and those in the NP and the SP are both present at the lower reaches. The near bottom water convergence in the NCS, the NP and the SP are relatively greater at the middle and lower reaches, the middle reaches and the upper reaches, respectively. During the neap tide, the stagnation points in the NCS, the NP and the SP are present at the lower reaches, the middle reaches and the lower reaches, respectively. The near bottom water convergence is relatively greater at the same reaches as that during the spring tide, and the convergence in the NCS and the NP during the neap tide is greater than that during the spring tide.2) The net seaward water transport and the salinity fronts at the river mouth vary with the change of the runoff, causing the variation of the stagnation points’ positions. The stagnation point is not present in the NCS during the spring tide as the runoff varies. As the runoff increased (decreased) by3000m3/s in the dry season, the stagnation point in the NCS moves about10km downstream (about2km upstream) during the neap tide. The stagnation point in the NP moves about16km downstream during the neap tide after the runoff increased, and it in the SP moves downstream (upstream) slightly during the spring tide as the runoff increased (decreased). As the runoff increased by10000m/s in the flood season, during the spring tide, the stagnation point is not present in the NP, and it in the SP moves4km downstream. During the neap tide, the stagnation point in the NP (the SP) moves about5km (about4km) downstream. As the runoff decreased by10000m3/s in the flood season, the stagnation point in the NP (the SP) moves about7km (about10km) upstream during the spring tide, and about3km (about17km) upstream during the neap tide. The near bottom water convergence in the three channel increases (decreases) after the runoff increased (decreased) in both dry and flood seasons.3) The variation of the wind speed impacts the stagnation points in different ways in different seasons. As the northerly wind enhanced in the dry season, the horizontal wind-driven circulation and the landward Ekman transport in the Changjiang Estuary are enhanced, and the stagnation point is not present in the NCS. The stagnation point in the NP is present at the lower reaches, and that in the SP moves about2km downstream during the spring tide. The stagnation point in the NP moves about8km downstream during the neap tide. In the flood season, as the southerly wind intesified, the saltwater intrusion from the south of the Changjiang Estuary is enhanced, and the seaward net unit width residual water fluxes in the three channels are decreased. The stagnation point is not present in the NCS, and those in the NP and the SP move about10km and10km upstream during the spring tide, respectively, and those in the three channels moves about6km,5km and15km upstream during the neap tide, respectively.4) The DWP impedes the southward water transport caused by the northerly wind in the dry season, and the northward water transport caused by the southerly wind in the flood season. It restricts the seaward water transport in the South Channel and the overflow during the spring tide, while enhances the seaward water transport in the NP. After the DWP, the stagnation point in the NCS moves slightly, and it in the SP moves about21km upstream during the spring tide in the dry season. After the DWP in the flood season, the stagnation points in the NP (the SP) moves about12km downstream (about6km upstream) during the spring tide, and about6km downstream (about6km upstream) during the neap tide. The near bottom water convergence in the NCS and the NP increases after the DWP.