simulation by delft 3d sediment transport according to ... qua du an... · (con dao, my thuan, vung...

Report on Task

Simulation by Delft 3D sediment transport

according to three situations: Northeast,

Southwest winds, and inter-monsoon season and

comparison with ROM3D

LMDCZ project: 3D modelling for sediment transport in the LMDCZ (WP2)

3

1. Introduction

The Mekong River is ranked as the 8th in terms of water discharge and as the 10th

in terms of sediment load in the world. During the last 4500 years, its delta prograded

more than 250 km to the south due to a tremendous amount of sediments deposited,

and turned from a “tide-dominated” delta into a “wave-and-tide dominated” delta.

This study aims at completing our knowledge on the fate of sediments that may be

stored in estuarine or coastal systems, or dispersed over the continental shelf and

slope. Sediment transport in the Mekong River Delta (MRD) coastal area was

studied by numerical simulations using the Delft3D model.

The model configuration was with 3 dimension, calibrated and validated from data

collected in situ and satellite image data.

This report will presented these results. The structures of the report:

- Introduction: give a summary of the goals of the study

- The seconds, supplied information about methods and material what is used

to implement the study.

- The third of report show the results about hydrodynamics condition, sediment

transport in the study area.

Finally, conclusions and recommendation of the report were presented in the

seventh of the report.

- Modeling hydrodynamics, wave transition, and sediment transport with some

scenarios in the study area.

- Setting up the model with some scenario: SW monsoon, NE monsoon and

transitional season

2. Data and methodology

2.1. Data

In order to implement the target of this study, data below was collected:

- Hydrometeorological data: wind speed and direction, water elevation at station

(Con Dao, My Thuan, Vung Tau, An Thuan, and Rach Gia). These data were

measured during 2013-2017 by National Hydro-Meteorological Central.


4

- Wind, pressure air, temperature (6h, 2.5 deg., NCEP) also will be used for

model.

- Harmonic constant group: group of harmonic constant in the coastal zone from

database Fes2014b (Lyard et al., 2016).

- Bathymetry group: bathymetry was digitalized from bathymetry map of

Mekong coastal provinces with scale: 1:50000 (provided by SIWRR). Bathymetry in

offshore area was used bathymetry database of GEBCO 2014 grid (30 arc-second

spacing)

- Measured water river discharge, SPM concentration at My Thuan Station (Tien

River) and Can Tho (Hau River) were used for river boundaries condition (Figure 1).

- Others data included water salinity, temperature from World Ocean Atlas 2013

(WOA, 2016).

- Measured data from the project in two survey campaign: flood season (October

and November 2016, dry season (February and March 2017). In each survey, wave

high, currents and SPM were measured at Go Cong and Uminh (Figure 1).

Figure 1. Position of hydrometeorology Station and measurement point of the project

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5

2.2. Model setup

Delft3d model

In this study, hydrodynamics (resulting from tides, currents and waves) and

suspended sediment transport were simulated using the Delft3D-Flow module and

the Delft3D-Wave module (based on the SWAN model). The Delft3D-Flow model

developed by Deltares (Delft, The Netherlands) is a 3D modeling suite to investigate

hydrodynamics, sediment transport and morphology, and water quality for fluvial,

estuarine and coastal environment (Deltares, 2014). Delft3D solves the Reynolds-

averaged Navier-Stokes equations, including the k‐ turbulence closure model, and

applies a horizontal curvilinear grid with σ-layers for vertical grid resolution.

SWAN is used as wave model (Booij et al., 1999; Ris et al., 1999, Deltares,

2014). In the action balance equation (see e.g., Jouon et al., 2009), the JONSWAP

expression (Hasselmann et al., 1973) is chosen to express the bottom friction dissipation,

with the parameter Cjon = 0.067 m2·s−3 proposed by Bouws and Komen (1983 for fully

developed wave conditions in shallow water. The model from Battjes and Janssen

(1978) is used to model the energy dissipation in random waves due to depth-induced

breaking waves.

Overall gird

In order to create open boundary condition for Mekong coastal area model, a

overall model was setup. The overall gird and bathymetric grid of the model are built

base on bathymetry database from GEBCO. Model frame covers all South China Sea.

The horizontal gird is orthogonal curvilinear co-ordinates with model frame split to

136 x 314 grid points. Grid size varies between 853 to and 6300m (figure 2). The

coordinate was used for vertical gird. Number layers of vertical gird are 24

(2%,2%,3%,3%,3%,3%,4%,4%,4%,4%,4%,4%,5%,5%,5%,5%,5%,5%,5%,5%,5%,

5%,5%,5%,5% of depth H for each layer). In order to assess influenced of layer

number on the modelling results, a model with 12 vertical layer (very thin in the

surface) was setup: 2%,3%,5%,7%,9%,11%,14%,15%,12%,10%,7% and 5% of

water depth. Except of the vertical layer, others parameters of this model was setup

same as model of 24 vertical layer

Mekong coastal area model- detailed model

The Mekong coastal area model cover whole all the coastal zone of Ba Ria-Vung

Tau (east part) to Kien Giang (west part). The horizontal orthogonal curvilinear grid


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encompasses 424 x 295 points with grid size between 44 m and 11489 m (figure 2).

Along the vertical, 24 layers in σ-coordinate are considered, similar to the parent

model. The hydrodynamics model takes into account the influences by water

temperature, salinity, and sediment transport and wave actions.

