In recent years, the erosion of beaches and estuaries in Bac Lieu has gradually intensified. Especially, foreshore lowering phenomenon has been happening at area adjacent Soc Trang province. This has severely affected the safety of local inhabitants, the quality of infrastructure, the degradation of environment, and the economic development. This is the critical reason that the author do the research The effect of foreshore lowering on Bac Lieu sea dike.
MINISTRY OF EDUCATION AND TRAINING MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT THUY LOI UNIVERSITY LE THI HIEN THE EFFECT OF FORESHORE LOWERING ON BAC LIEU SEA DIKE Major: Coastal Engineering and Management Code: 62580203 MASTER THESIS SUPERVISER 1: Dr LE HAI TRUNG SUPERVISER 2: Assoc Prof Dr TRAN THANH TUNG HANOI - 2016 i ACKNOWLEDGEMENTS First of all, I would like to thank to my supervisors, Assoc Prof Dr Tran Thanh Tung and Dr Le Hai Trung for his great contribution in this thesis I would like to express my deep gratitude especially to my main supervisor Dr Le Hai Trung for being so patient with me and for all of his supports and inspiration My gratitude also goes to colleagues from Southern Institute of Water Resources Research who have supported me in my research work I also deeply indebted to Assoc Prof Dr Dinh Cong San and Dr Nguyen Duy Khang, who have significant contribution to my research I also would like to thanks to M.Sc Nguyen Quang Duc Anh and Mr.Nguyen Cong Phong for providing whole-hearted assistance to my thesis Finally I would like to thank my family and friends for all of the time we’ve been through together Ha Noi, October 2016 Le Thi Hien ii DECLARATION I hereby declare that is the research work by myself under the supervisions of Assoc Prof Dr Tran Thanh Tung and Dr Le Hai Trung The results and conclusions of the thesis are fidelity, which are not copied from any sources and any forms The reference documents relevant sources, the thesis has cited and recorded as prescribed The results of my thesis have not been published by me to any courses or any awards Ha Noi, October 2016 Le Thi Hien iii TABLE OF CONTENTS LIST OF FIGURES iii LIST OF TABLES .vi ABSTRACT CHAPTER LITERATURE REVIEW 1.1.Introduction 1.2.Foreshore and mangrove 1.3.Design of sea dike with regard to foreshore and mangrove 1.4.Discussion 10 CHAPTER FORESHORE LOWERING AND ITS IMPACTS .12 2.1.Introduction 12 2.2.Description of the Bac Lieu sea dike 12 2.2.1 Topography and geomorphology .12 2.2.2 Hydrodynamic characteristics 14 2.2.3 Foreshore and mangrove 19 2.2.4 Bac Lieu sea dike system 23 2.3.Governing factors and classification of the foreshore lowering mechanism 26 2.3.1 Governing factors of the foreshore lowering mechanism 26 2.3.2 Classification of the foreshore lowering mechanism 31 2.4.Sea dike under impact of foreshore lowering .32 2.5.Summarise 33 CHAPTER MODELling of the FORESHORE LOWERING mechanism 34 3.1.Overview .34 3.2.Model set – up, calibration and verification 34 3.2.1 Introduction to Mike 21 34 3.2.2 Model set –up for a large domain 36 3.2.3 Model set - up for a small domain .38 3.2.4 Model calibration and verification 40 3.3.Boundary conditions and simulation scenarios 49 3.3.1 Boundary conditions 