Viet-Thanh NGUYEN, Jin-hai ZHENG* Ji-sheng ZHANG

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

2. College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, P. R. China

3. Faculty of Civil Engineering, University of Transport and Communications, Hanoi, Vietnam

Mechanism of back siltation in navigation channel in Dinh An Estuary, Vietnam

Viet-Thanh NGUYEN1,2,3, Jin-hai ZHENG*1,2, Ji-sheng ZHANG1,2

1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

2. College of Harbor, Coastal and Offshore Engineering, Hohai University, Nanjing 210098, P. R. China

3. Faculty of Civil Engineering, University of Transport and Communications, Hanoi, Vietnam

The Dinh An Estuary is one of the Nine Dragon estuaries of the Mekong River. An international navigation channel was built in the estuary for vessels traveling from the South China Sea to the southwestern area of Vietnam and then to Phnom Penh, Cambodia. The morphological evolution of the navigation channel is complicated and unstable. The back siltation intensity in the navigation channel has largely increased and been concentrated in the curvature segments of the channel since 1980. In this study, based on simulation results and measured data, five key factors that influence the back siltation in the navigation channel were systematically analyzed. These factors included the increasing elevation gap between the channel and the nearby seabed, the disadvantageous hydrodynamic conditions, sediment transport, mixing of saltwater and freshwater, and wave effects in the navigation channel. It is shown that the back siltation to a large extent results from the low current velocity of the secondary ocean circulation, which often occurs in the curvature segments of the channel. Suspended sediment also settles in the channel due to the decrease of the current velocity and the sediment transport capacity when flow passes through the channel. The changes of hydrodynamic conditions are responsible for the majority of the severe siltation in the curvature segments of the navigation channel.

back siltation; hydrodynamics; navigation channel; sediment transport; Dinh An Estuary

1 Introduction

The Dinh An Estuary is one of the Nine Dragon estuaries of the Mekong River. In the estuary, an international navigation channel was built for vessels traveling from the South China Sea to the southwestern area of Vietnam and through the Vam Nao Pass to the Mekong River and Phnom Penh, Cambodia (Fig. 1). Can Tho Port is the largest port on the Bassac River for passing vessels with 10 000-dead weight tonnage (DWT) berths, and it is about 120 km from theDinh An Estuary. The morphological evolution of the navigation channel in the Dinh An Estuary is unstable. The channel consists of many shoals and crescent bars. At the current stage, only vessels of less than 5 000 DWT can pass through the navigation channel. The channel needs to be dredged twice a year and a significant budget is allocated to maintain the navigation conditions. Sometimes, sediment is deposited in the navigation channel immediately after dredging (An 2005; Nguyen et al. 2011).

Fig. 1Location of Dinh An Estuary (Portcoast Consultant Corporation 2006)

The morphological evolution of the navigation channel in the Dinh An Estuary is very complicated. Nguyen et al. (2011) showed that the morphological evolution of the navigation channel depends on the crescent bar systems. Back siltation in the navigation channel plays an important role in the channel evolution, attracting a large amount of attention from coastal researchers (Hung 1988; An 1994). The main features of back siltation in the navigation channel during the period from 1980 to 2005, including the influence of monsoons, were described in Nguyen (2012) and Nguyen et al. (2011). The back siltation intensity in the navigation channel of the Dinh An Estuary has largely increased and been concentrated in the curvature segments of the channel since 1980. Back siltation can be induced by various physical processes. Investigation into the depth-integrated sediment flux suggests that at least 95% of the sediment is transported to the sea and deposited within a distance of 20 km from the coast. During the flood season, a fraction of sediment (approximately 5%) returns to the estuary with salt wedges. The particle size of suspended sediment with a median diameter of 0.003 mm at ebb tide is smaller than that at flood tide, and the clay portion of suspended sediment in the saltwater region (20% to 30%) is larger than that in the freshwater region (15% to 20%) in the estuary. Wolanski et al. (1996) investigated the pathway of sediment transport in the Mekong River’s estuaries. In the high-flow season, the bed is deposited and eroded periodically by the tides in the freshwater region, but the sediment is still transported downstream at a mean velocity of 0.5 m/s to 1.0 m/s. In the low-flow season, the partially well-mixed estuarine processes prevail and enable the sediment to move from the coastal zone to the maximumturbidity zone. Such upstream pumping of sediment is further enhanced by wave actions in shallow coastal waters.

