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Sep 22, 2015 - DEPARTMENT OF (IU t ENGINEERING, TOHOKU UN IUERSITY. UIETNAM ..... Reasonable distance between groynes placed on concave bank. Nguyen The .... Cell 3 (central zone) and cell 4 (South zone) extend from landmark B16 to B23 and B23 to B46, ...... e-mail: vietthanh@utc.edu.vn. Abstract.
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TABLE OF CONTENTS Page Preface By Nguyen Trung Viet ................................................................................................................. 3

Keynote Lectures 1. Morphological change on Cua Dai Beach, Hoi An, Vietnam Hitoshi Tanaka and Nguyen Trung Viet................................................................................. 7 2. Overview of status and prevention solutions for erosion and sedimentation at estuaries in central Vietnam Tran Dinh Hoa, Nguyen Khac Nghia and Mac Van Dan ....................................................... 9 3. Erosion control in Vietnam using Japanese traditional river engineering Hirotada Matsuki ................................................................................................................ 20

Technical Sesssions Session Coastal Engineering 1 4. Morphodynamics and Evolution of the Barrier Islands in Southwestern Taiwan and Their Implications for Future Coastal Hazard Management Tsung-Yi lin and Vo Le Phu ................................................................................................ 29 5. Improvement of artificial reef for countermeasure against coastal erosion Toshimitsu Takagi and Yuko Saitoh..................................................................................... 34 6. Morphological changes gate Dai - Tra Khuc river Quang Ngai province after the typhoons No11 and No12, 2013 Vu Van Ngoc, Truong Van Bon and Doan Tien Ha.............................................................. 40 Session Coastal Engineering 2 7. Estimation of shoreline changes of the Cai river estuary in Viet Nam Thanh Luan Nguyen, Thanh Tung Tran, Hoang Son Nguyen, Van Van Than and Yves Lacroix ......................................................................................... 49 8. Situation of coastal erosion, accretion in Thua Thien Hue province and countermeasures Nguyen Huu Ngu and Duong Quoc Non .............................................................................. 56 9. Study on the mechanism of sand bar deposition at Cua Lap estuary and proposed countermeasure for stabilizing Nguyen Van Giap and Nguyen Trung Viet ........................................................................... 62 10. Mapping technology with two spherical cameras and its applications Syoji Nakamura, Licheng Zheng, Toshihiro Ichihashi ......................................................... 68 v

11. Estimation of total sediment volume for beach nourishment in the northern part of Nha Trang Nguyen Trong Hiep, Nguyen Xuan Tinh, Nguyen Trung Viet and Hitoshi Tanaka ............... 73 Session Coastal Engineering 3 12. Study of rock running and propose solutions against concrete plate erosion of embankment on cat Pham Van Lap, Le Xuan Roanh and Le Tuan Hai ............................................................... 81 13. Study on cohesive sediment transport modeling Nguyen Xuan Tinh, Nguyen Thi Le Quyen, Nguyen Manh Cuong and Ho Duc Dat .............. 87 14. Recent advances in coastal structures in Japan M. Hanzawa, A. Matsumoto, N. Hirose, M. Tanaka, J. Mitsui, S. Maruyama and S. Suga ......................................................................................................................... 92 15. Efficiency analysis of river entrance construction works in central Vietnam Nguyen Quang Duc Anh and Luong Phuong Hau................................................................ 99 16. Physical study with a wave flume on the impact of wave returning nose on wave overtopping at sea-dikes Nguyen Van Dung, Le Xuan Roanh and Thieu Quang Tuan .............................................. 106 17. Study on statistics of wave characteristic caused by storm at Cua Dai river mouth – Hoi An city Huynh Cong Hoai and Le Duc Vinh .................................................................................. 112 18. A proposed construction solution for recovering of Ba Lang beach, Nha Trang, Khanh Hoa province Le Thanh Binh, Duong Hai Thuan, Nguyen Trung Viet, Hitoshi Tanaka............................ 119 19. Change of shoreline around artificial headlands in Misawa coast Mikio Sasaki ..................................................................................................................... 124 Session Coastal Engineering 4 20. Numerical modelling of coastal erosion: a case study at the western tombolo in giens Van Van Than, Yves Lacroix and Trong Tu Nguyen .......................................................... 135 21. Forecasting the erosional trend of the northern shoreline of Ly Son island (Quang Ngai, Vietnam) due to climate change and sea-level rise Tung, Tran Thanh, Tuyen Kieu Xuan, and Dung Le Duc ................................................... 141 22. Investigation of hydrodynamic regimes for Nha Trang bay using the 3D open-source EFDC Model Nguyen Viet Duc, Nguyen Xuan Tinh, Nguyen Trung Viet and Bui Minh Hoa ................... 147 23. Pre-and post-tsunami morphology changes on Kodanohama beach Hoang Dong Hai, Yuta Mitobe, Vo Cong Hoang, Nguyen Trung Viet and Hitoshi Tanaka ................................................................................................................. 152 24. Study on sediment deposition in estuaries and coastal zones in Ben Tre province by numerical modeling Doan Thanh Vu, Nguyen The Bien and Nguyen Thanh Minh ............................................ 158 vi

25. The effect of coastal foreshore lowering on wave height growth in Bac Lieu province, Vietnam Le Duc Dung, Nguyen Quang Chien and Tran Thanh Tung .............................................. 165 26. Numerical analysis of overtopping-type wave power generation equipment Masato Minami ................................................................................................................ 171 27. Modelling the effect of geotextile submerged breakwater on hydrodynamics in La Capte beach Yves Lacroix, Minh Tuan Vu, Van Van Than and Viet Thanh Nguyen ............................... 177 28. East sea wave climate simulation from 1979 to now using MIKE 21 SW Model Dang Quang Thanh and Le Quan Quan ........................................................................... 184 29. Micromechanical modelling of internal erosion by suffusion using DEM-PFV coupled Model Tong Anh Tuan and Bruno Chareyre ................................................................................ 189 30. Impact of Tri An dam-break on inundation in the lower Saigon-Dongnai river basin, Vietnam Dang Dong Nguyen and Le Thi Hoa Binh ........................................................................ 195 Session River Engineering 31. Performance evaluation of water turbine for applying to circular open-channel Shota Koyahata and Masato Minami ................................................................................ 203 32. Comparison of sediment movement due to ship wave and tide La Vinh Trung and Doan Van Binh .................................................................................. 209 33. Reasonable distance between groynes placed on concave bank Nguyen The Hung and Huynh Lam Nguyen....................................................................... 216 34. Proposed solutions for the protection of the rivermouth of Cai Lon river – Kien Giang province Nguyen Thi Phuong Mai and Le Trung Thanh .................................................................. 226 35. Impacts of contracted bridge on the river bed deformation Mai Quang Huy................................................................................................................ 232 36. Effect of energy head on scour downstream of Overtopped-embankment flow Doan Van Binh and La Vinh Trung .................................................................................. 236 Session Environmental and Hydrological Engineering 37. Development of a decision support system for Dau Tieng reservoir, Sai Gon river basin: initial results Nguyen Hong Quan, Le Viet Thang, Nguyen Tran Quan, Nguyen Van Lanh, Le Thi Khuyen, Trieu Anh Ngoc, Nguyen Tuan Duc, Mai Toan Thang, Nguyen Duy Hieu, Le Thi Hien, Vo Thuy Trang and Do Quynh Nga .............................................................. 247 38. A practical scheme to promote global warming countermeasures in coastal area A case study of “Yokohama blue Carbon project” Jun Iwamoto, Satoru Yoshihara, Yasusuke Nakata, Koshi Yamada, Katsuhiko Suzuki, Tomoyuki Dan, Takashi Ooshima and Masato Nobutoki ....................................... 259 vii

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NUMERICAL MODELLING OF COASTAL EROSION A CASE STUDY AT THE WESTERN TOMBOLO IN GIENS Conference Paper · September 2015

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Vietnam-Japan Workshop on Estuaries, Coasts and Rivers 2015, 7-8 September, 2015, CKT, Hoi An, Vietnam

SESSION COASTAL ENGINEERING 4

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Vietnam-Japan Workshop on Estuaries, Coasts and Rivers 2015, 7-8 September, 2015, CKT, Hoi An, Vietnam

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Vietnam-Japan Workshop on Estuaries, Coasts and Rivers 2015, 7-8 September, 2015, CKT, Hoi An, Vietnam

NUMERICAL MODELLING OF COASTAL EROSION : A CASE STUDY AT THE WESTERN TOMBOLO IN GIENS VAN VAN THAN1*), YVES LACROIX2) and TRONG TU NGUYEN3) 1*) Laboratory LATP, AMU and Faculty of Civil Engineering, TLU (WRU), Hanoi, Vietnam 39, rue F. Joliot Curie, 13453 Marseille Cedex 13, France corresponding author to provide phone: 0033(0)762164318 e-mail: [email protected] 2) SEATECH, UTLN and MEMOCS, Università Degli Studi dell’Aquila, Italy avenue G. Pompidou, 83162 La Valette du Var, France e-mail: [email protected] 3) Faculty of Civil Engineering, TLU (WRU) 175 Tay Son, Dong Da, Ha Noi, Viet Nam e-mail: [email protected] Abstract The Almanarre beach, located on the western tombolo of Giens peninsula in the town of Hyères (France), presents an important coastal erosion, the result of coastal hydrodynamic processes. The Salt Road parallel to the Almanarre beach has faced various threats from coastal erosion. Thus, the constrain of the coastal erosion is necessary for the protection of the Salt Road. This paper presents an analysis of the coupled hydro-sedimentological numerical model, in order to determine the main factors for this erosion. Mike 21 was applied in order to simulate the hydro-sedimentological processes. The model is calibrated with good fitting coefficients by using a Nikuradse roughness of 0.0656 m, a Smagorinsky coefficient of 0.9, and a Manning number of 24 m 1/3/s. This work shows an increase of the wave parameters, sand transport capacity, and beach evolution occurs in northern part of Almanarre beach. An average wave height range of 0.5-1.8 m was estimated mostly from the South-West direction, corresponding to the peak wave period range of 3.9-7.8 s. The highest energy density is at landmark B13 or B17. The South-Western waves are the most important for the beach erosion in the Western tombolo. Keywords: Almanarre beach, beach erosion, sand transport, morphological evolution. 1. INTRODUCTION The double tombolo of Giens is formed from western and eastern arrows. It is located in the Hyères Township (Var region, South of France). The Almanarre beach in the gulf of Giens belongs to the Western tombolo, 4 km long, along the Salt Road. It is subject to coastal erosion. The dynamics of sediment transport, and geomorphological dynamics have been mentioned in many studies (Blanc, 1973; Jeudy De Grissac, 1975; SOGREAH, 1988a, 1988b; IARE, 1996; Courtaud, 2000; Serantoni & Lizaud, 2000-2010; ERAMM, 2001). These works show that the northern part of Almanarre beach is the most affected by the erosion process. This erosion is predominantly controlled by the hydrodynamic condition. The high beach erosion happened mostly during southwestern extreme events conjugated to atmospheric depression in winter time. These studies help the coastal managers to establish a coastal protection plan which has important economical, social and environmental impacts. The hydro-sedimentary cells should be separated by their limiting landmarks (Fig. 1A). Cell 1 (North zone) spreads from north boundary to the landmark B03. Cell 2 (North-central zone) is between the landmarks B03 and B16. Cell 3 (central zone) and cell 4 (South zone) extend from landmark B16 to B23 and B23 to B46, respectively. In the present study we gathered all available data on waves, sea level, current, wind, geomorphology, sediments, and biocenosis. They are compiled to numeric format compatible with

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MIKE 21. We have implemented, and calibrated a coupled model using MIKE 21 to simulate the hydro-sedimentological processes.

(B)

(A)

(C)

(D)

Fig. 1 (A) Western tombolo which was separated in four hydro-sedimentary cells, namely from cell 1 to 4. (B) Regional computational mesh for Eastern gulf of Giens. (C) Bathymetry of regional mesh. (D) Local computational mesh for the Western tombolo of Giens 2. MATERIALS AND METHODS The MIKE model was chosen because it is capable of reproducing the hydro-sedimentology in shallow waters and unstable flows in 2 and 3 dimensions. The main conditions defining the model were presented. The model scenarios were then investigated. 2.1. Defining the Model The model domain was developed in two scales. In a regional scale including gulf of Giens, we established a spectral wave (SW) model. Then, the SW model is coupled with the hydrodynamic (HD) model on the same scale. The local scale was set up at Almanarre beach. It coupled the SW, HD, and sand transport (ST) models. The regional mesh includes 6 027 nodes and 11 508 elements (Fig. 1B,C). A local mesh was used containing 2 276 nodes and 4 365 triangular elements in (Fig. 1D). The HD, SW, and ST models were applied together by using MIKE 21/3 Coupled FM(DHI, 2014). An overall time step interval of 1 800 s (or 30 minutes) was chosen for all models. The overall number of time steps in the calculation is 370 corresponding to a one-week period simulation time. The durations of the simulation in annual and storm conditions are from 01 to 08 January 2008 and from 21 to 28 January 2007, respectively. Two principal wind regimes were defined as western and eastern regime. The western regime is dominated by Southwest and Northwest. The eastern regime has no impact on the Almanarre beach which is protected by the Eastern tombolo and the Giens peninsula. The wind forcing is specified as constant in space and variable in time. It was extracted from SYNOP 1 station of Hyères. The Almanarre beach is composed of rock, gravels, sand, and fine sand (Blanc, 1960; SOGREAH, 1988a). An average grain diameter of 0.5 mm can be used for sediments at the local scale.Porosity of the bottom sediment of 0.4 can be applied in the calculations. An estimated relative sediment density of the sand is 2.65 g/cm3. The sediment grading was specified as 1.1. The rocks bed were modeled as sand bed with a median diameter D50 = 20 mm. 1

Synoptic – data coding adopted by WMO

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Vietnam-Japan Workshop on Estuaries, Coasts and Rivers 2015, 7-8 September, 2015, CKT, Hoi An, Vietnam

For the regional boundaries, the waves and sea levels vary in time and are constant in space along the northwestern and southeastern boundary based on wave and sea level data at MEDIT-2185 and MEDIT2610 spots, respectively. The boundary conditions are linear in time and space along the southwestern boundary provided by wave and sea level data collected at these spots. For the local boundaries, the results of regional domain were then used as boundary conditions to be used in the calculations for site of focus with a finer mesh. The northern and southern boundary conditions were assumed as the closed boundaries and land zero normal velocity. Zero sediment flux gradient was applied at all the boundaries. A specific short time period was chosen to calibrate the model. The best estimate of the extreme values corresponds to a Nikuradse roughness of 0.0656 m; this value of roughness is used as a constant over the domain. It was seen that simulation with a Smagorinsky coefficient of 0.9 and a Manning number of 24 m1/3/s gave the best match. The results of calibrating the models conform to the general knowledge of Western tombolo. Once the model is calibrated, the calibration factors can be applied to model the scenarios for the Western tombolo. 2.2. Model Scenarios The aim of model scenarios was to understand the hydrodynamic conditions prevalent. According to the analysis of the wind and wave data, the history of storms, the model performs two driving forces scenarios: annual and storm conditions. The annual condition is defined by the statistics of wave data available from 1999 to 2012 at the buoy Porquerolles. Than (2015)shows that southwestern regime is major factor of erosion in the northern part of the Almanarre beach. Therefore, this paper concentrates on the southwestern wind and wave in both annual and storm conditions. All hydrodynamic condition scenarios are shown in a summary table (Table 1). Table 1. A summary of hydrodynamic conditions for numerical model for the Western tombolo of Giens Model Boundary Climatic conditions Wave height, H1/3 (m) Wave period , T (s) Wave direction, MWD (°) Sea level (m) Wind speed (m/s) Wind direction (°)

Regional scale Northwestern Annual Storm 0.84 4.33 5 12 160-200 200 0.2-0.7 1.1-1.5 5.57 17.71 228 250-310

Southwestern Annual Storm 0.84-1.77 4.33-6.43 5 12 160-250 200-250 0.2-0.7 1.1-1.5 5.57 17.71 228 250-310

Southeastern Annual Storm 1.77 6.43 5 12 200-250 250 0.2-0.7 1.1-1.5 5.57 17.71 228 250-310

Local scale Western Annual 0.19-1.19 4-5 218-270 0.2-0.7 5.57 228

Storm 1.06-2.57 5-10 215-320 1.1-1.5

17.71 250-310

3. RESULTS AND DISCUSSIONS The hydro-sedimentological simulation was conducted over the entire Western tombolo for two scenarios: annual and storm condition. The wave parameters (wave height, direction, period, and energy) were extracted at the points close to the coast at 1 m depth from E01 to E12 and one offshore at 3 m depths from Point 1 to 8 along the Western tombolo in Fig. 2A.. The results of hydrodynamic, sand transport, and morphological evolution were summarized in Table 2. An increase of the wave parameters, sand transport capacity, and beach evolution was observed in cell 2 (Table 2). However, cell 4 represents a decrease of these parameters (Table 2). The maximum onshore wave energy density and peak wave period were represented in Table 3. The energy densities were concentrated in cells 2 and 3. They are small in cells 1 and 4. The current speed in north part is greater than the one in south part. The sand accretions were concentrated in the Posidonia meadows and rock beds Fig 2B,C). In the following, hydrodynamic, sand transport, and morphological evolution were analyzed for each cell and scenario. 3.1. Hydrodynamics During the annual condition, the waves have a direct incidence in cell 1. The model indicates that the wave height, period, and direction are 0.69 m, 3.9 s, and 234degrees from true North on average in cell 1, respectively (Table 2). At the point E01 (landmark B01), the maximum wave energy density is 0.295 m2/s/rad, with a peak wave period of 5.2 s (Table 3). The north-south currents are 0.2 m/s in average corresponding to the sea level of 0.439 m above LAT in cell 1 (Table 2).

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Vietnam-Japan Workshop on Estuaries, Coasts and Rivers 2015, 7-8 September, 2015, CKT, Hoi An, Vietnam

(A)

(B)

(C)

Fig. 2 (A) Location of extraction points close to the coast (-1 m) from E01 to E12 and one offshore (-3 m) from Point 1 to 8 in the Western tombolo of Giens. (B) Maps of the rates of bed level change in annual condition. (C) Maps of the bed level change rates in storm condition. Table 2. Summaries of hydrodynamic, total load sediment transport, and evolution of the Western tombolo in case of southwestern regime for annual and storm conditions. Condition Annual

Storm

Cell 1 2 3 4 1 2 3 4

Wave Hs 0.69 0.79 0.71 0.5 1.7 1.8 1.5 1.3

Tp 3.9 4.2 4.2 4.0 7.8 7.5 7.2 7.2

MWD 234 249 258 268 174 191 209 220

Sea level (m) 0.439 0.437 0.438 0.438 0.492 0.490 0.490 0.491

Current Speed Dc 0.2 123 0.1 166 0.07 185 0.06 193 0.2 145 0.1 164 0.1 197 0.1 188

Sand transport Q Ds 7.21 96 43.5 131 16.6 148 18.9 168 24 94 263 103 212 119 131 128

Morphology Rv Rb -52 0 -663 -1 4 0 -13 0 -1 162 1.4 -10 141 -8.4 -1 477 -3.2 -3 297 -2.2

Hs – Significant wave height (m); Tp – Peak wave period (s); MWD – Mean Wave Direction (°); Speed – Current speed (m/s); Ds – Current direction (°); Q - Magnitude of the total load sediment transport (10 -7 m3/s/lm - cubic meters per second per linear meter); Ds – Direction of the total load sediment transport (°); Rv – Rate of global volume change of bathymetry (m3/day - cubic meters per day); Rb - Rate of bed level change (mm/ day - millimeter per day).

