WAVE OVERTOPPING AT SEA DIKES WITH CROWNN WALLS IN THE NORTHERN COASTAL DELTA OF VIETNAM

MINISTRY OF EDUCATION AND TRAINING MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT WATER RESOURCES UNIVERSITY NGUYEN VAN THIN WAVE OVERTOPPING AT SEA DIKES WITH CROWNN-WALLS IN THE NORTHERN COASTAL DELTA OF VIETNAM Specialization: Hydraulic Engineering Code No : 62-58-40-01 SUMMARY OF DOCTORAL DISSERTATION HA NOI, 2014 This scientific work has been accomplished at Water Resources University Advisor 1: Assoc. Prof. Nguyen Ba Quy Advisor 2: Prof. Ngo Tri Vieng Reviewer No. 1 Prof. Dinh Van Uu Reviewer No. 2 Prof. Tran Dinh Hoi Reviewer No. 3 Prof. Tran Dinh Hoa This Doctoral Thesis will be defended at the meeting of the University Doctoral Committee in room No………… on ……. This dissertation is available at: - The National Library - The Library of Water Resources University 1 INTRODUCTION 1. Rationale Viet Nam is a nation that is seriously affected by climate change and sea level raise. There are many centres of economy and culture along the coastline. In the north, sea dikes are relatively low with narrow crests, steep seaward and landward slopes; most dikes are directly exposed to wave attack. Historical records show that wave overtopping often causes damage to dike crests and landward slopes. One of the effective measures to reduce overtopping is the use of (low) crown-walls, because heightening dikes or constructing outer berms are costly and not feasible, especially dikes with narrow margins. Till now, studies into the influence of low crown-walls and interaction between wave-wall and overtopping are limited. A better understanding of the influence of crown-walls and promenade on overtopping is necessary to improve the present dike design guidelines in Viet Nam. These facts lend the foundation for this thesis that is to investigate “Wave Overtopping at sea dikes with crown-walls in the northern coastal delta of Vietnam”. 2. Research objectives The thesis aims to investigate the influence of low crown-walls on overtopping discharge and the behaviour of overtopping flow at sea dikes. By doing so, the reliability of overtopping estimation is increased to improve the dike design guidelines currently applied in Viet Nam. 3. Scope of the study Investigation of wave overtopping at sea dikes with low crown-walls in the north of Viet Nam. 2 4. Research contents Review on overtopping on sea dikes with low crown-walls; Physical model to investigate the effects of low crown-walls on wave overtopping at sea dikes; Numerical and physical examination of the wave-wall interaction and overtopping flow on sea dikes with low crown-walls; Case study-Overtopping at Giao Thuy dike, Nam Dinh province 5. Approach and study methods 5.1. Approach To obtain the objectives, the author carried out a literature study on wave overtopping at sea dikes with low crown-walls to select an approach that is inherent and also creative, suitable for Viet Nam. 5.2. Study approaches Literature review; Physical and numerical modelling; Application. 6. Implications Building low crown-walls on seadikes to heighten the crest level, reduce wave overtopping is considered technically and economically viable in Viet Nam. There exists limited research into overtopping at sea dikes with low crown-walls, especially the interaction between wave and wall. Therefore, better understanding of the wall influence on wave overtopping discharge and the overtopping flow characteristics will help improve the reliability of design for this type of sea-dikes in Vietnam. 3 7. New contributions - Insights into the influence of low crown-walls on wave overtopping and the merit of wall promenade through a detailed examination of the wave- wall interaction; - Empirical equations to determine the overall influence factor of low crown-walls for regular waves (Figure 2-12); - Relationship between splash height and wave parameters and wall geometry (Figure 2.13); - Proposal on the use of a new sea-dike cross-section with crown-walls and promenade, appropriate for the northern coastal detla of Viet Nam (Figure 4.8). 8. Thesis contents In addition to the Introduction, Conclusions and Recommendations, the thesis consists of 04 chapters Chapter 1: Literature study on overtopping on sea dikes with low crown-walls; Chapter 2: Physical model to investigate the effects of low crown-walls on wave overtopping at sea dikes; Chapter 3: Wave-wall interaction and overtopping flow on sea dikes with low crown-walls; Chapter 4: Case study-Overtopping at Giao Thuy dike, Nam Dinh province. CHAPTER 1 REVIEW ON OVERTOPPING ON SEA DIKES WITH LOW CROWN-WALLS 1.1 Introduction to research into overtopping at sea dikes Due to significant changes of climate and environment, the frequency and intensity of natural hazards gradually increase especially storms, tide and sea level rise. As a result, overtopping at sea dikes remains as a risk to countries with 4 sea. There exists limited research on overtopping at sea dikes with low crown- walls and no one is comprehensive yet. Therefore, studies on overtopping at sea dikes are essential in Viet Nam and elsewhere 1.2 Causes, failure mechanisms of sea dikes and measures 1.2.1 Causes of damage to sea dikes There are many failure mechanisms of sea dikes but historical records show that wave overtopping mainly causes damage to the dike crest and landward slope and dike breaching as a consequence. 