Tomohiro Takagawa

PROPAGATION AND INUNDATION CHARACTERISTICS OF THE 2011 TOHOKU TSUNAMI ON THE CENTRAL SANRIKU COAST

Propagation and inundation characteristics of the 2011 Tohoku tsunami on the central Sanriku coast are investigated through field surveys and numerical simulations using offshore wave recordings as incident wave conditions. The numerical model successfully reproduces the extent of flood areas as well as the distribution of tsunami heights along the intricate coastline except for run-up of extreme heights over steep slopes. The survey and computed results suggest significant variations of tsunami heights along the coastline. Their positive dependency on topographic slopes implies that the incoming tsunami propagates in standing wave mode to precipitous sites while in progressive wave mode accompanied by wave breaking over gentle slopes. Temporal-spatial analysis of wave properties in different bays reveals that the inner bay topography provides a clear contrast to inundation characteristics. The impacting waves have extreme heights due to the funnel effect and local wave resonances causing highly transient flooding in narrow V-shaped bays whereas tsunami surges over longer periods across innermost shores of U-shaped bays to produce large horizontal velocities during both run-up and backwash phases.

Investigation of the sediment movement along the Tenryu-Enshunada fluvial system based on feldspar thermoluminescence properties.

Abstract The thermoluminescence (TL) properties of feldspar were used to investigate the sediment movement processes in a fluvial–coastal system. Field samples were collected at various locations along the Tenryu River and the Enshunada Coast in Japan. After a series of pretests, an appropriate TL measuring sequence was proposed for this study. Applying this approach, the natural TL intensity of feldspar grains was measured. Owing to the young age of the research area, it was assumed that the natural TL difference was ascribed to the sample depositional environment under which different possibilities of sunlight exposure exist. Surface sediment particles in the target area were classified into three groups in terms of TL intensities, i.e., river sand with large TL signals, coastal sand with medium TL signals, and dune sand with small TL signals. Stratified configuration of the Nakatajima Coastal Dune was observed from the underground-sample TL glow curves with a top, windblown, dune-sand layer; a bottom, wave-induced, coastal sand layer; and a mixing layer in between. A rather complex sediment-movement pattern in front of the Tenryu River mouth was revealed after investigation of the underwater samples. Because of the seawater influence, acting as an ultraviolet filter to sunlight, underwater samples present a larger, high-temperature TL peak than the low-temperature peak; whereas these two are almost the same for ground samples.

DYNAMIC TOPOGRAPHY CHANGES OF SAND SPIT OF THE TENRYU RIVER MOUTH DUE TO OVERTOPPING WAVES

In October 2009, the category-5 typhoon T0918 hit the Japan Pacific Coast and significant topographic changes occurred along the Enshu-nada coast due to the storm waves. The significant wave height was more than 10 m and rapid landward migration of the sand spit of the Tenryu river mouth was observed. Based on X-band radar image analysis, spatial and temporal changes of overtopping waves and corresponding sediment flux across the sand spit were revealed. Spatial concentration of the sediment flux can be explained by the nonlinear relationship between wave height of overtopping waves and corresponding sediment fluxes. A sediment flux model was developed based on the relationship. Maximum flux was estimated by the model to be 51 m/m/hour, which was achieved when high waves and high tide were observed simultaneously. Introduction Coastal topography changes rapidly due to storm waves. In October 2009, the category-5 Typhoon T0918 (Melor) hit the Japan Pacific Coast (Fig. 1). The central atmospheric pressure of the typhoon reached 910 hPa and maximum wind speed recorded 55 m/s. The Tenryu River mouth area, shown in Figure 1, is one of areas that suffered the most significant topographic changes due to T0918. The significant wave height marked more than 10 m (Fig. 2). The sand spit at the mouth of the Tenryu River migrated landward about 50 m (Fig. 3). From the viewpoint of coastal sediment management, it is very important to reveal the sediment movement and corresponding topographic change around the sand spits of river mouths, because the sand spit morphology affects both flow and wave fields and sediment movements (e.g. Tajima et al. 2011). However, observationally-based analysis of sediment movement is limited in such extreme wave event because of the difficulty of installation of observation equipments under high-energy conditions. In this study, a remote sensing technology of Xband radar was utilized for wave observation. Overtopping waves across the sand spit were successfully captured in the X-band radar observation. This study aims to establish a sediment transport model on overtopping waves across sand

SELECTIVE MOVEMENTS OF SAND AND GRAVELS AND RESULTING DYNAMIC MORPHOLOGY CHANGES OBSERVED AROUND THE TENRYU RIVER MOUTH

This study focuses on the selective movements of sand grains and gravels around the sand spit of the Tenryu River mouth in Japan. Color sand and gravels were placed at the outer edge of the swash zone of both west and east sides of the sand spit and their changing spatial distributions were traced afterward. Observed different movements of sand and gravels were compared with the dynamic deformation of the sand spit and surrounding hydrodynamic characteristics such as waves and currents, all of which were continuously and quantitatively analyzed based on succeeding snap shot images recorded by field cameras and Xbanded radar. Numerical model based on Boussinesq-type non-linear dispersive wave with porous layer was finally applied to further investigate the hydrodynamic characteristics observed around the river mouth where waves, currents and complex river mouth morphology interact with each other.

57. LONGSHORE SEDIMENT MOVEMENT ALONG THE ENSHUNADA COAST INFERRED FROM FELDSPAR THERMOLUMINESCENCE

Investigation on the sediment movement characteristics was conducted in a broad research area with a 150-kilometer stretch along the Enshunada Coast and the Suruga Bay, Japan based on feldspar thermoluminescence (TL) properties. River sand grains (primary source) present a higher TL signal than beach sand. Along the nearshore area, local TL intensity peaks are observed at the river mouth and sample TL intensity gradually deceases with increasing distance from the river mouth, which indicates sediment longshore transport features. Far away from the river mouth, the decreasing trend on TL intensities terminates. Taking the spatial distribution of TL intensities into account, a quantitative estimation on the longshore sediment flux was carried out based on the total river sand discharge. A sunbath test was implemented to help to distinguish the beach sand constituents. Identification of coastal sand sources was achieved in terms of the profile and magnitude of the measured TL glow curves.