Table 3. The wave spectral described by maximum onshore wave energy density (E), frequency (f) and peak wave period (Tp) at 1 m depth in southwestern regime (Lacroix et al., 2015) Point

Landmarks

E01 E02 E03 E04 E05 E06 E07 E08 E09 E10 E11 E12 E13

B01 B04 B06 B07 B08 B10 B13 B16 B17 B18 B24 B33 B41

Annual condition E (m2/s/rad) 0.295 0.454 0.409 0.409 0.409 0.404 0.478 0.446 0.465 0.292 0.283 0.275 0.142

f (Hz) 0.193 0.193 0.193 0.193 0.193 0.193 0.19 0.176 0.176 0.176 0.176 0.193 0.193

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Tp (s) 5.2 5.2 5.2 5.2 5.2 5.2 5.3 5.7 5.7 5.7 5.7 5.2 5.2

Storm condition E (m2/s/rad) 0.787 1.467 1.308 1.308 1.308 1.272 1.562 1.933 2.041 1.24 0.981 0.971 0.4

f (Hz) 0.159 0.159 0.148 0.148 0.148 0.145 0.145 0.145 0.145 0.145 0.145 0.145 0.145

Tp (s) 6.3 6.3 6.8 6.8 6.8 6.9 6.9 6.9 6.9 6.9 6.9 6.9 6.9

Vietnam-Japan Workshop on Estuaries, Coasts and Rivers 2015, 7-8 September, 2015, CKT, Hoi An, Vietnam

The average wave height of 0.79 m and average peak wave period of 4.2 s were recorded at cell 2. The onshore waves are incident mostly from the west direction. The energy density is the highest in cell 2. The maximum energy density varies from 0.404 to 0.478 m2/s/rad, with a peak wave period range of 5.25.7 s in cell 2. The maximum value corresponds to the peak wave period of 5.3 s at landmark B13. The current speed has a low average value of 0.1 m/s at sea level 0.437 m above LAT in cell 2 (Table 2). An estimated wave height is about 0.71 m coming from 258 degrees of true North, with typical wave periods of 4.2 s in cell 3 (Table 2). The maximum wave energy density can reach 0.292-0.465 m2/s/rad corresponding to the peak wave period of 5.7 s in cell 3. The highest value was obtained at the landmark B17. A mean current speed of 0.07 m/s in cell 3 was found corresponding to the sea level 0.438 m above LAT in Table 2. The mean significant wave height in cell 4 reached 0.5 m with a peak wave period of 4.0 s. A mean wave direction of 268 degrees of true North in cell 4 was estimated. The maximum wave energy density in cell 4 oscillates from 0.142 to 0.283 m2/s/rad, with a peak wave period range of 5.2-5.7 s. The highest wave energy density was observed at the landmark B24. A low average current speed is about 0.06 m/s with sea level of 0.438 m above LAT in cell 4 (Table 2). During the storm condition, the wave model indicates strong wave focus in cell 1. The strongest wave was 1.7 m and 7.8 s in the direction 174° in cell 1. The maximum wave energy density can reach 0.787 m2/s/rad and corresponds to the wave period of 6.3 s at the point E01 (Table 2). Current speed can reach 0.2 m/s in southwestern direction at sea level 0.492 m above LAT in cell 1 (Table 2). A mean wave direction in cell 2 is 191 degrees from true North. Maximum wave heights of 1.8 m were estimated with an average peak wave period 7.5 s in cell 2 (Table 2). The maximum energy density in cell 2 fluctuates between 1.272 and 1.933 m2/s/rad. The highest energy density was reached at the peak wave period of 6.9 s at landmark B16. Current magnitudes reach an average value of 0.1 m/s in southeastern direction at sea level 0.490 m above LAT in cell 2 (Table 2). The mean significant wave height and peak wave period are 1.5 m and 7.2 s, respectively, provenance of 209 degrees of true North in cell 3. The maximum wave energy density range of 1.24-2.041 m2/s/rad, with peak wave period of 6.9 s has recorded in cell 3. The wave energy always is maximal at the landmark B17. The average current speeds are close to 0.1 m/s at sea level 0.490 m above LAT in cell 3. A mean significant wave height is about 1.3 m (direction 220 degrees of true North), with a peak wave period of 7.2 s in cell 4. An estimated maximum wave energy density range is between 0.4 and 0.981 m2/s/rad at the landmark B41 and B24, respectively, with a peak wave period of 6.9 s. An average current speed in cell 4 can reach 0.1 m/s associating a sea level of 0.491 m above LAT in Table 2. 3.2. Sand Transport For the annual condition, the maximum total load sediment transport to the east in cell 1 reaches 7.21 10-7 m3/s/lm in Table 2. A maximum sand transport of 43.5 10-7 m3/s/lm to the southeast was estimated in cell 2 (Table 2). The estimated sand transport to the southeast coast is about 16.6 10 -7 m3/s/lm in cell 3. A sand transport capacity in cell 4 was estimated as 18.9 10 -7 m3/s/lm to the southeast coast. For the storm condition, the total load sediment transport in cell 1 is characterized by the east direction at an average rate of 24 10-7 m3/s/lm (Table 2). Maximum sand transport in cell 2 reached 263 10-7 m3/s/lm towards the coast (Table 2). The total load sediment transport in cell 3 reaches 212 10 -7 m3/s/lm towards the southeast. A sand transport capacity of 131 10 -7 m3/s/lm in cell 4 was estimated towards the southeast. 3.3. Morphological Evolution In the annual condition, the rate of global volume change of bathymetry is approximately -52m3/day in cell 1 (Table 2). A mean rate of bed level change in cell 1 is very small approximately 0 m/day (Fig. 2B). Cell 2 was affected by erosion (Table 2). The estimated global volume change rate is about -663 m3/day in cell 2 (Table 2). Mean estimated rate of bed level change in cell 2 is -1 mm/day represented in Table 2. Table 2 shows no significant simulated bed level change rate and simulated global volume change rate (4 m3/day) in cell 3. A mean rate of the global volume change in cell 4 is low -13 m3/day (Table 2).. The beach evolution is negligible in cell 4. In the storm condition, the morphological changes show an accretion in cell 1. The rate of global volume change of bathymetry in cell 1 was estimated as -1 162 m3/day in Table 2. A mean rate of accretion is +1.4 m/day in cell 1 (Table 2 and Fig.2C). The morphological changes show the erosion

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between landmarks B03 and B08. The global volume change rate of -10 141 m3/day in cell 2 was calculated in Table 2. An average rate of erosion in cell 2 reaches -8.4 mm/day in Table 2. A mean global volume change rate in cell 3 can reach -1 477 m3/day Table 2. A mean rate of erosion in cell 3 goes up to -3.2 mm/day in Table 2. The rate of the global volume change in cell 4 was quantified as -3 297 m3/day in Table 2. The beach erosion is around -2.2 mm/day in cell 4. 4. CONCLUSIONS The study area is subdivided into four hydro-sedimentary cells 1-4 from north to south. The north part of the Western tombolo presents significant beach erosion due to sediments deficit. It is open to the South-Western events. MIKE 21/3 coupled model for hydrodynamic, sand transport, and morphological evolution has been applied to the Western tombolo.The best estimate of the model corresponds to a Nikuradse roughness of 0.0656 m, a Smagorinsky coefficient of 0.9, and a Manning number of 24 m1/3/s. Hydrodynamic simulations in cell 2 indicate an increase of the wave parameters, sand transport capacity, and beach evolution. However, a decrease of these parameters was shown in cell 4. The waves have a direct incidence on cells 1 and 2. An average wave height range of 0.5-1.8 m was recorded mostly from the south-western direction, with a peak wave period range of 3.9-7.8 s. The maximum energy density varies from 0.142 to 2.041 m2/s/rad, with a peak wave period range of 5.2-6.9 s at 1 m depth near to the coast. The waves present a small spread and long ridges. The highest energy density is at landmark B13 or B17. The current speed in south part is smaller than the one in north part. An average current speed was estimated from 0.06-0.2 m/s towards the south direction corresponding to the sea level range of 0.437-0.492 m above LAT. The sand transport capacity range of 7.21-263 10-7 m3/s/lm was calculated. The beach erosion is the most important between landmarks B03 and B08 in cell 2. The mean estimated global volume change rate is from -10 141 to 4 m3/day. The mean rate of bed level change range was found to be from -8.4 to 1.4 mm/day. It can be concluded that the South-Western waves are the most important on the beach erosion in the Western tombolo. The currents may play a role less important on the beach erosion. This is in accordance with the general knowledge of the Western tombolo. 2. ACKNOWLEGEMENTS The authors wish to thank to DHI Water and Environment who design the MIKE model. We kindly thank the following organizations for data possible: SHOM, IGN, EOL, ERAMM, SOGREAH, CETE, CEREGE, and HYDRO-M. 3. REFERENCES Blanc, J. J. (1960). Etude sédimentologique de la presqu l'île de Giens et de ses abords (pp. 35-52). Blanc, J. J. (1973). Recherches sédimentologiques sur la protection du littoral à la presqu’ile de Giens (Var). Courtaud, J. (2000). Dynamiques geomorphologiques et risques littoraux cas du tombolo de giens (Var, France méridionale). (Ph.D. dissertation), Université Aix-Marseille I. DHI. (2014). MIKE 21 & MIKE 3 FLOW MODEL FM - Sand Transport Module -Scientific Documentation. ERAMM. (2001). Etude sur la protection de la partie Nord du tombolo Ouest de Giens (Vol. phase I+II+III). IARE. (1996). Le Tombolo Occidental de Giens - Synthèse des connaissances- Analyse globale et scenarios d'aménagement et de gestion. Jeudy De Grissac, A. (1975). Sédimentologie dynamique des rades d'Hyères et de Giens (Var). Problèmes d'Aménagements. (Ph.D. dissertation), Université d'Aix-Marseille II, Marseille. Lacroix, Y., Than, V. V., Leandri, D., & Liardet, P. (2015). Analysis of a Coupled HydroSedimentological Numerical Model for the Tombolo of GIENS. International Journal of Environmental, Ecological, Geological and Geophysical Engineering, 9(3), 117 - 124. Serantoni, P., & Lizaud, O. (2000-2010). Suivi de l évolution des plages de la commune Hyères-les-palmiers. SOGREAH. (1988a). Défense du littoral oriental du golfe de Giens (Vol. Etudes pré, pp. 50 + annexes). SOGREAH. (1988b). Protection du tombolo Ouest (Vol. Rapport 2-). Than, V. V. (2015). Modélisation d'érosion côtière: application à la partie Ouest du tombolo de Giens.(Ph.D. dissertation), Aix Marseille Université, Marseille.

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FORECASTING THE EROSIONAL TREND OF THE NORTHERN SHORELINE OF LY SON ISLAND (QUANG NGAI, VIETNAM) DUE TO CLIMATE CHANGE AND SEA-LEVEL RISE TUNG, TRAN-THANH1), TUYEN KIEU-XUAN 2), and DUNG LE-DUC 3) 1) Faculty of Marine and Coastal Engineering, Thuy Loi University, 175 Tay Son, Dong Da, Hanoi, e-mail: [email protected] 2) Central Vietnam Institute of Water Resources, 132 Dong Da, Hai Chau, Danang, Vietnam, e-mail: [email protected] 3) Vietnam Institute of Sea and Islands, 125 Trung Kinh, Cau Giay, Hanoi, Vietnam, e-mail: [email protected] Abstract Many islands of various sizes exists along the central coastline of Vietnam; the islands play important roles in politics, national defense and sovereignty. In recent years, with higher population stress and impact of climate change and sea level rise (SLR), these islands withstand intensive disasters and frequent erosion with coastline retreat. This paper presents research output on erosional trends of the northern coastline of Ly Son island in Quang Ngai province, taking into account of sea-level rise. The research output, in programme KC.09/11-15, serves as a basis for planning, protection and sustainable development of Vietnamese islands in future. Keywords: Ly Son island, coastal erosion, SLR, MIKE 21, LITPROF. 1. INTRODUCTION With a coastline almost 2000 km long, accounting for over 2/3 the total length of Vietnam coast, the Central coastal zone with islands remarkably contributes to the socio-economic growth and security, national defense of Vietnam, and the Central region particularly. The location of these islands and distribution of local inhabitants plays an important role ine politics and country sovereignty. On several islands, the number of inhabitants reaches that equivalent to a district: Phu Quy island (Binh Thuan), Ly Son island (Quang Ngai), or equivalent to a commune: Cu Lao Cham island (Quang Nam), Con Co island (Quang Tri), etc. Therefore it is necessary to protect these island from adverse natural events, maintain safety and sustainable development of the local inhabitants, which is a key factor for developing a firm strategy of national defense. Under the increasingly apparent impact of climate change and SLR, the islands in the Central region have been withstanding remarkable stresses from nature, aside from population and development stresses. For local sustainable development, active measures must be taken to prevent and mitigate natural impacts; such actions include building coastal protection works, building harbours for effective marine transportation, fishery and sheltering vessels from frequent storms. This paper presents a research on effects of marine hydrodynamic factors e.g. wave, wind, littoral currents on Ly Son island (Quang Ngai, Vietnam) with prediction of erosional trend for this island, considering adverse effects of climate change and sea-level rise. The results from this research contribute to the scientific foundation for maintenance, protection, explotitation of natural environment as well as socio-economic development and national defense in Ly Son in the future. 2. MATERIALS AND METHODS 2.1. Study area and data used Ly Son is an island located eastward of Quang Ngai province (15°2251N, 109°07E), 18 km from the mainland. A district-level administrative division, Ly Son comprises 3 communes: An Binh, An Hai and An Vinh with a total area of 9.97 sq. km, accounting for 0.19% the area of Quang Ngai.

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The wave data used in this study is a long-term wave series measured at Capmia station (15°N, 109.5°E) by FUGRO company from 1-Jan-1996 to 31-Jan-2005 in the project of training My A inlet, Quang Ngai [4]. This wave series was analysed to find waves with equivalent energy [5] for various directions; the results are present in Table 1. Table 1: Equivalent wave energy for various directions, Capmia station (1996-2005). Direction (°)

Hs (m)

Ts (s)

tk (days)

Direction (°)

Hs (m)

Ts (s)

tk (days)

0-30

2.29

6.0

6

120-150

1.07

4.0

32

30-60

2.26

5.8

118

150-180

1.08

4.0

49

60-90

1.36

4.5

108

180-210

1.13

4.1

16

90-120

1.05

4.0

30

The wave data in Table 1 shows that the coastal sea of Ly Son is influenced by seven major wave directions, of which prevailent waves are characterizes for Northeastern and Southwestern monsoon. The NE and ENE waves which are classified as 30°→ 60° and 60°→ 90° occur with the frequency of 118 and 108 days/year respectively. SE waves (150°→180°) occurs on 49 days/year. Those waves take main responsible for littoral currents and sediment transport in a year. The wind time series measured at Ly Son station (1985-2012) was used for statistical analysis and calculation of equivalent wind energy similar to wave data. The results for equivalent wind energy can be referred to [1]. 2.2. Sea-level rise scenario used for simulation For simulating shoreline erosion, the water level data was obtained from various scenarios of sea-level rise due to climate change; these scenarios were made publicly available in 2014 by Ministry of Natural Resources and Environment [6]. This study uses the sea-level rise data for Hai Van Pass – Dai Lanh Cape (which is the nearest location from Ly Son), with high emission scenario (listed in Table 2), to calculate erosion for the island. The choice of a high emission scenario is to consider adverse effects of sea-level rise on Ly Son island. Table 2: Sea-level rise of the area by Hai Van Pass – Dai Lanh Cape, high emission scenario [6] Year

2020

2030

2050

2100

Scenario

KB1

KB2

KB3

KB4

KB5

Sea-level rise (cm)

9

14

29

74

97

3. MODEL SETUP To study the effect of marine hydrodynamic factors on Ly Son island, the research group conducted modeling major hydrodynamic processes for Ly Son island with the MIKE 21 software suite (Mike, 2007a) is used. 3.1. Computation domain and mesh and bathymetry The computational domain (Figure 1) is the coastal area surrounding Ly Son, 30 km × 30 km in extent. The computational grid is constructed using a bathymetry chart scaled 1:2000 which was surveyed in Dec-2012 by Centre of Environmental Fluid Mechanics (University of Natural Sciences, Hanoi) in the framework of governmentally-funded Research Project [1] KC.09/11-15 (Figure 2).The geographic extents of the model in UTM projection are: 722500 m – 736000 m Easting and 1870600 m – 1894600 m Northing. The grid is unstructured with 4851 triangular cells.

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Figure 1: Computational domain and grid

Figure 2: Bathymetry surrounding Ly Son island

3.3. Model calibration and verification The calibration and verification stage is necessary for producing a set of parameters suitable for area of interest. This will ensure better matching between simulation results and reality. Calibration of water level: The water level data used for calibrating the hydrodynamic model was measured at the pier on Ly Son island (15°2246.74N, 109°545.48E) from 0:00 1-Jan-2012 to 23:00 31-Dec-2012 with a sampling frequency of 1 hour. Boundary conditions, initial condition and model parameters are reported in detail in Ref [1]. The result of water level calibration is shown in Figure 3. The error between simulated and measured water level is evaluated through Nash index (0.91).

Figure 3: Result of calibration for water level of Ly Son Calibration of flow: The data of water current used for model calibration was measured offshore (15°23.3N, 109°10.4E), 5 km to the east of the island. The time of measurement was from 10:20 17Dec-2012 to 9:50 29-Dec-2012, with a sampling frequency of 10 minutes. The boundary condition used were water level, waves and wind.

The details of boundary conditions, initial conditions and model parameters are reported in Ref [1]. The result of verification for flow is shown in Figure 4. The calculated flow is similar in phase with the measured flow.

