Chapter 4 Assessment of Spatial-Temporal Distribution of Observed Salinity in Can Gio Bay
4.1 Introduction
Chapter 4 Assessment of Spatial-Temporal Distribution of Observed
In the Can Gio Bay area, the Soai Rap and the Long Tau Rivers run along the west side and through the middle of the forest, respectively. The Dua River is one branch on the Long Tau River. The Can Gio mangrove forest is located in the west side of the Thi Vai River. Thus, salinity in these rivers directly would affect the environmental health of the Can Gio mangrove forest. In this chapter, to assess the influences of river inflow discharge and tidal levels on the spatial distribution and temporal variation of salinity along the main rivers in the Can Gio mangrove forest, the field observations of salinity were conducted by using the salinity data loggers.
4. 2 Observation Method of Salinity
The periods of field observation were during the end of the rainy season from December 7 to 10, 2017 (Period I) and during the middle of the rainy season from September 13 to 14, 2018 (Period II). The Period I had a normal discharge from the upstream rivers, and the tide in this period was the spring tide as shown in Fig. 4.2.
The Period II had a high discharge from the upstream river, and the tide in this period was the middle tide as shown in Fig. 4.3.
Fig. 4.2 Observed water level at the Vung Tau Station and observed rainfall at the Nha Be Hydrological Station from December 03 to December 15, 2017 and the
Period I of field observation.
Water level at Vung Tau Station (m)
−2.0
−1.0 0.0 1.0
12/03 12/04 12/13
12/08
12/05 12/06 12/07 12/09 12/10 12/11 12/12
Time Year of 2017
12/14 12/15 Rainfall at Nha Be Station (mm) Period I
2.0
0
50
Fig. 4.3 Observed water level at the Vung Tau Station and observed rainfall at the Nha Be Hydrological Station from September 3 to September 15, 2018 and the
Period II of field observation.
4. 2. 1 Temporal Salinity Variation
There are many available instruments that provide salinity data by measuring electric conductivity and temperature, then deriving salinity from these measurements.
A HOBO U24 Conductivity Data Logger (U24-002-C) shown in Fig. 4.4 and Table 4.1 was used to collect salinity time series at fixed spots of two locations within the Can Gio Bay as shown in Fig. 4.1, to assess the salinity variation at the locations. The first station, the Phu Xuan Station was located in the upstream of the Can Gio Bay area where is the strongest area influenced directly by the discharge from the upstream rivers. The time series data of salinity at the station was observed for both the Periods I and II. The next stations were at the Nga Bay I for the Period I, and the Nga Bay II for the Period II. These stations were located in the middle of the mangrove forest and played an essential key for assessing changes in salinity regarding the tidal regime and the upstream discharge in this area. The instruments were set up approximately 2 m beneath the water surface, and recorded electric
Water level at Vung Tau Station (m)
−2.0
−1.0 0.0 1.0
09/03 09/04 09/13
09/08
09/05 09/06 09/07 09/09 09/10 09/11 09/12
Time Year of 2018
09/14 09/15 Rainfall at Nha Be Station (mm) Period II 2.0
0
50
conductivity and temperature continuously in ten-minute intervals. On December 7–
8, 2017 in the Period I, the data logger was set up at the Phu Xuan Station, and the Nga Bay I Station in Fig. 4.5 on December 8–10, 2017 in the Period I. On September 13–14, 2018, the instruments were set up at both the Phu Xuan and the Nga Bay II Stations, approximately 2 m beneath the water’s surface. The logger was kept in polyvinyl chloride (PVC) pipe housing and secured to a line that was anchored and connected to the station so as to maintain correct orientation and location.
Table 4.1 Specifications of the HOBO U24 Conductivity Data Logger.
Operating Range 2 °C – 36 °C
Memory 64 kbyte
Battery 3.6 V lithium
Battery Life 3 years (at 1-min logging)
Maximum Depth 70 m
Weight 193 g
Size 3.18 cm diameter × 16.5 cm length with 0.63 cm mounting hole
Fig. 4.4 HOBO U24 Conductivity Data Logger (U24-002-C).
Fig. 4.5 The Nga Bay I Station used to collect continuous salinity data.
