This research proposed a framework to investigate the social costs/benefits and applicability of a land use regulation strategy based on flood risk information. The framework was applied to the Neya River basin and showed that the social benefits exceeded the costs due to land use regulation which prohibited the use of area inundated with rainfall events of 2-30 years return periods. The land use regulation strategy was also compared to a flood fighting strategy with water control facilities. Results showed the two strategies to be largely comparable in terms of required costs. Another investigation on the effects of the future population decrease indicated that the total benefits of land use regulation would become larger and its applicability might be improved with decreased population.

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Nature-friendly porous hydraulic structures composed of natural stones have attracted attention in recent years. This paper aims to propose an accuracy improved model to simulate a flow passing a porous weir which is one of the nature friendly porous hydraulic structures. In the model, the VOF method which can well reproduce complicated free surface flow around the weir was used. In the basic equations, both the effect of the laminar and turbulent flow was introduced for the resistance term by the porous medium. The model was checked to confirm its applicability by comparing the analysis to both non-over flow and over flow state passing the porous weir, and found a good agreement with the experimental results.

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Throughput of a personal computer (PC) cluster, which is a parallel processing system, is approaching a super computer level due to network speed and PC performance improvements. This study evaluated the availability of a PC cluster on a mass transport numerical simulation in subsurface. Method of Characteristic (MOC) was used as a numerical simulation method. The sequential program of MOC was parallelized using Message Passing Interface (MPI), which is a library specification for message-passing in a distributed memory system. The computing speed of eight node PC cluster was six times faster than that of sequential processing. These result in that the parallelization technique using MPI was effective in shortening of the subsurface mass transport simulation by MOC.

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Internal tides were observed during summer in Nomi Bay in Kochi Prefecture, Japan, where extensive aquaculture has been carried out, and DO was sufficiently high even at the bottom. Meanwhile, slack water and anoxic water formation were observed during autumn. It is generally well known that anoxic water forms in summer due to strong stratification in vertical direction and it disappears in autumn. Contrary to this general expectation, however, the water quality in Nomi Bay was worse in autumn than that in summer. Thus, the effect of the internal tide is quantitatively investigated using a numerical model. It is concluded that internal tides enhance the exchange of seawater, being helpful for maintaining the good water quality of the inner area in Nomi Bay.

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In the tsunami numerical simulation, the dispersive wave theory is used for simulation of a trans-oceanic tsunami propagation and a near-field tsunami propagation with soliton fission. Many numerical schemes have been proposed to solve the dispersive wave theory, although we do not have a reliable criterion for selecting an adequate scheme at present. In this study, the exact solutions of the finite difference equations of linear dispersive wave theory are derived for some numerical methods on the basis of staggered Leap-Frog scheme. Characteristics of truncation error and numerical stability of schemes are discussed by use of the exact solutions. Leap-Frog implicit scheme appeared to be applicable to the practical problems due to its superior stability as well as high-accuracy of simulation.
A new numerical method of Leap-Frog implicit scheme is developed, in which the dispersive term in the equation of motion is solved as the differential equation of Poisson-type. The computation can be conducted effectively by a high-speed solver. We compare the model results with analytical solution of linear Boussinesq equation to validate the present model. Agreement of the present model is excellent, furthermore the present model requires less computing time with good stability than that of the conventional model. The present model is able to reproduce a solution of the nonlinear dispersive wave theory, and their accuracy is verified by comparing the numerical results with available experiment data.

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To protect beaches against erosions caused by storms without coastal structures on the beaches, we have developed a gravity drainage system that naturally drains groundwater offshore through a permeable layer installed under the beach. To examine the drainage capability of the permeable layer and its effect on the beach stabilization, model experiments have been carried out and field experiments have been conducted during storms. As a result, it is confirmed that this system drains the groundwater gravitationally into the surf zone and decreases the beach erosion rate in storms, and that eroded foreshores are recovered quickly in calms.

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We calculated the monthly residence time of seawater in the inner bay of Tokyo Bay during August 2002 to June 2003 and compared it with a monthly residence time of seawater averaged from 1947 to 1974. The residence time of seawater was calculated by dividing the volume of the inner bay by an exchange rate between the inner bay and the outer bay. The exchange rate was calculated by using the monthly averaged salinity in the inner bay and the outer bay, and freshwater discharge in the inner bay. Between the years of 1947 to 1974, estimations from past studies were used to calculate the average monthly salinity. During August 2002 to June 2003, insitu salinity measurements were conducted 22 times at 27 stations within Tokyo Bay. The freshwater discharge into the inner bay was calculated from the catchment area precipitation and water mass imported from the neighboring catchment area for both data sets. From the results, the residence time of seawater during 2002 consisted of an annual mean of 31 days, with seasonal residence times of 19 days during summer and 43 days during winter. On the other hand, during 1947-1974, the mean annual residence time of seawater was 48 days, with summer and winter seasonal durations of 28 days and 79 days respectively. Thus seawater residence time in the inner bay during 2002 was greatly reduced them previous long-term estimations, by approximately 35%.

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