3.1 Surface salinity

Figure 4 displays the observed surface salinity along the cruise tracks under the wind mainly in the northeasterly during the spring tide from 4 to 7 May (a) and during the neap tide under the wind mainly in the southerly from May 8, 2014 to May 11, 2014 (b). Under the northeasterly wind during the spring tide, freshwater flowed into the coastal sea mainly on the west side of the estuary due to the Coriolis force and the wind forcing, and high salinity water appeared in mid-estuary of the Lantau Channel, implying a high salinity intrusion along the channel. There was a strong salinity gradient in the cross-estuary direction. When the southerly winds dominated during the neap tide, the surface salinity in the estuary is lower than that under the northeasterly wind during the spring tide in the mid-estuary and on the west side, and the surface low salinity water may further spread to the west side. The asymmetry in the surface spatial distribution of the salinity suggests the influence of the wind and tidal forcing on the estuary stratification. In the spring tide and the easterly wind, the low salinity water is confined on the west side and the stratification is weak, while in the neap tide and southerly wind, the stratification is enhanced. Wind forcing is an obvious factor influencing the spatial surface salinity distribution and the stratification [8]. Another important factor is the tidal mixing that is higher in the spring tide and weaker in the neap tide. The stronger turbulent mixing in the spring tide might further decrease the stratification, resulting in higher horizontal salinity gradient in the cross-estuary direction. In the neap tide, the weaker mixing may facilitate the formation of the stratified water in the PRE.

#### Figure 4.

Observed surface salinity along the cruise tracks under the wind mainly in the northeasterly during spring tide from 4 to 7 May (a) and during neap tide under the wind mainly in the southerly from May 8, 2014 to May 11, 2014 (b).

#### 3.2 Cross-channel salinity distribution and velocity

Figure 5 shows the sectional salinity and current velocity along transects B and C. Transects B and C were under the northeasterly wind during the spring tide on the flood and ebb, respectively. In order to remove high-frequency noises, a 2D rotationally symmetric Gaussian low-pass filter with a (3 3) size is used to process the ADCP data. The positive directions of the coordinate system of the current field are in the northward (y) and the eastward (x). Both transects B and C are almost the cross-estuary survey tracks, crossing the Lantau Channel in the midestuary. On the flood tide during transect B, the higher salinity water occupied the Lantau Channel, and lower salinity water was located on the two sides (Figure 5a). The freshwater was confined on the west side of the estuary; therefore, Circulations in the Pearl River Estuary: Observation and Modeling DOI: http://dx.doi.org/10.5772/intechopen.91058

#### Figure 5.

The sectional salinity (practical salinity unit, psu) (a, d), cross-channel circulation (m s<sup>1</sup> ) (b, e), and along-channel circulation (m s<sup>1</sup> ) (c, f) along transects B (upper) and C (lower), respectively.

the salinity on the east side was higher than that on the west side. The isohalines were almost vertical on the west side with relatively strong stratification just in the Lantau Channel. The cross-estuary current was relatively weak with a westward flow at the surface, whereas a velocity divergence appeared in the Lantau Channel, with the eastward (positive) velocity on the east side and westward (negative) velocity on the west side of the Lantau Channel (Figure 5b). On the flood tide of transect B, the water flowed into the estuary (northward/landward) with the strongest current on the west side of the Lantau Channel; when the flood tide turned to the ebb tide, the northward current was weaker and reversed on the east side (Figure 5c).

When the tide was turning from flood to ebb during transect C, the salinity reached a maximum (more than 24 psu) in the Lantau Channel (Figure 5d) (vs. 21 psu for transect B). Furthermore, the turbulent mixing was enhanced in the Lantau Channel, resulting in weak stratification during transect C. On the ebb in transect C, the freshwater flowed out of the estuary with seaward current in the surface, especially on the west side; at the bottom layer in the Lantau Channel, a landward current appeared for the flood to ebb transition (Figure 5f). The crosschannel current was highly complex with alternating convergence and divergence (Figure 5e).

During both of the transect B and C periods, the high salinity water flowed into the estuary in the deep Lantau Channel, which could facilitate the appearance of the high-density water in the mid-estuary of the Lantau Channel even during the ebb tide. The survey data indicate that the salinity intrusion could exist on the early ebb for transect C at the bottom of the Lantau Channel during the spring tide and northeasterly wind.