4. 2. 2 Horizontal Spatial Salinity Distribution
Comprehensive salinity distribution were measured continuously in five-minute intervals along the Soai Rap and the Long Tau Rivers using the YSI ProDSS Data Logger, as shown in Figs. 4.6 and 4.7, Table 4.2, attached to the side of a moving boat to collect spatial salinity data of rivers along the Pathway I on December 10, 2017 in the Period I; from Phu Xuan Station through the upstream of the Long Tau River, the Dua River, the downstream of the Long Tau River and to the Soai Rap River as shown in Fig. 4.11. These data were combined with water depth and GIS location data. Besides, the HOBO U24 Conductivity Data Logger in Fig. 4.4 attached to the side of a moving boat was used for measuring salinity continuously in five-minute interval in two pathways: the Pathway II-1 on September 13, 2018 in the Period II; from the Phu Xuan Station through the upstream of the Long Tau River, the Dua River, the downstream of the Long Tau River and to the Soai Rap River as shown in Fig. 4.15, and the Pathway II-2 on September 14, 2018 in the Period II; from the Phu Xuan Station to the upper part of the Long Tau River, the Dong Tranh River, the Thi Vai River, and coming back to the lower part of the Long Tau River, the endpoint of the Tac Ong Tranh Creek as shown in Fig. 4.16.
Table 4.2 Specifications of the YSI ProDSS Data Logger.
Operating Range 0 °C – 50 °C
Memory > 100000 data sets
Battery Rechargeable lithium-ion Battery Life 20 hours
Maximum Depth 100 m
Weight 567 g
Size 8.30 cm width × 21.60 cm length × 5.60 cm depth, 4 port bulkheads with sensor guard: 45.36 cm × 4.75 cm
Fig. 4.6 YSI ProDSS Data Logger.
Fig. 4.7 Continuous data collection using the YSI ProDSS Data Logger attached to the side
of a moving boat.
4. 2. 3 Salinity Vertical Profiles
In addition, a CastAway CTD Data Logger as shown in Fig. 4.8 and Table 4.3 was used to collect vertical profiles of salinity along the Pathway I, the Pathway II-1 and the Pathway II-2 including the Soai Rap, the Long Tau, the Dua, the Dong Tranh, and the Thi Vai Rivers and some small branches in those two periods as shown in Figs. 4.11, 4.14, and 4.16. The aim of measuring these vertical profiles at the differential many spots was to grasp the infiltration process of salinity upward from sea or dilution of salinity due to the mixing of sea water and fresh water.
Table 4.3 Specifications of the CastAway CTD Data Logger.
Operating Range −5 °C –45 °C
Memory 15 Mbyte
Battery 2 AA alkaline Battery Life 40 hours Maximum Depth 100 m
Weight 450 g
Size 7.11 cm diameter ×
20.32 cm length
Fig. 4.8 The CastAway CTD Data Logger.
4. 3 Salinity Field Observation in 2017 4. 3. 1 Temporal Salinity Variation
The surface salinity ranged from 1.6 psu – 6.0 psu at the Phu Xuan Station (Fig.
4.9) and from 15.0 psu – 20.5 psu at Nga Bay I Station (Fig. 4.10). These fluctuations were clearly driven by tidal stage but also depended on station location:
the amplitude of change at the Phu Xuan Station about 40 km far from the sea was 4.4 psu while that at the Nga Bay I Station 20 km far from the sea was 5.5 psu.
Besides, the maximum and minimum salinities were affected by the hydrodynamic regime. The maximum water level at the Vung Tau Station occurred at 17:00 on December 7, 2017, but the peak salinity at the Phu Xuan Station occurred at 21:00, with a 4-hour lag from the peak tidal level at sea. This delay was also notable at the Nga Bay I Station, where peak salinity occurred shortly at 20:30, with a 2.5-hour lag after the peak tidal level at 18:00 on December 8, 2017.
4. 3. 2 Horizontal Spatial Salinity Distribution
Figure 4.11 shows the observed surface salinity levels along the Long Tau River and the Soai Rap River respectively. Salinity levels generally increased along the two rivers as the distance to the sea decreased. Observed salinity levels ranged from approximately 3 psu at the upstream near the Phu Xuan to 22 psu at the estuary.
The salinity was closely related and significantly affected not only by the distance to the sea but also by confluence flow entering from tributaries. Notable differences in salinity levels were observed at the intersection between the Vam Co and the Soai Rap Rivers. At this intersection, located nearly 17 km from the sea, the salinity dropped from approximately 10 psu to 7 psu. Along the Soai Rap River, observed salinity levels increased gradually to the sea, reaching 10 psu at 5 km from the sea. At the reach less than 5 km from the sea, observed salinity levels increased more rapidly, reaching 20 psu at the sea. These differences were attributed to the large cross-section of the Soai Rap River and high discharge of the Vam Co River.
