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Nishidissert.Pdf 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.
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