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The Red river is the largest river in the north of Vietnam, its water resources reserve many social economic sectors in the delta plain at the downstream. The Red river delta (RRD) is characterized by multi complex hydraulic channels and a bar-built estuary system. During the recent years, salinity intrusion is one of the most important factors that constrict the usage of water especially in the dry season. The strong seasonal variation that characterizes water exchanges and tidal mixing can play an important role in the distribution and transport of salinity in this river delta. Up to date, a large number of studies have been carried out depends on two major physical factors that driven salinity, e.g.: the tidal variation at the estuary mouth and the freshwater discharge entering the saline area. But the salinity intrusion is a result of a complex interaction between these forcing with topographic features at the estuary mouth. Valle-Levinson et al. (2000), Prandle (2003), Savenije (2005) summarized and established the relationships among tide, river discharge, and salinity with different estuary shapes. However, they have been derived for single estuary branch and for estuaries where the river discharge is small and the tide experiences only modest damping or amplification (Nguyen, 2006). Hibma et al. (2003), Nguyen (2006) investigated the morphological change and salinity intrusion for multi estuaries, but these estuaries are similar in shape and length. Furthermore, each estuary branch is separated from each other by the island and it can interact with another through junctions or small connecting channels.
The RRD estuaries are the typical type of alluvial estuary with a complex hydraulic system, including several rivers and canals connected each other. Their main topographical features are shallow waters with large width to depth ratio, funnel shaped cross section which is shallower and wider towards the estuary mouth. Each estuary has distinct characteristics of tidal forcing, river discharge rate and bathymetry that complicate the study of salinity intrusion in RRD estuaries. We know of no study on salinity in these multi estuaries under the effect of simultaneous physical factors. This paper sets out to analyze the tidal mixing, water exchange and salinity distributions in multi branches of RRD based on the field measurement conducted during 17 days in January, 2006 at six stations. In addition, we examine some bathymetrical parameters over a wide range of estuarine conditions. The aim of this contribution is to determine the correlations among density gradients, river flow and river bathymetry and to categorize RRD estuaries in term of each impact factors.
The paper will be organized in the following manner: Section 2 briefly describes the area of study. The details of data processing and salinity measurements including continuous water level and coherence measurement data are given in Section 3. The results of salinity analysis correspond to intra-tidal regime, river flows and variable forcing factors by river geometry are described in Section 4, and final discussion is presented in Section 5.
2. STUDY AREA
2.1. Red river delta
Shown in Figure 1, the RRD that is located in the west coast of the Tonkin Gulf. The RRD is concentrated by many important economic activities and the highest population density; the total area is approximately 16,644 km2, the population was 18.2 million in 2006 and entirely delta area is lying below three meter above the mean sea level (Minh et al., 2009). The delta is restricted landwards by Son Tay district and seawards by the coastline. RRD consists of two estuary systems located in the north-east and in the south-west. These two estuary systems are connected each other by the Duong river and the Luoc river in the upper part and the lower part of RRD, respectively. Before reaching the Tonkin Gulf, the main Red river in RRD branches out into nine estuary tributaries, e.g.: Da Bach, Cam, Lach Tray, Van Uc, Thai Binh rivers in the north-eastern part and Tra Ly, Hong, Ninh Co and Day rivers in the south-western part. This paper will concentrates on four estuary branches in the south-western part of the RRD.
The distinct monsoon climate is typical feature in this area, the wet season from May to October and the dry season from November to April. The annual rainfall varies spatially and seasonally from 700 to 4,800 mm/year. Here we have 80% of the total rainfall in the rainy season (MONRE, 1996-2006). The difference of the water discharge between two seasons is quite large (van Maren, 2007). Based on the hydrological data from 1996 to 2006 at Son Tay station, this station is located at the entrance of RRD, the average discharge in the dry season is around 1100 m3/s contrasting with the average one in the wet season of 14,000 m3/s. As a result, the river basin causes a shortage of water during the dry season and floods in the wet season.
At present, the total amount of water in the Red river is decreasing significantly as a result of climate change and large increase in the population in the delta (Lan et al., 2007). According to Cat et al., (2008) the decrease of river discharge in the dry season during the last ten years results in the increase of saltwater intrusion. Salinity intruded much further into the rivers. Salt concentration is observed throughout the delta for up to 30 km landwards from the Hong estuary, 34 km from the Tra Ly estuary, 35 km from Day estuary and 44 km from Ninh Co estuary (MONRE, 2009). The longitudinal distribution of salinity is subject to large variations during a year due to the influence of river discharge. The highest monthly salinity usually occurs in January and February, two driest months of the year. In Tonkin Gulf, salinity remains in a stable value during a tidal cycle and ranges between 25-30 psu during the dry season (based on the data observed at Hon Dau station).
