This paper describes a procedure of estimating the PMP (Probable Maximum Precipitation) using radar raingages and DAD (Depth-Area-Duration) analysis. Radar data obtained from two radars, which cover the Naka river basin in Japan, are calibrated with 45 ground raingages. The radar data indicated the local extreme rainfall which ground raingages could not catch. In obtaining DA relationship with spatially distributed rainfall data, constant area and fixed rainfall methods are proposed, and the difference of DAD equations given by the two methods is investigated. A nonlinear optimization technique determining parameters of DAD equation objectively is developed and applied to the records of maximum storms in Japan and the Naka river basin.

View full abstract

In flood control, the decision support information made by short-term severe rainfall prediction methods and flood control support systems are indispensable for quick and appropriate operation. On the other hand, it is difficult to forecast the weather of meso-β to γ scales numerically because of limited computational resources, stability of calculation, time and cost for computer and exactness of calculation. In this paper, based on these background, we develop the qualitative short-term severe rainfall prediction method using artificial intelligence for practical use in order to apply to river site by forecasting the time series variation of distribution of severe rainfall having a lead time of six hours, automatically.

View full abstract

Run-off analysis taking into account physical processes have been able to be carried out due to sophisticated computers. River basin is formed by a flow in a river channel, an infiltration flow and a groundwater flow that have different hydraulic characters. In order to pay attention to run-off analysis in a mountainous river, an infiltration flow model and a river flow model were proposed by using an unsaturated infiltration flow theory and a turbulent closure model, respectively. A whole basin was covered with rectangular grids and computations were carried out by finite difference method. Moreover, to carry out a long-term simulation during a thaw period, a snow-melting model was introduced by using heat budget equation. Run-off simulations were carried out in summer and a thaw period, and in both cases, good agreements were obtained even in long-term simulations.

View full abstract

A basin runoff simulation system is developed on the basis of a new digital topographic model which can describe both concentration and divergence of water flow. At first, the digital topographic model is briefly stated which is the basis of the runoff simulation system. At second, the runoff simulation system is elaborated, especially the data processing to extract the information used in a runoff model (slope length, gradient, connections among slope units, order of runoff computation, etc.) and the method to calculate runoff discharge from slope area by using kinematic wave model. Finally, the application results of the runoff simulation system are presented.

View full abstract

Model study is done to provide knowledge needed in a project of gas leakage prevention from unlined rock caverns for storage of compressed gas such as LPG. Rising velocities of bubbles with various diameters are measured in the water flowing through vertical narrow slits of 0.1mm-0.3mm thickness. The flow velocities in the slits are measured as well. With the data obtained, the minimum hydraulic gradient of flow required for the gas leakage prevention as well as drag force acting to the bubble are estimated. The minimum hydraulic gradient obtained is compared with those suggested by the existing studies. Effects of surface tension on the behavior of bubble and others are discussed as well.

View full abstract

Numerical calculation of unsteady open-channel flows was carried out using a low Reynolds number k-ε model, coupled with an anisotropic turbulence model, and VOF method. The distributions of ensemble averaged velocity, turbulence intensity, and Reynolds shear stress have been obtained in the whole flow region from the wall to the time-dependent free surface. The results of these calculations agree well with LDA database of Nezu et al. (1997, ASCE). Further, the turbulent energy budget in depth-varying unsteady flows, which is difficult to measure, has been calculated and discussed.

View full abstract

A higher-order computational scheme utilizing quintic spline interpolation, QSI method, has been proposed. The derivatives in the governing equations are evaluated by differentiating the quintic spline function. This computational method was applied to one- and two-dimensional pure advection problems. As a result, it was shown that the numerical accuracy in the QSI method is superior to a fifth-order upwind difference. In addition, two-dimensional cavity flows were solved with staggered and collocated grid systems. In the results of QSI scheme, it was shown that the accuracy of the QSI method is rather improved in the collocated system and that the accuracy is generally better than that of the 5th-order upwind difference.

View full abstract

Numerical simulation of sand wave formation was conducted in the present paper. Saltation model was extended in order to accomplish this simulation. One of the all bed particles which expose on the surface is checked up arbitraly whether it can be picked up and make an incipient motion or not. And its trajectory is traced every time step until it ceases its motion and rests on the bed. Time variation of flow velocity field over the bed surface is also evaluated simultaneously in order to reflect the effect of bed evolution. In the present model, any turbulence models were not introduced for simplicity to understand the mechanism of this formation process more easily and the space in future work is still left open.

