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

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.. Research contents Revie

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

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

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

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)

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

sea There exists limited research on overtopping at sea dikes with low walls and no one is comprehensive yet Therefore, studies on overtopping at sea dikes are essential in Viet Nam and elsewhere

crown-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 walls are applied where there is no more land to enlarge the dike cross-sections

Crown-or budget is constrained; Crown-or it is not allowed to heighten the crest level in Crown-order

to reserve residence and tourism areas

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 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

crown-of crow-walls on overtopping However, the influence crown-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

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)

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

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 Hb, flow thickness

on wall crest Ht, crest freeboad Rc

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

2.4.2 Wall influence on splash height

The splash height was estimated by image analysis with Matlab The author established the relationship between 𝐻𝑏

𝐻 and 𝑆.𝐻

𝑔.𝑊.𝑇2 , which reads y = 1.544e-30.9x with R2= 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

Figure2.10 Overall influence factore of low walls  v (measured - computed)

Figure 2.13 Relationship between 𝐻𝑏

2.5 Conclusions of chapter 2

The thesis successfully established empirical formulas estimating the overall influence factor by 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 (Non-Linear Shallow Water) and RANS-VOF (Reynolds Averaged Navier Stokes – Volume

Of Fluid)

NLSW modelling is simple and fast in calculation, e.g 1000 waves can be

simulated in 5 to 10 minutes This program can estimate relatively well mean overtopping discharge at dikes with mild slope and have no crown-wall It has several shortcomings when applied to structures with complicated shape, e.g dikes with 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 physical flumes However, the calculation efficiency is low It takes many hours to simulate some seconds in real time when run on a normal computer so that it is difficult to apply to regular waves

Therefore, NLSW of Tuan and Oumeraci (2010) was used to validate the estimated values of discharge The computations were compared to measurements with irregular waves Both numerical and physical flumes were

deployed to assess the interaction between wave – wall, i.e overtopping flow characteristics

3.2 NLSW modelling (Tuan and Oumeraci, 2010)

3.2.1 Basic formulations

The model by Tuan and Oumeraci (2010) is based on the flux-conservative form

of the NLSW equations solved with a high order total variation diminishing (TVD), Roe-type scheme:

in the surfzone on the mean flow

3.2.2 Wave overtopping of irregular waves

NLSW cannot model a vertical wall because the shallow water limit is violated,

a pragmatic manipulation of the wall geometry is necessary The author used two

pragmatic approaches: equivalent wall and equivalent freeboard (Figure 3.1 and 3.2)

Mean water levels were used in combination with wave signal recorded by gauges, which were positioned in front of the model dike toe (the closer these gauges to the toe, the higher accuracy the results of NLSW)

Figure 3.1 Conversion of crown-walls into slope (TAW-2002)

Figure 3.2 Conversion of crown-walls into equivalent freeboard method

On the landward side, the outflow boundary used a water level constant and very low with regard to crest level (in order to prevent any influence on overtopping) The simulation time was the same as the physical model experiments (1000.Tp

~ 10 minutes PC) In general, the computed results agree reasonably well with the experimental data, R2= 0.88 and 0.87 for the first and second approaches, respectively The mean prediction error is 39.8% with a standard deviation of 

56.2% However, discrepancies still exist for some particular cases of very low overtopping rates at high walls and walls without promenades This is because

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 freeboard

approach: measured versus computed