Analysis And 3D Modelling Of Tidal Circulations Within The North Passage Of The Changjiang River Estuary

The curved Deepwater Navigational Channel of the North Passage of the Changjiang River estuary, which encompasses channel units H-R, experiences highest siltation. To gain insight into the mechanisms for causing siltation within the curved channel, a 300 kHz ADCP was used to measure: (i) depth-averaged flow velocities at maximum ebb and flood tides at five north lines (LN-4, LN-3, LN2, LN^-1, and LN-0) and five south lines (LS^-1, LS-2, LS-3, LS-4, and LS-5); (ii) time series of current speed/direction over a flood-ebb tidal cycle at three hydrological gauging stations CS6, CSW and CS3'on spring tide in the flood season, on 13 August 2010. Analyses are made of tide-induced horizontal, longitudinal, and transverse tidal circulations within the curved Deepwater Navigational Channel, and their possible effects on erosion and siltation there. To examine the three-dimensional flow structure within the North Passage of the Changjiang River estuary, an improved three-dimensional hydrodynamic mathematical model, COHERENS, with orthogonal curvilinear coordinates (Lu 2008), is used to simulate tidal circulations.Depth-averaged tidal flow velocity data measured along the north and south sides of the curved Deepwater Navigational Channel show: (i) Current speeds range from 1.49 to 2.56 ms^-1 at maximum ebb tide, and from 1.02 to 1.92 ms^-1 at maximum flood tide. Current directions are focused on the particular range of 88°¹29°at maximum ebb tide, and of 268°³22°at maximum flood tide. (ii) At the landwards end of the curved Channel, current speeds measured at the north sections LN-4 (1.80 ms^-1), LN-3 (2.07 ms^-1), and LN-2 (2.20 ms^-1) are larger than those at the south sections LS^-1 (1.73 ms^-1), LS-2 (2.01 ms^-1), and LS-3 (2.06 ms^-1) at maximum ebb tide. At the seawards end of the curved Deepwater Navigational Channel, current speeds measured at the north sections LN^-1 (2.05 ms^-1) and LN-0 (1.82 ms^-1) are smaller than those at the south sections LS-4 (2.16 ms^-1) and LS-5 (2.24 ms^-1) at maximum ebb tide. This indicates the possible presence of shear flows between the north and south sides of the curved Channel. Both"Clockwise Vorticity (CV)"and"Anti-clockwise Vorticity (AV)"are formed, respectively. Thus the horizontal clockwise and anti-clockwise tidal circulations might occur at maximum ebb tide. (iii) At the landwards end of the curved Deepwater Navigational Channel, current speeds measured at the north sections LN-4 (1.30 ms^-1) and LN-3 (1.63 ms^-1) are smaller than those at the south sections LS^-1 (1.59 ms^-1) and LS-2 (1.71 ms^-1) at maximum flood tide. At the seawards end of the curved Deepwater Navigational Channel, current speeds measured at the north sections LN-2 (1.51 ms^-1) and LN^-1 (1.55 ms^-1) are larger than those at the south sections LS-3 (1.42 ms^-1) and LS-4 (1.52 ms^-1) at maximum flood tide. This again indicates possible shear flows could exist, with both"Clockwise Vorticity (CV)"and"Anti-clockwise Vorticity (AV)", and thus horizontal clockwise and anti-clockwise tidal circulations might be formed at maximum flood tide. (iv) A"Low Velocity Zone"apparently exists at the lower curved Deepwater Navigational Channel at maximum flood tide.Time series of tidal flow velocity data measured vertically at six layers at three hydrological gauging stations CS6, CSW, and CS3'within the curved Deepwater Navigational Channel show: (i) Tidal ellipses become much more rotational from the landwards end to the seawards end of the curved Deepwater Navigational Channel, and from bottom to surface. (ii) Current speeds and the turning time of tidal flow between flood and ebb tides differ for each of the six layers. (iii) During the turning of tidal flow from flood to ebb, rotational directions of tidal ellipses in the upper layer and the lower layer at stations are different, e.g., anticlockwise in the upper layer and clockwise in the lower layer at hydrological gauging station CS6, while clockwise in the upper layer and anticlockwise in the lower layer at hydrological gauging stations CSW and CS3'. This may suggest a pycnocline/stratification between the upper and lower layers. (iv) During the turning of tidal flow from ebb to flood, rotational directions of tidal ellipses at stations are different, i.e., anticlockwise in all layers at hydrological gauging CS6, clockwise in all layers at hydrological gauging station CS3', anticlockwise in the upper layer and clockwise in the lower layer at hydrological gauging station CSW. This may also suggest a pycnocline/stratification between the upper and lower layers at hydrological gauging station CSW. (v) Longitudinal and transverse (or secondary) tidal circulations apparently exist during the turning of tidal flow.Horizontal distributions of modeled tidal flow velocity vectors within the North Passage of the Changjiang River estuary show: (i) Current speeds reach a maximum value of 2.5 ms^-1 at maximum ebb tide and of about 2.0 ms^-1 at maximum flood tide. (ii) Current directions within the main channel of the North Passage are generally along the central axis of the Deepwater Navigational Channel at maximum ebb tide. (iii) Tidal currents at the seawards end of the curved Deepwater Navigational Channel cross Jiuduansha shoal in a northerly direction and then flow northeastwards into the main channel at maximum ebb/flood tide. However, there are flows at the head of curved Deepwater Navigational Channel transversely crossing the south jetty into the North Passage at maximum flood tide.Modeled tidal flow velocity vectors along three Transects Tl, Tm and Tr corresponding to each of three hydrological gauging stations CS6, CSW and CS3'show: (i) They all exhibit transverse velocity components (<0.3 ms^-1). (ii) Transverse tidal flow velocity components within the south intertidal zones of Transects Tl and Tr are northerly towards the main channel at maximum ebb tide, but not at Transect Tm. (iii) Transverse tidal circulations (the surface tidal currents flow towards the south while the bottom tidal currents flow towards the north) may exist at Transect Tl at maximum flood tide. (iv) Transverse tidal flow velocity components at Transects Tm and Tr are southwards towards the main channel at maximum flood tide.Future studies, together with additional salinity and suspended sediment concentration data, should further evaluate the implications of these findings for causing the areas of highest siltation in the units H-R within the curved Deepwater Navigational Channel.