The tidal regime in the Gulf of Tonkin is diurnal type with one cycle occurring every 25 h and tidal range decreases southwards gradually from 4 to 2 m. The tidal amplitude at Hong estuary, the main estuary in RRD, varies from 0.5 m during neap tides to 2.5 m during spring tides. The monsoon winds change from northeasterly in the dry season to southeasterly in the wet season. However, due to the topographic conditions of the delta, the fetch length for the stronger northeasterly winds is limited; as a result, wave height deceases northwards.
2.2 Shape of the Red River Delta estuaries
Most of the rivers in RRD are seaward extension. In the north-eastern delta plain, the deeper feature of the estuaries is dominated by tidal mixing (Tanabe et al., 2003). The estuaries are characterized by funnel-shaped type and an intricate tidal flat and creek system, which accelerate the penetration of tidal water into the rivers. While the wave-dominated is located in the low flatter land and widening estuarine system in the south-western part of the delta, where wave energy generated by monsoon winds is relatively strong. The estuaries in this part comprise tidal flats, marshes, and channels which are composed of meandering rivers. It can be seen clearly in Figure 2 the seaward expanding trend of the RRD estuaries. We attribute the tendency of larger opening and shallower water depth of the river mouth to the peculiar feature of the RRD estuaries by defining the cross-section area, storage width and river bed elevation at the tidal averaged water level. These bathymetry data was measured in the year 2000 in the "Red river delta flood protection program" organized by the Ministry of Agriculture and Rural Development, Viet Nam (MARD). The average depth near the mouth of RRD estuaries ranges from 4 to 6 m below mean water level, and the channel width at this location is about 800 m. The river channel gradually deepens and narrows upstream of the river mouth; 10 km upstream, the rivers are only 400 m wide and 8.5 m deep.
3. Data processing
3.1 Data sets of RRD
The data used in this paper consist of two sets: (i) the first set is from field measurement from January 1 to 16 in 2006 at 6 locations for salt concentration and water level, and (ii) the second set is from the collection by Vietnamese Institute of Meteo-hydrology (IMH) at fixed stations on the RRD.
The field measurements were conducted in the driest month in 2006 to obtain the salt intrusion curve at High Water Slack (HWS) and Low Water Slack (LWS). The measurements were conducted simultaneously at six locations including two locations on the Tra Ly river, two locations on the Ba Lat river, one location on the Ninh Co river and one location on the Day river.Â Salinity was measured consecutively in 17 days from 5 to 22 in January, 2006 at three points over the depth: 0.5 m from the surface, 0.5 m from the bottom and mid-water column. The actual recorded time was adjusted relatively to the time of high water (HW) and low water (LW). At HW period, we measured salt concentration during four hours: 1 hour before the HW, at HW and every one hour after water level peaked.Â At LW period, salinity was measured every two hours: before LW, at the lowest point, and after LW.Â The time steps for salinity measurement were selected to determine the variations of salinity during sub-tidal (spring-neap) corresponds to daily changing of a diurnal tidal regime at four estuary branches.
Table 1. Location of salt concentration stations in the field measurement.
Distance from sea
The second data set is based on the data collected from the IMH. The water levels are collected at 6 stations: e.g.: Hanoi, Thuong Cat, Pha Lai, Hung Yen, Trieu Duong and Chanh Chu. The tidal levels and salt concentrations at the mouth of estuaries are collected from measurements conducted by the IMH. For the purpose of this study, however, river discharge at the lower part of the delta is not available during our field measurements. In RRD, only three upstream stations (Son Tay, and Hanoi stations on the Red river; Thuong Cat station on the Duong river) measure daily discharge, no other systematic records of discharge exist in RRD during the dry season except the water levels. Therefore, we calculated the spatial distribution discharges over the river network in the south-western part of RRD using the MIKE 11 model. The MIKE 11 is a software package involving several modules and labs for the hydrodynamics, sediment transport and water quality in estuaries, rivers, irrigation systems and other water circumstances (DHI, 2003). It is being improved and becoming an effective design tool for engineers, hydrologists, ecologists and planners. The hydrodynamic module is the nucleus of the MIKE 11 and controls other modules. In the present simulation, we use the hydrodynamic and advection-dispersion modules. The former module treats the mass conservation and momentum conservation using the Saint Venant equation. They are solved numerically in a space staggered computational grid using a 6-point Abbot scheme (Abbott, 1979). The latter module deals with the advection-dispersion equation for dissolved material. It requires the discharge, water level and flow velocity data as the input values that are obtained from the computation using the hydrodynamic module in advance. The advection-dispersion equation is solved numerically using an implicit finite difference technique, which are devised unconditionally stable and less numerical dispersion.