Figure 2. The overall gird of the South China Sea model


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Figure 3. The gird and bathymetric gird of the Mekong coastal area model

Figure 4. The gird and bathymetric gird of the Mekong coastal area model

Boundaries and initial conditions for Mekong coastal model

There are four open river boundaries: Soai Rap, Vamco, My Thuan and Can Tho

(Figure 3). The Soai Rap and Vamco boundaries belong to the Dong Nai–Saigon

River catchment. River water discharge measured every hour in Can Tho and My

Thuan stations were imposed as river boundary conditions. Others river boundaries

were set to 546.9 m3·s−1 (Soai Rap R.) and 52.5 m3·s−1 (Vamco R.) in low flow season;

1310 m3·s−1 in the Soai Rap River and 177.8 m3·s−1 in the Vamco River in flood

season. The time serial SSC values during flood tide and ebb tide at Can Tho and My

Thuan stations were imposed as boundary condition for these two distributaries. SSC

in Vamco and Soai Rap rivers were almost the same and considered in this study to

be worth 55 mg·L−1 in low flow season and 70 mg·L−1 in flood season. At each river

boundary, the same SSC values were imposed from the surface to the bottom.

Sea boundaries condition of the detailed model were nested from the overall


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model results (figure 4).

For firstly running, initial conditions of the model are set with zero (0) value for

water sea level, salinity, suspended sediment concentration. Initial conditions for next

running are used to previously running results (restart file). The initial condition of

the each scenario run is the results one month previously before.

Other parameters

The bottom roughness was specified by a spatial distribution of Manning (n)

coefficients with values in the range 0.018–0.023 m−1/3·s (Arcement and Schneuder,

1989; Simon and Senturk, 1992). The background horizontal eddy viscosity and

horizontal eddy diffusivity were considered to be, after calibration, equal to 7.5 m·s−2.

The Horizontal Large Eddy Simulation (HLES) sub-grid, which is integrated in

Delf3D-Flow, is based on theoretical considerations presented by Uittenbogaard

(1998) and Van Vossen (2000). In this study, the HLES sub-grid was activated to add

calculated results to background values. Two sediment fractions were simulated in

the model: one non-cohesive and one cohesive.

Measured grain sizes of non-cohesive particles in the MRD coastal area range

from 29 to 252 μm (average 113 μm) in flood season (September 2013) and from 15

to 262 μm (averaged 103 μm) in low flow season (April 2014). They are results of

the survey from the Institute of Marine Environment and Resources in 2013-2014.

Therefore, median sand-sized particles of 113 μm and 103 μm were considered in our

simulations in flood and low flow season, respectively. A specific density of 2650

kg·m−3 and dry bed density of 1600 kg·m−3 were considered; all other sand transport

calibration parameters were kept at the default values proposed by Delft3D. Sand

fraction transport was modeled with the van Rijn TR2004 formulation (Van Rijn,

2007), which has been shown to successfully represent the movement of non-

cohesive sediment ranging in size from 60 μm to 600 μm.

Previous observational studies in the Mekong estuary indicated that most of

Mekong sediments are flocculated fine particles. In low flow season, measured

flocsize was 30–40 μm with about 20%–40% of clay content in volume (Wolanski)

(1998). In the flood season, the flocsize was 50–200 m with a 20%–30% clay

content in volume (1996). Recently, Gratiot reported that floc was 0-20% and flocculi

about 80%. In this study, three sediment classes were used for simulation: Sand:

D50= 200m, ws= 50mm/s; mud1: D50=15m, ws=0.03 mm/s; Mud-2: D50=100m,

Ws=0.1 mm/s.


9

For the cohesive sediments, if the bed shear stress is larger than a critical value,

erosion is modeled following the Partheniades’ formulation (1965), whereas if the

bed shear stress is less than a critical value for deposition, Krone’s formula (1962) is

used to quantify the deposition flux. The parameters required to simulate the cohesive

sediment transport include critical bed shear stresses for erosion τcre and deposition

τcrd, the erosion parameter M and the particle settling velocity ws (Doullet et al., 2001).

After calibration, τcrd was set to 1000 N·m−2, which effectively implied that deposition

was a function of concentration and fall velocity (Winterwerp and Kesteren, 2004).

τcre was set to 0.2 N·m−2, and M was set to 2 × 10−5 kg·m−2·s−1.

Scenario Simulation

In order to describe characteristics of sediment transport in the MRD coastal area,

scenarios were set up with different conditions of wave, wind and river discharge. In

this study, two groups simulation were setup:

Simulation group (2013-2014): dry season (December 2013 and January 2014)

and flood season (August – September and October 2014). Initial conditions of the

scenarios result from a previous simulation (either December 2013 for dry season of

August 2014 for flood season).

Validation group (2016-2017), to validation with measured data of the project:

flood season (August, Octobers and November 2016), dry season (January, Ferburary

and March 2017).

Calibration and Validation Process

Water elevation, wave high, current and SPM measure at some station (figure 1)

would be used to valid with model results. The discrepancy between results and

measurements was quantified, for each simulation, using the Nash-Sutcliffe

efficiency number E (Nash and Sutcliffe, 1970), calculated as follows:

𝐸 = 1 − ∑(𝑜𝑏𝑠 − 𝑐𝑎𝑙𝑐)2

∑(𝑜𝑏𝑠 − 𝑚𝑒𝑎𝑛)2

3. Results and discussions

3.1. Model validation


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With water elevation

Water elevation at hydrometeorological stations of An Thuan, Con Dao, Rach

Gia, Vung Tau were used to calibrate and then validate the model. After calibration,

comparisons show good agreement between model results and measurements. At An

Thuan, E coefficients are 0.77 (in January) and 0.72 in September 2014, Figure 5.

