1 MINISTRY OF EDUCATION AND TRAINING MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT THUYLOI UNIVERSITY PHAN DINH TUAN PROPOSING THE UPDATED CROSS SECTION AND WAVE OVERTOPPING CALCULATION FOR SEA DIKES[.]
MINISTRY OF EDUCATION AND TRAINING MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT THUYLOI UNIVERSITY PHAN DINH TUAN PROPOSING THE UPDATED CROSS-SECTION AND WAVE OVERTOPPING CALCULATION FOR SEA DIKES WITH HOLLOW QUARTER-CYLINDRICAL STRUCTURE ON THE CREST Major: Coastal Engineering Major code: 58 02 03 SUMMARY OF DOCTORAL DISSERTATION HANOI, 2022 This scientific work has been accomplished at Thuyloi University Scientific supervisor: Prof Tran Dinh Hoa Assoc Prof Nguyen Ba Quy Reviewer No.1: Prof Dr Thieu Quang Tuan - Thuyloi University Reviewer No.2: As.Prof Dr Phung Dang Hieu – Vietnam Institute of Seas and Islands – Ministry of Natural Resources and Environment Reviewer No.3: As.Prof Dr Pham Hien Hau -National University of Civil Engineering This Doctoral Dissertation will be defended at the meeting of the University Doctoral Committee at at ………………… on 8.30 Am, 24 December 2022 The dissertation can be found at: - The National Library of Vietnam; - The Library of Thuy loi University INTRODUCTION Rationale of the study Vietnam has a very large sea dike system, stretching from North to South, making an important contribution to protecting people's lives and property, and serving production and development of the country In recent years, climate change has become increasingly complicated and unpredictable, which has had a great impact on life and production The problem of coastal erosion has become more and more complicated, especially in the Mekong River Delta There have been many research projects proposing various solutions to enhance the stability of sea dikes In which, coastal protection works to reduce wave impacts and waves overtopping sea dikes have been widely studied and commonly applied Under the soft geological conditions in the Mekong River Delta, the increase of crest elevation necessitate a compact cross section, low self-load to limit ground subsidence The current common solution is to adopt a crest wall, which initially shows the efficiency in reducing wave overtopping by means of increasing crest elevation and decreasing the embankment height instead of enlarging the entire dike cross section Still, there have been numerous limitations due to the low wall height resulting in a large cross section and also the subsequent ground subsidence Besides, the crest wall generates high wave reflection with the coefficient Kr=0.7÷1, resulting in direct high pressure on the structure, and simultaneous erosion and the instability of the wall foot Based on the basis of analysis and assessment of the existing solutions and in order to meet the requirements of the safe crest level when allowing wave overtopping and reducing the cross-sectional load in the design of sea dikes, the author has proposed a new cross section for sea dikes with hollow structure (see Figure 1), overcoming the limitations of cross-sectional loads, wave reflection In recent years, hollow structures have been widely used for coastal protection works, especially detached breakwaters Hollow structures come in many different shapes, but they all have a common characteristic, which is a perforated seaward surface with design void ratio and a hollow chamber in the middle The research results have shown significant advantages such as: prefabricated concrete blocks, which are convenient in construction; effectively reduce wave transmission and reflection This is a solution with a new structural layout, which is suitable for coastal protection works in the Mekong River Delta Figure 1: Sea dike cross section including hollow structures When applying the hollow quarter-cylindrical structure to the sea dike crosssection, there are difficulty in wave reduction mechanism At the same time, the formula to determine the dike height according to the crest freeboard to ensure the design wave overtopping is not completed Therefore, the research topic "Proposing an updated cross section and corresponding wave overtopping calculation for sea dikes with the hollow quarter-cylindrical structure on the crest" has been chosen for this dissertation Research objectives - Proposing an updated cross-section of sea dikes with the hollow quartercylindrical structure on the crest - Deriving an empirical formula to calculate the average overtopping discharge for the sea dike cross-section with the hollow quarter-cylindrical structure on the crest Subject and scope of the study 3.1 Subject of the study Sea dike cross-section with the hollow quarter-cylindrical