Narrow Results

Propagation across the open ocean onto the continental shelf of Australia of tsunami waveforms generated by submarine earthquakes has been modelled using a coarse-grid depth-averaged hydrodynamic-numeric model. The initial conditions require the specification of the seafloor displacement field, which is determined analytically using the seismic parameters of the fault which generated the tsunami.

Focussing mechanisms are evident as the waveform propagation responds to bathymetric features. Near the land boundary, emphasis is upon the envelope of tsunami amplitudes and arrival times, which are compared with tide gauge data where available.

This model has been developed with a view to identify the areas of the Australian coast most vulnerable to tsunami damage due to seismic activity in the Indonesian region and large earthquakes south of Australia. The next phase of this project involves linking this model to a fine-grid coastal-sea model to simulate runup and inundation.

This chapter uses the Japan National Report on Disaster Reduction to the World Conference on Disaster Risk Reduction (DRR) (Kobe, Japan) in 2005 and the National Progress Report on the Implementation of the Hyogo Framework for Action (HFA) (2011–2013) premised on the HFA priority areas format to assess the status of DRR capacity in Japan. The assessment was conducted using components such as institutional arrangements, leadership, knowledge, and accountability: aspects drawn from the UNDP capacity assessment framework. The overall result showed a high DRR capacity. To examine this baseline capacity against a real disaster, a capacity gap assessment was done on the Great East Japan Earthquake with a focus on the Fukushima nuclear disaster. Several institutional, leadership, knowledge, accountability, psycho-socio-cultural and other gaps were identified. For a more effective future DRR response, the chapter then formulates and assembles a generic, systematic DRR capacity assessment framework factoring in the Basic Act for National Resilience, enacted two years after the Great East Japan Earthquake.

Increasing the tsunami disaster resilience of the Southwestern coastline of Sri Lanka is a pressing problem, due to the continuous presence of unprotected human settlements in tsunami-prone areas. Even though a variety of different tsunami countermeasures can be attempted, due to budgetary limitations early warning systems are typically used. Some types of coastal structures (such as coastal railway embankments, revetments etc.) have the potential to mitigate the impact of tsunami, which is often overlooked in research. The engineering resilience of these structures needs to be improved if they are to withstand a tsunami, though upgraded structures can offer a multitude of co-benefits to residents. This research assesses resident’s willingness to pay (WTP) for hard defensive measures, as well as the socioeconomic factors that influence residents’ WTP. WTP of residents to upgrade a coastal railway embankment and a revetment in Dimbuldooa and Wenamulla villages was measured by conducting a structured questionnaire survey of 200 residents. The results of the survey were triangulated through five expert interviews with representatives of government agencies, construction companies and academia, and two focus group discussions with residents. The findings suggest that it is necessary for disaster risk managers to pay special attention to socioeconomic factors to successfully enhance the resilience of community.

When a tsunami comes to seaside industrial areas, seawater would enter landside areas through underground pipelines and induce inundation disasters. In the present study, the authors aim to investigate the risks of this tsunami inundation via the pipelines using the numerical simulation model. The results indicated that the influence of the tsunami inundation via the pipelines on seaside areas is not small and especially it will likely to influence the initial phase of evacuation. It is therefore important for port authorities and city planners to consider such risks when developing tsunami mitigation plans.

Tsunami run-up through the river is one of important features for better predictions and estimations of inundation characteristics. This study carried out numerical investigations of observed tsunami inundation around the Kido river especially focusing on the prediction of run-up speed of tsunami along the river. Based on non-linear long wave model, a sensitivity analysis was first carried out by changing several computational conditions such as tsunami profile, bottom frictions and the river discharge. It was found through this analysis that these conditions, within the range of expected uncertain variations, have certain influence on predicted run-up speed of tsunami. Second, this study also investigated the influence of the different discretization schemes of the model on predictive skills of the speed of tsunami run-up. Difference of conservative and non-conservative forms of non-linear term was investigated through numerical experiments of non-viscosity Burgers equations and it was found that the difference of these forms has significant influence on the predicted propagation speed of the bore. The same analysis was applied in the case of the Kido river and it was found that the tsunami run-up speed was increased up to 40% by selecting appropriate discretization schemes of conservative form.