The E coefficients at Con Dao are 0.74 for dry season and 0.71 for flood season,

Figure 6. The E coefficients is lower at Rach Gia Station with E = 0.69 (dry season)

and 0.66 in flood season, Figure 7.

(a)

(b)

Figure 5. Comparison between simulations and measurements at An Thuan Station (Ham

Luong R.): (a) in January 2014; (b) in September 2014


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(a)

(b)

Figure 6. Comparison between simulations and measurements at Con Dao Island: (a) in

January 2014; (b) in September 2014


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(a)

(b)

Figure 7. Comparison between simulations and measurements at Rach Gia Station (a)

in January 2014; (b) in September 2014

Validation with measured data of the project

In the frame work of the project some data such as wave, current and suspended

sediment concentration were measured at Gong Cong and U Minh (Figure 1). The

validation results between model and measurement were on Figure 8 to Figure 23.

Validation with satellite image and CROCO model results


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The calculated suspended sediment concentration from the Delft3d model also

validated with satellite image and results of the CROCO model. Comparison

results showed a consistent between two models results and stelline image

data (figure 24 and Figure 25).

(a)

(b)

Figure 8. Comparison between wave high and direction simulations and measurements at

U minh from October 16 to November 1, 2016 ((a) wave high; (b) wave direction)


14

(a)

(b)


Go Cong from October 16 to November 1, 2016 ((a) wave high; (b) wave direction)


15

(a)

(b)


U minh from February 25 to March 12, 2017 ((a) wave high; (b) wave direction)


16

(a)

(b)


Go Cong from February 25 to March 12, 2017 ((a) wave high; (b) wave direction)

(a)

(b)

Figure 12. Comparison between current velocity simulations and measurements at U minh from October 16 to November 1, 2016 ((a) measure;

(b) model)


18

(a)

6

(b)

Figure 13. Comparison between current direction simulations and measurements at U minh from October 16 to November 1, 2016 ((a) measure;

(b) model)


19

(a)

(b)

Figure 14. Comparison between current velocity simulations and measurements at Go Cong from October 16 to November 1, 2016 ((a) measure;

(b) model)


20

(a)

(b)

Figure 15. Comparison between current direction simulations and measurements at Go Cong from October 16 to November 1, 2016 ((a)

measure; (b) model)

(a)

(b)

Figure 16. Comparison between current velocity simulations and measurements at U minh from February 25 to March 12, 2017 ((a) measure;

(b) model)


22

(a)

(b)

Figure 17. Comparison between current direction simulations and measurements at U minh from February 25 to March 12, 2017 ((a) measure;

(b) model)


23

(a)

(b)

Figure 18. Comparison between current velocity simulations and measurements at Go Cong from February 25 to March 12, 2017 ((a) measure;

(b) model)


24

(a)

(b)

Figure 19. Comparison between current direction simulations and measurements at Go Cong from February 25 to March 12, 2017 ((a) measure;

(b) model)

(a)

(b)

Figure 20. Comparison between SSC simulations and measurements at U Minh from October 16 to November 1, 2016 ((a) measure; (b) model)


26

(a)

(b)

Figure 21. Comparison between SSC simulations and measurements at Go Cong from October 16 to November 1, 2016 ((a) measure; (b) model)


27

(a)

(b)

Figure 22. Comparison between SSC simulations and measurements at U minh from February 25 to March 12, 2017 ((a) measure; (b) model)


28

(a)

(b)

Figure 23. Comparison between SSC simulations and measurements at Go Cong from February 25 to March 12, 2017 ((a) measure; (b) model)

Figure 24. Comparison monthly SPM distribution in January between (a) Satellite January

2002-2012, (b) CROCO model, (c) Delft3d model (kg/m3)

(c)

(a)

(b)


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Figure 25. Comparison monthly SPM distribution in October between (a) Satellite

October 2002-2012, (b) CROCO model, (c) Delft3d model

(a)

(b)

(c)


31

3.2. Flow velocity and suspended sediment distribution

Flow velocity in the Mekong coastal area are integrated of tide, wind driven and

river flow. The results showed the current in this area changing with tidal oscillation,

wind driven current and river driven current. Among them, wind and water river

discharge variation with the season. Therefore, integrated current showed a change

with tide and the season.

In the ebb tide, flows direction are almost from estuaries to the coastal zone and

northeast come to southwest. Results of the model showed magnitude currents are

prevalent variation between 0.2m/s and 1.2m/s. The current velocities are high in

estuary area (where intensified by water river) and Bac Lieu-Ca Mau coastal area

(Figure 26-a).

Otherwise, in the flood tide, flow direction are almost come from offshore in to

estuaries region and from southwest to northeast. Magnitude currents in this tidal

phase are prevalent variation between 0.2m/s and 1.1m/s. The current velocities are

high in Bac Lieu-Ca Mau coastal area (Figure 27-a, Figure 31-a).

At the high tide or low tide, integrated flows in this area are complicated

variation by the interaction of wind driven current and river driven current.

The integrated flow in the coastal zone of Mekong also show an interaction

between tidal current and wind driven current and river driven. If tidal current is

strongly, the integrated current is propertied of the tidal current. However, in some

case when wind driven current and river driven current become stronger, causing a

change of field current vector in this area. Ex. during in September, when wind

direction prevalent in SW, integrated current vector in the surface layer changed with

same direction of wind, even in the ebb tide (figure 30-a).

