Failure Mechanisms and Local Scour at Coastal Structures induced by Tsunamis

On March 11 2011, an exceptionally large tsunami event was triggered by a massive earthquake offshore, the northeast coast of Japan, which affected coastal infrastructure such as seawalls, coastal dikes and breakwaters in the Tohoku region. Such infrastructure was built to protect against the Level 1 tsunamis that previously hit the region, but not for events as significant as the 2011 Tohoku tsunami, which was categorized as a Level 2 tsunami [Shibayama et al. 2013]. The failure mechanisms of concrete-armoured dikes, breakwaters and seawalls due to Level 2 tsunamis are still not fully understood by researchers and engineers. This paper investigates the failure modes and mechanisms of damaged coastal structures in Miyagi and Fukushima Prefectures, following the authors' post-disaster field surveys carried out between 2011 and 2013. Six significant failure mechanisms were identified for the coastal dikes and seawalls affected by this tsunami: 1) Leeward toe scour failure, 2) Crown armour failure, 3) Leeward slope armour failure, 4) Seaward toe and armour failure, 5) Overturning failure, and 6) Parapet wall failure, in which leeward toe scour being recognized as the major failure mechanism in most surveyed locations. The authors also propose a simple practical mathematical model for predicting the scour depth at the leeward toe of the coastal dikes, by considering the effects of the tsunami hydrodynamics, the soil properties and the type of structure. The key advantage of this model is that it depends entirely on quantities that are measurable in the field. Furthermore this model was further refined by conducting a series of hydraulic model experiments aimed to understand the governing factors of the leeward toe scour failure. Finally, based on the results obtained, key recommendations are given for the design of resilient coastal defence structures that can survive a level 2 tsunami event

Development of ripple filter composed of metal mesh for charged-particle therapy.

In charged-particle therapy, a ripple filter (RiFi) is used for broadening the Bragg peak in the beam direction. A conventional RiFi consists of plates with a fine ridge and groove structure. The construction of the RiFi has been a time-consuming and costly task. In this study, we developed a simple RiFi made of multi-layered metal mesh (mRiFi), with which the Bragg peak is broadened due to structural randomness, similar to what occurs for the already proposed RiFi with porous material.. The mRiFi was constructed by stacking commercially available metal meshes at random positions and angles. The mRiFi was inexpensive to fabricate due to its high availability and low machining accuracy. The Bragg peak width modulated by the mRiFi can be uniquely determined by the wire material, wire diameter, wire-to-wire spacing of the metal mesh, and the number of mesh sheets. We fabricated four mRiFis consisting of 10, 20, 30, and 40 layers of stainless steel meshes with a wire diameter of 0.1 mm and a wire-to-wire spacing of 0.508 mm.Using the mRiFis consisting of 10, 20, 30, and 40 mesh sheets, we succeeded in broadening the Bragg peak following the normal distribution with the respective standard deviationvalues of 0.83, 1.15, 1.41, and 1.56 mm in water in experimental planar-integrated depth dose measurements with 140.3 MeV ucarbon-ion beams. The effect of range broadening with the mRiFi was independent of its lateral position, and the measurement of the surface dose using radiochromic films showed no severe inhomogeneity with a homogeneity index greater than 0.3 caused by the mRiFis.The developed mRiFi can be used as a RiFi in charged-particle therapy. The mRiFi has three advantages: high supply stability of the material for manufacturing it, easy fabrication, and low cost

Reconstruction of the Laser Compton Scattered gamma-ray Distribution in the Energy-Angle Phase Space from Crystal Diffraction Data

The distribution of laser Compton scattered (LCS) photons in the energy-angle phase space reflects the parameters of the laser and electron beams at the collision point and provides the spectral density and bandwidth available in the LCS source. Diagnosis of the energy-angle phase space is thus crucial for developing high-flux and narrow-bandwidth LCS sources. Such diagnosis has been realized only for photon energies below 100~keV, for which X-ray pinhole camera and double-slit systems are available. We demonstrate reconstruction of the energy-angle phase space distribution of an LCS photon beam at an energy of 1.7 MeV by utilizing photon diffraction with a silicon mosaic crystal and a silicon comb crystal, both of which have enlarged angular acceptances. The reconstructed photon distribution is well reproduced by numerical simulations. The demonstrated diagnostic technique is an efficient tool for the development of LCS photon sources and beneficial to the use of existing LCS facilities

Detached Breakwaters Effects on Tsunamis around Coastal Dykes

AbstractThe Japanese coastal areas have a lot of different types of coastal structures with different purposes, and it is important to understand each structure's effects on tsunami. In the present paper, the authors focused on detached breakwaters effects on tsunami flow around coastal dykes with a field data investigation and laboratory experiments using a tsunami basin. The field data obtained from the 2011 Tohoku Earthquake and Tsunami showed that tsunami flow was not uniform along a coastal dyke with the specific arrangement of detached breakwaters. The laboratory experiments performed in a tsunami basin showed that a detached breakwater with a small detached breakwater parameter (distance from the shoreline/length of the opening)had a tsunami mitigation effect along the shoreline just behind the main body of the breakwater, but did not have a tsunami mitigation effect along the shoreline just behind the opening. The results obtained from the laboratory experiments agreed with the field data of the 2011 tsunami

Generation of vector beam with tandem helical undulators

We propose a scheme to produce structured light in synchrotron light sources. In this scheme, light beams from two undulators are superposed by using a technique akin to the “cross undulator.” We demonstrate that a vector beam, in which the polarization direction varies with the azimuthal angle about the beam axis, is produced by superposing harmonic radiation from two helical undulators in tandem. Although this scheme is demonstrated in the ultraviolet range at the low-energy synchrotron UVSOR-III, it can be applied to high-energy synchrotrons to produce vector x-ray beams, which would open a new field in the application of synchrotron radiation

Enhanced diffraction of MeV γ rays by mosaic crystals

Diffractions of γ rays by Si mosaic crystals with thicknesses of 20, 40, and 80 mm and by a 2-mm thick prefect Si crystal have been measured using a high flux 60Co source with an intensity of 2.2 TBq. The measured diffraction intensities at 1.17 and 1.33 MeV using 40-mm and 80-mm thick mosaic crystals have been enhanced by a factor of 8.6 compared with that of the perfect Si crystal. The integrated reflectivity is well described in statistical dynamical theory

Experimental investigation of an optimum configuration for a high-voltage photoemission gun for operation at ≥500 kV

We demonstrated the generation of a 500-keV electron beam from a high dc voltage photoemission gun for an energy recovery linac light source [N. Nishimori et al., Appl. Phys. Lett. 102, 234103 (2013)]. This demonstration was achieved by addressing two discharge problems that lead to vacuum breakdown in the dc gun. One is field emission generated from a central stem electrode. We employed a segmented insulator to protect the ceramic insulator surface from the field emission. The other is microdischarge at an anode electrode or a vacuum chamber, which is triggered by microparticle transfer or field emission from a cathode electrode. An experimental investigation revealed that a larger acceleration gap, optimized mainly to reduce the surface electric field of the anode electrode, suppresses the microdischarge events that accompany gas desorption. It was also found that nonevaporable getter pumps placed around the acceleration gap greatly help to suppress those microdischarge events. The applied voltage as a function of the total gas desorption is shown to be a good measure for finding the optimum dc gun configuration