Figure 4: Result of calibration for flow of Ly Son

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In terms of velocity magnitude, the calculated and measured flows differ at certain phases, especially during ebb tide. This difference is due to the 2-D model used; the calculated velocity is verticallyaveraged over the whole domain, while the measured flow was obtained at a fixed depth. However, such difference is acceptable. 4. CALCULATING THE TREND OF FORESHORE PROFILE DUE TO SEA-LEVEL RISE

Figure 5: Representative profiles for calculation

For an overview picture of shoreline evolution and changes in foreshore profile of the northern coastline of Ly Son island, the LITPROF model was used in tandem with MIKE SW model to calculate the evolution of representative cross-shore profiles. To facilitate modelling shoreline of the study area, the northern shoreline of Ly Son island (which experience continuous erosion) is divided into 4 segments (Figure 5) based on geomorphic features, briefly described as following:

- Segment D1-D2 composed of a coral reef and a sandy beach. - Segment D2-D3 mainly composed of sand. At D3 there is a mountain cape. - Segment D3-D4 has fairly complex composition, alternating between rocky and sandy coast (mostly rock). The beach in this section is narrow and experiences little change; the seabed is mostly composed of coral. - Segment D4-D5 is composed mostly from coarse sand. In the vicinity of D5, the coast is rocky seaward and sandy landward.

Figure 6: Results of bathymetric changes of 3 profiles in 5 scenarios

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Therefore, except for Segment D3-D4 which is almost unchanged due to rocky shoreline, the other three segments are sandy coasts and frequently experience seasonal changes. Three profiles are selected for these three segment to analyse the temporal changes of the coast. The location of these profiles are shown in Figure 5. The representative profiles, being extended seaward to the limit of closure depth (depth contour -10 m), are 660 m long. Each profile is divided into 110 computational element, each having a size of 6 m. The simulation time for a profile is one year. The wave data used for this calculation is the equivalent wave energy listed in Table 1. Simulation results of the five sea-level rise scenarios are compared with present trend of shoreline evolution at the three repsentative cross-shore profiles of Ly Son (Table 3). The results mainly focused on evaluation of the maximum shoreline retreat, the erosion depth of the foreshore at both breaker zone and swash zone. The simulation results show a general picture on shoreline retreats in the five scenarios. Table 3: Summarize of simulation results on for 5 scenarios at 3 cross-shore profiles Present Shoreline retreat due to SLR (m)

Maximum erosion depth at breaker zone (m) Maximum erosion depth at swash zone (m)

SLR 9cm

SLR 14cm

SLR 29cm

SLR 74cm

SLR 97cm

MC1

12

18

30

60

100

MC2

12

18

36

60

100

MC3

6

12

18

33

36

MC1

-0.88

-1.12

-1.28

-1.25

-1.4

-1.19

MC2

-1.43

-1.98

-2.61

-1.98

-2.27

-1.4

MC3

-2.44

-2.2

-2.22

-2.2

-2.32

-1.22

MC1

-0.39

-0.47

-0.53

-0.42

-0.49

-0.38

MC2

-0.44

-0.54

-0.5

-0.44

-0.52

-0.38

MC3

-1.28

-1.37

-1.18

-1.27

-1.19

-1.12

Also from the simulation results, the shoreline retreat at profiles 1 and 2 are not much different, whereas Profile 3 shows much less retreat. On the contrary, the erosion depth and foreshore lowering at Profile 3 are much greater than those of Profiles 1 and 2. This difference is due to the fact that Profile 3 is composed of coarse sand with steeper slope and has different behaviour under the impact of sea level rise. The shift of shoreline at different profiles tends to form submerged sand bars of new equilibrium profiles adapting to the present local wave climate. Calculation results show maximum seabed change at breaker zone (bed elevation from -6 m to -8 m). For Profiles 1 and 2, due to mild bed slope, relatively small waves reach the shore, the erosion mainly occurs at the breaker zone, hence this zone experience the largest change. For Profile 3, the steep slope allows wave to penetrate close to shore, therefore not only at the breaker zone, waves also affect the swash zone wth greater impact compared to the case of a more gently beach slope as in Profiles 1 and 2. A more gentle slope and a protected foreshore are very important in wave reduction and mitigation of erosion at construction toes, especiall in the context of climate change and sea-level rise. 4. CONCLUSIONS The results of study show that the coastline erosion and retreat trends of Ly Son island is intensified due to climate change and sea-level rise. For the erosion trend, higher sea level causes the more impact of hydrodynamic factors to the beach which leads to more severe erosion and lowering of foreshore. For coastline retreat, both area and distance of retreat depend on sea-level rise scenarios and local bathymetry. At gently sloped and low sandy beaches, the effect of erosion and the amount of retreat are larger. Evaluating and predicting the trends of erosion and coastline retreat of Ly Son is very important to the future planning, protecting and sustainable development of the island when impacts of climate change and sea-level rise are taken into account. However, due to limited time and data, in this study only calculation, analysis and evaluation on erosional trend and shoreline retreat of Ly Son island had been performed; while there are 3000 island in Vietnam in need for protection and sustainable development, especially nowadays with growing impact

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of climate change and sea-level rise. Therefore further research is needed for the whole system of islands in the country. 5. ACKNOWLEDGEMENTS A part of this study was financially supported by a National research project “Evaluation of changes in extreme values of oceanographic factors; their effects on environment, socio-economic development, with recommendation of preventive solutions for populous islands in Central Vietnam (mainly Ly Son and Phu Quy islands). The authors thank the project managers for kind support 6. REFERENCES Tung T.T., Dung L.Đ., (2015) Technical Report Computation of shoreline changes and beach evolution for Ly Son island, Quang Ngai. Water Resources University. 2015. DHI. (2007). Mike 21 Flow Model FM, Hydrodynamic Module. User Manual. DHI DHI. (2007). Mike LITPACK- LITPROF. User Manual. DHI FUGRO Oceanor. (2006). Calibrated wave parameters off Cap Mia in Vietnam. Trondheim, Norway. Tung T.T., Dung L.Đ., (2012). Computation of nearshore wave energy for the Central coast of Vietnam. Journal of Water Resources and Environmental Engineering, Vol 39 , pp46-53. MONRE. (2012). Climate change, sea level rise scenarios for Vietnam. Ministry of Natural Resources and Environment, Hanoi.

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INVESTIGATION OF HYDRODYNAMIC REGIMES FOR NHA TRANG BAY USING THE 3D OPEN-SOURCE EFDC MODEL NGUYEN VIET DUC1), NGUYEN XUAN TINH2), NGUYEN TRUNG VIET2,3) and BUI MINH HOA2), 1) Central Vietnam Construction and Consultancy Address, No. 141A - Nguyen Du – Ha Tinh, Vietnam e-mail: [email protected] 2) Department of Civil Engineering, Thuyloi University Address, 175 Tay Son, Dong Da, Hanoi, Vietnam e-mail: [email protected] 3) Central Region College of Technology, Economics and Water Resources Address, 14 Nguyen Tat Thanh, Hoi An City, Vietnam e-mail: [email protected]

Abstract Nha Trang Bay is located east of the city of Nha Trang, Khanh Hoa Province. Northern is limited by Ke Ga and the southern by Dong Ba. Nha Trang Bay is the second largest bay after Van Phong Bay in Khanh Hoa Province with an area of about 500 km2. Nha Trang Bay is one of the 29 most beautiful bays in the world, a center of tourism and services have fast economic growth of Khanh Hoa province in particular and the South Central region in general. Nha Trang Bay has a length of about 16 km and a width of about 13 km. The main source of fresh water flowing into the bay is from the Cai River. However, Nha Trang beaches have been facing some issues such as coastal erosion causing the beach slope is steeper and narrower. In order to sustainable development in this area, it is therefore vital to further investigate the mechanisms of this existing problems and proposing the suitable countermeasure solutions to handle it. This study presents a detail study on hydrodynamic regimes, which is mainly caused the coastal erosion problems, in Nha Trang Bay area using the open-source EFDC model. The comparison of model results and measured data in 2013 have been shown a good agreement for the water level, current velocity and wave height. This calibrated model is a good management tool for the coastal authority. Keywords: Nha Trang Bay, coastal erosion, monsoon, hydrodynamic, EFDC model. 1. INTRODUCTION Marine environment morphology changes is always associated with hydrodynamic regime of the water body. Therefore, the main requirement is to identify the underlying factors causing the change, including hydrodynamic processes as tide, wave and river flows with sediment transport processes lead to deposition and erosion at the bottom, onshore and beaches. The study to evaluate the role of each process requires the continuous measurement data over long time that the current marine monitoring equipment is not allowed. Modeling methods, including numerical models, has become a key tool in research and help to meet the requirements to provide technical input during the planning, construction and operation of any coastal structure projects. The 3D open-source model EFDC (Environmental Fluid Dynamics Code) is chosen for simulating the hydrodynamic condition of Nha Trang Bay. This is a general-purpose modeling package for simulating three dimensional flow and transport processes in surface water systems including: rivers, lakes, estuaries, reservoirs, and near-shore to shelf-scale coastal regions. Recently, this code was improved to coupling with the SWAN model to take into account the wave impacts to hydrodynamic conditions. The EFDC model was originally developed at the Virginia Institute of Marine Science (Hamrick, 1992) for estuarine

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and coastal applications and is public domain software. The US EPA has continued to support its development and now EFDC is part of a family of models recommended by EPA. Simulation results show that the hydrodynamic process to turn the beach morphological changes in this research area is mainly due to waves and currents. Longshore current in the surf zone during the Northeast and Southwest Monsoon have a decisive role for seasonal sediment transport. 2. EFDC MODEL DESCRIPTION AND NHA TRANG MODEL SETUP 2.1. EFDC model description The formulation of the governing equations for ambient environmental flows characterized by horizontal length scales which are orders of magnitude greater than their vertical length scales begins with the vertically hydrostatic, boundary layer form of the turbulent equations of motion for an incompressible, variable density fluid. To accommodate realistic horizontal boundaries, it is convenient to formulate the equations such that the horizontal coordinates, x and y, are curvilinear and orthogonal. Details of the transformation may be found in Hamrick (1992) and Craig (2009, 2010). Transforming the vertically hydrostatic boundary layer form of the turbulent equations of motion and utilizing the Boussinesq approximation for variable density results in the momentum and continuity equations and the transport equations for salinity and temperature in the following form:

(1)

(2) (3) (4) (5) (6) (7) ( 8) In these equations, u and v are the horizontal velocity components in the curvilinear, orthogonal coordinates x and y, mx and my are the square roots of the diagonal components of the metric tensor, m = mx my is the Jacobian or square root of the metric tensor determinant. 2.2. Data collection and model setup The main data sets was collected from the two field observation surveys supported by the Phase 1 Project entitled as "Study on Hydrodynamics regime and sediment transport in estuarine and coastal zones of Nha Trang Bay, Khanh Hoa Province". These two survey were conducted to measure the hydrodynamic parameters such as water level, current velocity, and wave information as well as sediment concentration and shoreline change during the May 2013 and December 2013 which representative for the condition of summer and winter respectively. The bathymetry was also measured in this year and complied. There were 4 different measuring station deployed from the deep water to the surfzone area as shown in Figure 1. These detailed measurement data sets are used to calibrate the numerical results.

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Station D

Station C Station B Station A

Figure 1: Location of measurement station in May and December 2013 A model grid development is shown in Figure 2. The river is extended far upstream, where the tidal signal cannot be detect, to assign the river discharge from the Dong Trang station as a river boundary condition. The open boundary condition in the south is used the Cau Da tidal station and in the east open boundary applied the global tidal constants for water level. The wind was collected from the Khanh Hoa meteorological station. The model simulation time is set for the entire year of 2013.

Tidal level

Tidal level

Figure 2: Model grid development for Nha Trang area 3. RESULTS AND DISCUSSIONS 3.1. Hydrodynamic regimes and water level comparison Figure 3 shows the snapshot of model simulation velocity field in Nha Trang Bay. Figure 4 is the comparison of model water level results versus observed data in May and December 2013. The results showed a very good agreement with the Nash index is over 95%. Key dynamics mechanism significant impact on this area are the monsoon wave, the tidal current and river flow. Waves in two Northeast and Southwest monsoon as propagating to the coastal areas generated longshore sediment transport processes in swash zone. During the Northeast monsoon, this longshore sediment moves to the southwest direction, while during the Southwest monsoon period, the flow is directed to the northeast.

Figure 3: A snapshot of current velocity field in Nha Trang bay

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Even though the Cai river discharge is relatively small during the normal condition but it is also affected to adjacent coasts; during the flood event it contributes the sediment and deposit around the river entrance. During the lower tide period, the fresh discharge goes to the South direction while flow the North during the higher tide level. Data - Station A Model - Station A

Data - Station A Model - Station A

Data - Station A Model - Station A

Data - Station A Model - Station A

Figure 4: Comparison of model water level versus data at the Station A in May and December 2013 3.2. Current velocity comparison Figure 5 is the comparison result of model velocity versus measured data at 4 stations A, B, C, D located from deep water areas to shallow water areas, respectively. The results show very good agreement of time variable current velocity and its direction at the station located in deep water areas A and B during both surveys in May and December 2013. The vertical velocity profile at these stations are also well reproduced. In the surf zone stations C and D, the simulation results velocity magnitude are relatively lower than measured data. 250

1.00

Model Tram A Cell 2-Model Tram A Cell 2-Data Data

STATION A

150

Velocity: XY Magnitude (m/s)

Velocity: Direction (deg from E)

200

100 50 0 -50 -100 -150

Model Tram A Cell 2-Model Tram A Cell 2-Data Data

0.80

0.60

0.40

0.20

-200 -250 24-May-13

26-May-13

0.00

30-May-13

24-May-13

26-May-13

28-May-13

Velocity: XY Magnitude (m/s)

150 100 50 0 -50 -100 -150

30-May-13

Time (days)

1.00 Tram B Cell 2-Model Model Tram B Cell 2-Data Data

200

Velocity: Direction (deg from E)

28-May-13

Time (days)

250

STATION B

0.80

Model Tram B Cell 2-Model Tram B Cell 2-Data Data

0.60

0.40

0.20

-200 -250

0.00 24-May-13

26-May-13

28-May-13

24-May-13

26-May-13

28-May-13

Velocity: XY Magnitude (m/s)

150 100 50 0 -50 -100 -150

30-May-13

Time (days)

1.00

Tram C-Data Data Tram C-Model Model

200

Velocity: Direction (deg from E)

30-May-13

Time (days)

250

STATION C

0.80

0.60

Data Tram C-Data Tram C-Model Model

0.40

0.20

-200 -250

0.00 24-May-13

26-May-13

30-May-13

24-May-13

26-May-13

28-May-13

30-May-13

Time (days)

1.00

Data Tram D-Data Tram D-Model Model

200 150

Velocity: XY Magnitude (m/s)

Velocity: Direction (deg from E)

28-May-13

Time (days)

250

100 50 0 -50 -100 -150

STATION D

0.80

Data Tram D-Data Tram D-Model Model

0.60

0.40

0.20

-200 -250

0.00 24-May-13

26-May-13

28-May-13

30-May-13

24-May-13

Time (days)

26-May-13

28-May-13

30-May-13

Time (days)

Figure 5: Model current velocity versus measured data at 4 different station A, B, C and D in May 2013

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3.3. Wave height comparison As discussed, the wave is one of the main hydrodynamic parameters to cause the sediment transport in Nha Trang bay area. The SWAN model is used to simulate wave propagation and the model comparison results versus the measured wave data during May and December 2013 are shown in Figures 5 and 6, respectively. Time variation of wave height during December survey (Northeast monsoon) generally higher compared with the wave in the May (Southwest monsoon). Simulation results indicate that the model perform better for the event in December and a bit underestimation for May event. However, simulated time-varying wave height is quite consistent with observations.

Wave height (m)

1.60

Data - May 2013 Model - May 2013

1.20

0.80

0.40

0.00 23-May-13

25-May-13

27-May-13

29-May-13

31-May-13

Date

Figure 6: Model wave height result versus measure data in May 2013 1.60

Wave height (m)

Data - December 2013 Model - December 2013 1.20

0.80

0.40

0.00 3-Dec-13

5-Dec-13

7-Dec-13

9-Dec-13

Date

Figure 7: Model wave height result versus measure data in December 2013 4. CONCLUSIONS Building successful the 3D hydrodynamic model using open source software EFDC for Nha Trang Bay. Model comparison results versus very detailed measurements data of water level, current velocity and wave indicate that the model obtains a very good, consistent and highly reliable results. Hydrodynamic regimes in the Nha Trang Bay is mainly driven by the effect of monsoon waves whichs causes the longshore current. In addition, the typhoon events can increases the cross-shore s flow direction rates but it only happens in a short time of daily or weekly scale. This study also showed that the influence of the Cai river flow is very significant to change the hydrodynamic mechanism in the estuary area. This river flows can carry sediment and deposition in the coastal area adjacent during a flood event. 6. REFERENCES Craig, P.M., 2009, “Users Manual for EFDC_Explorer: A Pre/Post Processor for the Environmental Fluid Dynamics Code”, Dynamic Solutions, LLC, Hanoi, Vietnam. Craig, P.M., 2010, “Hydrodynamics of the Lower Nam Hinboun Floodplain Hydraulic Model”, Dynamic Solutions, LLC, Hanoi, Vietnam. Hamrick, J.M., 1992a: A Three-Dimensional Environmental Fluid Dynamics Computer Code: Theoretical and Computational Aspects. The College of William and Mary, Virginia Institute of Marine Science. Special Report 317, 63 pp. Nguyen Trung Viet, 2014. Study on Hydrodynamics regime and sediment transport in estuarine and coastal zones of Nha Trang Bay, Khanh Hoa Province". Summary Final Report, 349p.

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PRE- AND POST-TSUNAMI MORPHOLOGY CHANGES ON KODANOHAMA BEACH HOANG DONG HAI1), YUTA MITOBE2),VO CONG HOANG3), NGUYEN TRUNG VIET4)and HITOSHI TANAKA5). 1)Department of Civil Engineering, ThuyLoi University 175 Tay Son, Trung Liet, Dong Da, Hanoi 100000, Vietnam e-mail: [email protected] 2) Department of Civil Engineering, Tohoku University 6-6-06 Aoba, Sendai 980-8579, Japan e-mail: [email protected] 3) Department of Civil Engineering, Tohoku University 6-6-06 Aoba, Sendai 980-8579, Japan e-mail: [email protected] 4) Department of Civil Engineering, ThuyLoi University 175 Tay Son, Trung Liet, Dong Da, Hanoi 100000, Vietnam e-mail: [email protected] 5) Department of Civil Engineering, Tohoku University 6-6-06 Aoba, Sendai 980-8579, Japan e-mail: [email protected]

Abstract Kodanohama Beach is located on Oshima Island, Kessennuma City, Miyagi prefecture, Japan. The Tsunami in 2011 caused severe erosion. Kodanohama beach is a pocket beach with very little sediment supplies, that’s whythe recovery processes are expected to be different from other sandy beaches. The purposes of this study are: investigating the behaviors of the shoreline of Kodanohama Beach before and after the 2011 tsunami by analyzing the aerial photographs taken in this area. The results obtained show that before the 2011 tsunami, the constructions of the port and the breakwater had an influence on the morphology of Kodanohama beach. After the tsunami, severe erosions on the beach could be clearly observed. Recent photograph analysis shows that the beach has become stable.

