ABSTRACT
The coastal region has been developed for the human activity and industrial purposes particulaly during the last half century in Japan. During most of this period, many coastal engineering problems have emerged to harbor construction, reclamation, construction of artificial islands, and other projects. Along with the developing projects of the coastal area, coastal engineers have also faced the great responsibility of protecting the developed area which is located behind and on the coast, as well as to preserve the natural environment in neighboring coasts.
As mentioned, human activities and properties are located close to the seaside where violent destructive forces by fluid have the potential to damage the coastal area during a storm.
In addition, sand supply from rivers to nourish coasts has been decreasing due to the construction of hydraulic dams and river rivetment works. As a result, the width of beaches tends to narrow, and then storm waves tend to damage dune fronts in many coasts in Japan.
The empirical law suggests that the narrower the beach and dune system, the lower the protective potential of the coastal properties against natural disasters such as high waves, storm surges, and tsunami. In other words, the coast is a boundary where the sea and land meet, therefore the beach and dune system is the major element in dissipation of destructive wave force and is the last defence against a severe storm surge to protect human beings. Thus, research into the mechanism of beach and dune profile evolution due to waves was conducted.
To study the geomorphological character and mechanism of beach and dune systems subjected to random waves during a storm, firstly a literature survey on coastal processes and related cross-shore sediment transport have been conducted in chapter 1. Prior to the literature survey, a laboratory study on beach processes due to random waves in a three ii dimensional wave basin had been conducted, then, additional laboratory study had been carried out to evaluate the storm surge effect on erosion profiles and post-storm profile recovery. In chapter 2, since these two previous laboratory studies show that the energy dissipation by breaking waves is one of the dominant mechanisms in mobilizing the sediment particles and generating net sediment transport in a surf zone, a data analysis and numerical study of breaking waves, especially the probability of breaking waves, has been made to verify the performance and limitations of this wave model prior to a coupling with a sediment transport model. This wave model will later be applied to the simulation of two dimensional profile evolution. In chapter 3, the geomorphological characteristics of the beach and dune erosion at Fukiage and Kashiwabara beaches, Kagoshima Prefecture, in Japan are investigated by field surveys. Following the field surveys, numerical simulation of the generation of beach and dune scarps have been done, because the scarp is a distinct erosional feature. In chapter 4, the further field observation and laboratory study on the mechanism of beach and dune erosion have been conducted to study an effect of wave impact on erosional process.
Following this, the DEMAND model which is applied to simulate the beach and dune profile evolution in chapter 3 was improved to the SUPPLY and DEMAND model based on knowledge from field observations and large wave tank tests in chapter 4. In chapter 5, because the previous study in chapter 4 shows that compaction is an important geomorphological parameter to estimate the cross-shore sediment transport rate and the strength of beach and dune, which are in turn related to the potential risk of coastal properties behind a dune, the compaction of beach and dune has been investigated through field survey and laboratory study. In addition, the compaction parameter has been applied to study other geomorphological features such as beach cusps, and to the detection of erosional and depositional areas as well. Finally, in chapter 6, overall conclusions on geomorphological aspects of beach and dune profile evolution due to waves are drawn.
iii The earlier stage of this study reveals the facets of beach and dune profile evolution due to waves by means of field studies. Then, the accumulated knowledge is incorporated in the simulation algorithm and computer program to commence the quantitative engineering application, however it is emphasized that further study on some important mechanisms of profile evolution such as wave randomness should be undertaken. Therefore, in appendices
A and B, a statistical wave model and a SIWEH approach have been coupled with the cross- shore sediment transport model used in chapter 3 to incorporate the randomness of incident waves into the simulation of two dimensional profile change as a part of future study, respectively.
iv
ACKNOWLEDGEMENTS
The author would like to express his special appreciation to his supervisor, Prof. Michio
Sato for his long-time support, constructive inspiration and encouragement in academic and personal life. The author has learned an attitude to study the nature from him.