View full abstract

There are many flows with submersible large roughness elements in nature. In order to understand these flows, it is important to clarify resistance of the large roughness elements and flow around them. For this purpose, a numerical model together with physical model tests, which can estimate the resistance and the flow, is necessary. In this paper, drag forces acting on submersible large roughness elements were measured directly through experiments. A numerical computation method using a two-dimensional shallow water model which employs the general curvilinear coordinate system is developed and made possible to evaluate the resistance of submersible large roughness elements. The model is applied to simulate flows with a group of submersible large roughness elements and in an urban district, where submersible and non- submersible houses are arbitrarily arranged.

View full abstract

A dynamic response of a submarine pipeline buried in a sandy seabed under cyclic loading of water pressure is examined experimentally and theoretically. Characteristics of cyclic hydrodynamic forces acting on the buried pipeline is clarified by small scale laboratory tests, and a floatation process of the buried pipeline is also clarified by the method of distinct element analysis. A parametric study was carried out in order to investigate the influence of burial depth, specific weight of pipeline, permeability of sand bed, and frequency of pressure loading on the floatation of pipe. An analysis of the pipeline stability potential against floatation is presented, in which all the experimental test cases are verified.

View full abstract

A field investigation on sediment movement and beach evolution was performed in an alluvial system composed of the Same river and the Nakoso coast, Fukushima prefecture, which experienced significant beach erosion recently. Analyses on sand volume in the system revealed that the rate of decrease in sand on the beach is of the same order of the rate of accretion of sand in dam reservoirs and that the dredging of river bed near the rivermouth influenced nearby shoreline retreat. Sand movement processes were estimated from the distribution of mineral composition included in the sediment samples collected from the beach.

View full abstract

The particle/particle interaction is the key to investigate the sediment transport phenomena from the viewpoint of the computational mechanics. In this paper, the mechanism of graded sediment sheetflow are described experimetally and numerically under the oscillatory flow. Simulation model is the combined distinct elemet method (DEM) with k-ε turbulence model. The mechanism of the vertical sorting and resultant armoring have been investigated from the velocity profile, the concentration profile, snap shots of particles and so on. A comparison of experiments with numerial results shows good agreement.

View full abstract

An acoustic Doppler profiler operating at 1.5MHz, has been used to observe the variability of sound scatters and velocity in the northern Hiroshima Bay. While the variability of horizontal velocity is dominated by the semi-diurnal tide, diurnal variation of the acoustic backscattered energy (ABSE) is evidence. Since ABSE near the water surface increases at night, the diurnal variability of ABSE is probably caused by zooplankton. The ABSE near the bed highly correlates with the gravimetric concentration of suspended particulate matter. Also, the variability is in good agreement with that of turbidity from an optical backscatter sensor. In the bottom boundary layer, the maximum ABSE occurs at the high water slack of spring tide. Moreover, it seems that the residual trend of ABSE shows the same fortnightly variation as tidal range. The trend of ABSE near the bed is well explained by the current speed and residual current.

View full abstract

The impacts of Southern Oscillation on climate are widespread and extend far beyond the tropical Pacific. However, the concrete influence is not so clear, especially in middle to high latitudes, including Japan. In this study, the correlation between monthly values of on one hand Southern Oscillation Index (SOI) and on the other hand normally standardized precipitation in Fukuoka, Japan, which is constantly exposed to high drought risk is investigated in detail using 110 years of data. For the original time series, no significant direct correlation is detected. However, statistically significant correlation coefficients are obtained when using SOI data categorized into five groups according to their magnitudes.

View full abstract

A criticism of the DEM-based model for the simulation of sediment transport process is an empirical selection of tuning parameters. Here, the parameters of the DEM-based numerical movable bed simulator are tuned to agree the calculated sediment transport rate with previous experimental results. Very simple analytical expression of the tuning parameters are shown. Standard diagram of the calculating time step is given for various combination of the sediment diameter and the thickness of sediment depositing layer.

View full abstract