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LMDCZ project: Shoreline protection measures (WP6)

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Studying impacts of hard breakwaters and sandbars at Go-Cong using MIKE21 Model

Table of Contents

Studying the impacts of hard breakwater and sandbar measures at Go-Cong by

MIKE21 Model ......................................................................................................................................... 1

1. Introduction .................................................................................................................................... 4

2. Objectives ......................................................................................................................................... 4

3. Methodology .................................................................................................................................... 4

4. Impacts of breakwater ................................................................................................................. 5

4.1 Breakwater scenarios .............................................................................................................. 5

4.2 Simulation results of the non-structure measure .......................................................... 6

4.2.1 Flow regime .............................................................................................................. 6

4.2.2 Wave regimes ........................................................................................................... 9

4.3 Current impact .......................................................................................................................... 11

4.4 Wave impact results ............................................................................................................... 12

4.5 Impact on morphology ........................................................................................................... 15

4.6 Impact on sediment reduction ............................................................................................ 19

4.7 Sumary the impact of breakwater ..................................................................................... 20

5. Simulation results of the impact of sandbars .................................................................... 20

5.1 Methodology .............................................................................................................................. 20

5.2 Sandbar scenarios ................................................................................................................... 20

5.3 Calibrate the sandbar deformation in 21FM (HD&SW&ST) model ....................... 21

5.4 Morphological impact of sandbars .................................................................................... 25

6. CONCLUSION .................................................................................................................................. 26

7. REFERENCES .................................................................................................................................. 26


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LIST OF TABLES

Table 4-1 Protection measure configurations ............................................................................. 5

Table 4-2 Total accretion and erosion in study area after one month in the Northeast monsoon (25/12/2013 ÷ 5/2/2014) of scenarios ....................................................................................... 18

Table 4-3 Total accretion and erosion in study area after one month in the SW monsoon (25/8/2014 ÷ 5/10/2014) ........................................................................................................... 18

Table 4-4 Net volume (Mm3) and maximum erosion thickness (m) with various scenarios ....... 18

Table 4-5 Accretion and erosion volume variation after one NE monsoon month (January 2014) (from 25/12/2013 ÷ 5/2/2014)after sediment reduction of 75% ................................................. 19

Table 4-6 Accretion and erosion volume variation after one SW monsoon month (September 2014) (from 25/8/2014 ÷ 5/10/2014) ......................................................................................... 19

Table 4-7 Net volume (Mm3) and maximum erosion thickness (m) with various scenarios and sediment reduction 75% ........................................................................................................... 19

Table 5-1 Sandbar configuration for Go Cong study area ......................................................... 20

Table 5-2 Typical scenario for 21FM (HD&SW&ST) model calibrate ........................................ 21

Table 5-3 Morphological changes after one month in the NE monsoon (January 2014) ........... 22

Table 5-4 Morphological changes after one month in the SW monsoon (September 2014) ...... 22

Table 5-5 Morphological changes of sandbar impact after one month in the NE monsoon (January 2014) ......................................................................................................................... 25

Table 5-6 Morphological changes of sandbar impact after one month in the SW monsoon (September 2014) .................................................................................................................... 25

Table 5-7 Net volume (Mm3) and maximum erosion thickness (m) with sandbar scenarios ..... 26

LIST OF FIGURES

Figure 3-1 Model partitions ......................................................................................................... 5

Figure 4-1 Detail meshes and protection measure of T shape breakwaters ................................ 6

Figure 4-2 Distribution of current field at the estuarine and coastal studied area (a) at the falling tide and (b) at the rising tide (belows are the water level and current at Soai Rap at extracting time and corresponding to current field above) ........................................................................... 7

Figure 4-3 Extraction location in the study area for analysis ....................................................... 7

Figure 4-4 Current roses at positions P1 ÷ P4 with computational time period is from 25/8/2014 to 5/10/2014 (SW monsoon) ....................................................................................................... 8

Figure 4-5 Current roses at positions P1 ÷ P4 with computational time period is from 25/12/2013 to 5/2/2014 (NE monsoon) ......................................................................................................... 8

Figure 4-6 Wave climate in the NE monsoon (January 2014) ..................................................... 9

Figure 4-7 Wave roses at P1 and P2 in the NE monsoon (January 2014) ................................ 10

Figure 4-8 Wave roses at P3 and P4 in the NE monsoon (January 2014) ................................ 10

