Motohiko Umeyama

Eulerian-Lagrangian analysis for particle velocities and trajectories in a pure wave motion using particle image velocimetry

This paper investigates the velocity and the trajectory of water particles under surface waves, which propagate at a constant water depth, using particle image velocimetry (PIV). The vector fields and vertical distributions of velocities are presented at several phases in one wave cycle. The third-order Stokes wave theory was employed to express the physical quantities. The PIV techniques ability to measure both temporal and spatial variations of the velocity was proved after a series of attempts. This technique was applied to the prediction of particle trajectory in an Eulerian scheme. Furthermore, the measured particle path was compared with the positions found theoretically by integrating the Eulerian velocity to the higher order of a Taylor series expansion. The profile of average travelling distance is also presented with a solution of zero net mass flux in a closed wave flume.

Coupled PIV and PTV Measurements of Particle Velocities and Trajectories for Surface Waves Following a Steady Current

This paper describes kinetic aspects of surface waves propagating with or without a current in a constant water depth. Physical properties of the velocity and trajectory of a water particle during one wave cycle were investigated experimentally using intrusive and nonintrusive measuring techniques. Two optical devices were used for the nonintrusive measuring technique: particle image velocimetry (PIV) and particle tracking velocimetry (PTV). The techniques provided the velocity fields and distributions at various phases and the trajectory of the water particle over one cycle, respectively. The instantaneous velocity from the PIV measurement was compared with that given by a third-order Stokes wave theory. Suitable agreement between the two velocity profiles proves the ability of the PIV technique to accurately measure both temporal and spatial variations of the velocity. The PIV technique was applied to the prediction of particle trajectories in an Eulerian scheme. The significant novel element of our technique is the use of the velocities at surrounding Eulerian grid points to identify a Lagrangian point.

Investigation of Single and Multiple Solitary Waves Using Superresolution PIV

AbstractMeasurements of the characteristics of a solitary wave propagating alone or against a current were performed using superresolution particle image velocimetry (PIV). The velocity field was obtained when the solitary wave passed in front of a high-speed charged-coupled device camera. The instantaneous and spatial surface profiles were measured using four resistance wave gauges and the particle mask correlation method that was applied to monitor tracers seeded into the flow field. In addition, an algorithm was employed to track water particle paths in a two-dimensional scheme. The trajectory measured using the superresolution PIV was compared with the one modeled using the solitary wave theory. This flow visualization technique was applied to analyze the collision of two solitary waves propagating in the same and in opposite directions.

Shore Protection against Sea Level Rise and Tropical Cyclones in Small Island States

AbstractThe abrupt phase shift of the El Nino Southern Oscillation (ENSO) resulted in several cyclones during 2004–2005 in the Southeast Pacific and east of the International Date Line. During a four-week period in February 2005, cyclones Meena, Nancy, Olaf, and Percy struck the Cook Islands. Because these cyclones were stronger than Cyclone Sally in 1987, which is reputed to have caused the most damage to Rarotonga in the past, the government was well prepared for the storm and its effects. Evacuation shelters were set-up throughout the island and no injuries or casualties were reported. In contrast, the storm surge and high waves of Sally devastated houses, shops, and the airport on the seafront in north Rarotonga. Since Sally, the government of the Cook Islands has worked together with the Japan International Cooperation Agency (JICA) to specifically protect the shoreline from storm surge; the outcome of their work provides scientific information regarding vulnerability to future hypothetical cyclone e...

A new concept for lower-lying land areas and coastal villages safe from natural disasters

Over the next century or two, rising sea levels resulting from the global warming and the increased frequency of extensive natural disasters caused by global climate change will bring about serious problems. Such problems are especially foreseen in lower-lying coastal and riverside areas, located below sea level. In this paper, we introduced a new concept that utilizes floating foundations to create an urban community base in man-made lagoons and/or inlets. This paper proposes the establishment of secure bases by using floating structure to safeguard against any risk brought about by future natural disasters, especially flooding due to heavy rain, storm surges caused by typhoons, tsunamis, earthquakes and so on. In addition, and idea for reconstruction of coastal areas in northeast Japan which was devastated by the extraordinary tsunami is introduced. This idea will help realize a sustainable city and rural environment for a prosperous future.

