Characterization of incoming tsunamis for the design of coastal structures: A numerical study using the SWASH model

The Tohoku Tsunami of 2011 in Japan flooded a large part of the coastal area of Japan. The tsunami was caused by an earthquake with a magnitude of 9.0 just of the coast of Tohoku. The inundation height of the tsunami exceeded the design height of the tsunami barriers. This event led to thousands of fatalities. The aim of this study is to find the characteristics of the incoming tsunami waves for the design of coastal defence structures. This incoming tsunami wave close to the shore or on the shore is influenced by a lot of offshore factors that will change the wave and its behaviour. The tsunami wave will either develop into a bore or just run up the coast. This has large influence on the forces on the barrier. Potential influencing factors are examined on if and how they influence the tsunami wave when it travels to the coast. A numerical one-dimensional SWASH model is used throughout this study to simulate the tsunami wave. The tsunami factors and the factors that influence the wave were studied in several steps.The factors that have the most influence on the wave are used to simulate bores. From all these bores the important characteristics for the design of a barrier are investigated. These are the bore height, the bore velocity and the corresponding Froude number. With the simulations a new definition of the bore height is introduced. This is the height at the maximum velocity of the bore. The bore characteristics are also tested with an existing formula for the impact forces on a structure. The behaviour of the breaking wave is studied and a breaker parameter [ξtsunami] for tsunami waves is made. This breaker parameter defines if the tsunami wave develops into a bore before it reaches the coastline or that the wave runs up the coast without breaking. This is important for the location of the coastal structure. This breaker parameter and the Froude number of the bore give a relationship between the important parameters that influence the development of a bore and the characteristics of the incoming tsunami bore. Finally, physical tests were performed at the Waseda University in Tokyo, Japan, to simulate the bore attack on a coastal defence structure with a dam-break. The bore of the tests is compared to the bore from the SWASH simulations. This resulted that the velocities of the tests seem too high. However, with a new method to find the bore front characteristics is a Froude number constructed. This Froude number matches very well for the tests and SWASH simulations. The Froude numbers of the test represent a bore at the coastline.DIM

Formulation of a Surf-Similarity Parameter to Predict Tsunami Characteristics at the Coast

To calculate tsunami forces on coastal structures it is of great importance to determine the shape of the tsunami front reaching the coast. Based on literature reviews, analytical reasoning,video footage, and numerical modelling it is concluded that both the continental shelf slope and the bay geometry have a significant influence on the transformation of a tsunami wave near the coastline. After conducting 1D and 2DH wave simulations, a distinction is made between three types of tsunami waves; a non-breaking front (surging), a breaking front and an undular bore breaking front. Tsunami waves transform into these three wave types over a steep continental shelf, an intermediate sloped continental shelf, and a gentle sloped continental shelf, respectively. A new tsunami surf-similarityparameter is proposed to quantitatively predict the type of wave at the coastline, which was validated based on observations during the 2011 Tohoku Earthquake and Tsunami.Hydraulic Structures and Flood RiskEnvironmental Fluid Mechanic

Tsunami Bore Overtopping Of Coastal Structures

In the aftermath of the 2011 Tohoku Earthquake and Tsunami Japanese tsunami protection philosophy now dictates that coastal defences should prevent the land that they protect from being flooded under a Level 1 event (with a return period in the order of about 100 years). To ascertain the overtopping mechanism and leeward inundation heights of tsunamis as they hit coastal structures, the authors conducted physical experiments using a dam-break mechanism, which could generate bores that overtopped different types of structures. Three different types of structures were considered, namely a wall of “infinite” height, a coastal dyke, and a vertical tsunami wall. The results show that the velocity of the tsunami bore is crucial in determining whether the structure will be overtopped or not, and thus it is imperative to move away from only consideringthe tsunami inundation height at the beach

Tsunami Bore Overtopping Of Coastal Structures

In the aftermath of the 2011 Tohoku Earthquake and Tsunami Japanese tsunami protection philosophy now dictates that coastal defences should prevent the land that they protect from being flooded under a Level 1 event (with a return period in the order of about 100 years). To ascertain the overtopping mechanism and leeward inundation heights of tsunamis as they hit coastal structures, the authors conducted physical experiments using a dam-break mechanism, which could generate bores that overtopped different types of structures. Three different types of structures were considered, namely a wall of “infinite” height, a coastal dyke, and a vertical tsunami wall. The results show that the velocity of the tsunami bore is crucial in determining whether the structure will be overtopped or not, and thus it is imperative to move away from only consideringthe tsunami inundation height at the beach.Hydraulic Structures and Flood Ris

Overtopping of coastal structures by tsunami waves

Following the 2011 Tohoku Earthquake and Tsunami, Japanese tsunami protection guidelines stipulate that coastal defences should ensure that settlements are shielded from the coastal inundation that would result from Level 1 tsunami events (with return periods in the order of about 100 years). However, the overtopping mechanism and leeward inundation heights of tsunami bores as they hit coastal structures has received little attention in the past. To ascertain this phenomenon, the authors conducted physical experiments using a dam-break mechanism, which could generate bores that overtopped different types of structures. The results indicate that it is necessary to move away from only considering the tsunami inundation height at the beach, and also consider the bore velocity as it approaches the onshore area. The authors also prepared a simple, conservative method of estimating the inundation height after a structure of a given height, provided that the incident bore velocity and height are known.Hydraulic Structures and Flood Ris