On the other hand, the interaction between tidal currents were opposite direction

with wind driven – river current, creating flow converging region (in flood tide) or

flow diverging region (ebb tide) in Bac Lieu coastal area, Figure 28, Figure 32 and

Figure 29, Figure 33). These phenomena occurred not only in low flow season but

also in flood season. They also came in sometime in both ebb tide and flood tide

(figure 28 and Figure 29). These phenomena could be one of causes that made

strengthen the erosion process in coastal zone of Ganh Hai- Dam Doi (between Bac

Lieu an Ca Mau province).


32

Figure 26. Flow velocity vector and suspended sediment concentration (mg/l) at layer 1(a)

and layer 24(b), ebb tide, January 20, 2014 (dash line --- depth contour: 5, 10, 20, and

25m)

(a)

(b)


33


and layer 24(b), flood tide, January 16, 2014

(a)

(b)


34

Figure 28. The interaction of tidal current (flood tide) and wind driven –river driven at

layer 1(a) and layer 24(b), January 04, 2014

(b)

(a)


35

Figure 29. The interaction of tidal current (ebb tide) and wind driven –river driven at

layer 1(a) and layer 24(b), January 04, 2014

(a)

(b)


36


and layer 24(b), ebb tide, September 14, 2014

(a)

(b)


37


and layer 24(b), flood tide, September 16, 2014

(a)

(b)


38

Figure 32. The interaction of tidal current (flood tide) and wind driven –river driven at

layer 1(a) and layer 24(b), September 23, 2014

Interaction of tidal current (flood tide) and wind-river driver current, Sep

(a)

(b)


39

Figure 33. The interaction of tidal current (ebb tide) and wind driven –river driven at

layer 1(a) and layer 24(b), September 23, 2014

(a)

(b)


40

Similar to the flow velocity, the suspended sediments distribution in the Mekong coastal

area depend on sediment flux from the river and hydrodynamics in the coastal area.

Modelling simulation results showed SSC variation in large range from 10mg/l to bigger

than 100mg/l (Figure 26 to Figure 33). The variation of SSC was higher in estuaries region

where strong interaction between fresh (high SSC) and marine water (low SSC).

Figure 34. Flow velocity vector and salinity (psu) at layer 1(a) and layer 24(b), ebb tide,

January 28, 2014

(a)

(b)


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Figure 35. Flow velocity vector and salinity (psu) at layer 1(a) and layer 24(b), flood tide,

January 29, 2014

(a)

(b)


42


half last of Septemver, prevailing wind SW

(a)

(b)


43


half last of September, prevailing wind SW

(a)

(b)


44


half last of September, not prevailing wind SW

(a)

(b)


45


half last of September, not prevailing wind SW

(a)

(b)


46


half first of October

(a)

(b)


47


half first of October

(a)

(b)


48


half last of October.

(a)

(b)


49


half last of October

3.3. Monthly flow velocity distribution

The monthly flow velocities in the coastal zone of Mekong are results of the influence

of wind driven current and the fresh water. The distribution and direction of monthly flow

decides moving of water as well as sediment flux. Model results showed the averaged flow

in the Mekong coastal area were strong changed by season (wind direction and fresh water).

(a)

(b)


50

In the low flow season, the averaged flows were strongest in the surface layer, with

velocity change in 0.15-0.5m/s. They were higher in coastal area of Dinh An, Soc Trang,

Bac Lieu, and a part of western coastal of Ca Mau (Figure 44). The averaged flow directions

in this season were dominated from northeast to southwest, same direction with NE dominant

wind in the winter. The result also showed field current almost same among first half of

January, last half of January and all of January (Figure 44). They showed a stabilization of

wind direction as well as averaged flow direction in the low flow season. The model results

showed a small gradually decreased of flow magnitude from the surface layer (Figure 14) to

lower layer (Figure 45 to Figure 48). Flow magnitude at layer 16 changed in 0.05-0.15m/s

(Figure 48), it indicated a low stratification in the low flow season.

In the flood season, with the interaction of strong fresh water and the variation of wind

direction, the averaged flows were gap between regions both velocity magnitude and flow

direction.

- The averaged velocity in the flood season (September and October) was high in the

Mekong estuary region, with velocity change in 0.1-0.5m/s (at surface layer, Figure 49 and

Figure 52) and strong decreased at lower layer (Figure 50 and Figure 51, Figure 53-56), and

very small at lower layer. At the layer 8 (Figure 50, 54), the averaged velocity was very

small (almost below 0.15m/s), this differed from similar layer in the January (see Figure 51

and Figure 46). This showed a strong stratification occurring in the Mekong coastal area in

the flood season compared with in the low flow season.

- The directions of the averaged field current in the flood season changed with the

interaction of wind and fresh water. In the first half of September, when SW monsoon still

stronger, direction of the averaged field current were almost from SW to NE, fresh water

from the river also moved northeastern from the estuary region (Figure 49-a). Otherwise, in

the last half of September, with weak SW wind, direction of fresh water changed, and

moving straight from the river to the offshore (in SE direction), Figure 49-b. The change of

flow direction continued occurring in first half of October, fresh water moved to southward

(Figure 52-a). On the other hand, in the last of October, NE wind developed and because

their direction was almost same with flow direction, it strengthened moving of fresh water

from estuary region to southwest of Mekong coastal area (from Mekong estuary region to

coastal of Ca Mau), Figure 52-b.