49 3.3.2 Simulation scenarios 49 i 3.4.Simulation results 50 3.4.1 Wave 50 3.4.2 Current 52 3.5.Dicussion and conclusion 54 CHAPTER the EFECT OF FORESHORE LOWERING ON SEADIKE 57 4.1.Introduction 57 4.2.Changes of the bathymetry 57 4.3.Changes in the wave characteristics .59 4.4.The effect of foreshore lowering on sea dike .61 4.4.1 Sea dike design without mangrove forest 62 4.4.2 Sea dike design with mangrove forest in the case of a lowered foreshore 64 4.5.Discussion 66 CONCLUSION 69 APPENDICES 75 ii LIST OF FIGURES Fig.1-1: Energy dissipation of waves on higher floodplains with mangroves (Albers et al., 2013) 10 Fig.2-2: Map of the Mekong delta .12 Fig.2-3: The location of the study sea dike 13 Fig.2-4: The average long-term wave field of Con Dao(KHCN-BĐKH/11-15, 2015) 15 Fig.2-5: The average long-term wind field of Con Dao(KHCN-BĐKH/11-15, 2015).17 Fig.2-6: Direction and speed of the winter currents(KHCN-BĐKH/11-15, 2015) .18 Fig.2-7: Direction and speed of the summer currents(KHCN-BĐKH/11-15, 2015) 18 (i)The large eroded beach (ii)Deeply eroded offshore 20 Fig.2-8: The erosion phenomenon at the Northern area of windmill field in Bac Lieu ( Sources: Le Hai Trung) 20 Fig.2-9: The mangrove forest erosion at Ganh Hao town, Bac Lieu province 22 Fig.2-10:Mangrove tree left in front of Nha Mat embankment 22 Fig.2-11:The current status of forest beach at a canal gate out to the sea 22 Fig.2-12:Preliminary locations of survey Bac Lieu sea dike in Aug- 2014 23 Fig.2-13: The sea dike hardened of Nha Mat estuary 24 Fig.2-14:Two type of protection structures applied at Nha Mat 25 Fig.2-15:The construction current status of concrete dike from Lang Ong Hai Nam to Ganh Hao estuary 26 Fig.2-16:The current status of concrete dike defence Ganh Hao town .26 Fig.2-17: The sediment accumulation and the trend of sediment transport in Mekong Delta, which is affected dominantly by northeast monsoon (Sources: Marine economy.vn) 28 Fig.2-18:The section of tidal beach is surface eroded (Loi, 2015) .31 Fig.2-19: The section of tidal beach is background eroded (Loi, 2015) .31 Fig.2-20: The section of tidal beach is deep eroded (Loi, 2015) 32 Fig.2-21: The foreshore lowering process 33 Fig.3-22:The topography data of the study area(KHCN-BĐKH/11-15, 2015) 37 Fig.3-23:The large domain gird of the calculation area (KHCN-BĐKH/11-15, 2015) 37 iii Fig.3-24: The large domain topography of the calculation area (KHCN-BĐKH/11-15, 2015) 38 Fig.3-25:The topography data at the study area(KHCN-BĐKH/11-15, 2015) 39 Fig.3-26:The mesh of the calculation area (KHCN-BĐKH/11-15, 2015) 39 Fig.3-27: The topography of the detailed calculation area (KHCN-BĐKH/11-15, 2015) 40 Fig.3-28:The location of the water level measuring station .41 Fig.3-29:The calculation boundaries of the area 41 Fig.3-30:The calibrated result of measured water level and computed at Dinh An inlet 43 Fig.3-31: The calibrated result of measured water level and computed at Tran De inlet 43 Fig.3-32:The location of water level measuring stations 44 Fig.3-33:The verification result of measured water level and computed at My Thanh inlet 45 Fig.3-34: The verification result of measured water level and computed for Ganh Hao inlet 45 Fig.3-35:Wave