Haijiang Liu

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.

Equatorial electromagnetic emission with discrete spectra near harmonics of oxygen gyrofrequency during magnetic storm

Equatorial electromagnetic ELF emissions at a frequency range around the oxygen ion gyrofrequency were observed in the low altitude plasmasphere (L = 1.5∼2.5) during magnetic storms with high activity of the ring current by triaxial search coil and single component electric field instruments on board the Akebono satellite. We find that a particular type of the ELF emissions occur with a multi-band spectral structure around the local gyrofrequency (Fo+) of the oxygen ion (O+) during the main phase of magnetic storm. Our finding includes the following; The multi-band emissions are localized in the morning sector and often consist of several spaced frequency bands closely related the local gyro-harmonics of oxygen ion (O+) with a fundamental frequency of Fo+ or 2Fo+. The frequency of each band is dependent on the local magnetic field variation along the satellite orbit and the spacing between the bands is almost equal to Fo+ or 2Fo+. It is suggested that emissions are generated by terrestrial oxygen ion composing the ring current area during the main phase of magnetic storm.

MODELING SEDIMENT MOVEMENT UNDER SHEETFLOW CONDITIONS USING A TWO-PHASE FLOW APPROACH

A two-phase flow model based on the mass and momentum conservation is presented, which can simulate the fluid and sediment movement on a flat bed under sheetflow conditions. The governing equation in the vertical direction is modified by considering the influence of static normal intergranular stress. Horizontal pressure gradient is modified in terms of sediment concentration. A criterion is introduced to decide the temporal variation of the still bed level during a one-wave cycle. Vertically parabolic eddy viscosity and corresponding sediment diffusion coefficient are assumed. Numerical results include initial validation comparisons with the existing experimental data from Horikawa et al. [1982], All measured properties, such as the concentration, sediment velocity and sediment flux are reproduced quite well. Further investigations on the experimental data of Delft Hydraulics [Ribberink and Al-Salem, 1995; Dohmen-Janssen, 1999] cover a wide range of pure sinusoidal wave and combined wave/current flow conditions for different sediment sizes. The numerical results are satisfactory with respect to the measured time-varying and time-averaged concentration distributions both in the sheetflow layer and the suspended layer. Taking into account the various experimental measurements, the present two-phase flow model shows the significant superiority over an existing two-phase flow model [Mina and Sato, 2004], Comparison between the measurement and the present simulation for sediment flux and net transport rate is also performed.

LABORATORY STUDY ON SHEETFLOW SEDIMENT MOVEMENT IN THE OSCILLATORY TURBULENT BOUNDARY LAYER BASED ON IMAGE ANALYSIS

Laboratory experiments were conducted under sinusoidal sheetflow conditions. By using image analysis, which overcame the demerits introduced by intrusive measurements, the time-varying as well as maximum erosion depths for different flow conditions were estimated. The temporal variation of suspended sand concentration in the sheetflow layer showed high concentration during the deceleration phase and relatively low concentration during the acceleration phase. Rapid sand deposition was observed around flow reversals. The phase lag between the free stream velocity and the sand concentration increased with elevation. The temporal and the spatial distribution of suspended sand concentration revealed an asymmetry in the suspension process, namely relatively long suspension (erosion) and short sedimentation (deposition) reflecting the asymmetry in turbulence. The instantaneous sand particle velocities were estimated using a PIV technique. The mean particle horizontal velocity was found to decrease at the beginning of the acceleration phase, corresponding to the rapid deposition of sand.

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.

FIELD STUDY ON THE NEARSHORE SEDIMENT PROCESS AROUND THE TENRYU ESTUARY USING IMAGE ANALYSIS

A field investigation, based on the shore-based video technique, was conducted to estimate the temporal and spatial variation of the nearshore sediment process at Tenryu estuary, Japan. Image analysis with a new boundary detection technique and an improved rectification approach was used on the recorded images. A case study, by focusing on the time period from Jul to Sep, 2007 when significant typhoon and flood visited the relevant region, was carried out to reveal the qualitative and quantitative insights into the corresponding morphodynamic behaviors.

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

MONITORING THE NOURISHED SAND LONGSHORE MOVEMENT BASED ON FELDSPAR LUMINESCENCE MEASUREMENT

In this study, monitoring the nourished sand longshore movement was conducted based on the feldspar luminescence measurement, including both Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL). Nourished sand presents larger TL/OSL properties than the natural beach sand. Investigation on the spatiotemporal distribution of post-filling beach sand luminescence features at the Miyazaki coast and the Shounan coast, Japan, provides a new approach to evaluate the nourished sand movement characteristics. Comparing these two techniques, it is confirmed that the OSL is suitable for study on short-term sediment movement within a limited area; whereas, TL can be applied to the long-term and large-scale nearshore process assessment.

Experimental study on the dam-break hydrographs at the gate location

When studying the dam-break flow phenomenon, the basic hydrodynamic features of the dam-break flow at the gate location should be verified primarily. In this study, laboratory experiments were performed in a rectangular and horizontal flume with the same initial water head setting on the dry and wet downstream bed conditions. Water surface elevation was extracted through image analysis and validated by comparing with the data measured using a wave gauge. Temporal variation of the water surface elevation at the gate location, quantified in terms of high-speed video recorded images, can be divided into three stages, the sharp decreasing stage, the relatively steady stage, and the gradually decreasing stage. Applicability of several classic analytical solutions of the dam-break problem at the gate location was validated using present experimental data. Ritter’s solution is effective for the dry bed condition while Stoker’s solution could be applied to the wet bed case, and both are only applicable during the steady stage. Lin’ solution reproduces the gate-site hydrographs well during both the relatively steady and the gradually decreasing stages, especially for the condition under which the down-upstream water depth ratio is smaller than 0.138.

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.