Time-Averaged Distributions of Velocity and Sediment-Concentration under Irregular Waves and Currents

The velocity and sediment-concentration distributions in the case of irregular waves and currents are studied by using the theoretical model developed by means of a mixing-length concept for sediment-laden flow, and by improving the sediment-concentration formula based on the same hypothesis. The predicted results are compared with a set of velocity and sediment-concentration data collected previously in a recirculating wave flume. The time-averaging procedure is adopted, since it eliminates the random scatter of the instantaneous components of velocities and concentration. The general characteristic of the time-averaged velocity is greatly affected by the significant-wave height and the flow direction to the generated waves, although the mixing length is a function of the sediment concentration. To compute the time-averaged vertical distribution of suspended sediment, two different formulae are proposed. The agreement between the simulated velocity and concentration profiles and the measured ones is satisfactory for the flow containing fine sands.

Dynamic-pressure distributions under Stokes waves with and without a current

To investigate changes in the instability of Stokes waves prior to wave breaking in shallow water, pressure data were recorded vertically over the entire water depth, except in the near-surface layer (from 0 cm to −3 cm), in a recirculating channel. In addition, we checked the pressure asymmetry under several conditions. The phase-averaged dynamic-pressure values for the wave–current motion appear to increase compared with those for the wave-alone motion; however, they scatter in the experimental range. The measured vertical distributions of the dynamic pressure were plotted over one wave cycle and compared to the corresponding predictions on the basis of third-order Stokes wave theory. The dynamic-pressure pattern was not the same during the acceleration and deceleration periods. Spatially, the dynamic pressure varies according to the faces of the wave, i.e. the pressure on the front face is lower than that on the rear face. The direction of wave propagation with respect to the current directly influences the essential features of the resulting dynamic pressure. The results demonstrate that interactions between travelling waves and a current lead more quickly to asymmetry. This article is part of the theme issue ‘Nonlinear water waves’.

A Comprehensive Approach for Estimating Hydraulic Quantities in a Multi-branched Estuarine System

Despite many previous investigations, there is no theoretically justifiable equation to determine the freshwater discharge, tidal velocity, and salinity in a complex estuary that contains several branches. In this study, the longitudinal distributions of freshwater and salinity concentration in multi-branched estuaries are investigated using a mathematical model, considering the energy balance by frictional head loss and the salt balance by diffusion and advection in a one-dimensional steady-state condition. We attempt to obtain the time-mean freshwater discharge rate quantitatively at a junction where the main flow separates into two branches. The salinity distribution along each branch is calculated by following the dispersion coefficient equation previously proposed for a single estuary. The salinity field is divided into the near-coast region and the upstream region, because in each segment the inland velocity varies according to the tidal flux. For computing the tidal velocity, we propose the Burgers equation and the Hoph-Cole transformation. The theoretical results obtained by these equations were found to be in good agreement with a set of observed data in the Red River estuary system.

A concept for water-based community to sea level rise in the lower-lying land areas — Toward reconstruction of coastline area in disaster-stricken northeast Japan

Strenuous efforts have been made to restore the global environment and create a sustainable society. In spite of the widespread movement to diminish greenhouse gas however, a substantial result is still to be attained. Meanwhile, the situation has worsened for many rural residential areas and cities near sea and/or rivers, such as the tidewater region, faced grave danger. On Mar. 11, 2011, incredible disasters struck the coastlines of northeast Japan in the form of huge earthquakes and massive tsunami. A 10m-high breakwater, the strongest available at that time, was destroyed completely. We have now learned that whatever type of civil engineering structure is used, it is impossible to counter the immeasurable power of nature. In this paper, we propose a revolutionary concept for establishing innovative urban community bases in lower-lying lands. The living space is founded upon floating platforms riding the water in artificial inlets and/or lagoons, to realize a sustainable residential environment which ensures safety and prosperity. This paper presents concrete details as to the application of our concept to the proposed water-based urban community in the Kesennuma Harbor Area of northeast Japan. Using this concept plan was evolved for a floating city, with its economic efficiency having been evaluated; in addition, some engineering studies were conducted on assumption that an actual site in Kesennuma was to be redeveloped.

Measurements of Particle Velocities and Trajectories for Internal Waves Propagating in a Density-Stratified Two-Layer Fluid on a Slope

The existence of internal waves was first recognized by a measurement of water temperature by Helland-Hansen and Nansen (1926). Later, the vertical structure of internal waves has been detected by observation of temperature, salinity, or ocean current, while the propagation of internal waves has been identified by images from radar or acoustic Doppler and echo sounder. Shand (1953) found internal wave fronts appearing on aerial photographs. From observation of offshore temperature variations, LaFond (1962) found that time-dependent isotherms are flattened for a shallower thermocline and peaked for a deeper thermocline on the wave crests. Apel et al. (1975, 1976, 1985) reported a series of research results for the internal waves observed in pictures from satellites, space shuttles, and aircraft. In addition to these indirect photographs, they also used various instruments such as the expendable bathythermograph (XBT), acoustic echo sounding, and ship radar.