Figure 4-9 Wave roses at P1 and P2 in the SW monsoon (September / 2014) ......................... 10


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Figure 4-10 Wave roses at P3 and P4 in the SW monsoon (September / 2014) ....................... 10

Figure 4-11 Current roses at P1 in the Northeast monsoon (25/12/2013÷ 5/2/2014) of Baseline (KB0), KB1, KB2, KB3 scenarios .............................................................................................. 11

Figure 4-12 Current roses at P1 in the Southwest monsoon (25/8/2014÷ 5/10/2014) of Baseline (KB0), KB1, KB2, KB3 scenarios .............................................................................................. 11

Figure 4-13 Cross-sectional locations to check the impact of T-shaped breakwaters .............. 12

Figure 4-14 Wave heights at section 1 of KB0, KB1, KB2 and KB3 scenarios .......................... 12

Figure 4-15 Wave heights at section 2 of KB0, KB1, KB2 and KB3 scenarios .......................... 13

Figure 4-16 Wave heights at section 3 of KB0, KB1, KB2 and KB3 scenarios ......................... 13

Figure 4-17 Significat wave roses in the Northeast monsoon (25/12/2014 ÷ 5/2/2014) at P1 for KB0, KB1, KB2 and KB3 scenarios .......................................................................................... 13

Figure 4-18 Significat wave roses in the Southwest monsoon (25/8/2014 ÷ 5/10/2014) at P1 for KB0, KB1, KB2 and KB3 scenarios .......................................................................................... 14

Figure 4-19 The NE monsoon wave fields between scenarios ((a) Baseline (KB0), (b) KB1, (c) KB2, (d) KB3) ........................................................................................................................... 14

Figure 4-20 Distribution of erosion and accretion after one month (January 2014) of scenarios (KB0 (a) KB1 (b), KB2 (c), KB3 (d)) .......................................................................................... 16

Figure 4-21 Distribution of erosion and accretion after one month of Northeast monsoon (January 2014) of scenarios (KB0 (a) KB1 (b), KB2 (c), KB3 (d)) – from Tieu River to Rach Bun sluice (middle of the shoreline) ................................................................................................. 17

Figure 4-22 Distribution of erosion and accretion after one month of Southwest monsoon (September 2014) of scenarios (KB0 (a) KB1 (b), KB2 (c), KB3 (d)) ......................................... 17

Figure 4-23 Distribution of erosion and accretion after one month of Southwest monsoon (September 2014) of scenarios (KB0 (a) KB1 (b), KB2 (c), KB3 (d)) - from Tieu River to Rach Bun sluice (middle of the shoreline) .......................................................................................... 18

Figure 4-24 The zoning to calculate erosion and accretion volume in the study area ............... 18

Figure 5-1 Sandbars in the study model at Go Cong, Tien Giang province ............................... 21

Figure 5-2 Numerical model 21FM (HD&SW&ST) set up for sandbar deformation calibration .. 21

Figure 5-3 Calibration result of 21FM (HD&SW&ST) with typical scenario WP6-NOU-B5-R15-JSW2 ....................................................................................................................................... 22

Figure 5-4 The results of sand bar deformation (B = 50m) after one-month period of the Northeast monsoon (1/2014) (a) and one-month period of the SW Monsoon (9/2014) (b) ........ 23

Figure 5-5 The results of sand bar deformation (B = 70m) after one-month period of the Northeast monsoon (1/2014) (a) and one-month period of the SW Monsoon (9/2014) (b) ........ 23

Figure 5-6 Cross sections to analyze the sandbar deformation ................................................ 24

Figure 5-7 Morphological changes at section 1 after the one-month period of the Northeast monsoon (1/2014) .................................................................................................................... 24




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1. Introduction

This report presents the simulation of hydrodynamics and morphology impacts by protection measures in Go Cong area.

The protection measures are proposed based on the review of existing protection measures applied in the LMDCZ and suggestion of possible measures. Field surveys were carried out to examine the concrete breakwaters which were applied in West coast of Ca Mau. From that, a porous breakwater was proposed. From experiences of the same natural conditions in Demak, North Java, Indonesia a Building with Nature measure, the sandbar was proposed. Both porous breakwater and sandbar were testing their funtions in the Flume of SIWRR. These proposed protection measures will be studied their efficiency and impacts by numerical modeling.

2. Objectives

- To select shore protection measures for the coastal zones of Go-Cong. Selected measures should be suitable to the economic and tourist conditions of the coastal zones of Go-Cong.