The author also wishes to extend special thanks to Dr. Nicholas C. Kraus who was professor at the Conrad Blucher Institute for Surveying and Science, Texas A&M University-
Corpus Christi (currently research scientist at the CHL, U.S. Army), and Prof. Hsiang Wang at the Department of Coastal and Oceanographic Engineering, University of Florida for their kind discussion and advice to the numerical study and the American way of professionalism to conduct a research. The author acknowledges the kindness received from other members of both institutions
The author would like to appreciate Profs. A. Maeda and R. Kitamura, and Associate
Prof. T. Asano who are the members of doctoral thesis committee at the Graduate School of
Science and Engineering, Kagoshima University for their constructive criticism for the revision of this dissertation. Thanks are due to Mr. K. Nakamura and the previous members of Coastal
Engineering Research Group, for their hard labor work and help during the field survey trip. In addition, thanks go to my old friends, Dr. Venkataramana Katta of Kagoshima University, Mr.
MyoKhin of Dai-ichi Kogyo University, Dr. Kim Namhyong of Chejyu National University, Dr.
Taerim Kim at the Korean Oceanographic Research and Development Institute and Dr. Li-hwa
Lin at the Coastal and Hydraulic Laboratory, Waterway Experiment Station, U.S. Army for their academic and personal advises.
Finally, the author would like to extend his special appreciation to his wife Misuzu Nishi
i and his parents, Ikuo and Yosiko Nishi for their endless and significant support.
ii
Chapter 1 PREFACE
1.1 PREVIOUS RESEARCH ON PROFILE EVOLUTION
(a) Tombolos generation due to Detached (b) Pronounced beach scarp at Kashiwabara
breakwaters at Kawajiri beach. beach
Fig. 1.1 Coastal morphologies in Kagoshima Prefecture, Japan
Natural beaches are subjected to random sea and swell. These waves transport sediments in cross-shore and longshore directions to generate geomorphological features such as those shown in Fig. 1.1. Regarding cross-shore sediment transport, constructive calm waves carry much sediment onshore during the first stage of a post-storm event. In contrast, destructive storm waves erode dune and beach, then transport the sediment offshore. The combination of the calm waves and storm waves causes a beach cycle. These processes of accretion and erosion significantly influence a shoreline position and coastal properties as shown in Figs. 1.2, 1.3, and 1.4. Therefore, many morphologists, geologists, and coastal engineers have been involved in research on coastal processes to protect the shore. As a result, a great
1 amount of data concerning profile evolution has been collected through laboratory experiments, field observations, and theoretical studies.
Fig. 1.2 Damage of Route 262 by a typhoon. Maenohama beach locates at the edge of a caldera and surrounded by a coastal cliff.
Fig. 1.3 Damage of coastal property in Nagasakibana beach by a typhoon. The cliff height is nearly 6 m. Remains of gabion mat and sea wall can be seen.
Fig. 1.4 Damage of beach and dune at Kashiwabara beach. This dune scarp is created just behind a crescentic longshore bar system. The center of scarp corresponds to a rip-current area. Beach topography shows many geomorphological features, for instance; beach cusps, 2 giant cusps, tombolo, crescentic longshore bars, and other features that are caused by the presence of coastal structures. To reduce the complexity of coastal topography analysis, topography changes are often classified as either a cross-shore related processes or longshore related processes. Each process is related to cross-shore sediment transport and longshore sediment transport. In fact, both transport rates can be combined to generate quasi-three dimensional models such as conducted by Watanabe (1987) and Wang and Miao (1993). The sediment transport rates are referred to as wave-term and current-term in a three dimensional model. With regards to cross-shore process, a beach profile often shows a seasonality which consists of shoreline recession and progress, foreshore erosion and berm generation, and the appearance and disappearance of a longshore bar in a surf zone. Johnson (1949) drew the conclusion on the profile prediction that an "ordinal profile" is generated by waves with steepness lower than 0.025, and a "storm profile" is generated by waves with steepness higher than 0.03 based on his laboratory experiments. Later many researchers conducted laboratory studies on the beach profile predictor as shown in Table 1.1, then applied them to natural beach conditions.