A tsunami force has unknown features due to its complexity, and makes it difficult to be predicted accurately. As a typical example, in the 2011 off the Pacific coast of Tohoku earthquake with huge tsunami, many breakwaters slid or fell down by the unexpectedly terrible damages caused by the tsunami. For effective design of coastal structures, achieving a correct prediction of the tsunami pressure should be an urgent subject. In the existing studies, the tsunami force F with an overflow is estimated by using a compensation coefficient α based on the hydrostatic pressure as: F = αρgh. The compensation coefficient α is usually set as α = 1.1 for the front of the caisson and 0.9 for the rear of that constantly. However, in recent studies, it was found that Parameter α can be easily and randomly changed depending on the boundary condition, and unfortunately, estimation method of its effective has not been developed yet. To resolve this problem, in this study, hydraulic experiments targeting on tsunamis with their overflows on a breakwater are implemented to examine the variability of the compensation coefficient α. And from the experimental results, a simple and effective estimation method for the compensation coefficient α is newly proposed.

It is important to estimate the scouring depth for the design of coastal facilities against tsunamis. The past researches indicated that the maximum scouring depth was proportional to the overflow time. In this study, in order to clarify the relation between overflow time and scour depth behind the structure, experiments and numerical calculations were carried out and analysis of the past experiments was conducted and discussed. The results showed that the scour depth could be evaluated approximately using the total flow rate and the verification of the numerical simulations were confirmed

A two-dimensional numerical model using the fifth-order weighted essentially nonoscillatory (WENO) scheme is presented in order to estimate topography change due to tsunami with high accuracy. In the model, the Cartesian coordinate system is adopted, and the fractional area/volume obstacle representation (FAVOR) method is introduced into the governing equations in consideration of applying the estimation to such as harbor shape with complex topography. In order to verify the validity and applicability of the model, it is applied to small-scale laboratory experiments and to large-scale actual topography change. Consequently, although the model cannot reproduce local scouring around breakwater where three-dimensional flow is developed, it is clarified that the model can reproduce the topography change well by contracted flow around a harbor.

A tsunami pressure acting on an onshore structure is generally evaluated by a formula with using an inundation depth η and a water-depth-coefficient α, which is based on the Froude number Fr in many cases. The definition of the water-depth-coefficient α is variously proposed by a semi-empirical way, and it is noteworthy that they are commonly based on only incident waves. Considering the real phenomenon and the practical procedure of design of the onshore structures, it is clear that synthetic waves including reflected waves should be considered. However, since the existing studies do not focus on the influence of the reflected waves, their applicability to the real phenomenon is not guaranteed yet. This paper performs hydraulic experiments with targeting tsunami running up the land to examine the influence of the reflected waves for more appropriate definition of the water-depth-coefficient α. Our experimental results in the cases with subcritical flow show unexpected increments of the inundation depth due to the refection of the forward tip of the run-up tsunami on the land, which implies a difficulty in obtaining the pure incident waves for the water-depth-coefficient α and would bring about overestimation of tsunami forces. Moreover, this study newly proposes a simple and effective method to evaluate the tsunami force with focusing on standing waves in front of the wall.

We propose that flap gates to protect against storm surges and tsunamis and have begun studies on the hydraulic and hydrodynamic characteristics and on the design of such gates. When danger of a tsunami is predicted, the flap stands up due to the buoyancy by inhale of air in a couple of minutes. In this paper, we discuss two-dimensional hydraulic model tests that were conducted on a 1:50 scale in a flume tank using a bore wave generator and confirmed the wave pressure and gate behavior required for flap gate design. In addition, hydraulic characteristics due to tsunami were simulated using the VOF method to compare the results of the model tests. Through the test and calculation, flap gate for Tsunami is efficient for Tsunami invasion to port area. Both cantilever type and rod support type are well operated during Tsunami. Flap gate can stand up instantly at Tsunami arriving for any water levels. However impulsive wave pressure on the flap and impulsive force at support rod is generated when a bore wave strikes the flap. Sustained wave pressure that followed to impulsive wave pressure, no notable differences was observed with regard to flap angle or support system. The results of numerical analysis by VOF method generally tracked the results obtained in the model tests; but the wave pressure in numerical analysis did not agree with the results of the model tests.

The highly transient initial shapes of 1D forced long waves are described in analogy with the swinging up of a mass on a spring forced by a simple harmonic force. That is, the complete solution is seen as the asymptotic, steady forced wave plus free waves which cancel it in the initial state of rest. The highly transient, complete wave form subsequently grows as the free waves separate from the forced wave. This approach gives a clear understanding of many of the characteristics of storm surge, tsunami and surfbeat.