height (left) and wave period (right) on the Bien Dong area of Wave Watch III model 46 Fig.3-36: Wave model calibration 47 Fig.3-37:The location of wave measuring station of Nha Mat estuary .48 Fig.3-38: Wave model verification .48 Fig.3-39: The distribution of the characteristic wave for the northeast monsoon (left), the southwest monsoon (right) 50 Fig.3-40: The location of extract points of calculation result .50 Fig.3-41:The significant wave height chart for climate one year (May 2011 to April 2012) at positions P1, P2, P3 .51 Fig.3-42: The distribution of total currents (speed and direction) when tide going up (left), going down(right) in northeast monsoon scenario 52 Fig.3-43: The distribution of total currents (speed and direction) when tide going up (left), going down (right) in southwest monsoon scenario 52 iv Fig.3-44: The distribution of currents at the time of tide when going up (left) and going down (right) - the scenario only considers to effect of tide 53 Fig.3-45:The chart of distribution of currents speed in year climate (May 2011 April 2012) at P1,P2, P3 position 53 Fig.3-46: Simulated result of the average currents distribution due to wind in northeast monsoon season (left) and southeast monsoon (right) .54 Fig.4-47: The measured topography of March 2011 (left) and January 2015 (right) (KHCN-BĐKH/11-15, 2015) .57 Fig.4-48: The calculation topography of March 2011 (left) and January 2015 (right) (KHCN-BĐKH/11-15, 2015) .58 Fig.4-49: The foreshore lowering in 2015 compared to 2011 58 Fig.4-50:The location of cross sections .59 Fig.4-51: Wave propagation and evolution of cross section in 2011 and 2015 .60 Fig.4-52: Wave propagation and evolution ofcross section in 2011 and 2015 60 Fig.4-53: Wave propagation and evolution of cross section in 2011 and 2015 .61 Fig.4-54: The effect of the foreshore lowering on sea dike 68 v LIST OF TABLES Table 1-1: The area of mangrove forest in Mekong Delta (Loi, 2015) Table 1-2:The area of mangrove forest in Mekong Delta from 1973 to 2012 Table 3-3: The parameters of the tidal model 42 Table 3-4: The coordinates of measuring stations 44 Table 3-5: The parameters of the wave model .46 Table 3-6: Simulation scenarios 49 Table 4-7:The input parameters used for Wadibe 62 Table 4-8: The parameters of mangrove forest 62 Table 4-9: Design parameters of cross section 2, without mangrove 64 Table 4-10: Design parameters of cross section with topography in 2015 (with mangrove) 65 vi ABSTRACT Rationale In recent years, the erosion of beaches and estuaries in Bac Lieu has gradually intensified Especially, foreshore lowering phenomenon has been happening at area adjacent Soc Trang province This has severely affected the safety of local inhabitants, the quality of infrastructure, the degradation of environment, and the economic development This is the critical reason that the author the research "The effect of foreshore lowering on Bac Lieu sea dike" Study objectives The aim of study is to find out the effect of mangrove foreshore lowering to Bac Lieu province In order to achieve the purpose of research that should deal with the bellow problems: - Literature review - Foreshore lowering and its impacts - Numerical modelling