Satoshi Takewaka

Experimental Study of Blown Sand in a Vegetated Area

Abstract Wind-tunnel experiments were conducted to investigate the effects of vegetation, in particular beach grass, on blown sand in terms of elevation changes of a mobile sand bed and distributions of mean wind velocity for various vegetation canopies. For flexible vegetation layer displacement amplitude of vegetation leaf sway was also measured. The results indicate that a vegetation canopy with low height, high density, and vegetation flexibility is effective in reducing the sand-transport rate. Within and above a canopy of flexible vegetation, the vibrations of leaves increases the air turbulence, which creates a complex interaction between the mean wind velocity and the air turbulence, and thereby reduces the wind velocity in the vegetation area; consequently, the sand-transport rate decreases.

MEASUREMENTS OF SHORELINE POSITIONS AND INTERTIDAL FORESHORE SLOPES WITH X-BAND MARINE RADAR SYSTEM

An X-band marine radar system has been employed to determine shoreline positions and intertidal foreshore slopes over an area 1.9 km in the longshore direction at the research pier HORS in Hasaki, Japan. The X-band marine radar is an imaging radar that provides instantaneous distributions of wave crests and shorelines along the shore. Time-averaged radar images were analyzed to estimate the horizontal positions of shorelines. Simultaneously, the water surface level was measured at the pier and at the fishery port nearby to determine the elevation of the shoreline. Radar measurements were conducted from high to low tide or vice versa to trace the bottom profile and estimate the foreshore slope in the intertidal range. The horizontal positions of the shoreline were measured within an error of 10 m. The change of shoreline positions and intertidal foreshore slopes after attacks of high waves is depicted to demonstrate the potential of the radar measurements in capturing essential characteristics of coastal morphology.

OBSERVATION OF A STORMY WAVE FIELD WITH X-BAND RADAR AND ITS LINEAR ASPECTS

An X-band nautical radar system was employed to observe the wave field during a storm around the research pier HORS in Hasaki, Japan. The radar system provides instantaneous distributions of wave crests in the near-shore region, with image sequences comprising a clear spatial and temporal variation of the wave pattern. The spectra of radar echo signals and water surface elevations from wave gauge records were compared, and a relatively high coherence for a particular bandwidth was observed. A sequence of radar images was filtered at the dominant frequency, which was used to estimate the wavenumber distribution of the refraction field. The Fast Fourier Transform (FFT) and Maximum Entropy Method (MEM) were used to estimate the wavenumbers. The estimated wave angles were then compared with linear refraction analyses. The water depths along the pier were also estimated using the linear dispersion relationship and compared with the surveyed depths. The linear estimates of hydrodynamic parameters during the passage of the storm are presented in this paper to demonstrate the potential of radar measurements in capturing essential characteristics of coastal dynamics.

DYE DIFFUSION EXPERIMENT IN A LONGSHORE CURRENT FIELD

An instantaneous dye-release experiment was conducted in a coastal field study. Dye was released into a longshore current field from the research pier HORS located in Hasaki, Japan. The release point of the dye was placed in a wave reforming zone which lay between a bar, where limited breaking occurred, and shorewards final surf zone, where all waves broke. Longshore current was present between the bar and shoreline. Deformation of the dye patch was observed efficiently and effectively with a moored video system. Some essential characteristics of the surf zone hydrodynamics and shear flow dispersion are explained from the results of video image analyses of the temporal variation of the dye patch distribution.