- To check the efficiency of the selected shore protection measures for the coastal zones of Go-Cong.

- To check the impact of the selected shore protection measures to the neighbouring of Go-Cong.

3. Methodology

With nesting approach, MIKE21 has been calibrated well from the Regional model to Local Model (Figure 3-1) with water levels, discharges, tides, waves and currents, sediment transport and morphology especially the validation results based on the in-situ data of the LMDCZ project in October 2016 and February-March 2017, presented in WP5 Report.

This WP6 Report we discuss the efficiency and impacts of protection measures.

The mesh of the study area model is an unstructured mesh with the triangular element occupying most of the sea area but with the quadrilateral part in most of the rivers. To assess the impact of the protection measures in the area of Go Cong, the net areas are divided very smoothly with a grid step of about 10m ÷ 15m (Figure 3-1).


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Figure 3-1 Model partitions

4. Impacts of breakwater

4.1 Breakwater scenarios

The T breakwater and its configuration is presented in

Figure 4-1 and Table 4-1.

Table 4-1 Protection measure configurations

No SCENARIOS Senario

description

BREAKWATER CONFIGURATION

Note Lengh (Ls)(m)

Distance from

shoreline (Y)(m)

Gap between

two breakwater

(Lg)(m)

Crest elevation of breakwater

(m)

1 KB0 Baseline

2 KB1 T shape

breakwaters 600 300 30 2.2 The crest

elevation of cross shore breakwater is +0.5 m

3 KB2 T shape

breakwaters 600 300 50 2.2

4 KB3 T shape

breakwaters 600 300 70 2.2


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Figure 4-1 Detail meshes and protection measure of T shape breakwaters

4.2 Simulation results of the non-structure measure

4.2.1 Flow regime

With two southwest and northeast wind seasons in the area, there are two hydrological seasons upstream: the flood season and the dry season. On the river system of Sai Gon - Dong Nai, the main stream flows have been significantly regulated by upstream reservoirs, thus limitedly affecting the estuaries and tidal flow is dominated.

On the Mekong River, about 80-85% of water and 90-95% of sediments are drained to the sea every year in the flood season (Vu Kien Trung et al., 2011). In this flood season, river flows are prevailing in estuarine and neighboring areas. In the dry season, the influence from the sea (tide) predominates.

The impact of tides on the flow regime at estuaries is very high. With a tidal amplitude of 2-4 m, the flow velocity at the river mouth can reach up to 1.35 m/s at rising tide and 1.65 m/s at the falling tide (Figure 4-2). Interference between out and inflows at estuaries has formed areas of small velocity, contributing to the convex coastal line in the area.

The current roses at the locations P1 ÷ P4 is shown in Figure 4-4 and Figure 4-5. The currents along the coast of Go Cong are strongly influenced by Soai Rap and Cua Tieu Rivers. Therefore, the flow direction is mainly north and northwest at rising tide and south and southeast at falling tides. In this area, current values in the Southwest monsoon are higher than that in the Northeast monsoon.


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(a) (b)

Figure 4-2 Distribution of current field at the estuarine and coastal studied area (a) at the falling tide and (b) at the rising tide (belows are the water level and current at Soai Rap at extracting time and

corresponding to current field above)

Figure 4-3 Extraction location in the study area for analysis


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Figure 4-4 Current roses at positions P1 ÷ P4 with computational time period is from 25/8/2014 to 5/10/2014 (SW monsoon)

Figure 4-5 Current roses at positions P1 ÷ P4 with computational time period is from 25/12/2013 to 5/2/2014 (NE monsoon)


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4.2.2 Wave regimes

Figure 4-6 indicate wave climate in the NE mosoon of the study area.

Figure 4-7 and Figure 4-8 shows the results of wave calculated at positions P1 ÷ P4 (see Figure

2 20). This result again shows that the wave height during the Northeast monsoon (from

25/12/2013 to 5/2/2014) is much higher than the one in the southwest monsoon (25/8/2014 ÷

10/10/2014). The wave roses at P1 ÷ P4 in Figure 4-7 to Figure 4-10 shows that during the SW

monsoon the wave height of <0.1 m is more than 90% while is only about 10% to 25% in the NE

monsoon. Wave height of 0.2 m is only about 2% during the SW monsoon and 40% in the NE

monsoon. This means shoreline erosion due to waves during the northeast is much higher than

that during the southwest monsoon. According to this result, wave directions nearshore are

predominantly east and southeast. In other words, these are two possible waves dirrections that

have the greatest potential to impact on shore erosion in the study area.