structure on the crest and the corresponding wave overtopping 3.2 Scope of the study To propose the sea dike cross-section with the hollow quarter-cylindrical structure on the crest; to study the corresponding wave overtopping on the basis of current natural conditions in Mekong River Delta Research methodology Analyzing and assessing the existing sea dike systems, thereby proposing an updated sea dike cross section with the hollow quarter-cylindrical structure on the crest, which is suitable for Mekong River Delta On the basis of theoretical research combined with physical modeling experiments, an empirical formula for calculating wave overtopping in case of sea dike cross section with the hollow quarter-cylindrical structure on the crest has been proposed The obtained results were then applied to the design of an actual project Scientific and practical meaning 5.1 Scientific meaning The research has proposed an updated cross section for sea dikes and an empirical formula to determine the corresponding wave overtopping It has contributed to supplement and enrich the existing research results on sea dikes in general and wave overtopping in particular At the same time, it is the basis for the next studies on other unresolved issues for sea dikes 5.2 Practical meaning The updated sea dike cross section with the hollow quarter-cylindrical structure on the crest and the empirical formula to calculate the corresponding wave overtopping will make an important contribution to the analysis, selection and more effective design of sea dikes applied in practice Outline of the dissertation In addition to the introduction, conclusions and recommendations, the dissertation consists of chapters as follows: CHAPTER 1: Overview of studies on waves overtopping sea dikes; CHAPTER 2: Theoretical bases and research data; CHAPTER 3: Proposing an updated cross section of sea dike with the hollow quarter-cylindrical structure on the crest and deriving an emperical formula for calculating the corresponding wave overtopping; CHAPTER 4: Applying the research results for Nha Mat sea dike in Bac Lieu province CHAPTER OVERVIEW OF STUDIES ON WAVE OVERTOPPING AND APPLICATION OF HOLLOW STRUCTURES TO COASTAL PROTECTION WORKS 1.1 Overview of studies on waves overtopping sea dikes When carrying out the studies on sea dikes with inclined slopes, Saville (1955) was the first to lay the groundwork for the research on wave overtopping with a series of experiments for regular waves [2] Thereafter, Owen (1980) conducted the experiments on physical models with 500 scenarios for random waves, and proposed an empirical formula to determine the average overtopping discharge in case of smooth dike slope as follows [4] Rc q = a.exp −b T gH gH sTm m s (1.1) where, Tm is the mean wave period (s), Hs is the significant wave height (m) and Rc is crest freeboard (m) Owen (1980) mainly used a simple smooth sea dike model with seaward berm to conduct a few experiments The derived empirical coefficients a and b were determined for different slopes of the dike Owen (1980) also considered the influence of the slope surface roughness on wave overtopping by means of the reduction factor γr Owen (1980) then based on the additional experiments to re-verify the abovedmentioned coefficients for oblique incident waves De Waal and Van der Meer (1992) continued to study the waves overtopping smooth impermeable dikes, taking the deficiency of the crest freeboard (Ru2% - Rc)/HS into consideration when calculating the average overtopping discharges, where Ru2% is the height of the 2% wave run-up (corresponding to 2% of the waves exceeding this level on the non-overtopping dike slope) It can be seen that the applicable range of the formulae proposed by De Waal and Van der Meer (1992) has numermous limitations, such as: not taking into account the influence of the slope surface roughness, the influence of the berm and especially wave overtopping discharges are calculated by means of wave run-up Ru2% Therefore, Van deer Meer (1993) later improved the above-mentioned formula by relating the overtopping discharges directly to the relative crest freeboard Rc/Hs and using the results of Owen’s studies (1980) In addition, Van deer Meer (1993) added the influence the characteristic interaction between incoming waves and the structure to assess the wave overtopping TAW (2002) and EurOtop (2007) presents a fairly complete set of formulae for calculating wave overtopping applied to sea dikes, with a wide applicable range for a variety of sea dike geometries and taking into account various influence factors on wave overtopping Currently, these formulae has been widely used For vertical walls, Doorslaer