1. INTRODUCTION Kodanohama Beach is a sandy pocket beach (According to MacLennan et al. (2011), pocket beach is a beach that is contained between two bedrock headlands that essentially function as a closed system in term of littoral sediment transport) located on Oshima Island, off shores from Kessennuma City, Miyagi Prefecture, Japan (Fig. 1). This beach is about 500 m in length, facing the Pacific Ocean. In the Northern part of the beach, there are the port and the breakwater. The Southern part of the beach is adjacent to the bedrock headland. There is a sea-dyke built along the sandy coast. Before the tsunami, the harbor and the breakwater were constructed in the area. The roles

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Fig. 1 Study area

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of these structures on the morphological changes of Kodanohama Beach during this period will be discussed in the following chapters in this study. On 11th March 2011, a huge earthquake of magnitude 9.0 occurred off the coast of Japan, resulted in an enormous tsunami which the maximum wave height reached 39.7 m (Mori et al., 2012). This extreme event caused a significant number of coasts in Japan to severely erode. Kodanohama Beach was not an exception. The Great East Japan Earthquake and Tsunami changed the morphology of Kodanohama Beach. The erosions could be easily witnessed. Many researches have been conducted on morphological changes and recoveries of coastal and estuarine areas after disasters. Liew et al. (2010) and Choowong et al. (2009) studied about the damages and recovery processes in the coastal areas of Thailand using aerial images after the 2004 Indian Ocean Tsunami. Tanaka et al. (2014), Mitobe et al. (2013), Udo et al. (2012), Tanaka et al. (2012) and Tappin et al. (2012), Hoang et al. (2013), etc., conducted researches about the recoveries of coasts and estuaries in Japan after the 2011 Tsunami utilizing aerial photographs. In these studies, researchers investigated the recovery processes in long and straight sandy beach with river mouths as sediment supplies. There aren’t many researches about the recovery processes in small pocket beach with no or little sediment supplies. Therefore, this study aims to investigate the recoveries of this pocket beach after the tsunami (2011-2014) along with its long-term morphological changes before the tsunami (1966-2001) by analyzing aerial photographs taken in this area by airplanes and satellites. 2. DATA COLLECTION AND ANALYSIS This study utilizes aerial photographs taken by airplanes and satellites from 2 main sources: GSI (Geospatial Information Authority of Japan) and Google Earth. Google Earth provides images taken in 13 March 2011, 2 day after the tsunami, and a variety of photos taken in different time of the year 2011, 2012, 2014 and 2015 and GSI provides photographs taken in 1966, 1967, 1977, 1978, 1981, 2000, 2001, 2011 and 2013.

N

Googleearth

Fig. 2: Rectification and shoreline detection processes Collected images were rectified to the same coordinate system using a set of 55 Ground Control Points (GCPs). GCPs were selected in approximately the same elevation and close to the sea level. Due to the images qualities, the visibilities of the features of the beach at different times, etc., some aerial photographs may use more or less than 55 GCPs. The techniques of shoreline mapping are described in details by Moore (2000). All the photographs collected from Google Earth and GSI could not be justified for the tidal level due to the lack of exact time. The beach is divided into 3 sections. In each section, one baseline is chosen such that it is nearly parallel to the shorelines to minimize the errors when plotting temporal variation of shoreline positions.

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The baseline of each section is oriented an angle α clockwise from the North. In section 1, α equals 151 o. In section 2, α is 182o and in section 3, α equals 24o (Fig. 2).Shoreline can be defined as the physical interface between land and sea (Boak and Tunner, 2005). Shoreline positions ys(x,t) were extracted every 5m with respect to the baselines in each section(Fig. 2). 3. RESULTS AND DISCUSSIONS 3.1. Morphology changes before the tsunami (1966-2001) As a pocket beach (Fig. 1), the morphology of Kodanohama Beach is expected to be stable with time.Nonetheless the temporal variations of shoreline positions during this period show that the shoreline positionsweren’t stable.(Fig. 3) Shows the evolution of shoreline position in this period. As we can see, from 30th August 1966 to 9th October 1967in all sections, it retreated. From 9th October 1967 to 18th October 1977, it generallyadvanced. From 3 rd October 1978 to 24th June 1981, the shoreline retreated in section 2 and section 3 and forwarded in section 1. From 1981 to 2000, the shoreline advanced. From 31 th October 2000 to 15th October 2001, the shoreline advanced in section 1 and retreated in section 2 and 3. As can be seen from Fig. 3, the shoreline position in section 1 changed more dramatic than shoreline position in section 3. In section 1 the biggest difference in ys(x,t) is approximately 25 m while in section 3 the biggest difference is only10 m. There are many factors that may cause the phenomenon to happen. One of the possible causes can be the constructions of the harbor and the breakwater.

a)

a)

b)

b)

c)

c)

Fig.3. Temperal variation of shoreline position before the tsunami

Fig.4. Temperal variation of shoreline position after the tsunami

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3.2. Morphologicalchanges under the influence of the harbor and the breakwater From 9thOctober 1967 to 18thOctober 1977, a) 18-10the breakwater was constructed (Fig. 1, Red 1977 18-10-1977 square). During the same period, the shoreline 09-10-1967 was almost the same in section 1, retreated in and 3 and advanced in section 2 (Fig. 5a). From 3rdOctober 1978 to 24thJune 1981, the 100 GS breakwater was extended and the part of the m I harbor was built (Fig. 1, Blue squares). In the b) 24-06same period, it can be observed that the 1981 shoreline advanced in section 1 and retreated 03-10-1978 24-06-1981 in section 3 (Fig. 5b). The similar phenomenon was pointed out in Tanaka and Srivihok (2004); Tanaka (1983). From 24thJune 1981 to GS 31thOctober 2000, the harbor was extended to I its current size (Fig. 1, Green square). At the c) 31-10same time, it can be seen that the shoreline 2000 24-06-1981 advanced in section 1 and retreated in section 31-10-2000 2 and section 3 (Fig. 4c). The results obtained show that the constructions of the harbor and the breakwater, GS to some extent, influenced the morphological Fig.5. Rectified images with shoreline positionsI changes of the beach. The long-term before the tsunami morphological changes of the beach are related to the sediment transportation in the longshore directionwhich is caused by the constructions of the harbor and breakwater whereas the shortterm morphological changes are the changes related to the sediments transportations in cross-shore direction which are seasonal changes caused by tidal levels, wind and wave directions, storm events, etc.,(Horikawa, 1988). Due to the lack of photos taken in high frequency, tide and wave data from the study area, this research cannot explain the seasonal variations.

Fig.6. shoreline positions 3.3. The recovery of the Shoreline after the 2011 tsunami (2011-2014) The attack of the tsunami reformed the shape of Kodanohama Beach. Overall, more severe erosion can be observed in section 3. As can be seen from Figure 5, the shoreline positions are separated into 2 groups. The upper group consists of shoreline positions before the tsunami (dash line) and the lower group contains shoreline positions after that (continuous line). The separation is not clear in section 1 but

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becomes visible in section 2 and can be clearly seen in section 3. It also can be observed from Figure 6 that the lower group of shoreline positions is shorter than the upper group. After hit by the tsunami (13th March 2011), Kodanohama Beach was heavily damaged (Fig. 4 and Fig. 7a) the shoreline retreated from 10 m (C6, Fig. 4b) to more than 40 m (C9, Fig. 4c). At some points, the beach eroded so severely and shoreline reached to the position of the sea dike. In section 1, a part of the harbor and the sea-dyke were exposed (Fig. 7a). In section 3, shoreline retreated nearly 100m to the center of the main road and it can be seen that the road was ruined.76 days after the tsunami (26thMay 2011), the beach recovered in section 1 and section 3, and the shoreline already became smooth (Fig. 7b).From 26thMay 2011to 6th January 2015, the shoreline fluctuated but in general it reaches the dynamic equilibrium (Fig. 4 and Fig 7). With very little sediment supply, it can be said that the shoreline of the Kodanohama Beach will not recover to its prior state any time soon.

a) 13-03-2011

15-10-2001

100 m

b) 26-05-2011

Sea dyke

13-03-2011 GSI

26-05-2011

GSI c) 28-12-2012

28-12-2012

GSI d) 18-10-2013

18-10-2013

GSI e) 11-10-2014 11-10-2014

4. CONCLUSIONS

After analyzing data, the conclusions can be made as follows: Google Earth Before the tsunami, the constructions and extensions of the harbor and the f) 06-01-2015 breakwater influenced the 06-01-2015 morphological changes of Kodanohama Beach. After the 2011 tsunami, the beach was severely damaged. Morphological characters Google Earth of the area were changed dramatically. Three years after this Fig.7. Rectified images with shoreline position catastrophe, the shoreline recovered, after the tsunami but far from reaching its position before the tsunami. Results from most recent images show that the shoreline has reached the dynamic equilibrium state. Further studies need to be conducted in order to give more suitable explanations about the past behaviors of the beach and make more precise predictions of the future morphological changes. 5. ACKNOWLEGEMENTS We would like to deliver our deepest thank to JASSO scholarship for financing the first author of this research to study in Japan.

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6. REFERENCES Boak, E.B. and Turner, I.L.(2005). Shoreline definition and detection: a review.Journal of Coastal Research, Vol. 21, No. 4, pp. 688–703. Choowong, M., Phantuwongraj, S., Charoentitirat, T., Chutakositkanon, V., Yumuang, S. and Charusiri, P. (2009). Beach recovery after 2004 Indian Ocean tsunami from Phang-nga, Thailand. Journal ofGeomorphology, 104(3-4), 134-142. Horikawa, K.(1988). Near Shore Dynamic and Coastal Process.University of Tokyo Press. Hoang, VC., Mitobe, Y. and Tanaka, H. (2014). Changes inmorphology on Sendai Coast and its problems afterthe 2011 tsunami. Journal of Natural Disaster Science. 50(in press). Liew, SC., Gupta, A., Wong, PP. and Kwoh, LK. (2010). Recovery from a large tsunami mapped over time: The Aceh coast, Sumatra. Journal of Geomorphology, 114(4), 520-529. MacLennan, A., MS. and Williams, S. (2011). Pocket Beach Mapping in San Juan County. Coastal Geologic Services, Inc. Mitobe, Y., Hoang, VC., Adityawan, MB. and Tanaka, H.(2013). Beach recovery processes on Sendai Coast after the 2011 Great East Japan Earthquake Tsunami.Proceedings of the HYDRO 2013 International, 88-95. Moore, L.J., 2000, Shoreline mapping techniques, Journal of Coastal Research, Vol. 16, No. 1, pp. 111-124. Mori, N., Takahashi, T., Yasuda, T. and Yanagisawa, H.(2011). Survey of 2011 Tohoku earthquake tsunamiinundation and run-up. Geophysical Research Letters,38(7), L00G14 (6 pages). Pradjoko, E. and Tanaka, H. (2010).Investigation of Shoreline Change Trends around the Nakakita Rivermouth Using Aerial Photograph. Proceedings of 32nd International Conference on Coastal Engineering. Tanaka, H. and Srivihok, P. (2004).Impact of port construction on coastal and river mouth morphology: a case study at Sendai port.Proceedings of the 9th International Symposium on River Sedimentation, pp.406-415. Tanaka, H., Adityawan, MB. and Mano, A. (2014).Morphological changes at the Nanakita River mouthafter the Great East Japan Tsunami of 2011. CoastalEngineering, 86, 14-26. Tanaka, H., Nguyen, XT., Umeda, M., Hirao, R., Pradjoko,E., Mano, A. and Udo, K. (2012). Coastal andestuarine morphology changes induced the 2011 GreatEast Japan Earthquake Tsunami. Coastal EngineeringJournal, 54(1), 1250010 (25 pages). Tanaka, N. (1983). A study on characteristics of littoral drift along the coast of Japan and topographicchangeresulted from construction of harbors on sandy beach. Technical Note of the Port andHarbor Research Institute, Ministry of transport, no.453. (In Japanese). Tappin, DR., Evans, HM., Jordan, CJ., Richmond, B.,Sugawara, D. and Goto, K. (2012). Coastal changes intheSendai area from the impact of the 2011 Tōhokuoki tsunami: Interpretations of time series satelliteimages,helicopter-borne video footage and fieldobservations. Sedimentary Geology, 282(30), 151-174. Udo, K., Sugawara, D., Tanaka, H., Imai, K. and Mano, A.(2012). Impact of the 2011 Tohoku Earthquake and Tsunami on beach morphology along the northernSendai Coast. Coastal Engineering Journal, 54(01),1250009 (15 pages).

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STUDY ON SEDIMENT DEPOSITION IN ESTUARIES AND COASTAL ZONES IN BEN TRE PROVINCE BY NUMERICAL MODELING DOAN THANH VU1, NGUYEN THE BIEN2 and NGUYEN THANH MINH 3 1) Centre for Environmental Resources and Climate Change, Institute of Coastal and Offshore Engineering 658 Vo Van Kiet, District 5, Ho Chi Minh City, Vietnam e-mail: [email protected] 2) Department of Oceanography, Institute of Coastal and Offshore Engineering 658 Vo Van Kiet, District 5, Ho Chi Minh City, Vietnam e-mail: [email protected] 3) Department of Oceanography, Institute of Coastal and Offshore Engineering 658 Vo Van Kiet, District 5, Ho Chi Minh City, Vietnam e-mail: [email protected]

Abstract The objective of article aims to increasing comprehension to the complex patterns of cohesive sediments transportation, deposition, and re-suspension caused by wave effects involving tide, wind, sea currents in coastal zone by using numerical modeling. Knowledge of the behaviours of these systems are important. Managing authorities are therefore under strong pressure to develop and implement plans for sustainable development and management of these systems, and to compensate for infrastructure and other measures. In this study, Mike 21/3 Coupled Model FM has been used. The simulated results agreed fairly well with the available observed data. Furthermore, the present study provides a practical outline for application of numerical simulations in estuary areas, a complex geometry, topography, and hydrodynamic processes. Keywords: coastal zones, sediment deposition, numerical modeling, Mekong Delta estuaries.

1. INTRODUCTION Coastal zones and estuaries, encounter with continents and oceans, rivers and seas, are characteristically multiform, infinitely complex, quasi-fractal, always unpredictable change in many aspects (Dronkers, 2005). ̣Knowledge of the behaviours of these systems are important. Managing authorities are therefore under strong pressure to develop and implement plans for sustainable development and management of these systems, and to compensate for infrastructure and other measures. Sediment transportation in the coastal zone and estuaries governed by physical dynamics, tide, wave, wind and so on, corresponding to current, and their mutual interactions, are very complicated. Mekong River’s estuaries are subject to tidal motion superimposed by surface waves with windy condition. The tidal regime is semidiurnal macro-tides, tidal asymmetry, while waves and wind are driven by the monsoon. Field studies of fine sediment dynamics carried out in Mekong River estuary, Vietnam, during the low flow season in April 1996 show semidiurnal macro-tides and shallow water effects result in a tidal asymmetry with peak flood tidal currents 10% stronger than peak ebb tidal currents (Wolanski, Nguyen, & Simon, 1998). The combination of meteorological and oceanic effects produces strong variations in coastal areas, mainly in sea water level, as well as in coastal currents. These extreme shifting conditions lead to the cycle of deposition, consolidation, fluidization, erosion, flocculation, and deposition and so on of fine sediment. The complex interaction between those factors through processes as mentioned above directs morphological evolutions of the Mekong River’s Estuaries is inestimable processes.

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In general, although there are many researches on the coastal morphology, sea current, etc, along the Mekong River coast, For coastal zone of Bentre province, in recent years, we just have some projects that studying on the cohesive dynamics at the individual estuaries such as Ham Luong estuary, Ba Lai estuary, etc. A study that considering the overall effects and interactions between the estuaries along Ben Tre coastline is in need, because of the movement of sediment mass along the coastline pass each windy season. In addition, due to the complexity of the problem, the mechanism of Vietnamese East cohesive sediment transportation, deposition Sea and re-suspension, especially due to wave effects, still need further studies and careful formulation. Research on this topic is necessary in order to gain insight into the temporal and spatial variability of sedimentations. The purpose of study is to simulate the Figure.1: Study area in the Lower Mekong River hydrodynamics and cohesive sediment processes in the Ben Tre’s estuaries and delineate the cohesive sediment transportation, and deposition pattern that are mainly caused by wave effects in meso-temporal and spatial scale. The study areas are four main estuaries from North to South of Ben Tre Province namely Cua Dai, Ba Lai, Ham Luong, and Co Chien estuaries, respectively, which are characterized by complex interactions between hydrodynamics and sedimentation [see Figure.1]. 2. COASTAL MORPHOLOGY OF BEN TRE PROVINCE According to statistical data, Ben Tre is one of the third provinces in Mekong Delta that has the strongest coastal zone variation with the dominant deposition process.