Figure 4-6 Wave climate in the NE monsoon (January 2014)


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Figure 4-7 Wave roses at P1 and P2 in the NE monsoon (January 2014)

Figure 4-8 Wave roses at P3 and P4 in the NE monsoon (January 2014)

Figure 4-9 Wave roses at P1 and P2 in the SW monsoon (September / 2014)

Figure 4-10 Wave roses at P3 and P4 in the SW monsoon (September / 2014)


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4.3 Current impact

Figure 4-11 and Figure 4-12 illustrates the efficiency of breakwaters in reducing the flow

velocity. It can be seen that the breakwaters has significantly reduced the velocity at P1 in terms

of both the intensity and the duration of the high velocity. All scenarios increase the flow rate of

less than 0.1 m/s to over 90% for both northeast and southwest monsoon. The difference

between the scenarios can be seen by flow percentage less than 0.1 m/s reduced as the

distance between the two breakwaters increases. In general, KB1 is better than other scenarios.

Figure 4-11 Current roses at P1 in the Northeast monsoon (25/12/2013÷ 5/2/2014) of Baseline (KB0), KB1, KB2, KB3 scenarios

Figure 4-12 Current roses at P1 in the Southwest monsoon (25/8/2014÷ 5/10/2014) of Baseline (KB0), KB1, KB2, KB3 scenarios


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4.4 Wave impact results

In order to consider the effect of wave reduction between T-shaped breakwaters, sections as

shown in Figure 4-13 are presented for analysis. The distance between the cross section 1 and

section 2 is 50 m. The effect of wave reduction between the scenarios in sections 1, 2 and 3 is

shown in Figure 4-14 to Figure 4-16. It can be recognised dramatic change between the

baseline (KB0) scenario and protection scenarios. The wave heights at the section 1, 2 in the

northeast monsoon of KB0 are about 1 ÷ 1.2m. After protection by breakwaters, the wave height

between the two breakwaters are reduced to less than 0.6m.

The effect of wave reduction between scenarios are different. The smaller gap between the

breakwaters (Lg), the better the wave reduction effect. In this study, the reduction effect of KB1

(Lg = 30m) was highest compared to Lg = 50m (KB2) and Lg = 70m (KB3). However,

considering the significant wave height in section 3 (Figure 4-16), with KB3 (Lg = 70m) wave

height higher than 0.6m was about 50m from breakwater landward. Therefore, in order to

achieve the target of wave reduction of Hs<0.6 m, Lg should not be lower than 70m if not using

additional measures to combine such as bamboo fence for wave reduction inside the

breakwater. In general, KB1 is the better scenario among others.

Figure 4-13 Cross-sectional locations to check the impact of T-shaped breakwaters

Figure 4-14 Wave heights at section 1 of KB0, KB1, KB2 and KB3 scenarios


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Figure 4-17 Significat wave roses in the Northeast monsoon (25/12/2014 ÷ 5/2/2014) at P1 for KB0, KB1, KB2 and KB3 scenarios


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Figure 4-18 Significat wave roses in the Southwest monsoon (25/8/2014 ÷ 5/10/2014) at P1 for KB0, KB1, KB2 and KB3 scenarios

Figure 4-19 The NE monsoon wave fields between scenarios ((a) Baseline (KB0), (b) KB1, (c) KB2, (d) KB3)


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4.5 Impact on morphology

The morphological change with KB0 ÷ KB3 scenarios after 1 month of simulation of the Northeast monsoon (25/12/2013 ÷ 5/2/2014) were presented in Figure 4-20 and Figure 4-21. Even during the Northeast monsoon, the breakwater impacts are quite clear. The difference between the gap of two breakwaters (Lg) is not much.

The accretion impact is clearer after 1 month of simulation of the southwest monsoon (25/8/2014 ÷ 5/10/2014) as expressed in Figure 4-22 and Figure 4-23.