The response of composite caisson breakwaters against a partial failure of the armour layer is not well understood at present. Observations in the field have indicated that higher levels of damage can be observed in the caisson, and to try to understand the mechanisms at work laboratory experiments were carried out on a range of regular waves (including breaking waves) and solitary waves (to simulate tsunami type waves). The results show how for partially failed armour layers when a high energy wave acts directly onto the caisson much higher pressures can be expected than for when a full armour layer is present. Analysis of video recordings show how the wave changes shape by "piling-up" on top of the failed armour, and as a result their destructive potential is substantially increased.

Sediment transport caused by a tsunami was investigated by a simple experiment. We measured the horizontal velocities and suspended concentrations caused by one solitary wave, which is assumed to represent a tsunami passing on a planar sand bed without breaking or splitting. It was found that there are phase lags between peaks of suspended concentration and velocity, and the time variations of the vertical distribution of suspended sediment concentration are in good agreement with curve approximated with the exponential function. We found that Brown's formula is inconsistent with the time variations of sediment flux measured in the experiment because it cannot explain the phase lag between velocities or the total sediment flux. The bed load formula by Ashida-Michiue shows a close agreement with the amount of bed load.

A real-time tsunami prediction method is established in this study by an inverse method which only uses offshore observed water level variation without using the fault information. The analyzed results showed overall good performance and accuracy. Tsunami sources were estimated when compared with the initial displacement of tsunami calculated by the earthquake model. The tsunami profiles can be predicted with high accuracy fore inshore points of interests and information is easily updated by using longer observation data. The accurate prediction of initial tsunami source can estimate long term tsunami fluctuation, and it can serve as useful information for a decision making the release time of warning.

Tsunami flow and sediment transport is simulated using Delft3D, a three-dimensional coupled hydrodynamic/sediment transport/morphological model. Tsunami flow characteristics and sediment erosion and deposition are recorded in 1 second intervals, and the temporal and spatial variations are examined to determine which phases of inundation dominate sediment transport and morphological change. For the simulations conducted here, most of the sediment deposited onshore originates in shallow water. While some sediment is eroded by the passing wave front, sediment is predominantly eroded during backwash and advected onshore by the subsequent wave. The extent, composition, and structure of the onshore sediment deposit is a function of the tsunami wave form, as well as the local conditions, including the nearshore morphology, local topographic variations, sediment source characteristics, and bed roughness.

This paper describes the hydraulic characteristics of the flap gate-type breakwater subjected to tsunami and waves. The structural features represents the type that gates moored by tension-rods with stabilizer plate are sloped contrary to direction of the waves. In this study, we investigated the measurement of wave pressure and rotation moment that acted on the gate in tsunami and waves using the water tank test. In an experiment, the experimental results revealed a positive correlation between the rotational moment and wave pressure. Stabilizer plate is decreased the rotational moment and wave pressure. We concluded that the type with stabilizer plate is the most available for the structural strength of the gate.

During their propagation tsunamis often traverse continental slopes that are relatively steep compared to deeper oceans. Further due to the change of bed slope from offshore to near shore, there is every likely possibility that a tsunami might steepen and eventually break, thereby generating large pressure gradients that could enhance the likelihood of liquefaction of the seabed. In the drawdown, high shear stresses could trigger debris flow in submarine canyons and on steep ridges. Therefore estimation of the bed stress is important in estimation of the forces induced during tsunami wave propagation, both on the seabed as well as on the subsurface installations. Bed and shear stresses generated by wave forms that represent tsunami (a solitary wave and broken solitary wave in the form of a bore) are measured using a shear cell. This paper deals with the measurements and modeling of the bed stress under the solitary waves.

The coastal geotechnical structures along the waterfront of urban areas are typically constructed on soft alluvial deposits and hence are potentially vulnerable against earthquakes. These coastal structures have an important function to protect the land against flood due to high tide, typhoons, and tsunamis. The paper gives an overview of seismic performance of these coastal geotechnical structures during earthquakes and points out the importance of identifying the failure modes and degree of deformation. Based on this overview, the paper proposes a methodology for assessing the vulnerability of these coastal geotechnical structures and a design strategy for improving the seismic performance of these structures against tsunamis for mitigating disaster. The paper discusses further an alternative strategy to the extreme event like the Sumatra earthquake of 2004.