et al (2015) proposed the influence factor for the wall height, the front base and the wave-return structures on wave overtopping in case of vertical walls with steep front face Up to now, in the world, there have been a number of studies on the influence of crest walls on waves overtopping sea dike These studies mainly focus on the relationship between the wall height (W), the crest freeboard above the dike surface (or the freeboard above the wall top Rc) and the front base in front of the wall (S) and the influence factors γw, γs, γv On the other hand, the studies have not analyzed the simultaneous influence between factors such as the front base of the wall and the wave-return structure Based on the general formula, Franco et al (1994) determined the parameters a = 0.2 and b = 4.3 for deep water, while Allsop et al (1995) proposed the coefficients a = 0.05 and b = 2.78 in shallow water conditions Both formulae have been applied and compared with the same data set from CLASS project; the results shown by the theoretical lines are consistent with the formulae in the study range Franco et al (1994) obtained the convergent results in deep water; in shallow water, the results are derived correctly by means of the method proposed by Allsop et al (1995) 1.2 Overview of hollow structures applied to coastal protection works The idea of hollow structures was proposed by Jarlan in 1961 Since 1969 in Japan, a number of coastal protection works have been built with the application of this structure In addition to the dissipation of wave energy, the waveabsorbing chambers (BTS) in front of the caissons are also effectively used for fish farming and power plants taking advantage of wave energy In recent years, hollow structures with surface voids and wave-absorbing chambers have been increasingly studied and applied in offshore wave reduction works in Mekong River Delta In 2017, Institute for Hydraulic Construction, affiliated to Vietnam Academy for Water Resources, applied a hollow semicylindrical structure, rows of holes, each row of holes in contact with incident waves, rows of holes, each row of holes, the hollow cylindrical side facing landwards The diameter of each hole is 30 cm, with capacity of reducing waves, environmentally-friendly effects, and resulting in considerable accretion Current studies have been focusing on the solution of offshore hollow structures, which allow wave overtopping and are evaluated by wave transmission and reflection parameters Therefore, the current research results help to show the effective reduction of the wave reflection and recommend the surface void ratios as well as confirm the limitation and the necessity of studies on wave overtopping calculation for hollow structures Dhinakaran et al studied the detached wave dissipating hollow structures from 2009 to 2012 by means of physical models in wave flume The analysis results shown that the optimal value of void ratio in terms of wave reflection and transmission is 11% Taking the influence of foreshore water depth into consideration, Dhinakaran et al recommended that the model height should be 1.25 times higher than the water depth, the rubble mound layer should be 0.29 times as high as the physical model Some judgment on the magnitude of the effects can be obtained from one of the few studies (see Franco and Franco, 1999) for caisson breakwaters under nonimpulsive wave conditions Studies have been carried out for structures with round or rectangular holes with surface void ratio of 20% The effect of air venting was also taken into consideration Nguyen Trung Anh (2007) conducted experiments and studies on the caisson structure with wave-absorbing chambers and surface voids, and thereby evaluated the ability to reduce wave reflection with types of surface void (horizontal slit, vertical slit and round hole) and void ratios of 15%, 20%, 30% Structures with wave-absorbing chambers (BTS) have the best efficiency if B/L is in the range of 0.1÷0.27 for all types of surface voids The value B/L = 0.1 was recommended when designing the structure width The void ratios of 20% and 30% are better than that of 15%, but no recommendation was made for the selection of the design value In terms of surface voids, the results shown that round holes are better than horizontal and vertical slits 1.3 Current status of existing sea dike systems in Mekong River Delta Currently, two types of sea dike cross-sections have been being applied in Mekong River Delta: plain sloping dikes and sloping dikes with crest walls Both cross sections have advantages in reducing wave overtopping and partly meet the task of the structure However, there are still some problems