Figure.2: The variation of coastline and alluvial grounds in Ben Tre Province The aerial photograph in 21 years (1968-1989) showed that the total deposited area in the coastal zone is 61.17 km2, including 19.81 km2 in Binh Dai district; 16.99 km2 in Ba Tri district; and Thanh Phu district is 24.37 km2. During 21 years, the total area of Ben Tre increased 68.9 km2in the seaward side;

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with an average speed of 2.33 km2per year. The continuous development of the coastal alluvial grounds is one of advantages of agriculture, particularly feeding clams (Vu, 2007). Besides above-advantages, the development of the coastal alluvial grounds has been causing some disadvantages to local navigation in estuarine zone, drainage of floodwater from upstream, etc. Hence, a deep comprehension to process of shaping alluvial grounds in particular, sediment transportation in estuarine systems in general, pay a attention to local authorizes and scientists. 3. BACKGROUND OF SEDIMENT TRANSPORT The simulation includes: Forcing by waves; Salt-flocculation; Detailed description of the settling process; Layered description of the bed; andMorphological update of the bed. In the Mud Transport (MT) module, the settling velocity varies, according to the salinity, if included, and the concentration taking into account flocculation in the water column. Waves, as calculated by MIKE 21 SW for example, may be included. Furthermore, hindered settling and consolidation in the fluid mud and underconsolidated bed are included in the model. Bed erosion can be either non-uniform, i.e. the erosion of soft and partly consolidated bed, or uniform, i.e. the erosion of a dense and consolidated bed. They are described as layered and characterised by the density and shear strength. Governing equations: The sediment transport formulations are based on the advectiondispersion calculations in the Hydrodynamic module. The MT module solves the so-called advection-dispersion equation:

Where:

c : depth averaged concentration (g/m3); u,v : depth averaged flow velocities (m/s); Dx, Dy : dispersion coefficients (m2/s); h : water depth (m); S : deposition/erosion term (g/m3/s); QL : source discharge per unit horizontal area (m3/s/m2); CL : concentration of the source discharge (g/m3). In cases of multiple sediment fractions, the equation is extended to include several fractions while the deposition and erosion processes are connected to the number of fractions. Deposition: In the MT model, a stochastic model for flow and sediment interaction is applied. This approach was firstly developed by Krone (1962).Krone suggests that the deposition rate can be expressed by: SD = wscbpd Where: ws : settling velocity (m/s); cb : near bed concentration (kg/m3); pd : probability of deposition. The probability of deposition pd is calculated as: b : the bed shear stress (N/m2); cd : the critical bed shear stress for deposition (N/m2). 4. MATERIALS AND MODEL SET UP As de Vriend (1991) enlightened on the scales theory, the influence of bigger scaled processes act as boundary conditions for the smaller scaled processes, where influences from smaller scaled processes are considered ‘noise’ to the bigger scales. Processes on the same scale can have dynamic interactions. In this study, there are two models that will be built involving one extensive model that covering a part of the Vietnamese East Sea, and one main model of Ben Tre Province’s estuaries that covering the entire study area [Figure.3]. The extensive model is bounded on Ben Tre River mouths to Phu Quy and Con Dao hydrological station, which is appropriate for extensive models which limited wave data is available. The main model is bounded on Ben Tre Province’s coastal zone from continent to seaward side about 80 kilometers. This area is enough for considering all impacts of main factors on the wider area . The open

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boundary conditions, such as water level, sea current and waves of the main model, are driven by the extensive model. Different types of data are available through various sources. Bathymetric data is available from Institute of Coastal and Offshore Engineering in October 2009 and various projects from 1996 to 2009. Data on the water level boundaries and the wind parameters at the three open boundaries is extracted from Tidal Analysis and Prediction Module in MIKE 21. The sediment characteristics were used as follows: Table 4: Characteristics of computering fractions Parameters Types of mud Settling velocity coefficient 2

Fraction 1

Fraction 2

Consolidated mud

Consolidated mud

45

5

Critical shear stress for deposition (N/m )

0.03

0.07

Sediment density

2640

2638

Dispersion coefficient (m2/s) Concentration for hindered settling (kg/m3)

0.02 10

0.02 10

Table 5: Characteristics of sediment layers Parameters Erosion description Power of erosion value Erosion coefficient for erosion (kg/m2/s) Critical shear stress for erosion (N/m2) Density of the sediment layers (kg/m3)

Layer I Soft mud 10 10÷5 0.1 200

Layer II Hard mud 1 10÷4 0.3 400

Due to the climate features of the study area, apart from the two main monsoons are the North-East and SouthWest monsoons. there is another monsoon which occurred in a short time between the two monsoons earlier, with east dominant direction , local people usually call “gio Chuong”. However, since typical features of the two main monsoons, in this study just conduct simulation the wave, wind, current fields, and sediment transportation in these monsoons: the South-West monsoonin rainy season (September 2009) and the North-East monsoon in dry season (January 2010). Figure.3: The extensive and main study area 5. RESULTS AND DISCUSSIONS 5.1. Hydrodynamic simulation The simulated results will be extracted at two attentive periods including ebb and rising tide of the flood tide, as these periods present fully characteristics of a tidal period. In the South-West monsoon, the obtained results showed that the mean current speed was approximately 0.32 m/s at the ebb tide; it occurred in most estuaries along coastal zone, excepting forBa Lai estuaries. In the simulated time, the current speed distribution in Thanh Phu district was complex. In the north-east cap, the current speed was lower than in the south west-cap with an average of 0.7 m/s.

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Meanwhile, the current speed at the rising tidewas strong with an average of 0.65 m/s far 8 km toward the offshore of Thanh Phu district, and the current in this region was stronger than that in the remaining regions, the current speed decreased with the tendency toward the mainland. In estuaries, by affected remarkable interaction between river flow and sea current, the current speed dramatically decreased, thus, it evidenced as an enlightened advantageous condition for facilitating sedimentation processes. The average current speed at the Ham Luong estuary was 0.04 m/s, the Dai estuary was 0.08 m/s, and the Co Chien estuary was 0.07 m/s.

Figure.4: Current fields at the ebb tide of the flood-tide (6:30 PM on 19/9/2009)

Figure.5: Current fields at the rising tide of the flood-tide (1:30 AM on 20/9/2009)

Figure.6: Current fields at the ebb tide of the flood-tide (5:30 AM on 2/1/2010)

Figure.7: Current fields at the rising tide of the flood-tide (12:30 AM on 2/1/2010)

In the North East monsoon, the current speed was 0.55 m/s in area far from the shore (about 7 to 10 km) at the ebb tide. The average current speed at Dai estuary was 0.4 m/s, at Ba Lai estuary was 0.33 m/s, at Ham Luong was 0.57 m/s. In Co Chien estuary, the current drew out and deflected on the north-east side (0.52 m/s), while on the south-west side, the current was quite low (0.1 m/s).At the rising

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tide, the current speed was reduced with an average of 0.6 m/s, and increased at estuaries as well, such as at Dai estuary was 0.65 m/s, at Ba Lai was 0.5 m/s, at Ham Luong was 0.68 m/s, and at Co Chien was 0.75 m/s (see Figure.4-7). 5.2. Sediment transport simulation The MT Model was conducted in two typical monsoons as mentioned above. The computed results were extracted at specific times and places, depending on the evaluation purposes. To evaluate the total bed thickness change of the study area, data were extracted at the end of simulated time (time steps are 4,200), and results showed in Figure.8.

[B]

[A]

Figure.8: The change of total bed thickness due to sediment transportation in entire study area in the South-West monsoon (A) and North-East monsoon (B) The obtained results showed that Ben Tre coastal zone was entire deposited in the south-west monsoon, excepting for the south-west cap of Thanh Phu district. The strongest sediment deposition process occurred in the north-east cap of Binh Dai district, Dai riverside on estuary area. At the northeast cap of Thanh Phu district, alluvial grounds thicken reached to 1.4 m extending toward the sea nearly 3 km. In this time, the alluvial grounds appeared in large scale and extended toward the sea, especially tending to deflect to the north-east. It was suitable as compared to the actual state, and the computed results of current fields and wave fields at the main study area. Contrary to the south-west monsoon, in the north-east monsoon, the most of alluvial grounds in coastal zone were pushed back to the south-west due to the impact of the sea currents and wind direction, and they also were narrowed. In Binh Dai coastal zone, at the north-east cap occurred erosion instead of sediment deposition process like in the south-west monsoon. In addition, the sediment mass was pushed back toward the south and constrained to ashore, because of the strong near shore currents, hence still remained a narrow sediment bank. This phenomenon was also happened in Thanh Phu coastal zone. For Ba Tri coastal zone, due to stands back to the mainland, moreover, the cap of Thanh Phu and Binh Dai district obstructed the north and south of this area. That was the reason why the alluvial grounds in this area just were narrowed in the north-east monsoon. The thickness of sediment layers become denser and part of them was pushed into Ba Lai estuary, which created seriously sediment deposition. On the

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other hand, the upstream flow of Ba Lai River was limited due to Ba Lai irrigational systems; hence the upstream flow was not enough to push sediment out of estuary like other rivers. 6. CONCLUSIONS The coastline of Ben Tre is discontinues due to be separated by the estuaries system, hence, the nearshore current behavior is quite complicated in both magnitude and direction. The current concentrated greatly in some areas, while some other was inversed, whereas, the current field was governed by many natural factors such as the seasonal wind, the tidal regime, and topographical features, etc. These factors contributed an increase in complex level of accretion and erosion processes, and sediment transportation. For the low and medium water level cases, most of offshore wave direction approaching to the coast usually break far away from the shoreline. It means that although range of wave height is large, water level is not high, hence, the impacts of the wave the shoreline are inconsiderably and also result in erosion.For the high water level case, the wave propagate from the offshore usually breaks near the shoreline. Nevertheless, owing to bathymetric features, wave period and direction then the effects on different areas are not similar. The simulated results presented that the cohesive sediment processes have been reproduced delineate the cohesive sediment transportation and deposition pattern of Ben Tre coastal zone. To sum up, the influences of semidiurnal macro-tides, tidal asymmetry, river currents, sediment discharges, and saline intrusion on the coastal and estuarine zone are remarkable. The suspended sediment has a tendency to deposit in the distal zone, creating sand barriers and strengthening existing islets, this trend happens more powerful in the flood season, as the strong current from the upper river systems carried more suspended sediment out of the rivers and run into the sea. 7. ACKNOWLEGEMENTS I wish to express my sincere appreciation to the Joint Education Master Program between University of Liège - Belgium and Water Resources University - Vietnam for Sustainable hydraulic Structures. I am quite grateful for this cooperation program, which I am chosen the interesting topic inspired by Assoc. Prof. Trinh Cong Van, Assoc. Prof. Dr. Nguyen The Bien, and Assoc. Prof. Dr. Nguyen Huu Nhan. Many thanks to Msc. Le Van Tuan, Eng. Nguyen Thanh Minh, and colleagues from Oceanography Department for the help on the model software, and computer facilities. 8. REFERENCES DHI. (2007). Mike 21 Flow model FM Hydrodynamic module - Scientific documentation. DHI. (2007). Mike 21 Flow model Mud Transport module - Scientific Background. DHI. (2007). Mike 21 Spectral Wave Module - Scientific documentation. DHI. (2007). Mike 21/3 Coupled Model FM Scientific documentation. Doan, V. T. (2012). Study on sediment deposition in estuaries and coastal zones in Ben Tre Province by numberical modeling, and propose solutions for sustainable exploitation. Institute of Coastal and Offshore Engineering, Research Centre for Environment and Climate Change, Ho Chi Minh. Dronkers, J. (2005). Dynamics of Coastal Systems - Advanced Series on Ocean Engineering (Vol. 25). World Scientific. Vriend, H. J. (1991). Mathematical modelling and large-scale coastal behaviour. Journal of Hydraulic Research , 727-740. Vu, T. K. (2007). Study on the formation and development mechanism of coastal alluvial ground in Southern Vietnam and propose sustainable exploitation. Ho Chi Minh. Wolanski, E., Nguyen, N. H., & Simon, S. (1998). Sediment dynamics during low flow conditions in the mekong river estuary, Vietnam. Journal of Coastal Research, 14.

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THE EFFECT OF COASTAL FORESHORE LOWERING ON WAVE HEIGHT GROWTH IN BAC LIEU PROVINCE, VIETNAM LE DUC DUNG1), NGUYEN QUANG CHIEN2) and TRAN THANH TUNG3) 1) Vietnam Institute of Sea and Islands 125 Trung Kinh, Cau Giay, Hanoi, Vietnam e-mail: [email protected] 2) 3) Faculty of Marine and Coastal Engineering, Thuy Loi University, 175 Tay Son, Dong Da, Hanoi, Vietnam e-mail: [email protected], [email protected]

Abstract The paper presents results of our research on the effect of foreshore lowering on wave height growth along the Bac Lieu coast in Vietnam. The surveyed bathymetric data and long-term measured wave and water level data at site are used for numerical modelling with MIKE 21 software suite. Through calibration and verification, a suitable set of parameters was found and used for simulating wave propagation. The growth in wave height due to foreshore lowering had been estimated. This result shows that beach lowering has considerable effects on wave growth near the coastline. The increase in wave height has influence on littoral processes, e.g. increasing the longshore current and coastal erosion, with further implications on reinforcement and size (e.g. crest elevation) of coastal structures. It is recommended that for coastal structures in the context of climate change and sea level rise, design practice and guidelines in future incorporate this effect of wave height growth due to foreshore lowering. Keywords: Bac Lieu province, foreshore lowering, wave height growth, MIKE 21, sea dike

1. INTRODUCTION Among the 28 coastal provinces, Bac Lieu has 56 km coastline, accounting for 1.27% of the total length of Vietnam coast. In recent years, the erosion of beaches and estuaries in Bac Lieu has gradually intensified. This has severely affected the safety of local inhabitants, the quality of infrastructure, the degradation of environment, and the economic development. From the broader view, though, the coast of Bac Lieu experience both accretion and erosion in various sections. The research group has identified five sections of the Bac Lieu coastline from the border with Soc Trang province to Ganh Hao estuary, for which field data (Tung et al., 2014) showed that: - Section 1 (11 km long) is eroded during the whole year. - Section 2 (16 km) is eroded during several months and accreted during the rest of year. - Section 3 (22 km) is accreted during the whole year. - Section 4 (3 km) is eroded during several months and accreted during the rest of year. - Section 5 (4 km) eroded during the whole year. The main reason for this erosion is hydrodynamic forcings. Wave motions and littoral currents stir up and bring a large amount of sand away from the beach. This in turn lower the beach elevation and facilitate wave propagation closer to the beach face, hence intensify the effect of waves. It is therefore necessary to quantify the wave growth due to beach erosion, for local coastal zone management. In this reseach, numerical simulation using the MIKE 21 FM had been performed to analyse the change in wave heights along several coastal profiles in the area, which imply potentials for shoreline and beach erosion in Bac Lieu during the period 2011-2015, which had been identified in another work (Cat et al., 2015).

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2. MATERIALS AND METHODS 2.1. Study area The study area, ~ 1 km long, is a short coastline stretch bordering the East Sea (Bien Dong). Detailed measurements taken in two campaigns (Mar-2011 and Jan-2015) indicated that the bottom of surf zone had been eroded by ~ 1 m (Figure 1). Cat et al. (2015) had provided explanation for possible cause of this erosion; in this paper the authors instead analyse the possible consequence of such erosion. 2.2. Data used Bathymetry: The bathymetry data for the large computation domain (~ 240 km alongshore and 120 km offshore) had been obtained. Two bathymetric data sets were used for numerical modelling for the purpose of highlighting the foreshore lowering process. Waves and water levels data: The offshore boundary conditions (Waves data and water levels data) used for the model conforms to the Vietnamese Technical Standards in Sea Dike Design (MARD, 2012), i.e. a 5%-frequency design for Bac Lieu coast, corresponding to a significant wave height of 5.59 m and water level of 2.13 m.

Figure 1a: Sampled data points measured in Mar-2011 (red) and in Jan-2015 (blue)

Figure 1b:Erosion depth interpolated from the data points

3. MODEL SETUP The MIKE 21 spectral wave model (MIKE 21 SW) is based on the mild-slope equation, which is suitable for a number of typical scenarios relevant to coastal engineering. The model simulates wind-induced wave growth and transformation, including various wave dissipation phenomena, nonlinear wave-wave interaction, refraction and shoaling and wave-current interaction (Mike, 2007b) which are very relevant to the study area. The model is used to calculate wave propagation from deep water area to the shoreline of Bac Lieu. To complete the simulation of littoral hydrodynamic processes, the MIKE 21 Flow Module FM (Mike, 2007a) is used in tandem with spectral wave model. The former is a finite-volume based model capable of numerically solving the hydrodynamic equation on a triangular mesh, with wave forcing represented as radiation stresses. The flow-wave simulation is therefore coupled and is able to provide reliable estimation of the wave and flow field in the study area. 3.1. Computation domain and mesh and bathymetry To deliver adequate details for analysis of nearshore wave height variation, a nested grid system is used. The larger computation domain covering not only the coastal sea of Bac Lieu, but also neighboring provinces (Ca Mau, Soc Trang, and Tra Vinh), extending 150 km alongshore and about 120 km cross-shore (Figure 2). The fine domain for quantifying foreshore lowering contains much finer computational cells (Figure 3); this grid was generated for the bathymetric data of March, 2011 and that of January, 2015.

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The measured bathymetric data points, both sampled during the same dry season (with Northeastern prevailing wind) in 2011 and 2015, showed that the foreshore was lowered from 1 m to 1.5 m. This occurred along the whole coastal stretch being surveyed.

Figure 2: Large domain with bathymetric shading

Figure 3: Fine domain around structures

3.3. Model calibration and verification The calibration and verification stage is necessary for producing a set of parameters suitable for area of interest. This will ensure better matching between simulation results and reality. Calibration and verification of flow model: For calibrating the flow model, the measured water level in Sep-2009 at the Tran De estuary (for location, see Figure 2) was used. Measured data at the Nha Mat inlet served for model verification. The results (Figures 4 and 5) show good accordance between simulated outputs with the data.

Figure 4: Result of water levels calibration at the Tran De station

Figure 5: Result of water levels verification at the Nha Mat station

Calibration and verification of wave model: For these tasks, two sources of data were used: from nearshore wave buoys and offshore wave field obtained from the global WAVE WATCH-III model (NOAA, 2015). The measured data in Dec-2014 near Tran De estuary was used for calibration and data in Jan-2015 near Nha Mat inlet for verification.

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Figure 6: Result of wave model calibration

Figure 7: Result of wave model verification

4. RESULTS AND DISCUSSIONS The simulation of wave propagation was performed with two scenarios using bathymetric data of 2011 and 2015, both with parameters after being calibrated and verified. The simulated wave field (Figures 8 and 9) are compared; the increase in wave height (Figure 10) occurs along the study area, with a maximum rise of ~ 0.5 m. To analyse the wave height growth due to foreshore lowering, data was extracted along five representative cross-sections (Figure 11) of the study area.

Figure 8: Simulation result of wave propagation with the 2011 bathymetry

Figure 9: Simulation result of wave propagation with the 2015 bathymetry

Figure 10: Distribution of wave height growth due to foreshore lowering

Figure 11: Representative cross-section where data are extracted

A common pattern can be seen from the plots in Figure 12: remarkable increase of wave height associates with foreshore lowering. In five cross-sections, where the foreshore lowered ~0.4 m (on average) during the period 2011-2015, the wave height increases by ~0.25 m. A maximum lowering of foreshore on cross-section 1 results in a rise of wave height as large as 0.49 m. Generally wave height

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grows by 0.24 times the amount of foreshore lowering. The detailed data is given in Table 1. Also noted is that the growth in wave height take places further landward than where the profile lowers. This contributes to additional risk of damages to coastal structures and assets.