Laboratory studies showed that a certain type of beach profile was a function of wave condition (wave steepness), sediment property (either sediment diameter or the settling velocity), and beach profile condition (often chosen to be mean slope tan ). The beach profile predictors were first used just for classification and prediction of beach profile type, and then were coupled with a cross-shore sediment transport formula to estimate the direction of sediment transport in the numerical models. To date, the beach profile predictor is called a direction function in some profile evolution models.
3 Table 1.1 Beach profile predictors.
Author Parameter Note
Johnson (1949) H0/L0 H0/L0 > 0.03 Storm profile H0/L0 < 0.025 Ordinal profile Iwagaki and Noda (1963) H0/L0, H0/D Graphical method
Dean (1973) H0/L0,ρw/gT H0/L0 > Aρw/(gT), bar
H0/L0 < Aρw/(gT), berm Kriebel, Dally, and Dean A=1.7, lab scale (1987) A=4-5, prototype scale -0.27 0.67 Sunamura and Horikawa H0/L0, D/L0, tan b H0/L0>
Table 1.2 Classification of representative cross-shore sediment transport formulas.
(1) Concentration type h q = 0 c(z)u(z)dz (1.1) Dally (1982), Stive and Battjes (1984), Steetzel (1993), etc. (2) Shear stress type
( - cr ) q = Aw , q = f( ) (1.2) g Madsen and Grant (1976), etc. (3) Energy dissipation type q = K(D - ) Deq (1.3) Dean (1977), Moore (1982), Kriebel (1985), Larson and Kraus (1989), Nishi et al. (1994) (4) Velocity type (Energy type) n q = M u (1.4) Bagnold (1962), Bailard (1988), etc (5) Combined model
( - cr )u n f d E qw = Aw + Awb g g (1.5) Watanabe (1987), etc. (6) others
4
Fig. 1.5 Wave transformation on a sandy beach in Motte beach. These regular swells are forerunners of typhoon.
Fig. 1.6 Visualization of orbital motion of progressive waves. Stokes drift can be recognized even though a distorted orbital motion of a tracer. Right is a zoom up of the orbital motion.
Fig. 1.7 Orbital motion of a tracer in a middle layer of a water column. The orbit is elongated to both left and right sides. 5 In addition, when waves propagate over a sandy beach as shown in Fig. 1.5, orbital motions of waves as shown in Figs. 1.6 and 1.7 probably initiate sand particle motion and transport the sediment onshore and offshore. Coastal engineers revealed the sediment transport mechanism and enabled in the calculation of the transport rate by wave action. Table 1.2 shows a classification of some cross-shore sediment transport rate formula, where q is the sediment transport rate, h is the local water depth, c(z) and u(z) are the concentration of sediment and the horizontal velocity at elevation z, Aw and Awb are transport coefficients, is the density
2 of water, g is the acceleration of gravity, = wfu /2 is the shear stress, = /(s-1) gd is the
Shield's number, D and Deq are the energy flux and stable energy flux, M and n are empirical coefficients, u is the mean velocity, n=(1+2kh/sinh 2kh)/2, k=2 /L is the wave number, fd is the coefficient for energy dissipation, E is the wave energy, respectively. Most were mainly derived from regular wave and oscillatory flow conditions, but a few were derived from random wave conditions. As can be seen, most of the parameters in the transport formulas are related to a basic parameter, such as excess fluid velocity (u-ucr), excess bed shear stress ( - cr), and energy ( u) in which ucr and cr are the critical horizontal velocity and the critical shear stress, respectively.
