Vertical structure of tidal currents in the Xuliujing Section of Changjiang River Estuary
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Abstract: Three long-term fixed acoustic Doppler current profilers were first used for investigating the vertical structure of tidal currents in Xuliujing Section of Changjiang River Estuary. Moreover, three different periods (spring, summer and fall) were also considered for investigating seasonal variations. The semi-diurnal tides were the most energetic, with along-channel speed of up to 80 cm/s for M2 constituent, which dominates at all stations with percent energy up to 65%–75% during seasons. The shape of tidal ellipses of the most energetic semi-diurnal constituent M2 showed obvious polarization of the flow paralleling to the riverbank, with the minor semi-axis being generally less than 20% of the major one. The maximum velocity of mean current is appeared in top layers at all the three stations, and the velocity decreased with the depth. The seasonal variations of direction are also observed, which is probably caused by complex local topography since the erosion and deposition in riverbed. Observed vertical variation of four parameters of M2 ellipses, agreed well with the optimally fit frictional solutions in top and middle layers. However, there was an obvious difference between frictional model and observed data in the lower water column. Discrepancies are probably on account of stratification, which strengthens in summer and fall due to the freshening influence of the Changjiang River Estuary outflow.
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Figure 1. Area of study. Red circles indicate the deploy location of the acoustic Doppler current profilers, and the dashed blue line represents Xuliujing Section.
Figure 2. Time series of the discharges at Datong Station (thumbnail image in Fig. 1) in 2011. Datong is the last permanent hydrometric station with long discharge records on the main Changjiang River, and the discharge is difficult to observe owing to the tidal influence. So the discharge in Datong is commonly regarded as the net discharge into the East China Sea.
Figure 3. Shapes of Xuliujing Section on three different dates of 2011. A boat-mounted single-beam echo sounder transducer (sonar) was used for bathymetric surveying at Xuliujing Section (Fig. 1) once a month. The echo sounder was a 208 kHz HY1600, 8 degree transducer, with a depth accuracy of 0.01 m+0.1%D (D is measured depth) and sampling rate of 5 Hz.
Figure 4. Energy partition of vertical-averaged currents at three acoustic Doppler current profiler stations. Area of pie chart (top-row) indicates total kinetic energy variance
$\sigma^2_{{\rm{KE}}_{{\rm{dat}}}}$ , and the sector is the Kinetic energy variance ratio VE, which represents relative contribution of the tidal component to the total flow in each season. The largest pie appears at Station C2 in summer and it represents 0.67 m4/s4. Histogram bars indicate percent energy En in the individual constituents.
Figure 5. Vertical profiles of mean current for the three acoustic Doppler current profiler stations. Direction is counterclockwise angle from east.
Figure 6. M2 tidal constituent ellipses at different depths. Blue lines indicate measured data, red lines are solution of optimally fit frictional model with a constant eddy viscosity and corresponding value of cost function (detailed in Section 4.1).
Figure 7. Barotropic current (red line) and vertical distribution of baroclinic current (blue line) for M2 constituent at different stations.
Figure 8. The vertical distribution of M2 ellipse parameters and the frictional model for three stations. Dots indicate measured data, solid lines are optimally fit model solution with a constant eddy viscosity and corresponding value of cost function (detailed in Section 4.1). Horizontal lines denote the tidal-averaged water depth.
Figure 9. Frictional model solutions using four different eddy viscosity ω appropriate to the Station C2 in spring. Dashed lines indicate ω=2×10−4 m2/s, dots lines indicate ω=5×10−4 m2/s, solid lines indicate ω=10×10−4 m2/s, and thick solid lines indicate ω=20×10−4 m2/s.
Figure 10. Seasonal variation of mean currents at upper, middle and lower layers (0.2D, 0.5D and 0.8D, D is water depth) for three acoustic Doppler current profiler stations.
Figure 11. Seasonal variation of M2 ellipses at upper, middle and lower layers (0.2D, 0.5D and 0.8D, D is water depth) for three acoustic Doppler current profiler stations.
Table 1. Basic parameters of the three acoustic Doppler current profiler stations located in Xuliujing Section
Station Depth/
mTop unmeasured
length/mBottom unmeasured length/m Number of
valid binsC1 14.9 ~2 ~3 9 C2 51.5 ~4 ~4 45 C3 9.7 ~2 ~2 6 
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