Shigehisa Nakamura

Estimate of exceedance probability of tsunami occurrence in the Eastern Pacific

Abstract The exceedance probability of tsunami occurrence in the eastern Pacific was estimated as an extended Poisson process. Estimation was also undertaken for several blocks in the eastern Pacific. Some tsunamis that originated off South America hit the Japanese islands after crossing the Pacific.

Secular upheaval of datum level in relation to tsunamigenic earthquake

Abstract The relation between annual mean sea level and hazardous tsunamigenic earthquakes in the circum‐Pacific seismic zone is studied. A specific example is secular upheaval of datum level on the coast of the Japanese Islands Arc. At present, it is hard to frequently repeat geodetic surveys for finding the datum levels upheaval, and precision of satellite altimetry is hard to resolve this upheaval. On the other hand, an inversed annual mean sea level could be useful for detecting the upheaval, even though consideration should be given to climatological and oceanographical contamination in the variations of the datum level. The inversed annual mean sea level could be used to predict tsunamigenic earthquakes.

A tsunami model of Kelvin wave type

Abstract A linearized tsunami model of Kelvin wave type is introduced, and possible propagation speeds of fracture at tsunamigenic earthquakes are considered. This theoretical model shows that direction of the fracture line at the epicenter significantly affects the dynamical properties of the tsunami observed at a coast facing an ocean. Interest is focused on the amplitude of the tsunami at the coast in relation to the direction of the fault line formed at a tsunamigenic earthquake. A qualitative application of the model could be helpful for understanding past hazardous tsunami events in the northwestern Pacific.

An analysis of the 1985 Chilean tsunami

Abstract To understand the trans‐Pacific tsunamis, a study is undertaken with reference to the Chilean tsunami of 1985. The 1985 Chilean tsunami is a reminder of the severe coastal hazards in the northwest Pacific due to the 1960 Chilean tsunami. First an overview is given about the 1985 tsunami. One should designate the regions near the antipodes as a hazardous area in the planning of mitigation measures. The maximum amount of warning time available in Japan for Chilean tsunamis is less than 24 h.

Reliability of tsunami recordings from tidal wells

Abstract This article examines the reliability of tsunami recordings from tidal wells. Mareograms of tides, storm surges, and tsunamis recorded in tidal wells are introduced to determine their usefulness for observing variations in recorded sea levels and vertical speeds. A dynamic model is presented to discover whether the sea level near the coast can be well detected even in a tidal well. The sea levels vertical speed is affected by the structure of the well and by frequency characteristics of the sea level variations just outside it.

A concept of tsunami economics

A plan, or a design, of coastal structures to withstand tsunamis is considered in terms of economics. Nagao has been presenting this problem in his work since 1964, as a general problem of the design of the coastal structures. His work was reviewed by Iwagaki in 1971. If the coastal structures are large and strong enough to with‐, stand the tsunamis, the damage created by the tsunamis should be little or much less than if there were no structures. In such a case, an economic concept should be kept in mind, that an effective size of the structure from an economic point of view is one which protects the hinterland from the probable occurrence of tsunamis. This concept might be appreciated from the point of view of national economics, although its practice might be difficult without any skillful arrangements and effective cooperation.

A Note on Tsunami Bore Front

A note is presented on tsunami bore front. This tsunami bore front is an old dynamical problem but also a new problem to be understood. The tsunami event on 2004 December 26 has raised this is an urgent problem. The author introduces here a model in order to see a hydrodynamical specific property of the tsunami bore front. This modeling gives us a new understanding about what mechanics is for the interested tsunami bore front, especially, around a coastal zone. This work adds a new understanding about mechanics of water motions as the tsunamis generated by the earthquake undersea at a distant area from the coast. The model in this work points out a specific transitional pattern as a function of time and space of tsunami bore front. This modeling gives what is essential at considering tsunami bore front.

On local probability of invasive tsunami

Abstract Tsunami occurrence and invasive tsunami at a local area in the circum‐Pacific seismic zone were studied as a Poisson process. The tsunami height at Osaka, Japan, was related to tsunami magnitude. The exceedence frequency of invaded tsunami at Osaka showed a good fitness to the Poisson process. However, an adapted process should be introduced for exceedence frequency of tsunami occurrence in the western Pacific. The exceedence probability of invasive tsunami at Osaka was shown as a function of tsunami magnitude on a diagram with a parameter of the time period. The obtained result might be useful for the planning of coastal area, warning of invasive tsunami, and designing coastal structures as protection within the scope of tsunami economics.