Numerical models can also be classified in terms of their capability such as two dimensional (2-D) model, three dimensional (3-D) model, 1-line model, and N-line model. In general, 2-D and 3-D models require longer computational time and are, therefore, more suitable for short term prediction whereas 1-line, and N-line models are commonly used for long term prediction. The models also can be classified as profile model, planform model and topographic evolution model in engineering application. This doctoral thesis is focused mainly on the evaluation of profile evolution models which are related to the cross-shore sediment transport rate.
6 Most of the profile evolution models consist of three parts: (1) hydrodynamic modeling, (2) sediment transport modeling, and (3) profile response modeling. In hydrodynamic modeling, the primary concern is wave property, particularly within the surf zone, however there are exceptions because some models use mean sea level fluctuation as the main driving force for sediment transport. Current computation also becomes important when the models are for 3-D application. In general, it is safe to state that the wave transformation model together with the associated mean water level change governs the behavior of the beach evolution model. Since wave transformation in the vicinity and within the surf zone is sensitive to the bottom topography, and the ability to frequently upgrade the wave condition in response to the profile changes is essential for any profile evolution model. Profile response modeling almost universally uses the mass conservation equation for bed material. Some models restrict the profile response to a pre-determined shape known as the equilibrium beach profile (Dean, 1977) and are referred as "closed model" here. Others let the profile respond to the input forcing function without a pre-determined shape and are called "open model". Most open models incorporate some type of weighing functions, spread functions, residual angles, or soil stability criteria to modify the profile shapes to produce berm, bars or other bottom features, or to avoid numerical instability around the shoreline and bar. To give some insight, a few representative examples of these numerical models are shown in Table 1.3. Further information can be found in comprehensive reviews on sediment transport formulae by
Horikawa (1988).
In profile evolution modeling, the main sediment transport mechanism of concern is cross-shore transport. Based on the beach profile predictor and cross-shore transport formula, beach profile evolution has been simulated by solving the mass conservation equation for bed material. There are several schemes to solve the conservation equation for bed material (i.e. forward, backward, and central). To improve the stability of mass conservation equation in a
7 numerical approach, Larson and Kraus (1988, 1990) applied spatial gradients of sediment transport rate at two time-steps.
Table 1.3 Example of beach profile(topography) evolution model.
Model Type Note
Houston model (1990) Open-model Sediment transport is dependent on shear stress due to wave breaking, wave nonlinearity, and slope term.
SBEACH (1989, 1990) Semi-open model Beach profile is divided into four regions. In the wave broken region, sediment transport is computed by equilibrium condition and other sediment transport rates are connected exponentially and linearly. The mass conservation equation is based on two time-step sediment transport rates.
Wang model (1993) Open model Sediment transport is composed of wave and current term. Wave field is derived from mild slope equation. The identification of sediment transport is not cross-shore and longshore, but current and wave transports terms.
Dally model (1980, 1984) Open model Suspended sediment transport model and computed by concentration * velocity field. Quite physically based model, but spreading function is used.
Kriebel model (1985) Closed model Equilibrium beach profile model; sediment transport rate is Q=K(D-Deq) Principle force is sea level change, not wave climate; does not produce bar profile.
GENESIS (1989) 1-line model (closed model) The longshore sediment transport rate is a function of the angle of incident waves and the distribution of wave height. The beach profile is restricted to the equilibrium beach profile.
Nishimura model(1986) Open model Sediment transport is a function of Ursell’s number and Hallermeier parameter. The direction function is used for the determination of net sediment transport direction. (* See, the term "open model" refers to the models which accept any beach profile caused by waves and currents, and "closed model" restricts the beach profile to equilibrium beach profile h=ay(2/3) proposed by Dean (1977).) 8 In spite of the progress on numerical profile modeling which was made in the last decades, our knowledge on sediment transport under waves and currents is still rather rudimentary and it is impossible to incorporate every physical factor into mathematical formulation, yet. For example, some models shown in Table 1.3 have their own assumptions hence, limiting their own range of applicability. To establish the applications and limitations of numerical models, certain criteria are essential to serve as references.
