SaltWater Intrusion in the Changjiang Estuary


The Changjiang River (CR) is divided into a southern branch (SB) and a northern branch (NB) by Chongming Island as the river enters the East China Sea.The CR is a typical tidal estuary characterized by abrupt bathymetry, islands, and deep channels. Chongming Island, located in the river mouth, divides the CR into southern and northern branches. The southern branch (hereafter referred to as SB) follows the mainstream of the CR discharge, is 10-20 km wide, and connects to the East China Sea through three deep passages (with a water depth of 20 m), while the northern branch (hereafter referred to as NB) is like a funnel with a width narrowing from 10 km in the downstream area to 2 km at the upstream entrance perpendicular to the SB.

Observations reveal that during the dry season the saltwater in the inner shelf of the East China Sea flows into the CR through the NB and forms an isolated mass of saltwater in the upstream area of the SB. An example can be seen in the salinity measurements averaged over tidal cycles at anchor sites and recorded from in situ shipboard surveys on an along-river transect in the SB during spring and neap tidal cycles on 13-22 February 2004. A tidally averaged salinity of 2 was observed at Chongtou (121.2E, 31.75N) around the northwestern headland of Chongming Island, forming a saltwater bore in the SB.

This salting tendency of the CR water has received intense attention in both economic and scientific aspects. The CR is a key freshwater source for Shanghai (the largest city in China) and surrounding regions. Chenhang Reservoir, which connects to the SB of the CR and functions as the second largest freshwater supply for Shanghai, has had difficulty in providing high-quality freshwater during the dry season as a result of the recent salting of the CR water.

What are the physical processes controlling the water transport in the northern branch?

  • Freshwater discharge in the upstream
  • Tidal motion
  • Wind

Which one is the key process? or other processes?

Model study: FVCOM (Finite Volume Coastal Ocean Model)
High resolution Changjiang Estuary FVCOM was nested to East China Sea FVCOM:

The saltwater intrusion into the CR depends critically on the net water transport in the NB. Accurate estimation of the transport requires a model to resolve the spatial/temporal structure of the water currents and water level over complex irregular coastal geometry and abrupt-varying bathymetry inside both NB and SB as well as around the river mouth. In addition to the need for a high resolution model approach, we also require a better fitting of the coastal geometry of the CR with inclusion of Chongming Island. Failure to adequately resolve these geometric features can significantly underestimate the nonlinear interactions of physical forcings in this system, and thus makes it difficult to estimate the relative contribution of various physical processes to the saltwater intrusion into the CR.

The high-resolution FVCOM-CR was developed by configuring the updated code of FVCOM to the CR with inclusion of the wet/dry treatment to intertidal zones. The computational domain of this model covers the CR, Hangzhou Bay, Zhoushan Island Complex, and is bounded by an open boundary in the inner shelf of the East China Sea. The horizontal resolution varies from 0.1 to 0.5 km inside the CR and over the Zhoushan Island Complex to 1.0–3.0 km in Hangzhou Bay and 10.0 km in the inner shelf closest to the open boundary. The vertical resolution is determined by 10 uniform sigma layers, which is 0.1 m or less over the intertidal zone and 2.0 m in the inner shelf. The FVCOM-CR is driven by tidal elevation at the open boundary through nesting to FVCOM-ECS (with inclusion of four major tidal constituents: M2, S2, K1, and O1), freshwater transport at the upstream end of the CR, and given winds that characterize the dry and wet seasons. The external and internal mode time steps were 1.0 and 10.0 s, which took about 15 min on a 16-node Linux cluster to integrate for a M2 tidal cycle.

Simulated Saltwater Intrusion:

The saltwater intrusion often occurs in winter after the transport adjustment from summer. In this process-oriented study, after the freshwater discharge rate decreases from 60,000 m3/s to 10,000 m3/s, saltwater on the shelf is gradually pushed toward the CR . On the 15th model day, saltwater on the northern shelf off the CR enters the NB. This saltwater is advected northward in the NB and enters the SB along the northern tip of Chongming Island after 60 days. Two points are learned. First, the saltwater intrusion from the shelf into the NB can occur in the dry season owing to a significant reduction of the river discharge. Second, the time scale of the salinity adjustment to produce the saltwater intrusion from the NB to the SB after the wet season is about 1–2 months.