Figure 12: Left column – Foreshore lowering at cross-sections MC1-5: 2015 vs 2011 Right column: Wave height distribution at at cross-sections MC1-5: 2015 vs 2011

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Vietnam-Japan Workshop on Estuaries, Coasts and Rivers 2015, 7-8 September, 2015, CKT, Hoi An, Vietnam Table 1: Summary of wave height growth due to foreshore lowering at selected cross-sections Cross-section name

Maximum lowering (m)

Average lowering (m)

Width of lowered section (m)

Maximum wave height growth (m)

Average wave height growth (m)

MC 1

-1.3

-0.7

150

0.49

0.35

MC 2

-1.3

-0.3

550

0.45

0.28

MC 3

-1.3

-0.4

670

0.39

0.24

MC 4

-0.9

-0.3

590

0.29

0.14

MC 5

-1.1

-0.3

650

0.45

0.2

5. CONCLUSIONS The study on foreshore lowering along Bac Lieu coastline with surveyed data in 2011 and 2015 shows that the local beach is eroded and this allows waves to propagate further landward, at the same time raises the wave height. Results show that with an offshore design wave of 5.59 m, when the foreshore lowers by 1.3 m, the nearshore wave will grow by 0.49 m at most. In general, the study delivers a quantitative result of wave height growth due to foreshore lowering: this wave growth is remarkable and roughly proportional to the amount of foreshore lowering. The rise of waves has major influence on littoral currents and coastal erosion. In addition, it greatly affects the reinforcement and crest elevation of coastal structures e.g. sea dikes, groins, breakwaters. Therefore, in studying coastal erosion processes, it is necessary to consider the lowering of foreshores and beaches, the causes of erosion by wave growth. In the design and maintenance of coastal structures, the same issues must also be addressed. The wave growth due to foreshore lowering is different for various locations in the study area. This increase in wave height depends on the bathymetric features e.g. beach slope and the decrease of foreshore elevation. Therefore the process must be studied for every coastline segments of Vietnam, especially those with high risk of erosion. However, due to limited time and data resources, the research is focused on the case of Bac Lieu province. 6. ACKNOWLEDGEMENTS A part of this study was financially supported by a National research project “Research on scientific foundation for technical design guidelines for Vietnam sea dikes and flood protection work in the context of climate change and sea level rise, with protective and mitigation measures” which belongs to the Program KHCN BDKH/11-15 from the Ministry of Science and Technology (MOST), Vietnam. 7. REFERENCES Cat, V.M, Tung, T.T, Doan N. K. Hien, L.T. (2015). Research and evaluation of erosion-induced foreshore lowering between locations K0 and K1+200 of Soc Trang sea dike route (2011 – 2015) (submitted to Journal of Water Resources and Environmental Engineering). DHI (2007a). Mike 21 Flow Model FM, Hydrodynamic Module, User Guide. DHI (2007b). Mike 21 SW, Spectral Wave FM Module, User Guides MARD. (2012). Technical Standards in Sea Dike Design. Ministry of Agriculture and Rural Development, Vietnam. NOAA. (2015). Wave Watch III GriB dataset, online access http://polar.ncep.noaa.gov/waves Tung T.T., Trung, L.H., Doan N.K. (2014). Dataset on water level, waves, and bathymetry collected along Soc Trang – Bac Lieu coastline under Project “Research on scientific foundation for technical design guidelines for Vietnam sea dikes and flood protection work in the context of climate change and sea level rise, with protective and mitigation measures”. Thuy Loi University

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NUMERICAL ANALYSIS OF OVERTOPPING -TYPE WAVE POWER GENERATION EQUIPMENT MASATO MINAMI Department of Industrial system engineering, National Institute of Technology, Hachinohe College 16-1 Uwanotai Tamonoki Hachinohe City Aomori Prefecture, 039-1192, Japan e-mail: [email protected]

Abstract There are many systems which transform wave energy into electrical energy about wave power generation. In these, over topping-type wave power generation is the method of generating using the potential energy of the water surface. However, it is expensive in installation cost and maintenance cost, and cannot expect a production of electricity. By these reasons, it is still in the situation of a pilot phase. This research aims at attaining efficient-idolization of exploitation of wave force energy. The contents are an experiment and numerical computation. First, the model of the single-stage type over topping-type wave power generation equipment was produced. The calculation formula of the amount of over topping and the judgment type of the existence of over topping were computed. Next, reappearance calculation was carried out using heat current object analysis software. The main results are listed below. The over topping discharge is relation with an incident wave gradients and the height of collecting basin. When water level change used the VOF value 0.2, accuracy with wave height and a cycle comparable as an experimental value was acquired. Finally, the 3dimensional numerical computation which used the amount of spots is performed, and reappearance of breakwater and overtopping-wave phenomenon was performed. Keywords: Wave power, overtopping type, Overflow, Turbulence model, Numerical computation 1. INTRODUCTION The development and the popularization of the renewable energy become urgent business as the global warming prevention countermeasure. As the renewable energy, solar, wind velocity, biomass, geothermal and so on was practicalzed. There are various energies in the ocean. For example, they are a tide, a current, a difference in temperature, etc. The wave energy was also one of them. For the ocean energy, studies in wav-power generation in Japan started as early as in 1978 when a two-year research program on its commercial viability was carried out onboard an experiment ship Kaimei (Masuda,1987), but despite the numerous experimental studies that followed, have not yet found practical application. Meanwhile, in Europe and North America, a variety of wave-power generating systems have been developed and connection with local electricity transmission and distribution networks realized.But here in ocean energy was limited to be discussed and wave power. Also wave power generation is roughly divided from the method which change wave energy into the electric energy to six kinds (Nature, 2008), this research belong to the wave overtopping type. The overtopping type wave power development at home and abroad, the company Wave Dragon Aps (Denmark) Wave Dragon, the company WAVEenergy AS (Norway) The in-SSG (Sea-wave Slot-cone Generator) and research and development, the authors the overtopping type wave power device that has been fixed (Tanaka, 2010&2013). The system of the wave power generation which transforms wave energy into electrical energy is various.Overtopping type wave power generation is the method of generating using the potential energy of the surface wave of the water surface. For Japan which is an island country, this power generation method is suitable. Figure 1 shows the spot of setting point. Moreover becoming a new source of renewable energy is expected.

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Wave power generation has a merit which can obtain energy cleanly and semi permanently. However, it is still in the situation of a pilot phase. The reason is because a production of electricity cannot be expected comparatively with installation cost or high maintenance cost. For the purpose of attaining the increase in efficiency of exploitation of wave force energy, this research performed production of an ascension part model, and collection of water level data, and asked for the calculation formula of the amount of overtopping. Furthermore, numerical computation was carried out and reappearance calculation of the overtopping quantity was tried.

Figure 1. The bird's-eye view of a power generator and Principle of generation of electricity 2. OVERTOPPING EXPERIMENT The experiment was carried with a model using open channel with a wave generator machine in a hydraulic laboratory. The water tank is 0.55m-height, 10m-length and 0.8m-maximum depth. Model scale λ was considered as 1/21 and the amount of many was computed using the Froude rule in some numbers. The angle of the slope referred to the experimental result in Tokai University of Tanaka and others.Figure 2 shows an example of the experiment. The capacitance type wave height meter and the thermal recorder were used for measurement of a water level. The model was manufactured using Styrofoam and plywood. The Froude rule was used for the amount of many in some numbers. The amounts of water veins of water level change and storage time were measured. Time until a wave ascends a slope part, goes into collecting part and fills was measured. This was storage time.

Figure 2. Hydraulic model experiment (Overtopping) The definition of the used sign was shown in Figure 3.The flow discharge equation at the overtopping was examined. The overtopping quantity was calculated by multiple linear regression analysis. An overtopping quantity equation (Q) was described below

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The coefficient of relative of depth (h/L) is the largest, and the coefficients of d/L and wave steepness (H/L) were also larger than this equation. On the other hand, the height of collecting basin has seldom influenced overtopping conditions. Figure 4 was showing the relation between the experiment and prediction of overtopping quantity (Q) using Equation 1.The correlation coefficient R was about 0.56. This graph shows that a prediction quantity (Qpre) was 0.92 times as an experiment overtopping quantity (Qexp).

Figure 3. The definition of mark (single type)

Figure 4. Comparison with prediction equation and experiment

Q  2532

h d H R  500  317  1.45  240 (cm3/s) L L L L

(1)

3. TURBULENCE MODEL In order to calculate an overtopping quantity, run-up and overtopping were reproduced by numerical computation. The heat current analysis software used for calculation has the following features. By a cut cell method, creation of a mesh was easy. Flow analysis of movement and the rotating object can be conducted easily. Moreover, customization was easy. It was solving preservation formulas, such as mass, quantity of motion, and energy, using the iteration method using the finite volume method. And regular or irregular transient phenomena analysis was possible. The various turbulent flow models were incorporated and numerical computation was carried out. By trial calculation, the turbulent flow model with high reproducibility was chosen. They were a low Re model, a laminar flow model, etc. Verification of the accuracy of a model compared water level change in a top of slope position. Moreover, the dispersion range of splash was compared with the experiment.This figure indicated Volume of Fluid (VOF). Prandtl’s model has the highest reproducibility.

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Figure 5. The calculation result of run-up and breaking phenomena (VOF:Prandtl’s-model) 4. NUMERICAL ANALYSIS The reproducibility of the flow was examined. The calculated value of the flow was about 80% of reproducibility of the experimental value. Moreover, water level change of the water surface was using the VOF value 0.2, and it turned out that accuracy with wave height and a cycle comparable as an experimental value was acquired. Figure 6 shows comparison with water level change and the calculation result of an experiment. The conditions of the incident wave were wave height H = 12cm and period T = 1.2s.

Figure 6. Comparison of water level change An overtopping type was the method of generating by the fall of water mass which carried out the overtopping. The situation of run-up and overtopping was calculated by three dimensions. Three-dimensional form and grids are shown in Figure 7.A total of the number of calculation grids was 234,000 of 130x40x45. The conditions of an incident wave were the same as a two-dimensional case. Figure 20 shows the volume which ascended the slope and flowed into collecting basin. Here volume was computed by multiplying the flow velocity to the VOF value. Overtopping volume was almost constant after 10 waves

Figure 7. The 3-dimentional model shape of structure and grid

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Figure 8 shows the ratio of overflow volume to collecting basin volume. It was full of five waves. Overtopping was produced on this condition. Moreover, input-output at collecting basin was repeated like change of a surface elevation change. Thus, it was possible to predict overtopping volume by numerical computation.

Figure 8. The ratio of the volume of collecting basin and the volume of calculated over flow volume A clear presentation of experimental results obtained, highlighting any trends or points of interest. Figure 9 shows the spatial distribution of the water surface elevation after 20 wave action. Runup,overtopping, and overflow to the back wall were reproduced.

Figure 9. The ratio of the volume of collecting basin and the volume of calculated over flow volume 5. CONCLUSIONS This research aims at utilization of overtopping type wave power generation. The calculation formula of the existence of overtopping and the calculation formula of the amount of overtopping were obtained from incident wave conditions and structure form. Moreover the turbulent flow model suitable for run-up calculation was determined. It was necessary to take in the irregular wave characteristic of a field, and to examine the calculation formula of an overtopping volume.

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6. ACKNOWLEGEMENTS This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 24360360. And it should be noted that the study was an outcome of the New Energy and Industrial Technology Development Organization (NEDO). 7. REFERENCES H. Tanaka, M. Yodokawa and A. Suzuki (2010). “Studies on the wave run-up height for the development of wave overtopping type power generation”, JSCE, Proceedings of Coastal Engineering, Vol.66, pp.1271-1275. H. Tanaka, M. Yodokawa, N. Nikawadori and O. Yamanashi (2012). “Development of Wave Overtopping Type Wave Power Generation Devices”, Proceedings of the Twenty-second International Offshore and Polar Engineering Conference, Greece. M. Minami and H. Tanaka (2013). “Research on Wave Overtopping Quantity of Overtopping Type Wave Power Generation”, Proceedings of the International Workshop on OCEAN WAVE ENERGY (India), pp.181-188. Y. Masuda (2008). “The wave power generation in Japan”, Kasumi publishing company. NATURE DIGEST (Japanese edition). “To catch a wave”, Vol.5, No.1, pp.12-17.

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MODELLING THE EFFECT OF GEOTEXTILE SUBMERGED BREAKWATER ON HYDRODYNAMICS IN LA CAPTE BEACH YVES LACROIX1) 2), MINH TUAN VU1) 4), VAN VAN THAN3)and VIET THANH NGUYEN4) 1)

SEATECH, UTLN Avenue G.Pompidou, La Valette du Var, 83162, France e-mail: [email protected] 2) MEMOCS, Università Degli Studi dell’Aquila, Italy e-mail: [email protected] 3) Laboratory LATP, AMU and Faculty of Civil Engineering, TLU (WRU), Hanoi, Vietnam 39, rue F. Joliot Curie, 13453 Marseille Cedex 13, France e-mail: [email protected] 4) Faculty of Civil Engineering, University of Transport and Communicatons, Hanoi, Vietnam e-mail: [email protected]

Abstract The nice and attractive beach located south of La Capte port is subject to coastal erosion. It gradually disappears under the impact of waves and storms, especially in the autumn and winter. To limit this erosion, geotextile submerged breakwaters were established in February-March 2008 accompanying beach nourishment. The implementation of these submerged structures highly modified local hydrodynamics, sediment transport and the evolution of coastline of La Capte beach. The paper presents modeling this area before and after the installation of breakwaters. The simulation is implemented using Mike coupled models which takes into account the presence of Posidonia seagrass. The results of this work will be compared with those of measurements and monitoring which were conducted for many years. Thenceforth, the general overview of the site shall be presented, the effect of this breakwater on the proximity of La Capte beach will be also analysed and uncovered. Keywords: La Capte, geotextile, coupled models, wave transmission coefficient. 1. INTRODUCTION La Capte beach is located in the town of Hyères, South East of France (Figure 1 a). This is a shallow sandy beach which lies on the south of the eastern part of Giens tombolo. This tombolo was mainly formed due to the refraction of waves by the islands (Meulé, 2010). This is one of the most famous and attractive touristic places in France, especially in the summer. The beach of La Capte extends over more than one kilometer from the southern breakwater of La Capte marina to the end of urbanization. The coastline of La Capte suffered a decline under the impacts of waves and storms for many years which lead to a gradual disappearance of its beach. The primary reason of this disappearance pointed out by the researchers and specialists is that of the change of sediment transport mechanism. The main sediment input in Hyères bay comes from Gapeau river 6 km north and Pansard-Maravenne river 11 km north-east. The current of longshore drift from the north to the south allows sediment redistribution on the coast. However, some of the sediment is trapped in the Posidonia seagrass while most of the sediment flux is stopped by the port facilities (Meulé, 2010). Futhermore, many groynes were constructed between Gapeau river and the beach of La Capte. To sustain La Capte beach, to limit the loss of materials as well as to stabilize the coastline, two geotextile submerged breakwaters (GSB) of 100 and 150 m long were built. This installation took place during the spring of 2008 together with sand reloading to restore and nourish the beach (Richard, 2009).

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These breakwaters are composed of two geotextile tubes, side by side, 1.80 m diameter and final height of about 1 m. The two breakwaters were installed at approximately 120 and 160 m respectively from the shore, at a depth from 1.8 to 2.2 m, and from 2.5 to 2.8 m respectively. After 8 years, many studies and surveys have been conducted to evaluate the efficiency of GSB. Koffler et al. (2009)indicate that the beach was protected against erosion after the storms in December 2009. The scientific monitoring of the beach of La Capte before and after the implementation of GSB is conducted by S. Meulé. The results of measurements show that the geotextile tubes impact on the geometry of the beach, the sustainability of sand, and hydrodynamics in the area as well as the dynamics of Posidonia meadows significantly. The coastline in 2009 was enlarged 10 meters compared to the initial state established in 2007. Most of authors agreed that the geotextile works prove to be a solution for stabilizing in part the coastline and limit erosion but they did not stop completely, because they are often too immersed (Lenoble, 2011). They play the role in mitigating storm waves, but little attenuate moderate waves which also participate in sediment transport. Therefore, to provide an up-to-date overview, it is necessary to model the effect of GSB on the status quo of La Capte beach.

(a)

(b)

Figure 1: (a) Location of the La Capte beach, (b) Computational mesh of regional model. 2. MATERIALS AND METHODS 2.1. Data processing Firstly, the bathymetric data obtained from EGB (European Marine Observation and Data Network Gridded Bathymetry), Litto3D (edited by SHOM, Service Hydrographique et Océanographique de la Marine) and EOL (Etude et Observation du Littoral, an association) were processed and analysed. The result shows a mean slope of 0.5% for the area study and contours of 0.5 m from the coast to the 15 m depth (200 m wide). Next, the data of water level and storm surge are obtained from these sources. The range of astronomical tide on the SHOM map is less than 0.5 m. In addition, the rise of mean sea level (MSL) due to global warming is also estimated about 35 cm between 2010 and 2060 (Lenoble, 2010). From all this, the retained water levels for the scenarios are +1.00, +1.30, and 1.50 m above NGF (Nivellement Général de la France) for the annual, decadal, and fiftieth storms, respectively. The off-shore wave data is measured and recorded by the buoys 08301 and 08302 located approximately 1.8 km south of Porquerolles island (42°58,00'N and 6°12,29'E), at 90 m depth. Afterward, this data is processed statistically by CETMEF (Centre d’Etudes Techniques Maritimes et Fluviales). Moreover, ANEMOC (Atlas Numérique d'Etats de Mer Océanique et Côtier) also supplied the off-shore wave data via the points of MEDIT-2185, MEDIT-2610, and MEDIT-6975. The off-shore waves come from two main directions. The most frequent direction is south-western (frequency of about 40%), but these waves generally have low energy with heights of 0.5 to 1 m and periods of less than 6 seconds in 80% of cases. The second direction is southeastern waves. They are less frequent (25% of total annual duration). However, they have the heights of more than 2 m in 40% of cases, with periods of more than 6 seconds over 25% of cases (Capanni, 2011). Regarding to the wave data in La Capte beach, it was

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measured by Samuel Meulé in February 2008. The exact location of the measurement is 43°3.8'N and 6°9.17'E in water at a depth of 3-4 m. The measurement was carried out in the period from 13 to 19 February 2008. The wind has very different statistical characteristics, both in terms of overall average of extreme values and frequency. The east winds are responsible for the majority of morphogenesis events, which has the maximum speed up to 23 m/s, frequency of 11.4% and an average speed of 5.84 m/s. The current data used in our model comes from the thesis of Courtaud (2000). The wave-related currents play a key role in sediment transport. In the normal sea condition, the long-shore currents were measured in the order of 0.4 m/s on average Southeastern wind with maxima observed at 0.8 m/s. In the stormy sea condition, the flow velocities can exceed 1.3 m/s. Finally, the most famous characteristic of marine biocenosis in Hyères bay is that Posidonia seagrass. It plays an important role in reducing the hydrodynamic forces and the trapping of sediments (Meulé, 2010). Hence, the distribution of Posidonia affects the evolution of the coastline. 2.2. Implementation of numerical simulation First of all, all available information and data concerning the study area such as water level, waves, winds, currents, bathymetry, coastal line, sediments, and marine biocenosis was gathered. Afterwards, this huge datum was classified and processed by using some tools. Moreover, this datum was divided into two sets: the first set used to be input parameters for the Mike 21 coupled model, and the second set devoted to calibrate a numerical model simulated in Mike 21 software. Second of all, this area before and after constructing the GSB is simulated and modeled by applying Mike 21 coupled model. It is coupled by one spectral wave model (SW), one flow model (FM), and one sand transport model (ST) in Mike 21 package. Thus, it allows to simulate and depict the mechanism of hydrodynamic factors.