The Dinh An Estuary can be characterized as both a delta- and trumpet-shaped estuary (Hau 2005; An 2005; Nguyen et al. 2011; Nguyen 2012). It has the potential to become a delta estuary with a prismatic shape for which the tidal influence is smaller compared to the runoff influence.

According to the classification of tidal range, the Dinh An Estuary is a meso-tidal estuary with a tidal range of between 2.5 m and 4.14 m (Thuy 1989; An 2005; Nguyen et al. 2011). As discussed by An (2005), Nguyen et al. (2011), and Nguyen (2012), the Dinh An Estuary is mainly dominated by tidal hydrodynamics, freshwater runoff, and sediment transport from the Bassac River with a high transport rate of about 22 million tons per year.

The wind season in the Dinh An Estuary can be classified as three different seasons. The northeast wind season is from November to March, the transition season is from March to May, and the southwest wind season is from May to October. The monsoons may cause large differences in river discharge and wave characteristics between the flood and dry seasons. The flood season begins in late May and ends in October. At the Can Tho monitoring station on the Bassac River, the maximum monthly average discharge in the flood season during the period from 2000 to 2004 was about ten times that in the dry season (Nguyen et al. 2011; Nguyen 2012).

Waves generated by the summer southwest monsoon are mostly directed from the south to the southwest, whereas the winter northeast monsoon results in relatively strong waves directed from the north to the northeast. The bimodal distribution of the wave direction in spring and autumn shows the waves are in transitional periods. The waves from the north and northeast are generally higher than those from the south and southwest. The directions of ocean currents show similar seasonal trends compared with those of waves. Sediment transport is mainly southwestward- and northeastward-directed in dry and flood seasons, respectively.

In this study, to investigate the mechanism of back siltation in the navigation channel in the Dinh An Estuary, we systematically analyzed the influence of five key factors, including the increasing elevation gap between the channel and the nearby seabed, the disadvantageous hydrodynamic conditions, sediment transport, mixing of saltwater and freshwater, and wave effects, on back siltation in the navigation channel in the Dinh An Estuary.

2 Data and methodology

2.1 Back siltation data

The temporal variation of back siltation in the navigation channel from 1981 to 2004 is shown in Table 1 (Nguyen 2012). The results show that the back siltation intensity in the flood season is mostly larger than in the dry season.

Table 1Temporal variation of back siltation in navigation channel in Dinh An Estuary

Measurement data from 2000 to 2005 were used to investigate the spatial variation of back siltation. As shown in Fig. 2, the channel is divided into 17 segments (S1 to S17), separated by 18 measuring points (C1 to C18). The spatial variation of the back siltation intensity in the navigation channel is shown in Fig. 3. Fig. 3(a) shows that the back siltation intensity ranges from 0.12 to 6.55 mm·m-2·d-1, with an increasing tendency in the offshore direction. The peak of back siltation intensity occurred in segment S10 in 2000, and it moved to segment S12 in 2001 and from 2003 to 2005. In 2002, the peak of back siltation took place in segment S11. It is shown that the most serious back siltation was mainly concentrated in segments S11 to S13, and its peak value occurred in segment S12.

2.2 Methodology

Both measured data and simulated results were used to study the mechanism of back siltation in the navigation channel in the Dinh An Estuary. In the numerical simulation, a coupledmodel combining MIKE 21HD and MIKE 21SW and a model of cohesive sediment transport in the Dinh An Estuary were adopted. As shown in Fig. 4, the computational domain covered the Dinh An Estuary and its adjacent areas. The reference coordinate system used in the model was the projected Universal Transverse Mercator (UTM) coordinate system. Thex-origin was located 500 km west of the central meridian, and they-origin was at the Equator. The numerical mesh consisted of 16 364 nodes and 22 355 elements based on quadrangular grids in rivers, the coastline, and the navigation channel, and triangular grids in coastal areas. The models were calibrated and verified by field data collected at seven locations (T1 to T7) in May and September, 2004 (Nguyen 2012).