1.2.2 Damage mechanism due to wave overtopping There are many mechanisms leading to dike failure, from local damage to overall collapse; the reasons, influence factors, consequences are very various. Analysis indicates that dike failures due to wave overtopping are the most common. 1.2.3 Measures to reduce overtopping at sea dikes in the north Nowadays, there are several ways to reduce overtopping at sea dikes, ‘hard’ and ‘soft’ solutions such as submerged breakwaters, concrete blocks, seaward berms, high crests and mangrove … However, the conditions of construction space and economy are limited low crest-walls are popular and effective in reducing overtopping at sea dikes in Viet Nam. 1.3 Sea dikes with low crown-walls in the northern coastal delta Sea dikes with low crown-walls (W/Hs ≤ 0.5) located near the seaward edge are very popular in Viet Nam. This is considered as a simple and effective method to increase the crest level, reduce overtopping in the present situation. Crown- walls are applied where there is no more land to enlarge the dike cross-sections or budget is constrained; or it is not allowed to heighten the crest level in order to reserve residence and tourism areas. 5 1.4 Research into overtopping at sea dikes with low crown-walls 1.4.1 TAW 2002 In TAW (2002), the influence of crown-walls on overtopping discharge is not clear because it is an unknown variable; crown-walls increases the equivalent slope that the discharge becomes greater. However, the discharge is then corrected by a factor v . This overall influence factor only takes into account the seaward inclination but not the interaction between wave-wall, overtopping flow and the wall dimensions. 1.4.2 Viet Nam Till now, there is limited research into overtopping at sea dikes with low crown- walls in Viet Nam. Recent works do not cover all aspects of overtopping at Vietnamese dikes. Tuan et al. (2009) proposed a new method to assess the effect of crow-walls on overtopping. However, the influence of promenade was not discussed (S = 0). Tuan (2013) investigated the influence of crown-walls and promenade on overtopping. Though, he did not consider the interaction between wave-wall and the behaviour of overtopping flow when the wall exists. Furthermore, reduction effect was not determined for regular waves. 1.5 Conclusions of chapter 1 Overtopping is a danger to sea dikes. Damage due to overtopping is the most important. Crown-walls are effective to heighten the crest level and reduce overtopping. Studies on overtopping from TAW (2002) to Tuan (2013) are not complete. In line with Tuan (2013), the author performed tests with physical model to investigate the influence of crown-walls on regular waves, especially the interaction between wave-wall and the flow behaviour at walls. The thesis used numerical models from NLSW to RANS-VOF to consider the wall effects with regard to the interaction between wave-wall and the flow behaviour at walls. 6 The obtained results will provide insight into the characteristics of overtopping at dikes with low crown-walls, partly improve the dike design in Viet Nam. CHAPTER 2 PHYSICAL MODEL TO IVESTIGATE THE EFFECTS OF LOW CROWN– WALLS ON WAVE OVERTOPPING AT SEA DIKES. 2.1 Study objective Consideration of the influence of low crown-walls on overtopping discharge, the interaction between wave-wall and the behaviour of overtopping flow at sea dikes with walls. 2.2 Model similitude For the similarity between model and prototype, three criteria are required geometry, kinematic and dynamic. For the similitude of wave, model has to be geometric similarity; the scale has to follow the Froude criterion. In short wave tests with geometrically undistorted models, the Froude criterion is automatically satisfied. 2.3 Experiments with regular waves 2.3.1 Wave flume The Holland wave flume is 45 m long, (effective length of 42 m), 1.2 m high, and 1.0 m wide. The wave maker, which is equipped with an advanced automated system of Active Reflection Compensation, is capable of generating regular and irregular waves (JONSWAP) up to 0.3m in height and 3.0 s in peak period (Figure 2.1). 7 2.3.2 Dike model and parameters The dike model dimensions and testing parameters are selected according to a model length scale of 1/10. The dike slopes were smooth and impermeable, 70 cm height with a seaward slope of 1/3. The low crown-walls were 4, 6 and 9 cm. The walls were made detachable to allow varying wall height (W) and promenade width (S) with regard to test scenarios. The wall could be moved back and force to change the promenade from 0 to 10 and 20 cm. The foreshore was 24.5 m long and 1/100 steep (Figure 2.5). 2.3.3 Test programme A water depth of d = 0.60m was chosen for testing. A group of 3 wave gauges was positioned at the dike toe and another gauge was 24.5 m away from the toe. A camcorder with high resolution was mounted normal to the flume to capture 50 frames/second in order to observe the interaction between wave-wall and overtopping flow. Each test consisted of 10 regular waves and stopped before these got disturbed by reflected ones (Table 2.1). Figure 2.1 Overview of the wave flume. Hình 2.1 Toàn cảnh máng sóng sử dụng thí nghiệm Figure 2.5 Test set-up for regular waves. 8 2.3.4 Test procedures and measurement parameters Preparation time was from June to August 2012 and test duration lasted from August till September 2012. Measurement parameters includes: wave height H, wave period T, mean overtopping discharge q, splash height H b , flow thickness on wall crest H t , crest freeboad R c 2.4 Data analysis 2.4.1 Influence of crown-walls on wave overtopping discharge The overall influence factor by crown-walls is the product of component factors due to wall height and wall promenade, respectively.                              (2-11) Determine the overall influence factor  v (measured) and  v (computed), perform regression analysis to establish the relationship between these parameters, derive two coefficients c 1 =1.26, c 2 = 1.44 (Figure 2.12)                           (2-12) 2.4.2 Wall influence on splash height The splash height was estimated by image analysis with Matlab. The author established the relationship between    and    , which reads y = 1.544e -30.9x with R 2 = 0.624. Based on this curve, one may roughly predict splash height with regard to wave and wall characteristics (Figure 2.13). Table 2.1 Test programmes with regular waves Number of tests Wave parameter R c (m) W (cm) S (cm) H (m) T (s) 40 0.16- 0.24 1.5 – 2.5 0.10 0; 4; 6; 9 0; 10; 20 [...]... crown -walls on mean overtopping discharge for regular waves (2-12) and functions of splash height with regard to wave and wall (Figure 2-13) CHAPTER 3 INTERACTION BETWEEN WAVE – WALL AND OVERTOPPING FLOW ON SEA DIKES WITH LOW CROWN -WALLS 3.1 Problem definition At different levels of detail, the thesis modelled overtopping flow at sea dikes with low crown -walls using several programmes including NLSW... The resulting splash height and wave overtopping are therefore considerably smaller than in the former case (S=0) To conclude, walls with promenades (S > 0) are clearly more advantageous than those without promenades (S =0) in terms of wave overtopping and splash height In the situation of high wave splashes in combination with storm surges, it is difficult to evaluate the increase in overtopping discharges... follows The process of wave overtopping starts as the wave tongue collides with the wall and results in a violent jet (splash) into the air (Figure 3.13) In the second phase, the splash collapses as it reaches a certain height and falls over the wall At the end of this phase, a green flow over the wall is formed (Figure 3.14) If wave continues to thrust wave overtopping in the third phase simply follows the. .. particular cases of very low overtopping rates at high walls and walls without promenades This is because 12 the strong interaction between wave- wall could not completely be resolved in NLSW by using pragmatic wall schematizations Figure 3.3 Wave overtopping computation with wall schematization according to (TAW 2002): measured versus computed Figure 3.4 Wave overtopping computation with equivalent... formulations to determine the overall influence factor of low crown -walls for regular waves (212) and the splash height as a function of wave parameters and wall geometric configuration (Figure 2.11) Both physical and numerical models were carried out to examined in detail the influence of the crown -walls from in light of the wave- wall interaction and overtopping flow The thesis reveals the merit of wall... promenades in improving splashing condition and consequently recommends the 23 use of a wall promenade in a new sea- dike cross-section A simple computer tool was also developed for predicting overtopping characteristics The obtained results in the present study provide insight into wave overtopping at sea dikes with low crown -walls As a result, sea dikes with low crown -walls can better be designed in Viet... Snapshots of wave splashing showing the wall influence with and without promenade Figure 3.19 shows comparative snapshots of the wave- wall interaction at various instances before and at maximum splashing, under the same wave condition (herein Hs= 0.24m,Tp = 2.5 s), for two test cases of the same wall height with and without promenade (REW9S0_4 and REW9S10_4) The particle velocity of 20 the wave tongue... earlier buffer flow in the form of green overtopping to obtain a maximum depth above the wall crest (Figure 3.15) Green wave overtopping continues until wave retreats on the seaward slope in the last phase (Figure 3.16) Snapshots of the flow computed by the model at all computed instances were also reviewed to determine the corresponding maximum splash height for comparison with the physical experiments... crown -walls The thesis used NLSW (Tuan and Oumeraci, 2010) to simulate and compute with regular waves The obtained results were compared to experiments in wave flume RANS–VOF modelling (COBRAS-UC, numerical wave flume) is able to simulate the interaction between wave – wall and flow at structures of any shape (vertical walls, hollow walls …), from wave generation at boundary to wave propagation as in. .. by walls, thus resulting in high splash into the air The effectiveness in reducing the flow momentum or reducing wave overtopping is thus not very high When the promenade exists (S>0), the run-up tongue is diverted to flow along the dike crest (i.e more or less in the horizontal direction) before hitting the wall In this way, the run-up energy is more effectively dissipated during impact with walls The . to investigate Wave Overtopping at sea dikes with crown -walls in the northern coastal delta of Vietnam . 2. Research objectives The thesis aims to investigate the influence of low crown -walls. low crest -walls are popular and effective in reducing overtopping at sea dikes in Viet Nam. 1.3 Sea dikes with low crown -walls in the northern coastal delta Sea dikes with low crown -walls (W/Hs. applied in Viet Nam. 3. Scope of the study Investigation of wave overtopping at sea dikes with low crown -walls in the north of Viet Nam. 2 4. Research contents Review on overtopping on sea dikes