Above results showed that wind is main factor contributed to the averaged field current

in the coastal zone of Mekong in the low flow season. Otherwise, fresh waters were main

factor that created residual current in the flood season. However, direction of residual current

were sensitively with change of wind direction in the flood season.


51

Figure 44. Averaged flow velocity at layer 1: first half of January (a), last half January

(b), and all of January

(a)

(b)

(c)


52



(a)

(c)

(b)


53



(a)

(b)

(c)


54



(a)

(c)

(b)


55



(a)

(b)

(c)


56

Figure 49. Averaged flow velocity at layer 1: first half of September (a), last half

September (b), and all of September

(a)

(c)

(b)


57


September (b), and all of September

(a)

(c)

(b)


58


September (b), and all of September (c)

(a)

(c)

(b)


59

Figure 52. Averaged flow velocity at layer 1: first half of October (a), last half October

(b), and all of October

(a)

(b)

(c)


60



(a)

(b)

(c)


61



(a)

(b)

(c)


62



(c)

(c)

(c)


63



(c)

(a)

(b)

(c)


64

3.4. Profiles distribution of current, salinity and suspended sediment

In order to assess stratification of current, salinity and suspended sediment,

profiles distribution of current, salinity and suspended sediment were extracted at

Tien Giang, Cung Hau estuary and Dinh An estuary (Figure 57). The results

simulation showed distribution of profile current, salinity and suspended sediment

strongly change with tidal oscillation and seasons.

In the dry season (January), influences of fresh water (low salinity) were limited

in about 25km to offshore at Tien Giang (Figure 58-b and Figure 59-b), 30km at Cung

Hau (Figure 64-b and Figure 65-b), and 35km at Dinh An transect (Figure 70-b and

Figure 71-b). Meanwhile, high SSCs area (from river) also were limited in about

30km at Tien Giang (Figure 58-c and Figure 59-c), 35km to offshore from Cung Hau

estuary (Figure 64-c and Figure 65-c), and 40km from Dinh An estuary (Figure 70-c

and Figure 71-c). The stratification of current, salinity and suspended sediment in the

dry season at these estuaries were weak with a small variation in the water column

(Figure 58-59, Figure 64-65 and Figure 70-71).

In the flood season (September and October), under strong influences of river

discharge from the river, fresh water with low salinity and higher SSC could be

reached more further to offshore, especially in the upper layers. At Tien Giang

transect (Figure 57-a), fresh water (with low salinity and higher SSC) from the river

could reach to more than 35km from estuary to offshore (Figure 60- 61 and Figure

62-63). At Cung Hau estuary transect, freshwater from the river in the flood season

could go to offshore further more than 45km (figure 66-67 and figure 68-69).

Meanwhile, at Dinh An estuary transect, fresh water from the river in the flood season

could go to offshore about 60km from the river (figure 72-73 and figure 74-75). The

results showed having a strong stratification of salinity and suspended sediment in

the flood season. The fresh water showed influence on upper layers with 1.5-2m

surface. However, sphere of influence of freshwater in the flood season changed with

intensity of water river discharge: they are higher in September (Figure 60-61, Figure

66-67, Figure 72-73) compared with October (Figure 62-63, Figure 68-69, and Figure

74-75).

The results also a strong stratification of flow in the flood season. There are a

different both flow direction and magnitude between in upper layers and lower layers

(Figure 60-75).


65

Figure 57. Position of along estuary transect: Tiền Giang (a), Cung Hầu (b), Định An (c)

(a)

(b)

(c)


66

Figure 58. Profile velocity (a), salinity (b), and suspended sediment –c (mud 1) in

ebb tide, Jan., Tien Giang transect

(a)

(c)

(b)


67


flood tide, Jan., Tien Giang

(a)

(b)

(c)


68


ebb tide, Sep., Tien Giang transect

(a)

(b)

(c)


69


flood tide, Sep., Tien Giang transect

(a)

(c)

(b)


70


ebb tide, Oct., Tien Giang transect

(a)

(b)

(c)


71


flood tide, Oct., Tien Giang transect

(a)

(b)

(c)


72


ebb tide, Jan., Cung Hau transect

(a)

(b)

(c)


73


flood tide, Jan., Cung Hau transect

(a)

(b)

(c)


74


ebb tide, Sep., Cung Hau transect

(a)

(b)

(c)


75


flood tide, Sep., Cung Hau transect

(a)

(b)

(c)


76


ebb tide, Oct., Cung Hau

(c)

(b)

(a)


77


flood tide, Oct., Cung Hau transect

(a)

(b)

(c)


78


ebb tide, Jan., Dinh An transect

(a)

(b)

(c)


79


flood tide, Jan., Dinh An

(c)

(a)

(c)

(b)


80


ebb tide, Sep., Dinh An transect

(c)

(a)

(b)


81


flood tide, Sep., Dinh An

(a)

(c)

(b)


82


ebb tide, Oct., Dinh An

(c)

(a)

(b)


83


flood tide, Oct., Dinh An transect

(a)

(c)

(b)


84

3.5. Sediment transport

The sediments transport in the Mekong coastal area were influenced of hydrodynamics

processes and sediment flux from the river. Model results showed that in the low flow

season, the averaged suspended sediment concentration on (SSC) with higher value was

distributed with large region (see Figure 76 to Figure 80) than the flood season (figure 81 to

Figure 88). Meanwhile, higher SSC in flood season almost appeared in closed estuaries or

coastline (see Figure 81 to Figure 88).