For erosion and accretion analysis, the study area is separated into two areas (Figure 4-24). Area 1 is from the shoreline to 2km offshore. Area 2 is the rest of the study area. To analyse the impacts of breakwater, the net volume (V accr. – V ero.), average accretion thickness over the whole area 1 and the maximum erosion thickness (normally at the gap area) are considered. In general, the appropriate scenario is the one of high accretion of combined net volume for the two typical NE and SW months and low erosion thickness. The calculation results of erosion and accretion volume, after 1 month in the NE and SW monsoon are presented in Table 4-2 and Table 4-3. Table 4-5 combined the NE and SW monsoon results.

Due to the impacts in the whole area (2) is not significant, only the area 1 is considered in detail. The effect of accretion in Area 1 combining 2 months in the NE and SW monsoons in KB0, KB1, KB2 and KB3 are -0.087, 0.2996, 0.2891 and 0.2913 Mm3 respectively. For the average erosion in the gaps, they are -0.196, -0.653, -0.618 and -0.566 respectively (equivalent to -0.003, 0.01, 0.009, 0.009 m respectively). From these data, it can be seen that KB1 (with gap of 30 m) is better than among other scenarios in total accretion. It means that wave attenuation impact is the most important factor. All scenarios have erosion in the gap areas but not much different. KB1 is the better option among others interm of trapping sediment. All scenarios have erosion in the gap areas but KB3 is better than others. Therefore, KB1 and KB3 are the options.

Figure 4-20 Distribution of erosion and accretion after one month (January 2014) of scenarios (KB0 (a) KB1 (b), KB2 (c), KB3 (d))


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Figure 4-21 Distribution of erosion and accretion after one month of Northeast monsoon (January 2014) of scenarios (KB0 (a) KB1 (b), KB2 (c), KB3 (d)) – from Tieu River to Rach Bun sluice (middle of the

shoreline)


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Figure 4-22 Distribution of erosion and accretion after one month of Southwest monsoon (September 2014) of scenarios (KB0 (a) KB1 (b), KB2 (c), KB3 (d))

Figure 4-23 Distribution of erosion and accretion after one month of Southwest monsoon (September 2014) of scenarios (KB0 (a) KB1 (b), KB2 (c), KB3 (d)) - from Tieu River to Rach Bun sluice (middle of the

shoreline)


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Figure 4-24 The zoning to calculate erosion and accretion volume in the study area

Table 4-2 Total accretion and erosion in study area after one month in the Northeast monsoon (25/12/2013 ÷ 5/2/2014) of scenarios

January 2014

Scenarios

Area 2 Area 1

Vol. (106 m3) Aveg THK (m) Vol. (10

6 m3) Aveg THK (m) Net

Vol. (Mm3)

Max Ero. THK (m)

Accr. Ero. Accr. Ero. Accr. Ero. Accr. Ero.

KB0 11.062 -17.632 0.009 -0.013 0.3871 -1.0697 0.013 -0.037 -0.683 -0.196

G30(KB1) 11.337 -17.715 0.008 -0.013 0.4839 -0.8094 0.017 -0.028 -0.325 -0.653

G50(KB2) 12.242 -17.712 0.009 -0.013 0.4832 -0.8115 0.017 -0.028 -0.329 -0.618

G70(KB3) 11.339 -17.744 0.008 -0.013 0.4682 -0.7885 0.016 -0.027 -0.321 -0.566

Table 4-3 Total accretion and erosion in study area after one month in the SW monsoon (25/8/2014 ÷ 5/10/2014)

September 2014

Scenarios

Area 2 Area 1


6 m3) Aveg THK (m)

Net Vol.

(Mm3)

Max Ero. THK (m)


KB0 34.4229 -11.7568 0.025 -0.008 0.7560 -0.16 0.027 -0.006 0.5960 0.000

G30(KB1) 34.4080 -11.7638 0.025 -0.008 0.7946 -0.17 0.028 -0.006 0.6246 -0.020

G50(KB2) 34.4075 -11.7641 0.025 -0.008 0.7881 -0.17 0.027 -0.006 0.6181 0.000

G70(KB3) 34.3827 -11.7644 0.025 -0.008 0.7823 -0.17 0.027 -0.006 0.6123 0.000

Table 4-4 Net volume (Mm3) and maximum erosion thickness (m) with various scenarios

Scenarios

Area 1 Combined January and September

2014 in area 1

Jan.2014 Sep.2014 Vol. (Mm3) Everage thickness (m)

Net Vol. (Mm3)

Max Ero. Thickness

(m)

Net Vol. (Mm3)

Max Ero. Thickness

(m)

Net Vol. (Mm3)

Max Ero. Thickness

(m)