Smaller fluctuations in salinity were observed along the Long Tau River attributable to its smaller tributary flows. Along this river, observed salinity levels increased from 3 psu at the upstream near the Phu Xuan to 21 psu near the sea, with
Fig. 4.9 Calculated tidal level at the Vung Tau Station and the observed time series of surface salinity at the Phu Xuan Station from 09:30 December 7 to 08:30 December 8, 2017 (Period I).
Fig. 4.10 Calculated tidal level at the Vung Tau Station and the observed time series of surface salinity at the Nga Bay I Station from 14:00 December 8 to 12:00 December 10, 2017 (Period I).
0 2 4 6
−2
−1 0 1
Salinity (psu)
Phu Xuan Station
Water level (m)
09:00 15:00 21:00 03:00 9:00
Suface salinity Water level at Vung Tau
2017/12/07 2017/12/08
Time
15 20
−2
−1 0 1
Salinity (psu)
Nga Bay I Station
Water level (m)
14:00 01:30 13:00 00:30 12:00
Suface salinity Water level at Vung Tau
2017/12/08 2017/12/10
2017/12/09Time
significant fluctuations occurring at intersections with small tributaries. At the Dong Tranh Estuary, observed salinity level increased approximately 24 psu.
Fig. 4.11 Continuous observed salinity time series along the Pathway I recorded on December 10, 2017 in the Period I by the YSI ProDSS Data Logger. A black circle
shows an observation time and location of vertical profile of salinity using the CastAway CTD Data Logger.
0 10 20
−2
−1 0 1
Salinity (psu)
09:11 11:12 13:13 15:14 17:16
Suface salinity
2017/12/10 Time 2017/12/10
Water level (m)
Water level at Vung Tau
1 4
7 10
13
16
19
0 6 12 18 24
Salinity (psu)
Suface salinity
Phu Xuan Ly Nhon
Long Tau R. Soai Rap R.
Dong Hoa R. Dong Tranh E.
Ganh Rai E.
Location
Nga Bay S.
Dua R.
4. 3. 3 Salinity Vertical Profiles
Salinity was strongly influenced by the water depth and the mixing of seawater and freshwater; in almost all vertical profiles salinity was higher toward the riverbed and lower near the surface as shown in Fig. 4.12, Table 4.4. The points 1, 2 and 3 of Fig. 4.12 pointed out that the observed salinity around the Phu Xuan Station had a significant variation at the almost same recording time. The point 1 of Fig. 4.12 showed the salinity slightly changed from 3.0 psu to 4.0 psu from surface to bottom, the point 2 fluctuated in the range of 3.1 psu - 4.1 psu. Meanwhile, the vertical profile of the point 3 had sharply changed of 4.3 psu - 7.3 psu. This pointed out that the Soai Rap River contained most of the downstream flow compared to the smaller Long Tau River.
Along the upper Long Tau River up to Nga Bay I Station from the points 3 to 8, the salinity difference between surface and bottom ranged mostly between 2 psu and 4 psu, though each vertical profile followed a similar increasing trend with water depth. One exception occurred at the point 4, where the salinity increased from 7.2 psu to 13.1 psu because this location is the intersection point of the upstream branch of the Long Tau River which received a massive amount of water from the Saigon-Dongnai River system and two downstream branches of the Long Tau River. Overall, the maximum salinity increased in a seaward direction, except at the point 5 and the point 6 where the salinity of the point 6 was lower than that of the point 5. The reason for this difference was that the point 6 was located at a small branch of the Long Tau River and so received less seawater than the point 5. The salinity at the points 7 and 8 changed only a little from surface to bottom because these points were located in an area of high velocity which mixed the freshwater and seawater more thoroughly, thus there was not marked the difference of salinity between surface and bottom of the water.
Along the lower Long Tau River, the downstream from Nga Bay I Station, the salinity difference between surface and bottom at the points 10 to 12 reached 8 psu, and the vertical profiles changed downstream. The river’s cross-section underwent significant change over this stretch and the confluence of many tributaries caused hydrodynamic changes resulting in the salinity variations. This was particularly true
for the points 11 and 12 located along the Ganh Rai Estuary at the intersection of the river and the sea, where the river was especially wide and the velocity was low.
Thus the mixing of freshwater from rivers and seawater from the sea was not strong, leading to more segregated salinity vertical profiles as the denser and more saline seawater remained below the less dense freshwater.