Lowest mode of oscillations in a narrow‐mouthed bay

Abstract In order to analyze tide level around a narrow‐mouthed bay, a theoretical model was introduced. Two simple models, a square root model and a linearized model, were considered, as the tides in the bay were governed by the tide level difference on the outside and inside of the bay. An analytical solution for the linearized model was studied to provide a basic dynamical understanding. As an example of the application, Osaka Bay in Japan was studied. Tides in the bay at Osaka were estimated by referring to the tides observed at Wakayama, outside of the bay (Problem I). Inversely, estimated tides at Wakayama, referring to the observed tides at Osaka, were studied to find how close the agreement was with the observed tide at Wakayama (Problem II). The linearized model was also applied to the 1960 Chilean Tsunami. By using the model, a short‐range prediction of about one hour may be possible at the intrusion of a tsunami into Osaka Bay. The speculative model seems to give a physical understanding for th...

TRANSFORMATION OF TSUNAMIS IN A COASTAL ZONE

A significant portion of the damage by hurricanes is the storm surges. The National Weather Service has developed a dynamical-numerical model to forecast hurricane storm surges. The model is used operationally for prediction, warning, and planning purposes. The model requires fixed oceanographic and real time meteorological input data. The oceanographic data were prepared for the Gulf and East coasts of the U.S. and are stored as an essential part of the program. Meteorological data for any tropical storm are supplied by the forecasters or planners using the model. The model was applied to hurricane Camille 1969. Comparison between the observed and computed surges for Camille was satisfactory for prediction purposes.The main consideration in harbor master planning is to maximize the amount of time that the harbor can be used. The potential level of harbor utilization can be evaluated by analyzing vessel performance during harbor operations in terms of the range of imposed environmental conditions. The harbor utilization level is expressed statistically as the probable amount of time that the harbor can be used as planned.The design and construction of a major ocean outfall and diffuser system for disposal of wastewater effluents is a complex process involving an interplay of requirements originating from various disciplines. These include, among others, considerations of physical oceanography, mixing and dispersion, treatment processes, regulatory requirements, marine geology, economics and construction. The recently completed Sand Island Outfall and the newly designed Barbers Point Outfall are both on the southern coast of the island of Oahu, Hawaii, and are designed for treated sewage effluents from the densely populated portion of the City and County of Honolulu. In this paper, some design considerations of these outfalls will be examined. The emphasis in this paper is on the hydrodynamics, although other design aspects are also discussed briefly.The height of dikes and other coastal structures can only be calculated after determination of the wave run-up. Several formulas for the calculation of wave run-up are developed after model tests as a rule. But the influences of scale effects and natural wind conditions are practically unknown. To clear these questions further investigations and especially field measurements should be carried out. By measuring the markerline of floating trash on the slope of the seadikes the maximum wave run-up could be found out after four storm surges in 1967 and 1973In two graphs it will be shown that on the tidal flats the run-up depends on the waterdepth. The run-up was higher than it could be expected after model tests of 1954. With a newly developed special echo sounder the run-up could be measured in January 1976. The waves and the run-up could be registrated synchronously during two severe storm surges. As shown in Fig. 9 it was found a logarithmic distribution of the wave height, wave period and the higher part of the wave run-up. The found wave run-up is considerably higher than estimated before. The measured 98 % run-up is found about twice the computed value. That is an interesting and important result of the first synchronous recording of wave run-up on sea dikes.In March 1972 the authors firm in association with two Portuguese firms of consulting engineers, Consulmar and Lusotecna, were appointed by the Portuguese Government agency Gabinete da Area de Sines to prepare designs for the construction of a new harbour at Sines on the west coast of Portugal. The location is shown in Figure 1. The main breakwater, which is the subject of this paper, is probably the largest breakwater yet built, being 2 km long and in depths of water of up to 50 m. It is exposed to the North Atlantic and has been designed for a significant wave height of 11 m. Dolos units invented by Merrifield (ref. 1) form the main armour. The project programme required that studies be first made of a wide range of alternative layouts for the harbour. After the client had decided on the layout to be adopted, documents were to be prepared to enable tenders for construction to be invited in January 1973. This allowed little time for the design to be developed and only one series of flume tests, using regular waves, was completed during this period. Further tests in the regular flume were completed during the tender period and a thorough programme of testing with irregular waves was commenced later in the year, continuing until August 1974 when the root of the breakwater was complete and the construction of the main cross-section was about to start. The model tests, which were carried out at the Laboratorio Nacional de Engenharia Civil in Lisbon, were reported by Morals in a paper presented to the 14th International Coastal Engineering Conference in 1974. (ref. 2)Estuaries may be sequentially classified into highly stratified, moderately mixed and vertically homogeneous. An important difference between moderately stratified or vertically homogeneous estuaries, and highly stratified estuaries (salt wedges) is that, in the former, tidal currents are sufficient to cause turbulent mixing of fresh water and sea water over the full depth of the estuary. In the latter, a distinct interface or interfacial layer exists which separates the two nearly homogeneous layers. The vertical advectlon of salt in this two-layer flow is the dominant process in maintaining the salt balance. This paper presents an analytical model describing this process. Experiments have been conducted in the laboratory to compare with the developed theory.