of the foreshore lowering - The effect of foreshore lowering on sea dike Simulation of the hydrodynamic regime, the erosion and accretion mechanism of the area Evaluating the influences, effects of the foreshore lowering on sea dike The study is applied to calculate specifically for km of sea dike in Vinh Trach Dong commune, Bac Lieu city, Bac Lieu province Study approaches and methodology Evaluation for the effect of foreshore lowering on Bac Lieu sea dike by using Mike21 model for hydrodynamic regime simulation and then design a typical cross section Methodology is shown in lower figure http://czmsoctrang.org.vn/Publications/EN/Docs/Shoreline%20Management %20Guidelines%20EN.pdf [3]Besset M, Anthony E.J,Brunier G, Dussouillez P.“Shoreline change of the Mekong River delta along the southern part of the South China Sea coast using satellite image analysis (1973-2014)” Proceedings of the 16th Young Geomorphologists Days Nantes, 29-30 January 2015 [8]DHI (2007a) Mike 21 Flow Model FM, Hydrodynamic Module, User Guide [9]DHI (2007b) Mike 21 SW, Spectral Wave FM Module, User Guides [14]Lu X X, Siew R.Y (2005) Water discharge and sediment flux changes in the Lower Mekong Rive Hydrology and Earth System Science Discussion, 2, pp 2287– 2325 [16]MARD (2012) Technical Standards in Sea Dike Design Ministry of Agriculture and Rural Development, Vietnam [17]MRC, (2010) “Assessment of Basin-wide Development Scenarios”, main report Basin Development Plan Programme, Phase [19]NOAA.(2015).Wave Watch III GriB dataset, online access.http://polar.ncep.noaa.gov/waves [33]World Bank (2004) Modelled Observations on Development Scenarios in the Lower Mekong Basin Mekong Regional Water Resources Assistance Strategy, prepared for the World Bank with MRC cooperation [34]Xue Z., He R., Liu J.P., Warner J.C (2012) “Modeling transport and deposition of the Mekong River sediment” Continental Shelf Research, 37, 66-78 74 APPENDICES APPENDICES A – CHAPTER The wave rose at position P2 in northeast monsoon (the left), the southwest monsoon (the right) The distribution of total currents at the east direction in northeast monsoon scenario 75 The currents rose at P2 point one yearly cycle of climate: the northeast monsoon (the left), southwest monsoon (the right) The currents rose at P2 point in one year: northeast monsoon (left), the southwest monsoon (right) – The scenario only considers to effect of tide APPENDICES B – CHAPTER The frequency line of synthesis water level at Ganh Hao town, Gia Rai, Bac Lieu 76 Element Zone Hsig [meter] T [s] Hsig [meter] T [s] Hsig [meter] T [s] Hsig [meter] T [s] Period (Year) 10 20 50 100 125 150 200 8.22 8.64 9.19 9.61 9.74 9.85 10.03 10.7 11 11.4 11.7 11.8 11.9 12 5.32 5.59 5.95 6.22 6.31 6.38 6.49 8.4 8.7 9.2 9.3 9.3 9.4 4.7 4.94 5.25 5.49 5.57 5.63 5.73 7.9 8.1 8.4 8.6 8.7 8.7 8.8 4.35 4.57 4.86 5.08 5.15 5.21 5.3 7.6 7.8 8.2 8.3 8.3 8.4 Calculation result of deep water parameter Exponent related to the interaction process between waves and revetment type (roughness, porosity/permeability etc.) (0,5 ≤ b ≤ 1,0); + b = 0,5: for rough and permeable revetments as riprap; + b ≈ 1: for smooth and less permeable revetments; + b ≈ 2/3: Common representative value for other systems (i.e more open blocks and block-mats, mattresses of special design etc.) Safety height increment (a) 77 Current design policy is that the protective layers for outer dike slope is designed in terms of layer thickness instead of the weight of protective elements For sea dike design, the thickness of slope protection layers can be determined by Pilarczyk’s formula (Pilarczyk, 1990) Hs cosα ≤ Ψ u Φ b ∆ m D ξp or D≥ (for cotgα > 1,5) Hs ξ pb Ψ u Φ.