Along the Soai Rap River, from the points 16 to 21, the salinity at bottom was always higher in upper layer, but the vertical profile did not follow a similar trend as that in the Long Tau River. The salinity changed dramatically at the upper layer but increased insignificantly in the bottom layer. At the points 16, 17 and 18 at the mouth of the Soai Rap River, where the channel was wider than the Long Tau River, the salinity difference between surface to bottom was more than 10 psu that were higher than those observed in the Long Tau River. The point 19 and 20, located at the intersection of the Soai Rap and the Vam Co Rivers, had higher velocity, so the salinity difference between surface and bottom was lower than those at the points 16, 17 and 18.
On the other hand, the general trend of salinity rising with water depth was not applicable at the points 13, 14 and 15, along the branch connecting the Ganh Rai Estuary to the Dong Tranh Estuary. At these points, the salinity changed only slightly from surface to bottom; the point 15 actually showed decreased salinity vertical profile from surface to bottom over 3 m depth. This suggests that, the salinity vertical profile changing only slightly would be resulted from the strong mixing due to strong current and / or wave motion.
The amplitude of salinity between surface and bottom of the river depended on the flow regime and magnitude of velocity as well as water depth. The salinity in the sea exceeded 30 psu, declining to 23 psu at the mouth of the Soai Rap River and 25 psu at the mouth of the Long Tau River. The salinity in the latter was higher than that in the former because the latter was deeper and narrower. The salinity in the Soai Rap River declined to about 9 psu at the intersection with the Vam Co River and further decreased to 3 psu at the upstream of the Phu Xuan Station. The salinity in the Long Tau River declined to about 20 psu at Nga Bay I Station and reduced to 3 psu at the Phu Xuan Station.
Fig. 4.12 Vertical profiles of salinity at the 21 observation points along the Pathway I recorded on December 10, 2017 in the Period I by the CastAway CTD Data
Logger (see also Fig. 4.11).
3 3.5 4 18
15 12 9 6 3 0
3 4 5
5 4 3 2 1 0
4 6 8
20 16 12 8 4 0
7 10.5 14 25
20 15 10 5 0
12 14 16 15
12 9 6 3 0
12 13 14 24
18 12 6 0
15 16.5 18 16
12 8 4 0
18 19.5 21 25
20 15 10 5 0
18 19.5 21 10
8 6 4 2 0
2022.525 15
12 9 6 3 0
21 24 27 15
12 9 6 3 0
21 24 27 30 12
9 6 3 0
24 24.5 25 3
2 1 0
24 24.5 25 5
4 3 2 1 0
23 23.5 24 3
2 1 0
15 20 25 8
6 4 2 0
15 20 25 10
8 6 4 2 0
10 15 20 12
10 8 6 4 2 0
6 9 12 10
8 6 4 2 0
6 9 12 8
6 4 2 0
6 7.5 9 8
6 4 2 0
Depth (m)
Salinity (psu)
(1) (2) (3) (4) (5) (6)
(7) (8) (9) (10) (11) (12)
(13) (14) (15) (16) (17) (18)
(19) (20) (21)
Table 4.4 Observed salinity values at the water surface and the bottom for the 21 observation points along the Pathway I in the Period I (see also Fig. 4.11).
Point Time (December
10, 2017 )
Observed depth (m) Salinity at observed depth (psu)
Depth-averaged salinity (psu)
1 09:02 Surface 0.15 3.0
Bottom 16.67 4.0 3.5
2 09:16 Surface Bottom 0.15 4.80 3.1 4.1 3.5
3 09:32 Surface 0.15 4.3 6.3
Bottom 18.43 7.3
4 10:00 Surface 0.15 7.2
10.7
Bottom 24.67 13.1
5 10:26 Surface 0.15 13.5 15.3
Bottom 13.09 16.3
6 11:18 Surface 0.15 12.5
13.3
Bottom 21.26 13.6
7 11:33 Surface Bottom 15.13 0.15 15.3 17.7 16.8
8 11:53 Surface 0.15 18.5 19.3
Bottom 24.46 20.4
9 12:11 Surface 0.15 18.1
20.0
Bottom 9.86 20.5
10 12:22 Surface 0.15 19.5 22.6
Bottom 15.11 24.3
11 12:43 Surface 0.15 20.1
25.6
Bottom 19.84 28.0
12 13:00 Surface Bottom 11.77 0.15 22.2 28.7 25.0
13 13:16 Surface 0.15 24.1 24.1
Bottom 2.73 24.1
14 14:51 Surface 0.15 24.1
24.3
Bottom 4.86 24.3
15 15:17 Surface 0.15 23.7 23.7
Bottom 2.90 23.6
16 15:37 Surface 0.15 14.2
20.6
Bottom 6.73 25.4
17 15:47 Surface Bottom 0.15 9.40 13.9 27.1 21.5
18 16:20 Surface 0.15 11.0 17.3
Bottom 11.21 21.7
19 16:50 Surface 0.15 6.8
Bottom 9.30 12.1 9.9
20 16:56 Surface 0.15 6.7 8.9
Bottom 8.17 10.6
21 17:09 Surface 0.15 6.3
Bottom 7.83 7.9 6.8
The field observation in the Period I showed that the salinity declined from upstream to downstream (toward the sea) at about 0.5 psu/km. However, the range of salinity in the vertical profile differed by location and water depth.
