Masafumi Matsuyama

The effect of bathymetry on tsunami characteristics at Sisano Lagoon, Papua New Guinea

The 1998 Papua New Guinea tsunami was greater than expected from its earthquake magnitude. The area of significant impact was small, approximately a 30 km stretch near the mouth of Sissano Lagoon, Papua New Guinea. To explain the localized nature of the event, a submarine landslide has been conjectured to be responsible. Our study indicates that offshore bathymetry is critical to predicting tsunami coastal behavior. Model runs with newly obtained bathymetric data indicate that an earthquake fault source combined with the existing seafloor geometry may also explain the concentrated tsunami. Although the definitive cause of the Papua New Guinea tsunami remains uncertain, local bathymetry had a notable effect on the wave behavior.

ASPECTS OF INUNDATED FLOW DUE TO THE 2004 INDIAN OCEAN TSUNAMI

This paper reports the results of field surveys of the 2004 Indian Ocean Tsunami in Southern Thailand and Northern Sumatra. After outlining the general state of the tsunami strike, tsunami height and distribution in Sec. 3, aspects of inundated flow such as current velocity (water particle velocity), fluid force, relation between inundation depth and the degree of damage to reinforced concrete buildings, shape and height of surge front, inundated flow with floating bodies, effects of coastal forest on reducing tsunami energy and force are discussed. Damage estimation problems are also discussed. A simplified theory for predicting the moving velocity of floating bodies with inundated flow is presented.

Damage to coastal villages due to the 1992 Flores Island earthquake tsunami

A field survey of the 1992 Flores Island earthquake tsunami was conducted during December 29, 1992 to January 5, 1993 along the north coast of the eastern part of Flores Island. We visited over 40 villages, measured tsunami heights, and interviewed the inhabitants. It was clarified that the first wave attacked the coast within five minutes at most of the surveyed villages. The crust was uplifted west of the Cape of Batumanuk, and subsided east of it. In the residential area of Wuring, which is located on a sand spit with ground height of 2 meters, most wooden houses built on stilts collapsed and 87 people were killed even though the tsunami height reached only 3.2 meters. In the two villages on Babi Island, the tsunami swept away all wooden houses and killed 263 of 1,093 inhabitants. Tsunami height at Riang-Kroko village on the northeastern end of Flores Island reached 26.2 meters and 137 of the 406 inhabitants were killed by the tsumani. Evidence of landslides was detected at a few points on the coast of Hading Bay, and the huge tsunami was probably formed by earthquake-induced landslides. The relationship between tsunami height and mortality was checked for seven villages. The efficiencies of trees arranged in front of coastal villages, and coral reefs in dissipating the tsunami energy are discussed.

Three-dimensional sediment transport processes on tsunami-induced topography changes in a harbor

A three-dimensional hydrostatic numerical simulation on tsunami-induced topography changes near a harbor is carried out, and sediment transport processes on a significant local deposition near the center of the harbor caused by a tsunami, which was observed in an early experimental study, are investigated. This local deposition has not been well predicted by a vertically averaged hydrodynamic model. The results show that velocities, water levels and topography changes in the harbor predicted in this study agree with the experimental data. The local deposition has relations with a vortex generated in the harbor when the tsunami attacks the harbor. At areas near the vortex center, a secondary flow of the first kind develops, and it plays the role of transporting suspended sediment to the vortex center, located near the center of the harbor, and causes the local deposition there. In order to predict deposition areas with high accuracy, the secondary flow effects should be incorporated in prediction methods of tsunami-induced topography change. Key words: Tsunami, tsunami deposits, sediment transport, hydrodynamic model.

Response of Residents at the Moment of Tsunamis-The 1992 Flores Island Earthquake Tsunami, Indonesia-

The number of loss of lives due to the 1992 Flores island tsunamis was about 1,000. The maximum tsunami runup height 26.2m was measured at Kroko. At Babi island, reflected tsunami with the height of 5 to 7m attached two villages so that every structure was broken and swept away. At Waring, mooring fishing boats were easily carried to the dense-housing areas by tsunamis so that wooden-made houses were swept away by dominos. Through the questionnaire, tsunami characteristics and residents’ behavior are also described. Most residents have no information on tsunamis. After looking at tsunami coming, they run away to safe places. Therefore, the disaster weak such as children and old people might fell victim to the tsunami disaster. As disaster lessons, spread of tsunami information such as local tsunami characteristics and refuge programs are very important to mitigate the damages in tsunami-prone areas.