Fig. 2Arrangement of segments in navigation channel in Dinh An Estuary

Fig. 3Spatial variation of back siltation intensity in navigation channel in Dinh An Estuary

Fig. 4Numerical model domain of Dinh An Estuary

3 Results

3.1 Elevation gap between channel and nearby seabed

After dredging, the two banks of the channel tend to form a natural slope. The elevation gap between the channel and the nearby seabed is one of the key parameters affecting the back siltation intensity. A large elevation gap leads to a high intensity of back siltation. The measured elevation gaps during the period from 2001 to 2004 are shown in Fig. 5. It can be seen that large elevation gaps mainly occurred in segments S10 to S16 for the left bank and segments S12 to S15 for the right bank.

Fig. 5Elevation gap between channel and nearby seabed

3.2 Hydrodynamic factors

3.2.1 Ocean circulation and flow convergence

The transport path and residence time of dissolved constituents and suspended sediment in estuarine water are influenced by residual circulation patterns. These circulation patterns arecontrolled by tidal currents, which interact with the geometry, bathymetry, wind, geophysical rotation, freshwater inflow, and balance between barotropic (water surface) and baroclinic (density) pressure gradients (Warner et al. 2006). In the South China Sea, the ocean circulation is mainly dominated by monsoon winds. The main direction of ocean circulation is from the north-northeast to the south-southwest in winter and from the south-southwest to the north-northeast in summer.

In the dry season, the tidal current normally flows from the south-southwest to the north-northeast. The secondary ocean circulation is induced by the general summer circulation from the south-southwest, mixed with the ebb current. This circulation often occurs in the curvature segments of the navigation channel and moves to the south-southwest. The current velocity of the secondary ocean circulation is very small, leading to the back siltation in the navigation channel (Fig. 6(a)). In the flood season, the secondary ocean circulation is mainly due to the convergence of the high ebb tide flow from the Dinh An, Co Chien, and Cung Hau estuaries and the long-shore current from the southwest to the northeast on the shallow beach in front of the Cu Lao Dung Island (Fig. 6(b)).

Fig. 6Flow convergence and secondary ocean circulation in Dinh An Estuary

3.2.2 Current flow passing through navigation channel

When the flow passes through the navigation channel, its velocity decreases with the increase of the water depth. This reduces the transport capacity of the bed load and suspendedload. As a result, the bed-load particles and a certain amount of the suspended sediment particles are deposited in the channel (van Rijn 2004). When waves co-exist with currents, this process is considerably complicated due to the wave-current interaction (van Rijin 2004; Zheng et al. 2008). The waves may largely enhance the stirring of sediment in the near-bed region, and the sediment can be subsequently transported by the flow (Zhang et al. 2011). The navigation channel in the Dinh An Estuary often suffers from the formation of bars and shoals at the transition region of the flood-dominated and ebb-dominated channels. A large channel depth or width, a channel orientation that is almost normal to the flow, strong flows and large waves, and fine bed material are the main factors enhancing the sedimentation in the channel (Fig. 7).

Fig. 7Flow passing through navigation channel in Dinh An Estuary

3.3 Mixing of freshwater and saltwater

The mixing of freshwater and saltwater is an important process for most estuaries (Zheng et al. 2002). The observed data show that the salt intrusion in the Dinh An Estuary exhibits a dome-shaped intrusion curve, as commonly observed in funnel-shaped estuaries (Thuy 1989; Savenije 2005). Nguyen and Tanaka (2007) studied the relation between morphology variation and salinity distribution in the Dinh An Estuary. They found that the change of salinity is influenced not only by hydrodynamic parameters (e.g., river discharge and tidal level), but also by the changes of the estuarine morphology. For instance, the mean salinity was found directly proportional to the river mouth width. Therefore, the influence of morphology variation onsalinity distribution should not be ignored.