Dally (1980) cited the expected capability of the numerical model as follows: (1) The ability to generate general profiles of both normal and storm types depending on the wave conditions and sediment characteristics, (2) The ability to predict the proper shape of these profiles; i.e., (a) the normal should be monotonic and concave upwards, and (b) the bar(s) of the storm profile should have the proper spacing and shape, (3) The ability to accurately predict the rate of profile evolution, (4) The ability to respond to changes in water level due to tides, storm surges, or long term fluctuations, (5) The stability of the model as judged by its tendency to approach a stable profile asymptotically, if all the relevant parameters are held constant. An asymptotically stable profile is a profile approaching equilibrium.
Basically, these criteria are supposed to be used in evaluating the numerical models.
As mentioned above, a sandy beach is the most flexible natural structure to dissipate wave energy, however a dune will be a last defender against the destructive force of large storms and storm surges. Once a dune is breached or completely removed by storm, local communities and property will suffer huge damage. The dune protects human beings against tsunami waves, too. Therefore, many coastal engineers especially in the Netherlands and
U.S.A. have conducted research on dune erosion to save hinterlands such as shown in Table 1.4.
9 Table 1.4 Dune erosion studies
1. Steady state ( or graphical ) method Contents
Van de Graff (1977) Erosional profiles by field surveys
Hughes and Chiu (1981) Laboratory experiments
Sargent and Birkmeier (1985) Verification of Vellinga method(1983) by field data from Gulf of Mexico and
Atlantic Ocean
Hallermeier and Rhodes (1988) Generic treatment of dune erosion for 100-year event
2. Time-dependent model
Kriebel and Dean (1985), Kriebel Numerical model based on the equilibrium beach profile by Dean (1977)
(1990)
Larson and Kraus (1989) Numerical model based on the equilibrium profile approach with avalanching
and a generation of longshore bar
Wang and Miao (1993) Application of three dimensional model
Steetzel (1993) Numerical model with suspended sediment approach under random wave action
Nishi et al. (1994) Energy flux approach under random wave action
3. Swash wave approach
M.F.Overton and J.S.Fisher (1988a, b) Estimation of dune erosion by wave uprush
Nishi and Kraus (1996) Dune erosion by wave impact of swash waves
4. Theoretical approach
Kobayashi (1987) An analytical solution for dune erosion by storms
5. Observations
Edelman (1968, 1972) Dune erosion during storm conditions
Kana (1977) Beach cut during minor storm
6. Dune protection
Auerbach et al. (1988) Dune stabilization with a sand/gel composite system
Dette and Raudkivi (1994) Dune protection by mats
10 Even with these valuable studies, artificial and natural dune systems occasionally fail to survive during a storm event. To strengthen knowledge of dune erosion and improve design procedure of artificial dunes, a fundamental study on characteristics and the mechanism of dune erosion by storms should be conducted in more detail.
In addition to field study and small-scale laboratory tests as well as a numerical study, large wave tank facilities have been established at several institutions and have been used for the study of profile evolution. For instance, Saville (1957) conducted the first full-scale experiment of it kind at the CERC. Moreover, large wave tank experiments have been carried out by Kajima et al. (1982) at the Central Research Institute of Electric Power Industry (CRIEPI) in Japan, by Vellinga (1986) at the Delft Delta Flume in the Netherlands, by Dette and Uliczka
(1987) at the large wave flume (GWK) in Germany, and by Kraus et al. (1992) at the
SUPERTANK in the U.S.A. These full-scale experiments enable coastal engineers to estimate damage to beaches and dunes by wave and storm surge action, and to design a dune and beach profile which is reliable against storm activity. Full-scale experiments can provide valuable information on hydrodynamic conditions, the mechanics of transport, the resultant transport rate, and geomorphological features of profile without scale effects under controlled conditions. It is expected that release of these data by such as Shimizu et al. (1996) will facilitate the calibration and verification of the numerical models.