Flow Strength on Land and Damage of the 1998 Papua New Guinea Tsunami

Runup and inundation data on the sand spits of Sissano lagoon are described with discussions of flow state, current velocity, and degree of damage to houses. Sand erosion data on the sand spits are also described with discussions of their relation to the inundation depth and the current velocity. Laboratory experiments were carried out to confirm the flow state and to discuss effects of vegetation and so on.

Coastal Current Observation in the Area of Abrupt Topographic Change with DBF Ocean Radar

A DBF ocean radar can detect the surface current pattern every 15 minutes. The tidal current and the residual current were investigated through the continuous observation of the DBF (digital beam forming) radar in the middle-west area of Ariake Bay in autumn 2005. For the validation of the surface current data by the DBF ocean radar the ship-board ADCP measurements were also conducted at spring tide. As the result of the comparison of both surface current data, the correlation coefficient and the standard error of the current speed are 0.89 and 7.23cm/s, and those of the current direction are 0.92 and 35.89 degree. The harmonic analysis of the long-term data by the DBF ocean radar clarifies the spatial pattern of the principal tides and the residual currents. The residual currents in 15days flow south to southeastward caused mainly by the tidal residual currents due to the topographic asymmetry.

Short-time observation of coastal currents with DBF radar

We have developed a DBF (Digital Beam Forming) radar which can complete one round observation of surface current every 15 minutes by a digital beam forming system. The predominant frequency is 41.9 MHz, range-directional resolution 500 m, maximum reach 25 km, azimuthal resolution 13-17/spl deg/ and velocity resolution 2.13 cm/s. We applied the DBF radar for current observation in the northern area of Ise Bay and compared the data with those collected synchronously by current meters set at 1 m and 2 m depth at two stationary points for eight days. The correlation coefficient between the DBF radar and the current meter at 1 m depth was 0.7-0.8 and the standard error 5-10 cm/s. In view of the fact that the measurement depth for the DBF radar was about 30 cm they seem to be coincided well. Next, we assimilated the continuous observation data of the DBF radar into a quasi three dimensional model of coastal currents by the nudging method in order to infer spatial and temporal changes of the currents. When the inferences of three-dimensional current were compared with the data gained by concurrently implemented ADCP spot observation at 4 measurement points, the current distribution for the surface-to-3 m depth layer was improved well as to speed and direction.

A basic study of utilization of DBF ocean radar with VHF BAND for tsunami detection in open sea

After 2011 Tohoku-Pacific Ocean earthquake, Chubu Electric Power Co., Inc. has been voluntary addressing to build the tsunami observation network at Hamaoka nuclear power station mainly utilizing an ocean radar, the GPS-mounted Buoy, the submarine cabled real-time seafloor observatory system and a highly sensitive camera. The purpose of this study is to evaluate the basic usability of the VHF ocean radar through field observation in front of Hamaoka nuclear P/S, and to investigate short time variability of coastal currents under the natural condition for construction of tsunami warning system. High acquisition area of valid current data of the ocean radar varied from 13 to 17 km off the observation site in the radial direction, which suggests the maximum detection range of tsunami by the ocean radar in this sea area. The comparison of the current data by the ocean radar with the current meter resulted in the correlation coefficient of about 0.8 in radial direction, and the correlation improved after reducing the wind-driven effect by the multiple regression analysis with the wind data. As the temporal variability of coastal currents to 1-hour interval was almost about 5 to 10 cm/s, the current anomaly excited by tsunami greater than 10 cm/s seems to be detectable by the ocean radar in this area.

Practicality of VHF-band ocean radar to surface current observation in open sea area

The fundamental characteristics of the high-resolution ocean radar system with the VHF band under severe winter conditions in the open sea area were evaluated through the field observations in Wakasa Bay. As the results, over 60 percentage of the detection ratio of the horizontal currents by the radar was confirmed in the extent of 15km as to the maximum observation range of 25km. The comparison between the current data of the radar and that of the current meter in one-month observation at the close point to Monjyu Power Station showed good correlation with a correlation coefficient over 0.7 and the standard error of 5 to 10cm/s. It was also indicated that the currents from surface to 2.5m below the surface were mainly driven by the wind fluctuation component of the 25.3 hours with the phase of 30 minutes. The ocean surface radar could detect the horizontal current patterns in the irregular current field under the sever weather and oceanic condition, which might largely contribute to comprehend unknown ocean characteristics in an open sea.