LONGSHORE MIGRATION OF SHORELINE MEGA-CUSPS OBSERVED WITH X-BAND RADAR

Intertidal morphology was monitored continuously with an X-band radar at the research pier HORS in Hasaki, Japan. Hourly-averaged radar images were processed to observe longshore distributions of shoreline positions. Variations of longshore mean shoreline positions and their fluctuation intensities observed in the years 2005 and 2006 showed a seasonal change which followed the so-called beach-cycle. Longshore pixel intensities close to the waterline were extracted from time-averaged images for every hour of the two years to process longshore time-stack image. Longshore migration speeds of shoreline mega-cusps were estimated by cross correlation analysis of the time-stack image, and the reliability of the method was checked. Migration speeds were compared to measured longshore current speeds at the pier and the longshore component of the wave power, showing that they are highly synchronized for most conditions. Finally, the migration statistics were related to the wave data, and the results showed that the northwards migration rates were typically larger than southwards rates, which was consistent with the statistical results for wave forcing variables. Also, the relationship between the migration speeds and the forcing variables indicates that the migration was more active when the wave incidence angle was close to 45°.

WAVE RUN-UP ANALYSES UNDER DISSIPATIVE CONDITION USING X-BAND RADAR

An X-band nautical radar system was employed to examine wave run-up during a typhoon around the research pier HORS in Hasaki, Japan. Analyses on radar echo images were done to estimate the spatio-temporal variation of water fronts by manually digitizing cross-shore time stack images. Digitized instantaneous water fronts were validated with wave gauge measurements with an acceptable agreement. Longshore distribution of mean shoreline positions and inter-tidal foreshore slopes were then estimated using time-averaged images. Run-up, the height of discrete water level maxima, was estimated from the digitized water fronts with the help of foreshore profile. Run-up variations under dissipative condition were parameterized with surf similarity parameter. Low frequency variances in the run-up motion were observed, which travelled in the longshore direction.

Surf Zone Imaging with a Moored Video System

Measuring devices that are fixed to definite points are employed in the usual surf zone observation which provide accurate time records. The deployment of the instruments, however, is limited in most field experiments, which restricts the spatial understanding of the surf zone dynamics. In this context, a moored video system is developed to obtain spatial images of the surf zone. Field experiments were conducted at HORS, a research pier located in Hazaki, Japan. Results of the video record processing demonstrate the potential of the system. There are some foregoing studies existent in coastal studies using video techniques (for example: Lippmann and Holman, 1989; Holland and Holman 1997), which are mainly concentrating on inferring the sea bottom profiles by image analyses. This study is a new approach, which tries to reveal spatial and temporal variation of the surf zone hydrodynamics.

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

PIV Analysis of Filtered Radar Images to Derive Nearshore Wave Properties and Bathymetry at Hasaki, Japan

An X-band marine radar system was employed in the vicinity of Hazaki Oceanographical Research Station (HORS) located at Ibaraki prefecture of Japan. The nearshore region around HORS exhibited a complicated morphological feature represented by a barred beach as a result of frequent exposure to large storm events. The radar system provides subsequent wave images having a clear variation of wave pattern during a storm event. These images were analyzed with an improved approach based on spectra analysis and PIV technique. Both wave period and spatial distribution of wave celerity vectors were determined based on a refined approach to Bell (1999). A linear bathymetric inversion algorithm was initially examined and then applied to derive the nearshore bathymetry around HORS. The computed beach profile showed an appropriate agreement with the measured ones. However, there is a discrepancy existed due to ignorance of nearshore currents.

Intertidal Morphology and Wave Run-Up Observations during a Storm Event with X-Band Nautical Radar

A X-band nautical radar system has been employed to observe sea surface patterns of shallow coastal waters during a storm event. Wave gauges and current meters are employed in a conventional field test; however, the deployment of the instruments is limited, especially in the surf zone. In this context, a radar system is introduced here, which can remotely and broadly infer the sea surface state even during a severe sea conditions. The radar system was operated at the research pier HORS and radar echo images were collected during a passage of a storm in the vicinity of the pier. Analyses on radar echo images were done and the results demonstrate the potential of the system in observation of sea states, especially for discussion on morphological features and wave run- up motions. Analyses on radar echo images were done and (i) longshore distribution of mean shoreline positions and intertidal foreshore slopes, and (ii) temporal and spatial variation of wave run-up, are discussed.