as follows: with the sloping sea dike used to be built in the past construction of the dike crest elevation is relatively low Up to now, under the impact of climate change and the subsequent rise of sea level, the crest elevation has no longer met the task In addition, it will be difficult to increase the crest level by means of supplementary embankment due to weak subsoil For the sloping dike cross section with crest walls, the greatest disadvantage is the wave reflection The reflected wave at the foreshore of the dike will cause erosion of the seaward slopes and toes In addition, when the sea dike has a low crest wall, the overtopping waves interact with the dike slope due to the large wave runup energy, so when the waves hit the crest wall, high wave splashes will be generated, accompanied by storm winds from the sea with relatively high velocity; the water mass from splashing waves will directly hit the dike surface with a large kinetic energy, damaging the dike surface and leading to dike failure Figure 1.25: Flood tides in combination with high waves, strong winds resulted in waves overtopping the West Sea dike system on August 3, 2019 1.4 Conclusions for Chapter In Chapter 1, the overview of the issues related to the field of research on “waves overtopping sea dikes cross-section with hollow structures” is presented Thereby, the author has generalized and made certain conclusions on the research field as follows: The current studies of waves overtopping sea dikes are usually divided into two main types, which are sloping dikes and sea walls The studies on sea dike cross section including hollow structures at the crest are still limited and not mentioned in the world and in Vietnam Hollow structures are usually large precast concrete blocks and commonly applied to detached wave-dissipating works with large overtopping Studies on wave overtopping in case of hollow structures included in the cross-sections of sea dikes and sea walls are still very limited Analyzing the disadvantages and limitations of various cross sections and structures of current sea dikes and the governing wave parameters mainly related to dike failure such as wave overtopping, wave reflection, etc Thereby, a suitable cross section of sea dike and governing parameters were proposed for the next steps of the research; Therefore, the study of the updated cross section of sea dikes including TSD structure and the corresponding wave overtopping is a new research orientation that needs to be carried out The research is conducted in order to propose a new structure for sea dike cross section, which is suitable for soft soil areas Simultaneously, theoretical studies and experiments were carried out by means of physical models in wave flume to establish an emperical formula for calculating the corresponding wave overtopping where, q is the average overtopping discharge; Hm0 is the spectral wave height, d is the submerging level at the wall toe; h is the water depth at the foreshore; Rc is the crest freeboard above design water level; sm-1,0 = H m0 1.56Tm2 −1,0 is the wave steepness 2.3 Theoretical bases for physical model experiments 2.3.1 Law of similitude and scale factor To maintain the basic similarity of wave parameters, the model needs to be formalized; the model scale also needs to satisfy Froude's criterion F=V/(gL)0.5 (V is wave velocity; L is void diameter) The choice of NV = Nt = (NL)0.5 by means of dimensional analysis and Buckingham's law helps the model to ensure the Froude similarity index, by which Fm = Fn (m: model; n: prototype) 2.3.2 Dimensional analysis and determination of the general equations By means of the equivalent transformations included in the PI-BUCKINGHAM method, the general equation of the average discharge overtopping the sea dike section with the hollow quarter-cylindrical structure on the crest can be determined as follows: q g.H3m0 R d H = f c , , ε, m0 H m0 h h.sm−1,0 (2.22) It can be seen that the overtopping discharge is a function of the relative crest freeboard R c H m0 , relative submergence d h , relative wave steepness H m0 h.sm −1,0 and the surface void ratio ε 2.4 Bases for the selection of parameters and experimental scenarios Based on the synthesis of current sea dike data, wave conditions, water level in the study area, the crest elevation for experiments was chosen as +3.5m, with the foreshore elevation ranges from -1.5 to 0m and design high water level from +0.7 to +2.6m, in combination with various water depths: 1.5 m, 2.5m, 3m, 3.5m and 4m The wave height ranges from to 1.5m corresponding to the wave period determined by the formula proposed by Thieu Quang Tuan and Dang Thi Linh (2015) and SPM (1984) At the same time, taking the wave generation capacity of the laboratory into consideration, the prototype wave periods were selected as 4.1s; 5.5s; and 6.6s 2.5 Model setup and experimental layout and scenarios The model scale was determined as 1/10 according to Froude’s criterion, 11 ensuring the suitable roughness and dimensions The wave gauge system was arranged in order to measure the wave reflection using three wave probes (Mansard and Funke, 1980) estimating the incident and reflected waves based on the least squares technique Experimental scenarios were established based on the research objective to evaluate the effects of various wave parameters and water levels together with surface voids () on the wave overtopping in case of the hollow quartercylindrical structure (TSD) There were 79 scenarios by means of combining various boundary and cross-sectional conditions Figure 2.15: Experimental setup with wave gauges and data acquisition system 2.6 Conclusions for Chapter Chapter presented the basic parameters that need to be taken into account when studying the wave overtopping in case of sea dike cross sections At the same time, it focuses to the two categories of cross sections and the method of calculating wave overtopping, which serves as the bases for assessment of research on the proposed sea dike cross section The results of synthesis and theoretical analysis have built up the experimental input data set and the general equation of the average overtopping discharge in case of sea dike cross section with the hollow quarter-cylindrical structure on the crest With various methods of wave overtopping calculation, the current sea dikes are divided into types of cross sections: sea walls and sloping sea dikes Based on the results of dimensional analysis, it can be seen that the similarity of parameters governing the wave overtopping and also the functional characteristics and mechanism between the design cross section with that of sea walls Both types of cross sections include a large concrete block in terms of height and width, which directly interacts with the impact waves, unlike the dike crest wall (low height), which only interacts with wave runup and is not directly affected by incident waves 12 CHAPER PROPOSING THE UPDATED SEA DIKE CROSS SECTION WITH THE HOLLOW QUARTER-CYLINDRICAL STRUCTURE ON THE CREST AND ITS EFFECTS ON WAVE OVERTOPPING BY MEANS OF PHYSICAL MODEL IN WAVE FLUME 3.1 Bases for proposing the updated sea dike cross section with the hollow quarter-cylindrical structure on the crest Based on the evaluation results of the wave overtopping for the three cross sections, it can be seen that the hollow quarter-cylindrical structure (TSD) has certain advantages in terms of wave overtopping reduction The corresponding overtopping discharge is smaller than that of the plain sloping dike Depending on various voids and crest freeboards, the overtopping discharges in case of TSD structure were equal to or smaller than that of the sloping sea dikes in combination with crest walls Figure 3.8: Waves overtopping the cross sections with the same experimental boundary conditions 13 In terms of capacity to reduce wave reflection of perforated hollow structures as analyzed in the overview of research results of similar hollow structures applied to coastal protection works, the reflection coefficient Kr ranges between 0.25 and 0.65, which is smaller to that of the monolithic vertical sea walls (Kr = 0.7 - 1) Figure 3.9: Reflection coefficient Kr and relative crest freeboard Rc/Hm0 From the above-mentioned limitations and disadvantages in terms of wave overtopping and wave reflection of the two types of sea dike cross-sections, an updated cross section including the hollow quarter-cylindrical structure on the crest was proposed as shown 3.2 Evaluation of wave overtopping in case of sea dike cross-section with the hollow quarter-cylindrical structure on the crest and the method of calculating the wave overtopping for composite vertical wall cross-section 14 Figure 3.11: Wave overtopping in case of sea dike cross section with TSD structure related to composite vertical wall The comparison results show that many experimental points tend to be larger, locating outside the theoretical curve corresponding to the composite vertical wall, because the wavefront of the hollow quarter-cylindrical structure (TSD) creates more momentum for wave run-up and overtoppinging in some cases Various surface void ratios will result in different wave overtopping discharges, thus it is necessary to take into account the influence of the concave surface with design void ratio in the calculation of wave overtopping in case of the