11 1.2. PURPOSE OF THE RESEARCH
Waves on the natural beach often consist of a variety of wave heights and periods as shown in Fig. 1.8, as well as varying directions. This random sea is the dominant force in changing dune and beach profile and planform. To study this geomorphological character of dune and beach system ( such as shown in Fig. 1.9 ), a literature survey on coastal processes and related cross-shore sediment transport have been conducted in this chapter two.
Fig. 1.8 A record of random waves at Fukiage Coast, Kagoshiam Prefecture.
Fig. 1.9 Dune and beach profile evolution during a storm (SPM)
12 Prior to this doctoral research, laboratory study on beach profile change due to random waves and additional laboratory study of the storm surge effect on erosional profile and post-storm profile recovery had been carried out to obtain general knowledge of sediment transport under waves. These previous laboratory studies showed that the energy dissipation by wave breaking is one of the dominant mechanisms in mobilization of sediment particles.
In addition, these studies revealed that a single breaking of regular waves tends to generate a pronounced bar-trough feature, while multiple breaking of random waves cause a wide surf zone and generate a gentle beach topography.
Therefore, a numerical study of breaking waves, especially a probability of breaking waves, has been made to verify the performance and limitations of this wave model in chapter 2. This wave model will be applied to the simulation of two-dimensional profile change. In chapter 3, the geomorphological characters of the dune and beach erosion at
Fukiage and Kashiwabara beaches, Kagoshima Prefecture, in Japan are investigated by field surveys. Because the scarp is a distinct erosional feature and affects a coastal environment, numerical simulation of the generation of dune and beach scarps have been done. In chapter
4, the further field observation and laboratory study on the mechanism of dune and beach erosion have been conducted. Subcequently, the DEMAND model which is applied to simulate the dune and beach profile evolution in chapter 3 has been improved to the SUPPLY and DEMAND model based on knowledge from field observations and large wave tank tests.
In chapter 5, because previous study in chapter 4 shows that the compaction is an important parameter in denoting the strength of dune and beach and the amount of dune erosion, the compaction of dune and beach has been investigated by both field survey and laboratory study.
In addition, this compaction parameter has been applied to other geomorphological features such as beach cusps and to the definition of erosional and depositional areas as well. Finally, in chapter 6, overall conclusions on geomorphological aspects of dune and beach profile
13 evolution due to waves are drawn.
The work in this thesis reveals the facets of beach and dune profile evolution due to waves during a storm such as shown in Fig. 1.10 by means of field studies and numerical studies, because the dune has some important functions, i.e. (i) serving as soft structures against storm surge and severe wave attack including tsunami waves, (ii) filtering effect of wind blown sand and salt by the coastal forest on and behind the dune, (iii) sediment source to the foreshore and longshore bar in a severe storm condition, (iv) a habitat and nesting area for sea animals and vegetation, (v) a setback area for the sea-level rise as well as storm surge, (vi) a recreational area for local residents and tourists. The obtained knowledge from field studies is incorporated in the simulation algorithm and program to commence quantitative engineering applications.
Fig. 1.10 The maximum 30 waves since 1980 to 1996 at Biroujima.
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19
Chapter 6. CONCLUDING REMARKS
As seen in the previous chapters, the geomorphological character of beach and dune profile evolution due to short-term storm wave action is mainly studied in the laboratory and field. The obtained knowledge is then incorporated in the simulation algorithm and program to prepare a quantitative engineering application Beach and dune profile evolution have been studied based by field survey, laboratory study, and numerical modeling. Concluding remarks are given here.
Literature surveys and the development of data acquisition system have been conducted, and then physical modeling, numerical simulation, field survey, improvement of simulation algorithms, and further observation of erosion processes have been carried out to study the geomorphological character of dune and beach profile evolution when subjected to random seas during a storm. The overall conclusions are shown below.