(a)

(b)

Figure 2: Computational mesh and local bathymetry before (a) and after (b) installing the GSB. Subsequently, the model will be calibrated and validated by selecting specific short time periods, adjusting friction coefficients and dimension of sediment so as to satisfy the best fitting criteria of simulation towards the dedicated periods. If the results of this model meet the criteria as well as agree with the measurements and monitoring, analysis will be operated in the following step. In the worst case, the Mike 21 coupled model can be modified again. The first step of the simulation is that of generating two types of meshes: the regional and local scale. The regional scale spreads from Cap Brun to Cap Bénat (32 km East-West) (Figure 1b) with 8 147 nodes and 15 456 elements. The local scale comprises 2 529 nodes and 4 810 elements. In addition, the boundary conditions of this scale are calculated from the models in the regional scale. Both the regional

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and local scale employ the SW and FM models. The representation of geotextile breakwaters is showed by the change of local bathymetry (Figure 2a,b). After creating the computational meshes, the regional model is simulated with the change of MSL as defined in annual (A) and decadal (D) scenarios. The input data used in this model is described in Table 6. From the results of regional simulations, the area around La Capte beach will be modeled in local models. With each mean water level, two scenarios including the breakwaters (0) or excluding them (1) shall be taken into account by using the different bathymetries. Consequently, four cases are considered in this study, namely A0, A1, D0, and D1. Table 6: The characteristic parameters of hydrodynamic data in the regional model. H1/3 (m)

Tp (s)

MWD (o)

DSD (o)

Vw (m/s)

WL (m)

A

3.3

8

120

35

19

0.65

D

4.4

10

90

25

23

0.95

Scenarios

H1/3 - Significant wave height, T p - Peak wave period, MWD - Mean wave direction, DSD - Directional standard deviation, Vw - Wind speed, WL - Water level. 3. RESULTS AND DISCUSSIONS First of all, the water level in 2008 with the scenarios A of MSL +0.65 m is a little higher than those in 2007 approximately from 1 to 2 cm. Specifically, the water level along La Capte beach in 2008 is measured about +0.804 m. Meanwhile, the water level along this beach ranged from +0.792 to +0.798 m in 2007. It comes from the wave breaking when the wave passes the submerged breakwaters. This conclusion is also valid for the scenario D with MSL +0.95 m. The water level around the breakwater in 2008 is modified, but this change is negligeable. On the contrary, the water level behind the breakwater and near shoreline alters considerably when the comparison is done between 2007 and 2008. The water level above 1.085 m occurs along La Capte beach in 2008.

Figure 3: Maps of mean current speed near La Capte beach in the scenarios A0 (a), A1 (b), D0 (c), and D1 (d). On the other hand, the wave breaking also obviously causes the change of current speed and the disturbance of current direction in the MSL of +0.65 m. The current speed evidently reduces in the near shore area inside the breakwater when comparing with those in 2007 (Figure 3a, b). Before installing the breakwaters, a small area of current speed from 0.1 to 0.12 m/s occurs in the back of the upper breakwater. However, this area disappears in 2008. Mostly, the water area behind the breakwater usually has the current speed from 0 to 0.02 m/s. Their direction is changed and dispersed in the proximity of the breakwater. Similar to A scenarios, the speed of current and the direction of current in 2008 with D scenarios have difference when comparing with those in 2007 (Figure 3c, d). This difference is mainly focused on the rear area of the breakwater. Nevertheless, there is a large area of current speed from 0.1 to 0.14 m/s, which appears between the first and second breakwaters in 2008. The current speed of below 0.02 m/s distributes inside and close to the basin of La

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Capte marina. Along the end of La Capte beach, the current speed usually ranges from 0.06 to 0.1 m/s. Furthermore, in all cases, the current along beach below the breakwaters has the direction from the north to the south. One of the most important factors that will be used to analyze the results of local simulations is that wave field. The wave height in local model in 2007 and 2008 with the scenario A of MSL +0.65m is displayed in Figure 4a,b. Visually, the significant wave height near La Capte beach in 2008 (Figure 4b) is lower than that of 2007 (Figure 4a). Along with the wave height, the direction of wave behind the submerged breakwater is also changed significantly. In 2007, there is a small area with wave direction from 78o to 84o, which appears near La Capte beach. Nevertheless, this area almost vanishes in 2008. On the other hand, in the scenarios D of MSL +0.95m, the magnitude of wave height and wave direction in the simulation with and without breakwater is not clear. In Figure 4c,d, it is obviously seen that the contours of wave height do not change much. However, to assess the efficiency of the breakwaters more exactly, the wave transmission coefficient, Kt, is used in this paper. The wave transmission coefficient is defined as the ratio of the transmitted wave height at the shoreward toe of the breakwater to the incident wave height at the seaward toe of the breakwater. The wave transmission coefficient should range from 0 to 1, for which a value of 0 implies no transmission, and a value of 1 implies complete transmission (Pilarczyk, 2003). Four points at the shoreward toe of the breakwater and four points at the seaward toe of breakwater (Figure 2a,b) have been extracted from the local simulation. The values of these points are shown in Table 7.The wave transmission coefficients vary from 0.77 to 0.86 with the scenarios A of MSL +0.65 m and from 0.82 to 0.91 with the scenarios D of MSL +0.95 m. It is easily observed that K-I is the most efficient section for wave attenuation. Moreover, the values of Kt in D scenarios are greater than those in A scenarios. The increase of the transmission coefficient comes from the sea level rise which makes the breakwaters deeper. All above-mentioned comments reveal that the GSB does not keep the key role in wave-breaking and coastal protection at the water level of +0.95 m.

(a)

(b)

(c)

(d)

Figure 4. Maps of significant wave height near La Capte beach in the scenarios A0 (a), A1 (b), D0 (c), and D1 (d).

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In addition to the transmission coefficient, the significant wave heights of five points along La Capte beach in 2007 and 2008 are compared as Table 8. All of these points are 5 m far from the coastline (Figure 2a,b). According to this table, the significant wave heights at all points in 2007 (before installing the breakwater) are higher than those in 2008 (after installing the breakwater) from 1 to 8 cm. The coast points 1, 2, 3, and 4 present larger variations because they are in the area protected by both submerged breakwaters and the mound breakwaters of La Capte marina. Besides, the errors in D scenarios are smaller than those in A scenarios. Table 7: The transmission coefficients with annual and decadal scenarios Scenario A1

Scenario D1

Section

E-F

H-G

L-R

P-Q

K-I

E-F

H-G

L-R

P-Q

K-I

Ht (m)

0.99

0.95

1.05

1.02

1.02

1.19

1.12

t

1.15

1.13

Hi (m)

1.19

1.11

1.32

1.3

1.33

1.3

1.24

1.43

1.37

1.34

Kt

0.84

0.86

0.8

0.78

0.77

0.91

0.9

0.82

0.84

0.85

Table 8: The significant wave heightsalong La Capte with annual and decadal scenarios Scenario A1

Scenario D1

Point

Coast 1

Coast 2

Coast 3

Coast 4

Coast 5

Coast 1

Coast 2

Coast 3

Coast 4

Coast 5

2007

0.46

0.47

0.46

0.42

0.44

0.58

0.59

0.60

0.55

0.56

2008

0.39

0.39

0.38

0.36

0.42

0.52

0.53

0.52

0.50

0.55

E

6

7

8

6

2

5

6

7

5

1

Significant wave heights in meter, Absolute error (E) in centimeter.

4. CONCLUSIONS Firstly, the interim simulations of regional model have been done and calibrated with the experimental data. The simulations have reproduced correctly the real mechanism of current, wave and water level in the study area, especially, along the eastern Giens tombolo that has been mentioned in the previous works. They provide an overview of La Capte beach area before the GSB are installed. Secondly, the effect of these breakwaters is discussed and clarified more specifically by using the results of local simulations. The presence of the breakwaters has changed the current at La Capte beach in the positive direction. The significant wave height and current speed have been reduced to acceptable levels. Nonetheless, the effect of submerged breakwaters will be reduced when the sea level rises. Therefore, the expansion of these breakwaters could be a positive option to continue protecting La Capte beach area from the phenomenon of global warming and climate change. Finally, the above-mentioned simulations can be used to predict the development of hydrodynamic factors as well as coastline change in near future if the input data is being updated. Furthermore, they will be employed to optimize the dimension of the GSB in the next study. 5. ACKNOWLEGEMENTS The authors gratefully thank Danish Hydrological Institute for Mike 21 software. We thank IGN, EGB, EOL, CETMEF, IFREMER, SHOM, and ECMWF for the data provided. 6. REFERENCES Capanni, R. (2011). Étude et gestion intégrée des transferts sédimentaires dans le système Gapeau/rade d'Hyères. Aix Marseille 1. Courtaud, J. (2000). Dynamiques geomorphologiques et risques littoraux cas du tombolo de giens (Var, France méridionale). (Ph.D. dissertation), Université Aix-Marseille I.

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Koffler, A., Zengerink, E., Ascione, J.-C., & Birukoff, J.-M. (2009). Un atténuateur de houles en tube géosynthétique pour limiter l'érosion de la plage de la Capte à Hyères. Paper presented at the Rencontres Géosynthétiques. Lenoble, A. (2010). Etude pour la protection de la plage du Ceinturon et du secteur Sud du port SaintPierre - Phase 1 : Synthèse des connaissances - Rapport. Lenoble, A. (2011). Etude pour la protection de la plage du Ceinturon et du secteur Sud du port SaintPierre - Phase 3 : Définition des scénarios de protection - Analyse préliminaire. Meulé, S. (2010). IMplantation d’Atténuateur de Houle en GEOtextile: Suivi scientifique de la plage de La Capte, Hyères, Var : Instrumentation, Modélisation (pp. 230-230). Hyères. Pilarczyk, K. W. (2003). Design of low-crested (submerged) structures - an overview. 6th International Conference on Coastal and Port Engineering in Developing Countries, Colombo, Sri Lanka, 2003. Richard, L. (2009). Suivi de l’evolution de la plage de la Capte suite a la mise en place d’attenuateurs de houle.(Mémoire de stage d'initiation à la Recherche).

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EAST SEA WAVE CLIMATE SIMULATION FROM 1979 TO NOW USING MIKE 21 SW MODEL DANG QUANG THANH1) and LE QUAN QUAN2) 1) DHI Vietnam, 147-149 Vo Van Tan Str., Ward 6, Dist. 3, Ho Chi Minh City, Vietnam E-mail: [email protected] 2) The Southern Institute of Water Resources Research 658 Vo Van Kiet Str., Ward 1, Dist. 5, Ho Chi Minh City, Vietnam E-mail: [email protected]

Abstract Long-term wave climate data are a basic important information for hydrodynamics, shoreline moving, coastline and estuarine morphology study. However those data are rare, insufficient, and if those are measured, the measurement duration usually are not long enough for doing statistical analysis. Therefore, the hindcast approach method by using numerical tools to simulate wave from wind field has been widely applied nowadays. In the study, East Sea wave has been simulated from 1970 to now by using MIKE 21 SW model of DHI Group which is third generation spectral wind-wave unstructured grid model. Wind field data have been extracted from CFSR (Climate Forecast System Reanalysis) with the spatial resolution of 0.312 o for period form 1979 to 2011 and from CFS version 2 with the spatial resolution of 0.205 o for the period from 2011 to now. Two (02) set of data have time resolution of 1 hour. The model has been applied unstructured mesh with the equivalent spatial resolution of about 5km for the areas near the coastline and near some islands and group of islands of Vietnam. Other areas spatial resolution are varied from 5km to 50km. The model has been validated against some measured data also. The result of the study is the hindcast wave data of East Sea from 1979 to now which can be used for other related studies or used as boundary data for the detailed wave model of a certain project. Keywords: East Sea, wave, long-term wave climate, MIKE 21 SW model, wind, numerical model

1. INTRODUCTION Long-term wave climate data are a basic important information for hydrodynamics, shoreline moving, coastline and estuarine morphology study. However those data are rare, insufficient, and if those are measured, the measurement duration usually are not long enough for doing statistical analysis. The objective of this study is to develop a model in order to simulate hindcast wave of East Sea. The results of the study will helps for other related study such as hydrodynamics, sediment transport and coastline, estuarine morphology where wave climate plays an importance role in those processes. 2. MATERIALS AND METHODS 2.1. Wave model MIKE 21 Spectral Wave (SW) model has been used for the study. MIKE 21 SW is a state-of-the-art third generation spectral wind-wave model developed by DHI. The model simulates the growth, decay and transformation of wind-generated waves and swell in offshore and coastal areas. There are two different formulations in MIKE 21 SW which are fully spectral formulation and directional decoupled

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parametric formulation. The fully spectral formulation is based on the wave action conservation equation, as described in e.g. Komen et al (1994) and Young (1999). The directional decoupled parametric formulation is based on a parameterisation of the wave action conservation equation. The parameterisation is made in the frequency domain by introducing the zeroth and first moment of the wave action spectrum. The basic conservation equations are formulated in either for small-scale applications and polar spherical co-ordinates for large-scale applications. The discretisation of the governing equation in geographical and spectral space is performed using cell-centred finite volume method. In the geographical domain, an unstructured mesh technique is used. The time integration is performed using a fractional step approach where a multi sequence explicit method is applied for the propagation of wave action. 2.2. Model setup East Sea wave model based on unstructured mesh established from the depths extracted from the international C-MAP chart database. The model extends from 99oE to 124oE; -2oS to 26oN with total of about 60000 calculation points. The finest equivalent spatial resolution is about 5km for the areas near the coastline and near some islands and group of islands of Vietnam (Figure 1). Other areas spatial resolution are coarser and varied from about 5 to 50km. The fully spectral model has been applied including the following physical phenomena: • Wave growth by action of wind • Non-linear wave-wave interaction • Dissipation due to white-capping • Dissipation due to bottom friction • Dissipation due to depth-induced wave breaking • Refraction and shoaling due to depth variations

Figure 2. Calculation mesh (left) and bathymetry (right) 2.3. Boundary condition Wind field boundary data have been extracted from CFSR (Climate Forecast System Reanalysis) and from CFS version 2. The data from CFSR is available from 1979 to 2011 with the spatial resolution of 0.312o, whereas the data from CFS version 2 is available from 2011 to now with the spatial resolution of 0.312o. Two (02) set of data have time resolution of 1 hour. An example of wind field data is represented in Figure 2.

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Figure 2. An example of wind field data on 01/07/2015 (left) and on 01/01/2015 (right) 2.4. Model validation The model has been validated against some collected measured wave data which is shown in Figure 3. The model results performed quite well with three main wave parameters which are significant wave height, mean wave direction and peak wave period.

Figure 3. Comparison of simulated significant wave height (upper), mean wave direction (middle) and peak wave period (green solid line) with measured data (black dotted line)

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3. RESULTS AND DISCUSSIONS The result of the study is the hindcast wave data of East Sea from 1979 to now which some examples of the results are shown from Figure 4 to Figure 7. Main wave parameters are significant wave height, peak wave period, mean wave direction can be extracted from the results as 2D field or for further analyses at any points of interest in East Sea such as scatter diagram and/or wave rose drawing etc. The results also can be extracted for the boundary of the detailed smaller scale wave model for a local area.

Figure 4. Field of significant wave height and mean direction vector (left), Peak wave period (right) at a certain of time

Figure 5. Significant wave height at a certain location

Figure 7. Wave rose

Figure 6. Scatter diagram of Hs and Tp

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4. CONCLUSIONS The study has been focused on simulation hindcast wave from wind data since 1979. The results of the study are hindcast wave data which can be utilized as a basic information for various purposes related to coast and estuary area. The model has been also validated against some measured data but further validations against data are still needed to improve the reliability of the model. The model also is still limited to stormy wave condition which need finer time resolution of wind data. 5. REFERENCES DHI 2014. Mike 21 spectral wave module, Scientific documentation; Danish Hydraulic Institute (DHI) Komen, G.J., Cavaleri, L., Donelan, M., Hasselmann, K., Hasselmann, S., Janssen, P.A. E. M., 1994.Dynamics and Modelling of Ocean Waves.Cambridge University Press Saha, S., et al. 2010. NCEP Climate Forecast System Reanalysis (CFSR) Selected Hourly Time-Series Products, January 1979 to December 2010. Research Data Archive at the National Center for Atmospheric Research, Computational and Information Systems Laboratory Saha, S., et al. 2011, updated monthly. NCEP Climate Forecast System Version 2 (CFSv2) Selected Hourly Time-Series Products. Research Data Archive at the National Center for Atmospheric Research, Computational and Information Systems Laboratory. Young, I.R., 1999. Wind generated ocean waves, in Elsevier Ocean Engineering Book Series, Volume 2, Eds. R Bhattacharyya and M. E. McCormick, Elsevier.

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MICROMECHANICAL MODELLING OF INTERNAL EROSION BY SUFFUSION USING DEM-PFV COUPLED MODEL TONG ANH TUAN1) and BRUNO CHAREYRE 2) 1) Civil Engineering Department, University of Transport and Communications 3 Cau Giay  Lang Thuong  Dong Da, Hanoi, 100000, Vietnam e-mail: [email protected] 2) Grenoble University, Laboratory 3SR 1301 rue de la piscine, Domaine Universitaire, 38400 Saint Martin d’Hères, France e-mail: [email protected]

Abstract The numerical model of hydromechanical coupling DEMPFV presented in this paper uses a combination of the discrete element method (DEM) for the solid phase and a porescale finite volume of the flow problem (PFV). Comparison between the numerical and experimental results on internal erosion shows a good agreement of the model DEMPFV. Keywords: numerical model, hydromechanical coupling, DEMPFV, internal erosion, suffusion.

1. INTRODUCTION The description of flow and transport in porous media is one of the great interest in engineering in the recent years, particularly as ground water, seepage, settlement and stability of foundations. The statistical study of Foster et al. (2000) on 11192 hydraulic structures showed that 136 of them have had 46% accidents by internal erosion and 54% by sliding and overflow. The internal erosion phenomenon can be very serious. The complexity of phenomena, the extent of their demonstrations and the weakness of our knowledge to them obstruct the profession. Various numerical approaches have been attempted in order to model the mechanical behavior of media. The discrete element methods (DEM), proposed by Cundall et al. (1979) is now employed for studying complex aspects of the mechanical behavior as the complexity of granular structure and the interaction between solid and fluid phases. The model, presented in this study, the discrete element method had been adopted to model the behavior of the granular solid. The soil being represented by modeling the mechanics of the interaction between the particles. The flow problem is solved by a discretization of the void space in finite volumes. The fluid is then assumed to flow through such volumes in a pore network built upon such discretization. The numerical approach is considered to study the internal erosion in bi-dispersed saturated granular assemblies based on the combination of discrete element method and finite-volumes. Comparison between the numerical and experimental results in literature is presented and discussed in the last part. 2. NUMERICAL MODEL 2.1. Solid phase - The DEM method The DEM is essentially a Lagrangian mesh-free technique where each particle of the material is a sphere identified by its own mass, radius and moment of inertia. Interaction forces between particles, and resulting forces acting on each of them, are deduced from sphere positions through the interaction law. Newton’s second law is then integrated through an explicit second-order finite difference scheme to compute new sphere’s positions.