∆ m cosα In terms of strength – load: ∆ m D = H s ξ pb ψ u φ cosα where, Ψu - System-determined (empirical) stability upgrading factor + Ψu = 1.0 for riprap as a reference; + Ψu > for other revetment system; 78 Design wave run-up height (Rslp) is the vertical distance from Design water level to the highest point on dike slope that the waves can reach Rslp is calculated by the following formulae (TAW, 2002): Rslp / H m = 1, 75 γ β γ b γ f ξ m −1,0 Rslp / H m = γ β γ f (4,3 − or 1, ) ξ m −1,0 Rslp = 1, 75.γ β γ b γ f ξ m −1,0 H s R = γ γ (4,3 − 1, ).H β f s slp ξ m −1,0 where, Hs if 0,5 < γ bξm-1,0 < 1,8 if 1,8 < γ bξm-1,0 < ÷ 10 (0,5 < γ b ξ m−1,0 < 1.8) (1,8 < γ b ξ m −1,0 < ÷10) - Significant wave height at the dike toe (design wave height), Hs = Hm0 (m); ξm-1,0 - Surf similarity index (or Irribaren index); γβ - Reduction factor due to wave direction;; γf γb - Reduction factor due to the roughness of dike slope; - Reduction factor due to dike berms; The calculation of wave cross shore propagation 79 Wave simulation to toe of the structure with topography in 2011 Wave simulation to toe of the structure with topography in 2015 80 The calculation of wave cross shore propagation has mangrove forest Wave simulation to toe of the structure with topography in 2015 has mangrove forest Topography of cross section in 2011 and 2015 Topography of cross section in 2011 Topography of cross section in 2015 Distance Distance Distance Distance Elevation Elevation Elevation Elevation (m) (m) (m) (m) -41.37 2.13 5500.00 -4.07 -73.42 2.13 5500.00 -4.07 0.00 1.62 5600.00 -4.15 0.00 1.67 5600.00 -4.15 100.00 1.68 5700.00 -4.23 100.00 1.05 5700.00 -4.23 81 200.00 1.03 5800.00 -4.33 200.00 0.15 5800.00 -4.33 300.00 0.41 5900.00 -4.44 300.00 -0.05 5900.00 -4.44 400.00 0.16 6000.00 -4.55 400.00 -0.15 6000.00 -4.55 500.00 0.03 6100.00 -4.65 500.00 -0.22 6100.00 -4.65 600.00 -0.11 6200.00 -4.77 600.00 -0.28 6200.00 -4.77 700.00 -0.24 6300.00 -4.88 700.00 -0.33 6300.00 -4.88 800.00 -0.36 6400.00 -5.00 800.00 -0.40 6400.00 -5.00 900.00 -0.46 6500.00 -5.12 900.00 -0.48 6500.00 -5.12 1000.00 -0.52 6600.00 -5.21 1000.00 -0.52 6600.00 -5.21 1100.00 -0.52 6700.00 -5.30 1100.00 -0.52 6700.00 -5.30 1200.00 -0.49 6800.00 -5.40 1200.00 -0.49 6800.00 -5.40 1300.00 -0.47 6900.00 -5.51 1300.00 -0.47 6900.00 -5.51 1400.00 -0.47 7000.00 -5.62 1400.00 -0.47 7000.00 -5.62 1500.00 -0.60 7100.00 -5.73 1500.00 -0.60 7100.00 -5.73 1600.00 -0.85 7200.00 -5.85 1600.00 -0.85 7200.00 -5.85 1700.00 -1.25 7300.00 -5.99 1700.00 -1.25 7300.00 -5.99 1800.00 -1.62 7400.00 -6.13 1800.00 -1.62 7400.00 -6.13 1900.00 -1.94 7500.00 -6.26 1900.00 -1.94 7500.00 -6.26 2000.00 -2.24 7600.00 -6.39 2000.00 -2.24 7600.00 -6.39 2100.00 -2.51 7700.00 -6.52 2100.00 -2.51 7700.00 -6.52 2200.00 -2.67 7800.00 -6.65 2200.00 -2.67 7800.00 -6.65 2300.00 -2.74 7900.00 -6.78 2300.00 -2.74 7900.00 -6.78 2400.00 -2.82 8000.00 -6.90 2400.00 -2.82 8000.00 -6.90 2500.00 -2.92 8100.00 -7.02 2500.00 -2.92 8100.00 -7.02 2600.00 -3.02 8200.00 -7.14 2600.00 -3.02 8200.00 -7.14 2700.00 -3.09 8300.00 -7.24 2700.00 -3.09 8300.00 -7.24 2800.00 -3.11 8400.00 -7.33 2800.00 -3.11 8400.00 -7.33 2900.00 -3.14 8500.00 -7.42 2900.00 -3.14 8500.00 -7.42 3000.00 -3.17 8600.00 -7.52 3000.00 -3.17 8600.00 -7.52 3100.00 -3.20 8700.00 -7.60 3100.00 -3.20 8700.00 -7.60 82 3200.00 -3.22 8800.00 -7.68 3200.00 -3.22 8800.00 -7.68 3300.00 -3.25 8900.00 -7.76 3300.00 -3.25 8900.00 -7.76 3400.00 -3.27 9000.00 -7.83 3400.00 -3.27 9000.00 -7.83 3500.00 -3.29 9100.00 -7.89 3500.00 -3.29 9100.00 -7.89 3600.00 -3.32 9200.00 -7.94 3600.00 -3.32 9200.00 -7.94 3700.00 -3.34 9300.00 -8.01 3700.00 -3.34 9300.00 -8.01 3800.00 -3.37 9400.00 -8.08 3800.00 -3.37 9400.00 -8.08 3900.00 -3.39 9500.00 -8.15 3900.00 -3.39 9500.00 -8.15 4000.00 -3.41 9600.00 -8.21 4000.00 -3.41 9600.00 -8.21 4100.00 -3.43 9700.00 -8.27 4100.00 -3.43 9700.00 -8.27 4200.00 -3.45 9800.00 -8.33 4200.00 -3.45 9800.00 -8.33 4300.00 -3.48 9900.00 -8.37 4300.00 -3.48 9900.00 -8.37 4400.00 -3.51 10522.01 -9.00 4400.00 -3.51 10263.25 -9.00 4500.00 -3.54 11810.69 -10.00 4500.00 -3.54 11315.32 -10.00 4600.00 -3.57 13115.98 -11.00 4600.00 -3.57 12390.40 -11.00 4700.00 -3.61 14418.28 -12.00 4700.00 -3.61 13477.26 -12.00 4800.00 -3.64 15714.69 -13.00 4800.00 -3.64 14564.69 -13.00 4900.00 -3.68 17006.09 -14.00 4900.00 -3.68 15649.52 -14.00 5000.00 -3.72 18295.61 -15.00 5000.00 -3.72 16731.62 -15.00 5100.00 -3.77 19584.00 -16.00 5100.00 -3.77 17811.30 -16.00 4.4.1.Sea dike design without mangrove forest (i) Design wave parameters According to formula of Nguyen Xuan Hung (1999), the relation between wave height and wave period of sea region 62 (southern), ( statistics showed that for T Tm = 4.634×Hs0.914 = 4.634×Hs0.914 = 1.96 (s) 83 Tp = 1.2×Tm = 1.2× 1.91 = 2.35(s) Typical spectrum periodTm-1,0 = Tp 1.1 = 2.13 (s) Wave length with Tm-1,0: Lm-1,0 = 1.56×T2m-1,0 = 1.56×2.08×2.08 = 7.12(m) Hs Wave slope: Sm-1,0 = L = 0.054 m −1,0 +)Hs of topography in 2015 H = 3.64×10-4T5.164 =>Tm = 4.634×Hs0.914 = 4.634×Hs0.914 =2.36 (s) Tp = 1.2×Tm = 1.2×2.28 = 2.84 (s) Typical spectrum period Tm-1,0 = Tp 1.1 = 2.58 (s) Wave length with Tm-1,0: Lm-1,0 = 1.56×T2m-1,0 = 1.56×2.48×2.48 = 10.42 (m) Hs Wave slope:Sm-1,0 = L = 0.046 m −1,0 (ii) Calculating elevation of crest dike Based on geological conditions Bac Lieu region, design calculations for a representative cross-section in the case: sea dike without berm or sea wall and calculation following wave run – up standards: Slope angle factor: m = 3⇒tanα = 1/m = 1/3 α - Angle between the dike slope and horizontal line Breaker index (iribarren) with topography in 2011:ξm-1,0(2011)= tan α = 1.41 S m −1,0 tan α Breaker index (iribarren) with topography in 2015: ξm-1,0 (2015)= S = 1.52 m −1,0 The calculation of reduction factors - The sea dike without berm : Reduction factor for berms ɣb = - Incedent wave angle perpendicular with shoreline: γ β = – 0,0022×|β| = - Reduction factor for the slop roughness: ɣf = 0.90 Height of wave run - up calculation with topography in 2011 84 ɣb×ξm-1,0 =1×2.11 = 1.41< 1.8 => breaking wave Height of wave run -up in the case of wave breaking was calculated following formula: Rslp = 1.75γ βɣbɣf ξm-1,0Hs = 1.30 (m) Height of wave run - up calculation with topography in 2015 ɣb× ξm-1,0 = 1×1.53 = 1.53 breaking wave Rslp = 1.75γ βɣbɣfξm-1,0Hs = 1.76 (m) Crest dike level Crest level of incline roof type of none overflowing dike was calculated following height of wave run -up Zđđ = Ztk,p + Rslp + a Where: Zđđ - Design crest dike level Ztk,p - Design water level: which is the sea water level corresponding to the design frequency (combination of tidal water level frequency and storm surge frequency) Rslp - Crest freeboard above design water level, calculated with design wave run-up (m) a- Safety height increment (m) The research area is Bac Lieu area which has sea dike system with grade level of 4th That is equivalent to the design frequency is 5% According to the sea dike design standards, the safety height increment a = 0.3 m => Crest dike elevation with topography in 2011: Zđđ = 2.13+1.3+0.3 = 3.73 (m) => Crest dike elevation with topography in 2015: Zđđ = 2.13+1.76 +0.3 = 4.19 (m) (iii) Design of dike cross section - calculation of roof safety armour : Selecting safety armour is concrete blocks which can withstand the impacts of large wave and its construction is simple, easily installable Thickness of armour layer calculated by Pilarczyk formula (1990) 85 Hs cos α HS ≤ ψ uφ b => D ≥ ξ pb ∆m D ξp ψ uφ∆ m cos α ξp = tan α tan α = S0 Hs L0 - In which: ψ u : System – determined stability upgrading; choose ψ u = 1.5 φ : Function representing the material limit of movement/ stablemess; choose φ = 2.25 (Average value for incipient motion) Hs : Significant wave height [m] L0 : wave length [m] ξ p : Iribarren index with wave period Tp D: Specific size or thickness of protection unit [m] ∆ m : Relative density of a system –unit ρ 2,5 ∆ m = bt − = − =1,44 ρn 1, 025 + ρbt: Density of concrete (ρbt= 2,5 t/m3) + ρn: Density of water (ρn= 1,025 t/m3) b: Exponent related to the interaction process between waves and revetment type 0.5 Crest dike elevation with topography in 2015: Zđđ = 2.13+1.04 +0.3 = 3.47 (m) (iii) Design of dike cross section - calculation of roof safety armour : 87 Selecting safety armour is concrete blocks which can withstand the impacts of large wave and its construction is simple, easily installable Thickness of armour layer calculated by Pilarczyk formula (1990) Hs cos α HS ≤ ψ uφ b => D ≥ ξ pb ∆m D ξp ψ uφ∆ m cos α ξp = tan α tan α = S0 Hs L0 - In which: ψ u : System – determined stability upgrading; choose ψ u = 1.5 φ : Function representing the material limit of movement/ stablemess; choose φ = 2.25 (Average value for incipient motion) Hs : Significant wave height [m] L0 : wave length [m] ξ p : Iribarren index with wave period Tp D: Specific size or thickness of protection unit [m] ∆ m : Relative density of a system –unit ρ 2,5 ∆ m = bt − = − =1,44 ρn 1, 025 + ρbt: Density of concrete (ρbt= 2,5 t/m3) + ρn: Density of water (ρn= 1,025 t/m3) b: Exponent related to the interaction process between waves and revetment type 0.5