Figure 6 displays the sectional salinity and current velocity along transects D and F. Both transects D and F crossed the Lantau Channel in the lower estuary under the northeasterly wind during the spring tide. In the flood period for transect D, the water flowed into the estuary (northward/landward) with the strongest current appearing in Lantau Channel, reflecting the salinity intrusion along the deep channel, except at the surface on the west side, where the water flowed out of estuary (southward/seaward) influenced by the river discharge (Figure 6c). Due to

#### Figure 6.

The sectional salinity (psu) (a, d), cross-channel circulation (m s<sup>1</sup> ) (b, e), and along-channel circulation (m s<sup>1</sup> ) (c, f) along transects D (upper) and F (lower), respectively.

flood tide during transect D, the inflow might bring high salinity water into the estuary especially in the Lantau Channel. This flow pattern might cause the strong density gradient in the cross-estuary direction (Figure 6a). The low salinity water resided on the estuary west side, and the high salinity water was located on the east side. In the cross-estuary direction, the westward current appeared throughout the whole depth of the water column, implying that the cross-estuary surface current was induced by the northeasterly wind, while in the deep Lantau Channel, a velocity divergence appeared with eastward (positive) velocity on the east side and westward (negative) velocity on the west side of the deep channel (Figure 6b) [10].

On the ebb tide during transect F, the maximum seaward (southward) flow in the surface layer reached as large as 0.8 m s<sup>1</sup> with a weak current on the east side of the estuary. However, the bottom water flowed landward (northward) into the estuary, especially in the channel (Figure 6f). This was typical of a two-layer structure of density-driven estuarine circulation. Compared with transect D on the flood, the salinity gradient was weak and showed the two-layer structure, although the general pattern of the west low and east high salinity still appeared (Figure 6d). The cross-estuary velocity exhibited weak westward flow at the surface and strong eastward flow beneath the surface, related to the wind-driven current in the estuary (Figure 6e).

The observation suggests the salinity intrusion existed both for the flood (transect D) and ebb tide (F) same as that for transects B and C. However, different from the observation from transects B and C, the low salinity water further expanded to the east side of the estuary. This may be due to the fact that the seaward flow appeared on the west side both for the flood (transect D) and ebb (transect F) periods.

Transect H surveyed near the estuary mouth and south of Hong Kong Island under the easterly wind. Figure 7 shows the sectional salinity and current velocity along transect H. Near the mouth, low salinity water resided in the whole surface layer (west of 113.87°E) and high salinity water in the bottom layer, while east of 113.87°E, the vertical sectional salinity was well-mixed with the coastal water, indicating that the plume water was appearing most of the surface layer in the estuary mouth (Figure 7a). In the along-estuary direction, water flowed into the

Circulations in the Pearl River Estuary: Observation and Modeling DOI: http://dx.doi.org/10.5772/intechopen.91058

Figure 7.

The sectional salinity (psu) (a), cross-channel circulation (m s<sup>1</sup> ) (b), and along-channel circulation (m s<sup>1</sup> ) (c) along transect H.

estuary (northward/landward) during the flood tide (Figure 7c). At the surface in the estuary mouth, the water flowed out of estuary (southward/seaward), as it was influenced by the river discharge. In the cross-estuary direction, there appeared a strong westward current on the surface, indicating a wind-driven current induced by the easterly wind. However, a strong eastward current was found in the surface of the estuary mouth due to the plume water spreading, resulting in a velocity convergence at the plume water boundary, indicating a supercritical plume front at the plume boundary (Figure 7b).

During the neap tide period, the wind direction changed from the northeasterly to the southeasterly, and the easterly wind component was weaker than that in the spring tide (Figure 3b). The sectional salinity and current velocity along transects I and J are displayed in Figure 8. During the ebb tide of transect I, the upper layer water flowed out of the estuary (southward/seaward), with a strong current in Lantau Channel, while there was a bottom current flowing into the estuary (Figure 8c). In the cross-estuary direction, on the surface of the deep channel, there appeared a strong velocity convergence, an eastward current on the west side and a

#### Figure 8.

The sectional salinity (psu) (a, d), cross-channel circulation (m s<sup>1</sup> ) (b, e), and along-channel circulation (m s<sup>1</sup> ) (c, f) along transects I (upper) and J (lower), respectively.

westward current on the east side, with a weak velocity divergence on the left of the channel at the bottom (Figure 8b) [10]. The salinity had a sharp gradient at the velocity convergence location of 113.80°E. The salinity front was more apparent as compared with that during flood tide (Figure 8a).

In the early flood of transect J, the current was landward/northward (into the estuary) in Lantau Channel, while outside the channel on both sides, the water flowed seaward/southward (out of the estuary), especially in the surface layer (Figure 8f). In the cross-estuary direction, a velocity convergence appeared on the west side of the Lantau Channel in the bottom layer (Figure 8e). The density structure had still kept an ebb-like structure for a while until flood tide returned in this early flood.