KB0 (Baseline) -0.683 -0.196 0.5960 0.000 -0.0870 -0.196 -0.003

G30(KB1) -0.325 -0.653 0.6246 -0.020 0.2996 -0.653 0.010

G50(KB2) -0.329 -0.618 0.6181 0.000 0.2891 -0.618 0.009

G70(KB3) -0.321 -0.566 0.6123 0.000 0.2913 -0.566 0.009


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4.6 Impact on sediment reduction

In this part we assumes the sediment reduction by 75% at all river mouths (due to damming upstream). Morphological variation for different scenarios of breakwater are expressed in Table 4-5 and Table 4-6. Table 4-7 combined the NE and SW monsoon results. Table 4-7 showed that when sediment reduction by 75%, sediment deficit in the area 1 is changed from 0.1 Mm3 to 0.67Mm3. The breakwater reduced sediment deficit of 0.366 Mm3 when gap of 70 m and this gap is responded better than others.

Table 4-5 Accretion and erosion volume variation after one NE monsoon month (January 2014) (from 25/12/2013 ÷ 5/2/2014)after sediment reduction of 75%

January 2014

Scenarios

Area 2 Area 1



Vol. (Mm3)

Max Ero. THK (m)


KB0 11.062 -17.632 0.009 -0.0130 0.3871 -1.0697 0.013 -0.0370 -0.6826 -0.196

KB0-75% 7.9684 -17.8272 0.0057 -0.0128 0.3113 -1.0704 0.0108 -0.0372 -0.7591 -0.196

G30 -75% 7.8855 -17.9202 0.0057 -0.0129 0.3456 -0.8263 0.0120 -0.0287 -0.4807 -0.650

G50 -75% 7.8958 -17.9556 0.0057 -0.0129 0.3313 -0.8298 0.0115 -0.0288 -0.4985 -0.609

G70 -75% 7.8867 -17.9490 0.0057 -0.0129 0.3308 -0.8032 0.0115 -0.0279 -0.4724 -0.562

Table 4-6 Accretion and erosion volume variation after one SW monsoon month (September 2014) (from 25/8/2014 ÷ 5/10/2014)

September 2014

Scenarios

Area 2 Area 1



Vol. (Mm3)

Max Ero. THK (m)


KB0 34.4229 -11.7568 0.0248 -0.0085 0.7560 -0.1640 0.0274 34.4229 0.5920 -0.0058

KB0-75% 15.0812 -12.5520 0.0109 -0.0090 0.2824 -0.1949 0.0098 15.0812 0.0875 -0.0068

G30 (KB1) 34.4080 -11.7638 0.0248 -0.0085 0.7946 -0.1713 0.0276 34.4080 0.6233 -0.0060

G30 -75% 15.0708 -12.5581 0.0109 -0.0090 0.3132 -0.2019 0.0109 15.0708 0.1112 -0.0070

G50 (KB2) 34.4075 -11.7641 0.0248 -0.0085 0.7881 -0.1678 0.0274 34.4075 0.6203 -0.0058

G50 -75% 15.0702 -12.5582 0.0109 -0.0090 0.3073 -0.1984 0.0107 15.0702 0.1089 -0.0069

G70 (KB3) 34.3827 -11.7644 0.0248 -0.0085 0.7823 -0.1676 0.0272 34.3827 0.6147 -0.0058

G70 -75% 15.0702 -12.5578 0.0109 -0.0090 0.3046 -0.1982 0.0106 15.0702 0.1065 -0.0068

Table 4-7 Net volume (Mm3) and maximum erosion thickness (m) with various scenarios and sediment reduction 75%

Combined January 2014 and September 2014

Net Vol. (Mm3)in Area1 Scenarios

Area 2 Area 1

(106 m3) (10

6 m3)

V accretion

V erosion

V accretion

V erosion

KB0 45.483 -29.387 1.146 -1.234 -0.088

KB0+ Reduce SSC 75% 23.050 -30.379 0.594 -1.265 -0.672

G30 (KB1) 45.748 -29.474 1.275 -0.981 0.293

G30 (KB1) + Reduce SSC 75% 22.956 -30.478 0.659 -1.028 -0.369

G50 (KB2) 46.648 -29.474 1.268 -0.978 0.290

G50 (KB2)+ Reduce SSC 75% 22.966 -30.514 0.639 -1.028 -0.390

G70 (KB3) 45.723 -29.504 1.252 -0.958 0.295

G70 (KB3)+ Reduce SSC 75% 22.957 -30.507 0.635 -1.001 -0.366


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4.7 Sumary the impact of breakwater

The impacts of breakwater to the other areas are not the issue to be considered. However, all breakwater scenarios have reduced currents, waves and increased accretion in the protection areas. KB1 (with gap 30 m) is better than other scenarios. However, in case of sediment reduction by 75%, KB3 (with gap of 70 m) should be considered. Moreover, issue to be considered with breakwater scenarios is the increase of erosion in the gap and the offshore side of breakwaters.