The distribution of suspended sediment showed a strengthening moving to southwest

Mekong coastal area (Ca Mau) in the low flow season when dominant wind in NE direction

(Figure 76 to figure 80). However, in this season, sediments flux from the river were very

small. Therefore, source of sediment that transported to Ca Mau coastal area could

resuspension. Meanwhile, in the flood season with bigger sediments flux from the river to

coastal zone but by influence of dominant wind direction in SW (September), sediment

moved to NE and others were settled around estuaries region (Figure 81 to Figure 83). When

wind changed direction in the last of October, fresh water and wind direction were almost

same direction, as a result, sediment from the river strengthened moving to Ca Mau (Figure

84 to Figure 88).

Based on the measurement data and model simulation, sediment flux from the river (via

My Thuan and Can Tho gauge Station) and alongshore transport estimated (Table1, Figure

89 to Figure 91). The results showed in the net sediment flux changed by season and region.

- In the low flow season (January), the sediment flux from the river (via My Thuan and

Can Tho) was about 883277 tons. In the coastal area, the net alongshore sediments were

almost move from NE (estuaries region) to SW (Ca Mau coastal area). They increased from

NE to SW. This showed re-suspension (or lack of sediment) of sediment between transects

or indication of erosion processes (Table 1, figure 89, and Figure 92-a).

- In the flood season, the sediment flux from the river (via My Thuan and Can Tho) to

the coastal area were about 7486069 tons (in September) and 5938442 tons (October), Table

1. The net alongshore sediment transported change in wind condition. Ex., in the first half

of September sediment dominant moving from SW to NE (Figure 90-a), then change

something in the last half of September (Figure 90-b) and first half of October (figure 91-a).

Until the last half of October, the net alongshore sediment was fully moving from NE to SW

(Figure 91-b). Due to the large sediment from the river to the estuary region, deposition

phenomenon occurred in September (Figure 92-b) and October (Figure 92-c) much more


85

than in January (Figure 92-a).

Table 1. Sediment flux (ton) from river and alongshore transportation in the Mekong coastal area

Transect direction January (2014) September (2014) October (2014)

All first half Last half All first half Last half All first half Last half

My Thuan (A) to coastal

area

584177 320271 263906 4383378 2224638 2158740 3312090 2076899 1235190

Can Tho (B) 299099 176673 122426 3102691 1558055 1544636 2626352 1643430 982922

Vung Tau (1)

to NE 118851 65243 53608 144434 48766 95668 98347 38100 60247

to SW 87442 55993 31449 98364 58132 40232 105558 64042 41516

Balance* 31409 9250 22159 46070 -9365 55436 -7210 -25942 18732

Tien Giang (2)

to NE 295708 144394 151315 696106 234889 461217 369002 170275 198726

to SW 340692 201039 139653 448859 262671 186187 529051 313838 215213

Balance* -44984 -56646 11662 247248 -27782 275029 -160049 -143563 -16486

Ben Tre (3)

to NE 504359 245453 258905 1405817 672958 732859 799167 440125 359042

to SW 873115 506603 366512 962247 552268 409979 1037318 583000 454318

Balance* -368756 -261149 -107607 443570 120690 322881 -238151 -142875 -95276

Tra Vinh (4)

to NE 773211 406788 366424 2008021 1073849 934171 1445136 876953 568183

to SW 1711682 1005518 706164 1612733 813777 798955 1998291 1154963 843328

Balance* -938471 -598730 -339740 395288 260072 135216 -553156 -278010 -275145

Soc Trang (5)

to NE 689283 348313 340970 1170122 577524 592597 1068830 578860 489970

to SW 2337399 1374106 963294 1136396 515124 621272 2057468 950177 1107291

Balance* -1648116 -1025792 -622324 33725 62400 -28674 -988638 -371317 -617321

Bac Lieu (6)

to NE 727087 365529 361557 1300766 684342 616424 894197 500243 393954

to SW 2306398 1318879 987518 -1144189 -593036 551152 -1504650 670937 833713

Balance* -1579311 -953350 -625961 156578 91306 65272 -610453 -170694 -439759

Ca Mau I (7)

to NE 2705370 1416944 1288426 3648979 2048709 1600270 3497301 2257926 1239376

to SW -5212766 -2762856 -2449910 -2718358 -1413295 -1305064 -3834667 -2088482 -1746185

Balance* -2507395 -1345911 -1161484 930621 635415 295206 -337366 169443 -506809

Ca Mau II (8)

to N 1735000 886207 848793 907744 443557 464186 1087676 604454 483222

to S -2267720 -1034087 -1233633 -1168777 -545406 -623372 -1411043 -781128 -629915

Balance* -532720 -147880 -384840 -261034 -101848 -159185 -323367 -176674 -146693

Ca Mau III (9)

to N 447907 192087 255820 425261 215983 209279 319954 187935 132019

to S -493820 -190376 -303444 -463556 -224253 -239302 -346649 -202142 -144507

Balance* -45913 1711 -47624 -38294 -8271 -30023 -26695 -14207 -12487

Kien Giang

(10)

to N 607383 305351 302032 555364 327847 227516 744524 377188 367336

to S -602865 -296111 -306754 -597743 -348340 -249403 -755693 -364258 -391434

Balance* 4518 9240 -4722 -42379 -20492 -21887 -11169 12929 -24098

Rach Gia (11)

to N 564404 276904 287500 959453 505498 453955 680142 377115 303026

to S -622459 -306507 -315951 -998768 -519062 -479705 -758159 -428698 -329462

Balance* -58054 -29603 -28452 -39315 -13565 -25750 -78018 -51582 -26436

*: Minus sign (-): go from NE to SW or from N to S


86

Figure 76. Averaged suspended sediment concentration (kg/m3), mud1-fine flocs at layer