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Figure 1. The linear elastic-plastic model for contact interactions between solid particles (Chareyre et al. (2012)) Contact interactions between solid particles are modelled as a linear elastic-plastic law (Fig.1). Contact forces in the normal and tangential directions ( f n and f t ), are proportional to normal and .

tangential relative displacements ( U n and U t ) between two particles, and depend on normal and tangential stiffness ( k n and k t ) through particle radii and elastic modulus of the material (Fig.1). ..

..

..



sndS 

Global acceleration vectors X k   x k ,ω k  of particle k are governed by Newton’s second law: ..

mk x k 

k



nc

s gdV   f jc,k  mk g  Fc,k  mk g

(1)

j0

k

where m is the mass of the particle occupying the volume  k ,  s n is the stress applied at the particle surface in the direction of the unit normal n ,  s is the mass density, g is the gravitational acceleration; k

f jc ,k contact force between two particles j and k , F c,k total force exerted on particle k and nc is the total

number of contact points. For the 6 xN translational and rotational DOFs in the system, the global position vectors are defined: ..

X  J 1 (T c  W )

(2)

where J is the generalized inertia matrix, T is the generalized force vector, W is the global mass matrix including torques, and N is the number of particles. Positions of particles are obtained at each time step t by applying a centered second order finite difference scheme: c

X t t  2X t  X t t  J 1 (T c  W ) t 2

(3)

and then contact forces are updated by second order approximations at mid-step: .  c c Ft  t  Ft  B ( X t  t , X t  t / 2 )t . X  Xt X t  t / 2  t  t t 

(4)

The computational cycle of the explicit DEM model revolves around two main steps, including the computation of particle position and computation of interaction forces. 2.2. Fluid phase – The PFV model Regular triangulation and Voronoi graph are used for poral space discretization into tethraedrons. The vertices of tethraedrons are the centre of a sphere in the packing (Fig.2).

Figure 2. Poral space discretization in 2D and 3D (Catalano et al. (2011))

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Vietnam-Japan Workshop on Estuaries, Coasts and Rivers 2015, 7-8 September, 2015, CKT, Hoi An, Vietnam

Continuity equation gives a relation between the time derivative of the volume filled by the fluid .

f

and fluid fluxes exchanged qij from tetrahedron i to adjacent tetrahedron through four intersections of triangular surfaces S ij (Tong et a. 2012):

V

.

V if 

j4

  (u j  j1 S f

f

j4

 v c )dS   qij

(5)

j  j1

ij

where (u  v ) is the fluid velocity relative to the solid velocity. By analogy Stokes, Darcy equations, and Hagen  Poiseuille relation, the relation between fluid flux qij and the time derivation of the volume and the pressure field gives: f

c

.

j4

pi  p j

j  j1

lij

V if   qij  k ij

 K ij ( pi  p j )

(6)

k ij

is the hydraulic conductance of the surface S ij resulting the pressure difference ( pi  p j ) . lij Fluid-solid interaction forces results from the pressure field and can define as the sum of three terms: (i) Archimede’s force (buoyancy force) F b,k , (ii) pressure force F p,k and (iii) viscous force F ,k : (7) F k   gzndS   p * ndS   ndS  F b,k  F p ,k  F ,k where K ij 

k

k

k

where n is the unit vector in the normal direction of a surface integral on contour. At each time step, the local rate of pore volume change is computed, the pressure field is obtained from the equation (6), and new interaction forces are updated from the equation (7) at the next time step. 2.3. The model of hydromechanical coupling DEMPFV The solution of the DEM-PFV model is based on two global matrix relations liking the particle displacements to the pressure field: . (8) [G]{P}  [E]{X}  {Q q }  {Q p } ..

[M]{X}  {F c }  {W}  {F f }

(9) where the equation (8) describes the mass conservation of a compressible fluid in deformable porous media and the equation (9) corresponds to the movement of the solid particles. The pressure field is obtained by solving the implicit relation (10) by a first order explicit integration scheme:  X (t  t )  X (t )  [G]{P(t )}  [E]   {Q q (t )}  {Q p (t )} t  

(10)

Fluid-solid interaction forces are computed from the relation (11): {F f }  [S]{P}

(11) Finally, the explicit scheme of the DEM is integrated by the relation (9). The computation cycle of the coupling model DEM-PFV is shown on Figure 3.

Figure 3. Computation cycle of the DEMPFV (Catalano et al. (2011))

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3. RESULTS AND DISCUSSIONS The simulation procedure of internal erosion by suffusion is composed of steps (Tong (2014)): (i) A dense cubic sample was created by compacting isotropically using the growth algorithm REFD, by which an isotropic confining stress of 50kPa is applied, (ii) “no-slip” condition is considered on the lateral boundaries, (iii) Hydraulic gradients are imposed on the top and bottom boundaries, while no-flux conditions are imposed on the lateral boundaries, and (iv) Eroded mass, porosity, permeability, axial strain, water head, pore pressure and local hydraulic gradient are measured during the simulation. The main parameters of the simulation are reported on Table 1. Table 1. Parameters used in the internal erosion simulation Tested parameters Gap graded

G4C

Fines content

m1

40

[%]

Coarses content

m2

60

[%]

 D’15/d’85

30

[o]

8.3

[-]

Intergranular friction angle Terzaghi’s criterion

Both the lateral boundary conditions and the hydraulic gradients applied on the top and bottom boundaries are shown on Figure 4. The pressure field obtained at the beginning of the simulation for the number of grains N  2000 is presented on Figure 4.

Figure 4. Boundary conditions and pressure fields The results obtained on the test G4  C allow us to highlight the fact that, the internal erosion mechanism can be divided into three phases. In the first phase, a low mass eroded less than 1% at the bottom of the sample is accompanied by a redistribution of fine particles resulting in a small increase in permeability and low axial deformation of less than 1% (Fig.5). The mass eroded during this phase is insufficient to detect experimentally by weighting.

Figure 5. Eroded mass, permeability and axial deformation for i  1 ( 2   3  50kPa) During the second phase, a strong increase in mass eroded reaches 6% that is accompanied by a redistribution of fine particles progressing from downstream to upstream resulting an increase in porosity

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simultaneously with the increase in permeability by a factor of 1.2 (Fig.6). In the first two phases, internal erosion initially develops downstream and moves upstream which results in a rapid decrease in local hydraulic gradient i12 and an increase in local hydraulic gradient i45 and i56 (Fig.7).

Figure 6. Distribution of the water head compared to the experience of Moffat et Sail for i  1 and i  5 ( 2   3  50kPa) For the third phase, the eroded mass continues to increase by a factor less than 12 compared to that obtained at the end of the second phase. Both internal erosion processes were observed: suffusion and local instability. The suffusion appears, in the first two phases, approximately 14% eroded fines, which agrees closely with the results obtained by Sail et al.. Internal erosion appears as a diffuse process accompanied with a fine particle migration from upstream to downstream. A clogging causing a pore pressure increase preceded the realization of localized instability along the same vertical. Finally, there is an increase in the permeability by a factor of 6 to 7 compared with the beginning of the internal erosion, and an axial deformation of 3 to 7% .

Figure 7. Local hydraulic gradient for i  1 ( 2   3  50kPa) 4. CONCLUSIONS A series of numerical testing on internal erosion was performed on bi-disperses assemblies G4  C ( 40% by mass of fine particles) under different hydraulic gradients i  1  8 and the isotropic confining stresses  2   3  10  100kPa . The numerical results are in general agreement with the experimental results of Sail et al. (2011), of Moffat et al. (2011) performed on G4  C test glass balls a size ratio D15' / d 85'  8.3 . In this article, the DEMPFV coupling method has proved effective for the study of phenomena both fines migration than internal instability of the soil under the action of an internal flow structure at the microscopic scale with acceptable computation time and resources available CPU and RAM. Nevertheless, further research on the effect of geometric criterion and a large number of particles are

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needed to better validate coupled model DEM-PFV. We believe that the DEM method is very relevant to the study of internal erosion mechanisms in granular media. 5. REFERENCES Catalano E., Chareyre B., Cortis A., Barthélemy E. (2011). A Pore-Scale Hydro-Mechanical Coupled Model for Geomaterials Investigation of internal erosion processes using a coupled DEM-Fluid method, Particles 2011, II International Conference on Particle-Based Methods, Fundamentals and Applications, Barcelona, Spain, 26-28 October. Chareyre B., Cortis A., Catalano E., Barthélemy E. (2012). Pore-Scale Modeling of Viscous Flow and Induced Forces in Dense Sphere Packings, Transport in Porous Media92(2), 473-493. Cundall P.A., Strack O.D.L. (1979) A discrete numerical model for granular assemblies, Geotechnique 29, 47-65. Foster M., Fell R. and Spannagle M. (2000). The statistics of embankment dam failures and accidents. Canadian Geotechnical Journal, 37(5), 1000–1024. Moffat R., Fannin R. J. and Garner S. J. (2011). Spatial and temporal progression of internal erosion in cohesionless soil, Can. Geotech. J., 48(3), 399–412. Sail Y., Marot D., Sibille L. and Alexis A. (2011). Suffusion tests on cohesionless granular matter, European Journal of Environmental and Civil Engineering, 15(5), 799–817. Tong A.-T., Catalano E., and Chareyre B. (2012). Pore-Scale Flow Simulations: Model Predictions Compared with Experiments on Bi-Dispersed Granular Assemblies, Oil & Gas Science and Technology – Revue d’IFP Energies nouvelles, 67(5), 743–752. Tong A. Tuan. (2014). Modélisation micromécanique des couplages hydromécaniques et des mécanismes d’érosion interne dans les ouvrages hydrauliques. http://www.theses.fr.

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IMPACT OF TRIAN DAM-BREAK ON INUNDATION IN THE LOWER SAIGON-DONGNAI RIVER BASIN, VIETNAM DANG DONG NGUYEN1) and LE THI HOA BINH2) 1) Department of Civil Engineering, National Institute of Technology, Warangal, 506004, India Email: [email protected] 2) Water Resources Engineering, Thuyloi University- 2nd Base Hochiminh City, 700000, Vietnam Email: [email protected]

Abstract The Trian reservoir, one of the big reservoirs in the East South Area of Vietnam, was built for multipurposes such as agricultural irrigation, power supply, flood prevention and tourism, etc. Besides, it also brings many benefits for socio - economic sectors in this area and the surrounding. However, with the high head of water level between upstream and downstream as Trian reservoir exists many potential hazards, especially in the dam break case. When dams are collapsed that endanger to safety of human life and properties of inhabitants living in the downstream. The results of this study indicated that the hydraulic regime in the lower river basin has been significantly changed and cause the large inundation for Dongnai and Hochiminh province. Additionally, from this result, several suitable solutions should be given to minimize damages causing by dam collapsed. Key words: Trian reservoir, dam failure, flood simulation.

1. INTRODUCTION The Trian hydroelectric dam, located on the Dongnai River, is situated about 65 km far from Hochiminh city (HCMC) and belongs to the Vinhcuu district, Dongnai province (see Figure 1). Dam and hydroelectric station were built from 1984 to 1991 with installed electric capacity of 400 MW and producing about 1.7 billion kWh of electricity per year. The main dam parameters are 420 m in length and 40 m in height. The water surface and the total volume of Trian reservoir are 323 km 2 and 2.76 (106) m3 respectively. Trian dam not only plays a key role in providing electricity for the southern Vietnam but also contributes the amount of water for human activities, agriculture as well as for aqua-cultural activities of the surrounding areas. This study focus on the inundated areas that cause by overtopping and piping mechanism of Trian dam. 2. MATERIALS AND METHODS 2.1. Methods The methodology used to assess the effect of Trian dam-break in this study are numerical model and Empirical model. In fact, NWS DAMBRK and MIKE 11 ENERGY model will be applied to describe the dam-break mechanism while MIKE FLOOD, a model developed by DHI Group, Denmark, will be used in the floodplain studies. It combines the two numerical hydrodynamic models MIKE 11 (1-D) and MIKE 21 (2-D).

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Figure 5: The lower Saigon - Dongnai river basin 2.2. Materials: 2.2.2. Dambreak in MIKE 11 model In MIKE 11 implicit method and Finite Difference are used to solve the fluid equations in an unsteady state based on conservation of mass and Saint Venant momentum equations. The solution method to the above equations is 6 points Abbott scheme in which discharge and water level height are calculated at the nodal level namely H and Q. 2.2.3. Reservoir description In order to obtain an accurate description of the reservoir storage characteristics, the reservoir can be modelled as a single h-point in the model. This point also corresponds to the upstream boundary of the model where inflow hydrographs are specified. In this way the surface storage area of the dam is described as a function of the water level (see figure 2).

Figure 6: The Trian reservoir description

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2.2.4. Hydraulic model 1D The processing of the data for simulation in the MIKE 11 model including: the network, Cross section, Boundary condition and some these parameters. The data rainfall, water level and discharge are created in compatible MIKE 11 time series in a separate file as the input for the parameter editors. a) Network Trian dam belongs to The Saigon - Dongnai river system. The length of reach from Trian Dam to the East Sea is 150 km. This river is located in the large delta, with the deep, wide cross-section. These main tributaries consist of the Be, Dongnai, Saigon and Vamco River. Network is established in MIKE 11 including 9000 points and 287 branches. b) Cross section The cross section in the lower Saigon-Dongnai River consists of more 1000 cross section. This data based on the observation data of Thuyloi University Second Base in 2009 belongs the Water Resources Planning to Prevent Flooding for Hochiminh City Project. c) Boundary condition The upstream boundary includes 3 hydrograph which is release discharge from Trian, Dautieng and Phuochoa reservoirs and 2 hydrograph which is water level from Candang and Mochoa stations while the downstream condition is water level of Vungtau and Vamkenh stations. d) Calibration and validation hydraulic model 1D The calibration of hydraulic model is established based on the measured water level in 2000 (Bienhoa, Phuan station). Besides, in order to ensure the random and the agreement of model as validation process, the water level (2001) was chosen to valid in hydraulic modelling. The agreement between model result and observe data ensure the error standard (RMSE) (see figure 3 to 6).

Figure 3: Comparison of water level between model and observation data in Bienhoa 2000

Figure 4: Comparison of water level between model and observation data in Bienhoa 2001

Figure 5: Comparison of water level between model and observation data in Phuan 2000

Figure 6: Comparison of water level between model and observation data in Phuan 2001

2.2.5. Hydraulic model 2D MIKE 21 FM with HD module was used to simulate the water level, discharge and velocity variation for the whole study areas. The inputs of model are bathymetry (see figure 7) and some parameters including Manning’s number, Eddy viscosity and temperature (see table 1).

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Table 1: Variable HD module calibration Parameters

Value used

Manning's number

28 (m 1/3/s)

Smagorinsky coefficient

0.28

Temperature

27 (0C)

Figure 7: The bathymetry of the study areas 2.2.6. Floodplain model MIKE FLOOD is combined by hydraulic 1D and hydraulic 2D model to simulate the flow dynamic exchange between two models (see figure 8). MIKE FLOOD is set up as identifying the grid cells within MIKE 21 which correspond to cross section in MIKE 11 (see figure 9).

Figure 8: The link between hydraulic 1D and 2D

Figure 9: No. of grid cell in hydraulic 2D

3. RESULTS AND DISCUSSIONS The flood inundation results of the simulated by MIKE FLOOD model was generated at the time step of one-hour interval in two dimensional model. The detail results about flood time and inundated areas for both scenarios namely by overtopping and piping dam failure mechanism will be listed by table 2. Table 2: Comparison the inundated areas by piping and overtopping dam failure Period of time after failure (h) Inundated areas by piping (km2) 2

Inundated areas by overtopping (km )

1

3

5

7

9

11

20

30

17.3

61

122.8

157.8

194.3

233.5

360.2

412.4

14.3

56

113.7

144.6

182

223.9

344.5

406.8

3.1. Flood inundation that failure by overtopping These figures compare the water depth in downstream of Trian dam after 1,5,11 and 30 hour after the dam failure. It is clear that areas that impacted by Trian dam failure on the lower Saigon-Dongnai river are extremely serious, especially, when consider to the Dongnai province and HCMC. The highest water depth take place in the downstream of Trian dam, with the value is 12 m and gradually decrease to downstream areas about 0.3 m. It is easy to see that after 30 hour dam failure the inundated areas are greatest while the water depth has been significantly decreased (see figure 10 to 13).

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Figure 10. The water depth in downstream of Trian dam after 1h

Figure 11. The water depth in downstream of Trian dam after 5h

Figure 12. The water depth in downstream of Trian dam after 11h

Figure 13. The water depth in downstream of Trian dam after 30h

3.2. Flood inundation that failure by piping Figure from 14 to 17 describe the water depth in downstream of Trian dam after 1,5,11 and 30 hour after dam failure. It is clear that the inundated area by piping is more serious than by overtopping. In fact, 1 hour after dam-break with the same water depth but the inundated area by piping mechanism is larger than by overtopping around 17 %. After 30 hours dam collapse, the inundated area by overtopping is 406.8 km2 while this number for piping is 142.4 km2. 4. CONCLUSIONS AND RECOMMENDATIONS In case of the Trian dam-break, its impacts are extremely risk on the lower downstream. This study has been calculated the inundated areas by overtopping and piping failure mechanism and indicated the impacts of piping are larger than overtopping. The flood-forecasting tools, educational programs for residents within the floodplain area, evacuation plans, and evacuation notification systems are essential in mitigating the damage due to this incident. Climate change and extreme meteorological event as cause of dam-break incident should be considered.

Figure 14. The water depth in downstream of Trian dam after 1h

Figure 15. The water depth in downstream of Trian dam after 5h

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Vietnam-Japan Workshop on Estuaries, Coasts and Rivers 2015, 7-8 September, 2015, CKT, Hoi An, Vietnam

Figure 16. The water depth in downstream of Trian dam after 11h

Figure 17. The water depth in downstream of Trian dam after 30h

5. ACKNOWLEGEMENTS This research was carried out in the cooperation between Thuyloi University Second Based (TLU) and Power Engineering Consulting Joint Stock Company 2 (PECC2) and funded by PECC2. We thank to TLU for providing the climate, hydrology and topography data. 6. REFERENCES DHI Water & Environment, 2009. Dam and Reservoirs – Dam and Levee Failure Modeling and Mapping. MIKE Documentation. DHI Group, California. Coleman, S., Andrews, D., and Webby, M. G., 2002. Overtopping Breaching of No cohesive Homogeneous Embankments. In: Journal of Hydraulic Engineering. Foster, M., & Fell, R., 2001. Assessing embankment dam filters that do not satisfy design criteria. In: Journal of Geotechnical and Geo-environmental Engineering. Schmocker, L. and Hager, W. H., 2009. Modelling dike breaching due to overtopping. In: Journal of Hydraulic Research.

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