As a result of the monsoon climate in the East China Sea, northerly winds prevail in winter along the shelf off the CR. The northerly wind pushes the ocean water southward on the shelf but has little direct influence on the water movement inside the NB and SB. Blown by a northerly wind of 5 m/s (a southward wind stress of 0.036 N/m2), more saltwater is advected into the NB as the oceanic water on the northern coast off the CR is pushed southward. After 60 days, the entire NB is filled with relatively high-salinity water and the salinity concentration pumping into SB from the northern tip of Chongming Island could reach 4.24, significantly higher compared with a value of 1.97 found in the case without wind. An unusually high salinity level of >3.5 was observed in SB in the 2004 hydrographic survey [He et al., 2006], which was believed to be evidence of the effects of relatively strong northerly wind-forcing.

Simulation of particle tracking:

The saltwater intrusion is also evident in the trajectories of Lagrangian particles. In the wet season, simulated particles released in the upper part of the mainstream of the CR show a dominant group flowing through the SB and entering the shelf. Although only one particle moves into the NB, it is symbolic of the outflow tendency of water movement in that branch during the wet season. This is supported by the trajectories of particles released near the mouth of the NB: all of them quickly leave the NB and move onto the shelf. In the dry season, however, the particles released at the same upstream location of the CR all enter the SB and move toward the ocean, with several flowing into the NB by following the cyclonic flow around the southern end of Chongming Island. Particles released near the mouth of the NB show a main group flowing upstream and then entering the SB from the northern tip of Chongming Island, although three of those particles move onto the shelf. The trajectories of particles released in the dry season support our salinity simulation results and also demonstrate that the inflow to the NB can come from either the cyclonic return flow around the southern end of Chongming Island or saltwater inflow from the shelf.

Model's momentum balance:

To examine the physical mechanisms governing the saltwater intrusion, we looked at the momentum balance under various forcing conditions and then studied the related transport process. The analysis was done on the NB cross-branch section, where each term was directly computed using the FVCOM-CR time series output and averaged over a tidal cycle and in a cross-branch direction.We focused here on the along-branch component of the momentum balance since the NB is a narrow channel in which the along-branch Coriolis force term is dynamically negligible and the other terms in this direction control the along-branch volume flux. Four cases are discussed here: (1) with onlyM2 tidal forcing; (2) withM2 tidal plus river discharge for the wet season; (3) withM2 tidal plus river discharge for the dry season; and (4) with M2 tidal plus river discharge for the dry season and a northerly wind of 5 m/s.

The take home message: the intrusion results from a complex nonlinear interaction process of tidal currents, surface elevation gradient forcing, and vertical diffusion. The tidal rectification, resulting from the nonlinear interaction of tidal currents and bottom friction over abrupt topography, produces a net upstreamward volume flux from the NB to the SB. The saltwater intrusion occurs when the freshwater flow into the NB is smaller than the tidal rectified flow. In the wet season, the surface elevation gradient forcing is sufficiently large to maintain the transport into the NB. This inflow is driven through an along-branch momentum balance between the surface elevation gradient forcing, nonlinear current advections and bottom friction. In the dry season, reducing the surface elevation gradient forcing makes tidal rectification becomes a key process favorable for the saltwater intrusion. The fact that the model-predicted net flux of the saltwater intrusion is smaller than the sum of the freshwater discharge and tidal rectified transports in the NB suggests that the resulting surface elevation gradient forcing, tidal rectified flow and vertical diffusion tends to reduce an upstreamward flux. A northerly wind tends to enhance the saltwater intrusion by reducing the seaward surface elevation gradient forcing rather than either by the baroclinic pressure gradient forcing or local wind-driven Ekman transport.

To see the completed analysis of this research, click here