1: first half of January (a), last half January (b), and all of January

(a)

(b)

(c)


87



(c)

(b)

(c)


88



(c)

(a)

(b)


89



(a)

(b)

(c)


90



(a)

(b)

(c)


91


1: first half of September (a), last half September (b), and all of September

(a)

(c)

(b)


92



(a)

(b)

(c)


93



(c)

(a)

(b)


94


1: first half of October (a), last half October (b), and all of October

(a)

(b)

(c)


95



(a)

(b)

(c)


96



(a)

(b)

(c)


97



(c)

(a)

(b)

(c)


98



(a)

(b)

(c)


99

Figure 89. Total sediment transport (x 103 tons) alongshore Mekong coastal area: first half of January (a), last half January (b), and all of January

9250

56646

261149

598730

1025792

176673

320271 (1)

(2)

(3)

(4)

(5)

(6)

(7) (8)

(9)

(10)

(11)

(A)

(B)

22159

11662

107607

339740

622234

122426

263906 (1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(A

(B

31490

44984

368756

938741

1648116

584177

299099

(1)

(2)

(3)

(4)

(5)

(6)

(7) (8)

(9)

(10)

(11)

(B)

(A)

(a)

(c)

(b)


100

Figure 90. Total sediment transport (x 103 tons) alongshore Mekong coastal area: first half of September (a), last half September (b), and all of September

9365

27782

120690

260072

62400

2224638

1558055

(1)

(2)

(3)

(4)

(5) (6)

(7)

(8)

(9)

(10)

(11)

(B)

(A)

55436

275039

322881

135216

28674

2158740

1544636

(1)

(2)

(3)

(4) (5)

(6)

(7)

(8)

(9)

(10)

(11)

(B)

(A)

4607

247248

443570

395288

33725

4383378

3102691

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(B)

(A)

(a)

(b)

(c)


101

Figure 91. Total sediment transport (x 103 tons) alongshore Mekong coastal area: first half of October (a), last half October (b), and all of October

25942

143563

142875

278010

371317

1643430

2076889 (1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(B)

(A)

18732

16486

95276

275145

617321

1235109

982922

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(B)

(A)

7210

160049

238151

553156

988636

2626352

3312090 (1)

(2)

(3)

(4) (5)

(6)

(7)

(8)

(9)

(10)

(11)

(B)

(A)

(a)

(c)

(b)


102

Figure 92. Morphological change in MK coastal area after January (a), September

(b), October (c)

(c)

(a)

(c)

(b)


103

3. 6. Difference of 12 vertical layer and 24 vertical layer

In order to assess influenced of layer number on the modelling results, two kind

of model with differently vertical layer: 24 vertical layers (2%, 2%, 3%, 3%, 3%, 3%,

4%, 4%, 4%, 4%, 4%, 4%, 5%, 5%, 5%, 5%, 5%, 5%, 5%, 5%, 5%, 5%, 5%, 5%,

and 5% of water depth) and 12 vertical layers (2%, 3%, 5%,7%, 9%,11%, 14%,15%,

12%, 10%, 7% and 5% of water depth). Except of the number layer, others parameters

of two these model were kept intact. The modelling results showed that profile

distribution of salinity and suspended sediment at each estuary transect are almost

same between results of modelling 12 layers and 24 layers (Figure 93 to figure 128).

It could be explained that the stratification (salinity and suspended sediment)

occurred at upper layers, meanwhile thick layers of upper layers defined almost same

between two model. At lower layer, there are different of thick layer but salinity and

suspended sediment not much different in the water column.

The different spatial distribution of model with 12 vertical layers and model with

24 vertical layers were discovered by the comparison of mean field current and

suspended sediment. The results showed that there are a differently between field

current of each layer from model with 12 vertical layers and model with 24 vertical

layer (Figure 129 to figure 138) and suspended sediment (figure 139 to figure 151).

However, these comparison are relative because different of thick layer in the model

12 vertical layer and model 24 vertical layers.


104

Figure 93. Comparison of salinity profile in ebb tide, Jan., Tien Giang transect: a-

12 layers; b-24 layers

(c)

(a)

(b)


105

Figure 94. Comparison of suspended sediment profile in ebb tide, Jan., Tien Giang

transect: a- 12 layers; b-24 layers

(a)

(b)


106

Figure 95. Comparison of salinity profile in flood tide, Jan., Tien Giang transect: a-

12 layer; b-24 layer

(a)

(a)

(b)


107

Figure 96. Comparison of suspended sediment profile in flood tide, Jan., Tien

Giang transect: a- 12 layer; b-24 layer

(a)

(b)


108

Figure 97. Comparison of salinity profile in ebb tide, Sep., Tien Giang transect: a-

12 layer; b-24 layer

(a)

(b)


109

Figure 98. Comparison of suspended sediment profile in ebb tide, Sep., Tien Giang

transect: a- 12 layer; b-24 layer

(a)

(b)


110

Figure 99. Comparison of salinity profile in flood tide, Sep., Tien Giang transect: a-