The Long Tau River was deeper and narrower than the Soai Rap River and thus had higher salinity. The salinity vertical profiles were also affected by proximity to the sea; at the Dong Tranh Estuary the surface salinity ranged from 10–14 psu in comparison with the bottom salinity of 22–27 psu, with a sharp gradient between them showing less mixing than in the upstream areas; a similar pattern was observed at the Ganh Rai Estuary, despite a lower salinity range. Finally, the salinity vertical profiles were strongly influenced by the tidal regime of the area, with tidal effects particularly significant near the river mouths and weakening upstream toward the Phu Xuan Station.
4. 4 Salinity Field Observation in 2018 4. 4. 1 Temporal Salinity Variation
Field observations in the Period II took place in the middle of the rainy season.
With frequent heavy rainfall, the upstream discharge from rivers was especially high. This significantly diminished the salinity at the Phu Xuan Station in this period. Figure 4.13 shows that the range of salinity at the Phu Xuan Station in this period was between 0.1 and 0.4 psu. Salinity also varied depending slightly on the tide. However, despite peak tides, the salinity was still minimal. The Phu Xuan is located on the upper stream, which is the main drainage water path from the city center. Combined with the particularly high rainfall volume during the Period II, the topography of the drainage path instigated a large discharge of freshwater to be released into the Phu Xuan. This led to significant freshwater dilution, causing the salinity to decrease dramatically as seen in Fig. 4.13.
Therefore, a high discharge of freshwater released into the Phu Xuan leading significant freshwater dilution that has caused salinity to drop dramatically.
Though, salinity was low at the Phu Xuan Station in the Period II, salinity at the Nga Bay II in the Period II was much higher, reaching 18.9 psu at the peak tide on September 13, 2018, as shown in Fig 4.14. This showed that the salinity
at the Nga Bay II Station was less affected by upstream flow than that at the Phu Xuan. However, the minimum salinity decreased around 8 psu observed in the Period II.
The above-mentioned results reflected that the upstream river flow going to the Phu Xuan was divided into two parts: almost of water amount of the Saigon River going through the Soai Rap River before flowing into the sea, and the remaining part of water flowing through the Long Tau River and going to the Nga Bay II Station, then entering into the Ganh Rai Estuary. The freshwater amount coming to the Nga Bay II Station caused the decrease of 5 psu at the low tide, and the upstream flow dominantly stretched salinity amplitude wider in range of 8.5 psu to 18.9 psu as shown in Fig. 4.14. The smallest salinity at the Nga Bay II Station remained at 8.5 psu because of 20 km distance from the Phu Xuan and the influence of the upstream flow was faded.
4. 4. 2 Horizontal Spatial Salinity Distribution
Figure 4.15 shows the continuous observed salinity time series on 13 September 2018 in the Period II by the HOBO U24 Conductivity Data Logger along the Pathway II-1: the Phu Xuan, the Long Tau, the Dua, the Ganh Rai Estuary, the
Fig. 4.13 Calculated tidal level at the Vung Tau Station and the observed time series of surface salinity at the Phu Xuan Station from 08:00 September 13 to 08:00 September 14, 2018 (Period II).
Fig. 4.14 Calculated tidal level at the Vung Tau Station and the observed time series of surface salinity at Nga Bay II Station from 10:00 September 13 to 16:00 September 14, 2018 (Period II).
0 0.2 0.4
−2
−1 0 1
Salinity (psu)
Phu Xuan Station
Water level (m)
08:00 14:00 20:00 02:00 8:00
Suface salinity Water level at Vung Tau
2018/09/13 2018/09/14
Time
0 10 20
−2
−1 0 1
Salinity (psu)
Nga Bay II Station
Water level (m)
10:00 17:30 01:00 08:30 16:00
Suface salinity Water level at Vung Tau
2018/09/13 2018/09/14
Time