hollow quarter-cylindrical structure (TSD) is placed on the dike crest 3.3 Assessing the influence of the governing parameters The relationship between relative crest freeboard and wave overtopping discharge in case of the hollow quarter-cylindrical structure (TSD) is shown as follows: With the same wave height, the smaller the crest freeboard, the larger the overtopping discharge On the other hand, with the same crest freeboard, the higher the wave, the higher overtopping discharge, and the varying trend becomes more rapid when Rc/Hm0 ≤ 1, which is represented by a steep line in the diagram It can be seen that the crest freeboard is one of the important factors affecting the overtopping discharge The change of water level corresponds to the crest freeboard, the overtopping discharge increases rapidly as the water level rises In the case of low water level 15 d/h=0 (the structure is completely on the water surface), the capacity to reduce waves is effective The results in Figure 3.6 show that the correlation line has a steep slope when d/h > 0.35, in the design calculation, the above value can be considered as the value to be calculated as a reference Figure 3.12: The correlation between the relative crest freeboard RC/Hmo and overtopping discharge Figure 3.13: The correlation between the relative water depth d/h and overtopping discharge 16 Correlation of surface void ratio and overtopping discharge: in case of no perforation on the curvature of the hollow quarter-cylindrical structure (TSD) ( = 0), the overtopping discharge is largest, and in the case of complete perforation ( = 1), the structure will now be a plain vertical wall and the calculation method for composite vertical wall will be used With the limitations of the experiments, the two upper limits cannot be generalized precisely, because there are no experiments with cases = and = Therefore, when considering the void ratio, the parameter (1-) was selected to evaluate the inverse correlation between the surface void ratio ( = 10 % ÷ 20%) and the hollow quarter-cylindrical structure (TSD) and overtopping volume; 3.4 Deriving the empirical formulae Based on the dimensional analysis and evaluation of the method of calculating the overtopping discharge for the composite vertical wall, the general equation to determine the overtopping discharge in case of the proposed section is as follows: 0.5 q g.H3m0 0.5 R d H m0 = a exp b c h h.Sm−1,0 H m0 − ε (3.6) The results of determining the mean coefficients of the average line a=0.0136, b = -1.804, equation (3.1) can be rewritten as follows: 0.5 q gH m3 0.5 R d H = 0.0138 m exp −1.814 c H m0 − h h sm−1,0 (3.7) The confidence bands of equation (3.7) are given by σ(0.0138) = 0.0025 and σ(1.814) = 0.171 For the design or assessment approach, it is necessary to increase the average discharge by about one standard deviation Thus, equation (3.8) can be used in design and safety assessments with the efficiency and dispersion related to this prediction to be evaluated according to Figure 3.17 0.5 q gH m3 0.5 R d H = 0.0163 m exp −1.644 c H m0 − h h sm−1,0 17 (3.8) Figure 3.17: The regression line of the empirical function determining the average overtopping discharge in case of sea dike cross-section with the hollow quarter-cylindrical structure (TSD) The experimental formula is applicable to the experimental conditions such as the surface void ratio of the hollow quarter-cylindrical structure (TSD) of =10%÷20%, relative crest freeboard Rc/Hm0 of 0.7÷2.57; relative submergence d/h of 0÷0.45 3.5 Conclusions for Chapter Chapter provides quantitative bases for the advantages of reducing wave overtopping and wave reflection of the hollow quarter-cylindrical structure on the dike crest Thereby, the sea dike cross-section with the hollow quartercylindrical structure on the crest, which is suitable for the conditions of the Mekong River Delta, was proposed Simultaneously, the experimental formula for calculating the corresponding wave overtopping was determined by means of the regression analyses based on 60 experimental results 18 ... cross-section with the hollow quartercylindrical structure on the crest, which is suitable for the conditions of the Mekong River Delta, was proposed Simultaneously, the experimental formula for calculating... horizontal and vertical slits 1.3 Current status of existing sea dike systems in Mekong River Delta Currently, two types of sea dike cross-sections have been being applied in Mekong River Delta:... analysis have built up the experimental input data set and the general equation of the average overtopping discharge in case of sea dike cross section with the hollow quarter-cylindrical structure