(b)

(a)


111

Figure 100. Comparison of suspended sediment profile in flood tide, Sep., Tien

Giang transect: a- 12 layers; b-24 layers

(a)

(a)

(b)


112

Figure 101. Comparison of salinity profile in ebb tide, Oct., Tien Giang transect: a-


(a)

(b)


113

Figure 102. Comparison of suspended sediment profile in ebb tide, Oct., Tien


(a)

(b)


114

Figure 103. Comparison of salinity profile in flood tide, Oct., Tien Giang transect:

a- 12 layers; b-24 layers

(a)

(b)


115

Figure 104. Comparison of suspended sediment profile in flood tide, Oct., Tien


(a)

(b)


116

Figure 105. Comparison of salinity profile in ebb tide, Jan., Cung Hau transect: a-


(a)

(b)


117

Figure 106. Comparison of suspended sediment profile in ebb tide, Jan., Cung Hau


(a)

(b)


118

Figure 107. Comparison of salinity profile in flood tide, Jan., Cung Hau transect: a-


(a)

(b)


119

Figure 108. Comparison of suspended sediment profile in flood tide, Jan., Cung

Hau transect: a- 12 layers; b-24 layers

(a)

(b)


120

Figure 109. Comparison of salinity profile in ebb tide, Sep., Cung Hau transect: a-


(a)

(b)


121

Figure 110. Comparison of suspended sediment profile in ebb tide, Sep., Cung Hau


(a)

(b)


122

Figure 111. Comparison of salinity profile in flood tide, Sep., Cung Hau transect: a-


(a)

(b)


123

Figure 112. Comparison of suspended sediment profile in flood tide, Sep., Cung


(a)

(b)


124

Figure 113. Comparison of salinity profile in ebb tide, Oct., Cung Hau transect: a-


(a)

(b)


125

Figure 114. Comparison of suspended sediment profile in ebb tide, Oct., Cung Hau


(a)

(b)


126

Figure 115. Comparison of salinity profile in flood tide, Oct., Cung Hau transect: a-


(a)

(b)


127

Figure 116. Comparison of suspended sediment profile in flood tide, Oct., Cung


(a)

(b)


128

Figure 117. Comparison of salinity profile in ebb tide, Jan., Dinh An transect: a- 12

layers; b-24 layers

(a)

(b)


129

Figure 118. Comparison of suspended sediment profile in ebb tide, Jan., Dinh An


(a)

(b)


130

Figure 119. Comparison of salinity profile in flood tide, Jan., Dinh An transect: a-


(a)

(b)


131

Figure 120. Comparison of suspended sediment profile in flood tide, Jan., Dinh An


(a)

(b)


132

Figure 121. Comparison of salinity profile in ebb tide, Sep., Dinh An transect: a- 12

layers; b-24 layers

(a)

(b)


133

Figure 122. Comparison of suspended sediment profile in ebb tide, Sep., Dinh An


(a)

(b)


134

Figure 123. Comparison of salinity profile in flood tide, Sep., Dinh An transect: a-


(a)

(b)


135

Figure 124. Comparison of suspended sediment profile in flood tide, Sep., Dinh An


(a)

(b)


136

Figure 125. Comparison of salinity profile in ebb tide, Oct., Dinh An transect: a- 12

layers; b-24 layers

(b)

(a)


137

Figure 126. Comparison of suspended sediment profile in ebb tide, Oct., Dinh An


(a)

(b)


138

Figure 127. Comparison of salinity profile in flood tide, Oct., Dinh An transect: a-


(b)

(a)


139

Figure 128. Comparison of suspended sediment profile in flood tide, Oct., Dinh An


(a)

(b)


140

Figure 129. Comparison of monthly (January) flow velocity at layer 1: 12 layer (a), 24

layers (b)

(b)

(a)


141

Figure 130. Comparison of monthly (January) flow velocity: layer 2 of model 12 layer

(a), layer 4 of 24 layers (b)

(a)

(c)


142



(a)


143



(b)

(a)


144

Figure 133. Comparison of monthly (September) flow velocity: layer 1 of model 12 layer


(b)

(a)


145



(a)

(b)


146



(a)

(c)


147

Figure 136. Comparison of monthly (October) flow velocity: layer 1 of model 12 layer


(a)

(b)


148



(a)

(b)


149



(a)

(b)


150

Figure 139. Comparison of monthly (January) suspended sediment concentration: layer 1

of model 12 layer (a), layer 1 of 24 layers (b)

(a)

(b)


151



(c)

(b)


152



(a)

(b)


153



(a)

(b)


154



(a)

(b)


155

Figure 144. Comparison of monthly (September) suspended sediment concentration: layer

1 of model 12 layer (a), layer 1 of 24 layers (b)

(b)

(a)


156



(a)

(b)


157



(a)

(b)


158

Figure 147. Comparison of monthly (October) suspended sediment concentration: layer 1


(a)

(b)


159



(b)

(a)


160



(b)

(a)


161



(b)

(a)


162



4. Conclusions

A model system (based on Delf3d model) was setup to simulate hydrodynamics

and sediment transport in the Mekong coastal area. The model ran with some

scenarios for model validation and simulation.

The model validation results showed a consistent with in situ data, satellite image

data and CROCO model’ results.

(a)

(b)


163

The results of scenarios simulation in the low flow season (January), flood season

(September) and transitional season (October) showed typical characteristic of

residual current and sediment transport in the Mekong coastal area.

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