3.2 River network and model calibration
The computational network is established for the south-western part of RRD. The present numerical model requires a series of 2-dimentional points describing each cross-sectional profile along the river. It consists of 8 rivers with 280 cross sections. These data was collected from the bathymetry measurements in the year 2000. The most Vietnamese research institutes have depended on this database. The boundary conditions are given at one discharge station (Son Tay station) and two water level stations (Pha Lai and Chanh Chu stations) in the upstream and at 4 tidal water level locations at the estuary mouth in the downstream. We set the salinity zero at upstream boundary and the real time variations at all downstream boundaries. All hydrological monitoring stations are depicted in Figure 1.
The task of calibration is executed in two steps. First, the model parameters such as a Manning coefficient and an initial water level at each cross section are adjusted to obtain the best fit between the observed and computed values for both discharge and water level at the prescribed station. Second, the dispersion coefficient and initial salt concentration at each river segment is determined for the calibration of salinity.
Figure 3 shows the comparison of the observed and computed data at stations in the upstream for the water levels (Ha Noi, Thuong Cat, Hung Yen, Trieu Duong stations) and the discharges (Ha Noi and Thuong Cat stations). The time series observed data for water level and discharge at both Ha Noi and Thuong Cat stations are used for the calibration of the model while at Hung Yen and Trieu Duong stations only daily observed water level is conducted for that purpose. The calibration result of the river-system model proves that the calculated water level at the upstream stations agrees well with the observed one. Furthermore, the simulation also describes accurately the discharges measured at both Thuong Cat and Hanoi stations.
As the saline intrusion along a river is very sensitive to the initial conditions, boundary conditions and dispersion coefficients, the calibration of salinity is generally more complicated when compared with that of discharge or water level. Figure 4 shows computed and the average observed salt concentrations at four stations at the downstream, e.g.: the Ba Lat, Dong Quy, Phu Le and Nhu Tan stations. The water levels measured during field survey at these stations are combined to compare with the prediction of the model. Although there are several discrepancies between lower salinity concentrations, the model predicts the salinity variation reasonably well. Additionally, the in-situ water levels agree strongly with the tidal fluctuation from the model calculations. Therefore, the parameters obtained from the calibration are acceptable. These findings validate the approach we used to obtain the discharge from the limited data available at the IMH.
4. Discussion (Salinity in RRD estuaries)
4.1 Effects of tides
Salinity in RRD varies following the tidal regime. The lower salinities near the river head and higher salinities seaward are related to the tidal variations. At the indicated stations nearby the river mouths (6-9 km from the sea), the salinity measured did not seem to be in equilibrium. Salinity differences are high during a tidal cycle due to ebb-flood flows at all seaward stations where the maximum salinity measured at the HW and the minimum one obtained at the LW. The salinity structure was stratified at high water column of a tidal cycle but it was fully mixed at the end. The variation of salinity also well corresponds to the spring-neap variation. The highest salinity occurs during spring tides and decreases towards neap tides due to the mixing. According to Simpson et al. 1990, mixing rates are highest and smallest during spring and neap tides, respectively. Salt wedges can therefore intrude further into an estuary during the neap tide. Our observation of salinity variations reflects this trend at Phu Le and Nhu Tan stations (see Figure 4). The stratification occurring during the neap tide was related to mixing in combination with tidal straining or advection. However at the Ba Lat and Dong Quy stations, a contrast of better mixed type was observed during our measurement in the dry season. The measured vertical salinity structure at two stations suggests that baroclinic circulation is not important in the dry season, leaving tidal straining as the main stratifying process. Tidal advection also leaves sea water intruded further into rivers, however, salinity reduces quickly. In up-estuaries, as distance upstream increases, salinity dropped rapidly to values between 0.5 and 3.5 psu; the highest value fell about 3 hours after HW at Phuc Khe station and Duong Lieu station. Longitudinal density advection is supposed much influenced by river discharge and topographic characteristics of each river.