5. Simulation results of the impact of sandbars

5.1 Methodology

Due to the MIKE21 model cannot run cohesive and non-cohesive sediment in one scenarios, we treated them in two steps.

The first step we run the model of sand transport assuming that the morphological changes in the area without sandbar are negligible, then only the sandbar deformation is considered. Fortunately the deformation of sandbar is not high so that we can go to the second step. Otherwise, the sandbar scenarios are not considered further.

The second step we run the model of mud transport, assuming that the average cross section of the deformation sandbar in the first step are unchanged. That means the sandbars now become “concrete” breakwater when morphological changes are considered the impact of sandbars.

5.2 Sandbar scenarios

Two sandbar scenarios were studied as expressed in Table 5-1 and Figure 5-1.

Table 5-1 Sandbar configuration for Go Cong study area

No Scenarios Description Sandbar configuration/dimensions (m)

Leng Distance of 2 units

Distance from shoreline

Width of sandbar

Top elevation

1 SB1 Sandbar was made from 500 m from the shoreline offshore

1000 200 500 50 -2.4

2 SB2 Sandbar was made from 500 m from the shoreline offshore

1000 200 500 70 -2.4


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Figure 5-1 Sandbars in the study model at Go Cong, Tien Giang province

5.3 Calibrate the sandbar deformation in 21FM (HD&SW&ST) model

The deformation of sandbars in the physical models were presented the result of physical model test. In this section, numerical sand transport model is set up to calibrate sandbars deformation. Model set up is expressed in Figure 5-2. The smallest grid size is 2 m.

In general, the deformation rates of the numerical model and physical model were similar. One scenarios , for example presented in Table 5-2 and Figure 5-5.

Figure 5-2 Numerical model 21FM (HD&SW&ST) set up for sandbar deformation calibration

Table 5-2 Typical scenario for 21FM (HD&SW&ST) model calibrate

B

(m)

Rc

(m)

Hm0

(m)

Tp

(s)

Dura

(min)

B

(m)

Rc

(m)

Hm0

(m)

Tp

(s)

Dura

(hrs)

WP6-NOU-B5-R15-JSW2 5 -0.15 0.1 1.53 80 100 -3 2 6.84 9.32

MODEL PROTOTYPE

Tên kịch bản


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Figure 5-3 Calibration result of 21FM (HD&SW&ST) with typical scenario WP6-NOU-B5-R15-JSW2

The sandbars deformation after one month in the NE and SW monsoons were showed in Table 5-3, Table 5-4 and in Figure 5-5, Figure 5-5 .

In addition, three cross sections were checked the sandbar deformation as presented in Figure 5-6, Figure 5-7, Figure 5-8 and Figure 5-9. The deformation reduction southward of the study area can be recognized.

Table 5-3 Morphological changes after one month in the NE monsoon (January 2014)

January 2014

Senarios

Sand bar area (area 1) Shore area (area 2) Note

Volume ( 10

3 m3)

Average thickness H(m)

Volume ( 10

3 m3)


Initial sand

volume ( 10

3 m3)

V accretion

V erosion

Accretion Erosion V

accretion V

erosion accretion erosion

B50 3.561 -42.894 0.03 -0.32 36.628 0.000 0.01 0.00 345.694

B70 6.167 -46.117 0.02 -0.14 37.259 0.000 0.01 0.00 466.258

Table 5-4 Morphological changes after one month in the SW monsoon (September 2014)

September 2014

Senarios

Sand bar area (area 1) Shore area (area 2) Note

Volume ( 10

3 m3)


Volume ( 10

3 m3)