DEVELOPMENT OF HIGH PRECISION TSUNAMI RUNUP CALCULATION METHOD COUPLED WITH STRUCTURE ANALYSIS

The 2011 Great East Japan Earthquake (GEJE) has shown that tsunami disasters are not limited to inundation damage in a specified region, but may destroy a wide area, causing a major disaster. Evaluating standing land structures and damage to them requires highly precise evaluation of three-dimensional fluid motion – an expensive process. Our research goals were thus to develop a coupling STOC-CADMAS (Arikawa and Tomita, 2016) coupling with the structure analysis (Arikawa et. al., 2009) to efficiently calculate all stages from tsunami source to runup including the deformation of structures and to verify their applicability. We also investigated the stability of breakwaters at Kamaishi Bay. Fig. 1 shows the whole of this calculation system. The STOC-ML simulator approximates pressure by hydrostatic pressure and calculates the wave profiles based on an equation of continuity, thereby lowering calculation cost, primarily calculating from a e epi center to the shallow region. As a simulator, STOC-IC solves pressure based on a Poisson equation to account for a shallower, more complex topography, but reduces computation cost slightly to calculate the area near a port by setting the water surface based on an equation of continuity. CS3D also solves a Navier-Stokes equation and sets the water surface by VOF to deal with the runup area, with its complex surfaces of overflows and bores. STR solves the structure analysis including the geo analysis based on the Biot’s formula. By coupling these, it efficiently calculates the tsunami profile from the propagation to the inundation. The numerical results compared with the physical experiments done by Arikawa et. al.,2012. It was good agreement with the experimental ones. Finally, the system applied to the local situation at Kamaishi bay. The almost breakwaters were washed away, whose situation was similar to the damage at Kamaishi bay.

Results of Post-Field Survey on the Mw 8.3 Illapel Earthquake Tsunami in 2015

On 16 September 2015, a Mw 8.3 earthquake occurred off the coast of Illapel, Central Chile, generating a tsunami that caused moderate damage along the coast of Central Chile. To investigate tsunami damage and evacuation responses, we conducted a post-tsunami field survey between La Serena and Concón. The average tsunami inundation height in the survey area was 3–4 m; however, in the innermost corner of Coquimbo Bay, which was shielded from the tsunami source by the Coquimbo Peninsula, the tsunami inundation height was 6.0–6.2 m, decreasing gradually along the coast away from the corner. This phenomenon was also confirmed around Tongoy Bay, which is geographically similar to Coquimbo Bay. Based on our results, we hypothesize that the tsunami was diffracted and refracted by the peninsula, and then converged on the innermost corner of the bay, located behind the peninsula. Interviews with local residents confirmed that most people evacuated immediately after feeling strong seismic motion, but before the tsunami warning and evacuation orders were issued. However, several people did not evacuate, either because they believed that they were located in a safe place, or because they wanted to protect their property.

Sand Transport and Sedimentary Features Based on Feldspar Thermoluminescence: A Synthesis of the Tenryu–Enshunada Fluvial System, Japan

ABSTRACT Liu, H.; Takagawa, T., and Sato, S., 2014. Sand transport and sedimentary features based on feldspar thermoluminescence: a synthesis of the Tenryu–Enshunada fluvial system, Japan. On the basis of the feldspar thermoluminescence (TL) measurement, sediment transport features were investigated in the entire Tenryu River watershed and the corresponding littoral sediment zone of the Enshunada coast. Subsequently, the sedimentary configuration and sand provenance in the river mouth area were studied using subsurface samples. In the Tenryu River watershed, TL intensities of the riverine samples fluctuated with respect to the local geological characteristics. Comparing with samples collected from the main river route, branch samples generally present larger TL intensities. Nevertheless, sediment supply from branches is considered to be rather limited in terms of their small discharges. Increase in the riverine TL property can be detected in the immediate dam downstream area, as well as the region with mass sand movements. Taking into account the Enshunada coastal samples, measured sediment TL intensities present a local peak value around the river mouth and a decreasing trend with the increasing distance away from the river mouth. This is attributed to the river sand supply and the gradual sunlight exposure during the longshore sediment transport. In general, riverine samples present much larger TL intensities than coastal samples, and a criterion on the basis of a TL intensity value of 5 was recognized to distinguish these two types of sands in the study area. Applying this criterion to the subsurface samples collected in the river mouth sand spit, a sedimentary interface at Tokyo Peil −1 m was confirmed with coastal sands (small TL intensity with wave experience) located on the top and riverine sands (large TL intensity without wave experience) deposited at the bottom, which is in agreement with estimations from other physical and geomorphological approaches.

Analysis of flooding behaviors due to the 2011 Tohoku Tsunami focusing on its flow direction

While most of post-tsunami studies focus on tsunami inundation heights, this study mainly focuses on inundating tsunami flow, which may have more direct impact on damages of the inundated area. Based on tilted poles and trees in the inundated area, this study collected data of tsunami flow directions at Rikuzen-takata, Ootsuchi, and Naraha. Obtained data totaled more than 850. Numerical tsunami inundation model was then applied to each of these sites to link simulated time-varying flow patterns with observed flow directions, which may represent the direction at which the largest force acted at each location. Proposed methodology is useful to deepen our understandings on the dynamics of the inundation tsunami flow.