It is found that in RRD both partial mixed and well-mixed estuary types are existed during the dry season (Figure 5). The varying salinity responds rapidly to the change of tidal variations in the latter type (Hong and Tra Ly estuary branches) while it reacts more slowly in the former one (Ninh Co and Day branches). Decreased tidal mixing in well-mixed estuaries decreases the exchange flow, and hence lower salinity was obtained at these branches during our measurements. During low river discharge in January, HWS salinity at the Ba Lat and Dong Quy stations was around 24 psu compared to 29 psu at Phu Le station. Lower salinity was observed at Nhu Tan station, however, we expect fresh discharge and river bathymetry are significantly diluted salt water. After LWS, salinity increased rapidly to sea value throughout the estuary. High salinity was obtained at all four seaward stations during HWS. At two stations Phu Le and Nhu Tan, salt concentration was relatively high at LW even through the tidal range during the survey was almost the same. We cannot rule out that a change in the sub-tidal salinity regime over that period could account for the local bathymetry change or the dismissing of fresh discharge. Both probably play a role in higher salinity. However, without any available data it is difficult to assess the salinity variation at these stations.
4.2 Effect of fresh discharge
The river discharge, together with relevant parameters defining estuary shape and tidal forcing, is the key parameter determining salt intrusion in alluvial estuaries (Nguyen, 2006). In alluvial estuaries during periods of low flow, when salinity is highest, the river discharge is generally small compared to the tidal flow. This makes the determination of the freshwater discharge a challenging task. Even if discharge observations are available during a full tidal cycle, the freshwater discharge is seldom much larger than the tidal discharge. Observations further upstream, outside the tidal region, do not always reflect the actual flow in the saline area due to withdrawals or additional drainage. Discharge computation is even more difficult in a complex system such as the RRD, which is a multi-channel estuary consisting of many branches, over which the freshwater discharge distribution cannot be measured directly.
Before reaching the Tonkin Gulf, the main Red river distributes its flow through six branches with 25% of the flow of the Red River discharging into the sea via the Hong estuary, 10% via the Tra Ly estuary, 6 % via the Ninh Co estuary, 22% via the Day estuary and the other water volume is discharged into the Duong and the Luoc rivers (Pruszak el al., 2005). The annual discharge of the Red river is about 137 Ã- 109 m3 however less than 20% of that water is supplied for the RRD in the dry season. During our measurement, the average discharge at the Son Tay hydrological station is 1500 m3/s. However, the distribution of the discharge over the branches downstream depends on a complex interaction of topography, tide, hydraulic channel network. Therefore, it is difficult to obtain a reasonable estimate of the discharge distribution over the branches of the system.
The new approach to use the salinity measurements to estimate the discharge distribution is tested against the results of a hydraulic model that uses the observed upstream discharge as the upstream input and the observed tidal variation at the downstream boundary. For this purpose, we have used the MIKE1 model. The data for the schematization of the hydraulic model, including topographical data, has been obtained in the year 2000. Based on the model results, we are able to derive the freshwater discharge values in the branches of the RRD. The computed results are presented in Table 2, as a percentage of the observed discharge at Son Tay station in the upstream.
Table 2. Distribution of discharge rate over RRD branches
Rivers and estuaries
(at Hanoi station)
(at Thuong Cat station)
(at Trieu Duong station)
Tra Ly estuary
Ninh Co estuary
It can be seen clearly from this table:
Highest discharge distributed via Hong estuary, and then followed by Tra Ly estuary.
Well-mixed salinity observed at two stations Ba Lat and Dong Quy. Discharge flush out salt water seawards, salinity decrease rapidly (Figure 6)
Smallest rate obtained in Ninh Co estuary.
Highest salinity value observed at Phu Le station on this estuary branch. Partial mixed salinity obtained during neap tide period.
Apparently the discharge rate at Day estuary is relatively small
It is noted that the sum of the discharge values of all estuaries is not necessary equal to the total discharge (see Figure 4). This is understandable since a certain amount of water can enter or leave the inland channel system and a certain amount of water can go into the estuary branch.
At Ba Lat station, large amount of sea water flowing into the Hong estuary, fresh water is relatively large here, because of being the largest estuary in the RRD system.Â But only outflow observed during the neap tide. The similar trend also obtained at Dong Quy station in Tra Ly estuary. This may explain why in two estuaries salinity is relatively lower.Â During the neap tide period, the outflow is much higher than inflow, most turbulent energy is supposed for the mixing that leads to well mixed type of salinity.
The contrast feature can be seen in the Day and Ninh Co estuaries. The partial mixed obtain at Nhu Tan and Phu Le stations. This may be due to the influence of bathymertry.
We have seen that the discharge distributed into RRD branches performs well with the salinity observed in our measurements, salinity measured simultaneously over every branch within the same period. A good overview of how salinity and discharge distribute over the branches of the delta, this implies that a simple model can provide a good insight into a complicated system like the RRD.