The characteristics of the suspended load are significantly related to the sediment deposition (Zhang et al. 2012; Shao et al. 2011). In the upstream region of the Dinh An Estuary, the median diameter of suspended sediment is about 0.001 to 0.06 mm with a fluid salinity of 0.1% to 1.5% (Wolanski et al. 1996; Nguyen 2012). The upstream area is preferable for flocculation, saltwater wedges, and current stagnation, and it is also found to be the area of maximum estuarine turbidity. Therefore, back siltation is concentrated at the curvature segments of the navigation channel with higher siltation intensities in flood seasons than in dry seasons.

3.4 Sediment transport

Bed-load transport was investigated from September 2003 to April 2004 by the radiotracer method. It is shown that bed-load transport affected the back siltation in the navigation channel, especially in the offshore segments of the channel. However, the induced back siltation intensity was not very high. The effective thickness of transport substrate was 0.05 m, and for six months the central mass of the radiotracer elements could not pass through the navigation channel (An 2005; Nguyen et al. 2011; Nguyen 2012).

As shown in Fig. 8, the simulation results for the dry season (May 2004) and the flood season (September 2004) indicate that the sediment transported from the river to the estuary is deposited in front of the Cu Lao Dung Island. Meanwhile, due to the secondary ocean circulation, the mud from the river was firstly transported to the northeastern estuary, and then to the shore in the southwest direction. This can be explained by the deposition area in the northeastern navigation channel.

Fig. 8Simulated change of bed thickness after 20 days in Dinh An Estuary

3.5 Wave effects

Nguyen (2012) investigated the influence of waves on the back siltation in the navigation channel in the Dinh An Estuary from 18 measuring points (C1 to C18) along the navigation channel. As illustrated in Fig. 9, the impact of waves on the bed thickness change is very small. This can be explained by the occurrence of small wave heights during both dry and flood seasons.

Fig. 9 Wave impact on back siltation in navigation channel

The back siltation in the navigation channel is essentially dominated by the waves with large heights and low occurrence frequencies. Previous studies show that waves with a height of 2.36 m, a period of 8.0 seconds, and an incident direction of 75° are responsible for 36% of the back siltation, but their occurrence frequency is only 9.1%. Although waves with a maximum significant wave height of 3.58 m and a period of 9.6 seconds have an occurrence frequency of only 1.1 %, they controls over 10% of the back siltation. On the other hand, waves with a height of less than 0.56 m and a period of 3.3 seconds have an occurrence frequency of 42.38%, but only dominate 6.0% of the back siltation in the navigation channel (Portcoast Consultant Corporation et al. 2009).

4 Conclusions

Topography, tide currents, sediment load, and salinity have significant impacts on back siltation in the navigation channel in the Dinh An Estuary. The mechanism of back siltation involves the increasing elevation gap between the channel and the nearby seabed, the disadvantageous hydrodynamic conditions, sediment transport, mixing of saltwater and freshwater, and wave effects in the navigation channel. The secondary ocean circulation with a low current velocity occurring in the curvature segments of the channel results in a large amount of back siltation. When flow passes through the channel, the decrease of the current velocity due to the increasing water depth reduces the sediment transport capacity of flow and leads to a large amount of suspended sediment settling in this region. The sediment transported from the river to the estuary is deposited in front of the Cu Lao Dung Island. Mixing of saltwater and freshwater has an effect on morphology variation and causes the back siltation to concentrate in the curvature segments of the navigation channel. It was also found that waves with large heights and low occurrence frequencies are much more effective in back siltation. The changes of hydrodynamic conditions are responsible for the majority of the severe siltation in the curvature segments of the navigation channel. This study suggests that a better understanding of sediment transport and navigation channel evolution in the Dinh An Estuary will be helpful for regulation work in the future.

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(Edited by Pei WANG)

This work was supported by the 322 Project of Vietnam International Education Development, Ministry of Education and Training, Vietnam (Grant No. 322), the Qing Lan Project, the 333 Project of Jiangsu Province (Grant No. BRA2012130), the Fundamental Research Funds for the Central Universities (Grant No. 2012B06514), and the 111 Project (Grant No. B12032).

*Corresponding author (e-mail: jhzheng@hhu.edu.cn)

Received Aug. 31, 2012; accepted Feb. 21, 2013