Average thickness H(m) Initial sand

volume ( 10

3 m3) V

accretion V

erosion Accretion Erosion

V accretion

V erosion

Accretion Erosion

B50 2.609 -15.253 0.03 -0.05 11.656 0.000 0.00 0.00 345.694

B70 4.163 -16.677 0.02 -0.05 11.560 0.000 0.00 0.00 466.258


23

Figure 5-4 The results of sand bar deformation (B = 50m) after one-month period of the Northeast monsoon (1/2014) (a) and one-month period of the SW Monsoon (9/2014) (b)

Figure 5-5 The results of sand bar deformation (B = 70m) after one-month period of the Northeast monsoon (1/2014) (a) and one-month period of the SW Monsoon (9/2014) (b)


24

Figure 5-6 Cross sections to analyze the sandbar deformation

Figure 5-7 Morphological changes at section 1 after the one-month period of the Northeast monsoon (1/2014)



25


5.4 Morphological impact of sandbars

This section we discuss the morphological changes of sandbars where sandbars are considered to be “concrete breakwater” and using MIK21FM-MT (as mentioned in part a)).

Table 5-5 and Table 5-6 showed the impacts of sandbars (both B=50 m and B=70m) in NE and SW monsoon. Table 5-7 combined the NE and SW monsoon results. It can be seen that sandbar with of 70 m is better in trapping sediment. Comparing with hard breakwater, the sandbar scenario is less effective with the accretion volume of 0.364 Mm3 whereas the hard breakwater can get 0.300 Mm3 for two typical NE and SW monsoon months of 2014.

Table 5-5 Morphological changes of sandbar impact after one month in the NE monsoon (January 2014)

January 2014

Senarios

Shore area (area 2) Sand bar area (area 1) Note

Volume ( 10

6 m3)


Volume ( 10

6 m3)


Initial sand

volume ( 10

3 m3)

V accretion

V erosion

Accretion Erosion V

accretion V

erosion accretion erosion

KB0 11.0623 -17.6321 0.009 -0.013 0.3871 -1.0697 0.013 -0.030

SB50 10.520 -10.507 0.010 -0.010 0.3900 -0.8240 0.014 -0.030 0.346

SB70 13.116 -10.972 0.010 -0.010 0.4720 -0.9960 0.016 -0.030 0.466

Table 5-6 Morphological changes of sandbar impact after one month in the SW monsoon (September 2014)

September 2014

Senarios

Shore area (area 2) Sand bar area (area 1) Note

Volume ( 10

6 m3)


Volume ( 10

6 m3)

Average thickness H(m) Initial sand

volume ( 10

6 m3) V

accretion V

erosion Accretion Erosion

V accretion

V erosion

Accretion Erosion

KB0 24.84 -9.09 0.018 -0.007 0.7560 -0.17 0.022 -0.006

SB50 30.370 -8.991 0.022 -0.006 0.8184 -0.202 0.024 -0.007 0.346

SB70 30.370 -8.992 0.022 -0.006 0.8202 -0.204 0.024 -0.007 0.466


26

Table 5-7 Net volume (Mm3) and maximum erosion thickness (m) with sandbar scenarios

Scenarios

Area 1 Combined January and September

2014

Jan.2014 Sep.2014 Vol. (Mm3)

Evaluation Net Vol. (Mm3)

Max Ero. Thickness

(m)

Net Vol. (Mm3)

Max Ero. Thickness

(m)

Net Vol. (Mm3)

Max Ero. Thickness

(m)

KB0 (Baseline) -0.6826 -0.030 0.59204 -0.01 -0.0906 -0.030

SB50 -0.5242 -0.035 0.79802 -0.01 0.2739 -0.035 Good

SB70 -0.4335 -0.029 0.79739 -0.01 0.3639 -0.029 Better

6. CONCLUSION

With nesting approach, MIKE21 has been calibrated well from the Regional model to Local Model with water levels, discharges, tides, waves and currents, sediment transports especially the validation results based on the in-situ data of the LMDCZ project in October 2016 and February-March 2017.

These results in the Local model of LMDCZ are ready for creating the boundary conditions for the detail study areas of Go Cong and Phu Tan.

The impacts of hard breakwaters are the reducing of wave, current and increasing accretion in the protection area.

The impact of soft measure, that is the sandbar which is more effective than hard breakwater in both of trapping more sediment and reducing erosion thickness at the gaps of the measures.

Sandbar scenarios should be considered more in term of economic aspect.


27

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Studying impacts of hard breakwaters and sandbars at Go ... qua Du an... · North Java, Indonesia a Building with Nature measure, the sandbar was proposed. Both porou s breakwater

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