4.2 Effect of bathymetry
Topographic features can have an effect on the mixing, based upon the geometry and forcing influences of the estuary. A particular case for a feature affecting mixing is the shape of the estuary entrance.
At the mouth of RRD estuaries, river channel is about 1000 m wide and the bed elevation is about 5 m below mean water level. The river channel in the Hong river and the Tra Ly river gradually deepens and narrows landwards within the distance of 15 km from the river mouth. While it keeps a flat feature in the Ninh Co river and the Day river, the bed level is about 6 m and 8 m below the mean water level, respectively.
At these two rivers, large tidal discharge flows back and forth through an identical cross sectional area and hence velocity difference between the surface and bottom layers is relatively small and mixing rates are low.
In the Hong and the Tra Ly rivers, the bathymetry has a complex change in bed elevation; it is shallow at the entrance of the estuary and deepens landwards. Additionally, the bathymetry at the Hong river is characterized by barrier-spit system close to the estuary mouth. The river flow decelerates as it enters the Tonkin Gullf, depositing sandy sediments that form the river mouth bar (Maren, 2007). Based on the previous studies on RRD, the period of the rising stage of the tide (inflow) is shorter than that of the falling stage of the tide (out-flow): 42% and 58% of time, respectively. The same tidal prism flows through an identical cross-sectional area in a shorter time span, and therefore there it can be expected that inflow velocities exceed outflow velocities, resulting in a flood-tide asymmetry. However, this flood-tide asymmetry might be compensated by river outflow which is in the direction of the ebb currents. In the dry season, this fresh water flow balanced the tidal asymmetry in the main channel resulting in equal inflow and outflow velocities, although the duration of outflow was longer.
Scenario 1: Changing river discharge in the upstream
Based on the hydrological database at the Son Tay station from 1996 to 2006, the averaged discharge varies from 900 to 1500 m3/s. In order to examine the sensitivity of longitudinal salinity distributions correspond to the ratio of water discharge in the RRD estuarin system, we divided the discharge at Son Tay station into three levels: 800 m3/s, 1200 m3/s, and 1600 m3/s. Numerical simulations are carried out throughout the estuarine system. According to this indicated ratio, the rate of discharge flows into RRD estuary branches in our computations proved the similar ratios that are highest in the Hong river, lowest in the Ninh Co river, and moderate in the Day and Tra Ly rivers, respectively.
When the fresh water increases (Figure 6), as predictable, the intrusion length by saltwater will be decreased. There is an upstream shift in the brackish saltwater due to the influx of river flow during the dry season. For example, if we define a discharge of 800 m3/s at Son Tay station as a threshold saline intrusion, the intruded length reduces 4% and 7% at the Tra Ly river, 3% and 5% in the Ninh Co river, 8% and 11% in the Day river, 21% and 34% in the Hong river when discharge increased by 50% and 100%. However the declines in saline intruded length are relatively small compared to the increased discharge rate except the Red river. The salinity profile for varying river discharges does not show a noticeable change in the Tra Ly, Ninh Co, and Day rivers. A closer look at these profiles reveals that the calculated result is not sensitive, the outflow discharges in the dry season are too small that can ease the saline intrusion.
In spite of small reduction of saltwater intrusion length, the adjustment of salinity profiles are relatively large in the distance of 10-25 km from the river mouths. One can find that at the high water in the spring tide, the water keeps the high saline density from the river mouth to the distance of 10 km, and decreases the density rapidly to the next 15 km and slowly to the end of calculation segment for the Hong river. In contrast, the density profile decreases gradually from the river mouth to the upper stream in the same condition for the Ninh Co river. The other trends of salinity intrusion look like although these patterns are not peculiar (see Figure 7).
The result of the analysis demonstrates that there are two types of estuary based on the trend of saline intrusion curve: bell shape and dome shape (Savenije, 2005). The former type can be seen in the Ninh Co river, the narrow width in the upstream cross-section but strongly funnel shaped near the river mouth. The latter type is typically obtained in the Hong river characterized by wide channel with a pronounced funnel shape and sand bars at the mouth. Both rivers have wider and shallower features at the river mouth, however the salinity differences are relatively high. The partially mixed and well-mixed estuaries in these rivers of the RRD estuary system probably due to the differences in discharge rate and shape of river bathymetry. Other factors as wind, wind-wave forcing are also important to turbulent mixing, but they will not be treated in this paper.
Scenario 2: Increasing sea water level at the Tonkin Gulf
Flow distribution in each tributary of the Red river