In chapter 2, numerical study of a breaking wave model has been carried out to extend the model to the prediction of dune and beach profile change. Since previous physical studies show that the energy dissipation by wave breaking is one of the dominant factors in estimating the cross-shore sediment transport rate. Therefore, the probability of random wave breaking has been investigated numerically. The conclusions are as follows:
(1) Field data obtained at DUCK85 is analyzed by visual inspection to keep the number of waves the same at different photopole positions. The representative wave height
distribution by this method is nearly equivalent to the result by Ebersole's computational
method. (2) Based on the numerical analysis, it is shown that the probability of breaking waves by
the
Goda method and by the M.C. method are qualitatively similar. Those by the B-J model
and the T-G model are similar to each other, but differ from the results of the other two models.
(3) As a possible application of the M.C. method, computations of the probability of break-
ing waves have been carried out over a non-uniform beach. The result shows a good agreement over beach profile except the trough region where the surface roller generates.
In chapter 3, the geomorphological characters of dune and beach erosion at Fukiage Beach and Kashiwabara Beach, Kagoshima Prefecture, were investigated by field surveys.
Because the scarp is a distinct erosional feature, numerical simulation of the generation of dune and beach scarps have been done. The conclusions are as follows;
(1) It is observed that once a vertical scarp face is generated by wave action, anteceding
waves directly attack the scarp face during high water and undermine the foot of the
scarp. As a result, blocks of sand on the vertical dune face collapse, and are deposited at
the foot of the scarp and transported offshore by successive waves.
(2) There was little evidence to show that the negative gradient of longshore sediment
transport dominated scarp generation on Kashiwabara beach, because the beach scarp
was also generated around the groin where the longshore sediment transport might
deposit the sand. It is also shown by a numerical study that the steeper the beach face,
the higher the resulting scarp. Therefore, a steep beach face enhances the generation of a
scarp.
In chapter 4, further observation on the mechanism of dune and beach erosion was conducted. Then, the DEMAND model which is applied to simulate dune and beach profile evolution in a previous chapter was improved to the SUPPLY and DEMAND model based on knowledge from field observations and large wave tank tests. The conclusions are as follows:
(1) Three types of dune erosion mechanisms by storm are identified through field
observations as: (a) layer separation, (b) notching and slumping, and (c)sliding and
flowing. The erosion by layer separation mechanisms was quantified by the dune
erosion test at the SUPERTANK project. Visual inspection also shows that collapsing
and slumping occurs while the impact wave backwashes seaward.
(2) The compaction of a dune is a significant parameter that decreases the erosion volume,
but the higher compaction produces a steeper dune face during an erosion process.
(3) The improved SUPPLY-and-DEMAND algorithm is capable of simulating dune and
beach erosion based on the hydrodynamics acting in each region. The computed results
show good agreement with the dune erosion at the SUPERTANK project.
(4) It is emphasized that the compaction of a dune face, which means the strength of the
dune to resist wave impact, is a key parameter in making a protective dune. Therefore,
the erosion volume by storm waves is dependent upon the compaction of the dune, so
the compaction coefficient has to be taken into account during the calculation of the
cross-shore sediment transport rate in future research.
In chapter 5, compaction of dune and beach has been investigated through field survey and laboratory study, because compaction is an important and inexpensive parameter to denote the strength of dune and beach, cross-shore sediment transport rate, and definition of erosion and deposition area. The conclusions are as follows:
(1) The depression-type method applied in this study can measure only the compaction of
the sand surface of beach and dune. The penetration-type can measure compaction
inside the dune so that it allows us to represent the durability and degree of protection of dune and beach.
(2) In general, compaction of the sand dune is lower than that of medium and fine sand
beaches. Compaction is higher in the erosion area that that in the accretion area.
Therefore, compaction can be a practical parameter to check the erosion and accretion regions over a long extend.
(3) The compaction of lower and middle positions of a scarp is higher than the compaction
of the upper portion of a scarp or the compaction in the deposition region in front of the
scarp. This is probably due to the wave impact on the scarp face and compression of the sand itself.
(4) Compaction should be taken into account in the conservation equation for calculating the cross-shore sediment transport rate especially around the bar and berm features.