A new optimal design framework including mitigation countermeasues taken in floodplains is proposed based on distributed risk assessment in urbanized areas. In order to take both of human damage and economic one into account and to incorporate non-facility-based countermeaseures, ‘damage level’ is defined. The damage level is computed based on the combination of external force level, which is derived from several aspects of inundation flow dynamics, and flood-proof level, which is defined by protection and mitigation ability at each point in a floodplain. With the clear reference to project time scale and feasibility, the planning problem is formulated as a permutation optimizing problem under the budgetary constraints. A solution search algorithm based on genetic algorithm is proposed to find optimal solutions of the large and complicated system design problem including non-linear numerical simulation.

View full abstract

A new optimal design framework including mitigation countermeasues taken in floodplains is applied to an actual river basin (the Yura river basin) and optimal solutions are obtained under three conditions. The target area is divided into 38 blocks and dam renovation, continuous levees, circle levees and rasing of houses are taken into consideration as alternatives for flood mitigation. Optimizations by genetic alogorithm are performed to obtain the combinations and construction orders of the alternatives which minimize the damage level of the basin under budget constraints. It is ascertained that realistic solutions can be obtained by the performance evaluation based on the quantization of damage level including human damage, damage on houses, and damage on important infrastructure. The analyses of optimal solutions reveals that in some cases some blocks remain with relatively high human damege level although the total damage level of the whole basin has been minimized. In order to cope with the situation, an optimal solution is derived for a new formulation with allowable human damage level as an additional constraint.

View full abstract

Hydrogeological parameters of field soil porous media are distributed with large heterogeneity. Evaluation of physical heterogeneity effects on soil water flow is important to understand basic flow characteristics of field soil. In this study, two types of water infiltration experiments were carried out to examine water flow under the heterogeneous distribution of hydraulic conductivities. Soil water change was measured by time domain reflectometry (TDR) using small printed circuit board probes (PCBP). To evaluate soil water distribution change in the experimental sand box, the simple image analysis of digital photo of the experiments was applied. In order to understand detailed water and air flow in the experiments, numerical simulations were carried out. In the numerical simulations, ordinary unsaturated-saturated seepage analysis and two-phase flow analysis were compared. Moreover, hysteresis effects on the soil water content change were also examined. Apparent differences between unsaturated-saturated and two-phase analyses were not observed in the results of water content change. For the ponding experiment, histeresis should be included in the simulation.

View full abstract

According to field results in a dam reservoir for the mechanism of convective circulation due to surface cooling at the water surface, we estimate both the flux profiles of temperature and dissolved oxygen. In the northern basin of Lake Biwa, buoyancy flux is only significant in the epilimnion with exceptions of turbid intrusion into the upper thermocline. Oxygen flux profiles are calculated assuming the zero-flux surface at a certain level in the thermocline. Oxygen capacity decreases during the heating season, however the oxygen does not decrease due to successive algae blooming events and the oxygen release from the epilimnion into the air is observed only in early summer, while oxygen is supplied from the air during cooling months and the inflow becomes moderate showing a linear flux profile in January. In contrast, the oxygen flux in the hypolimnion becomes negative consumed in the sediment, all the year except for full turnover in late winter. However the flux occasionally becomes positive, which is not due to the vertical exchange but the horizontal flux due to convective circulation and its final turnover. The surface process in the heating period is due to photosyntheses and bottom process is also biotic and thus the annual cycle is described.

View full abstract

A simple model for estimation of snowfall and snow melt was proposed. In this model, the amount of precipitation and air temperature were needed as input data. Seven parameters used in this model were identified by field observations, in-situ experiments and laboratory experiments. Furthermore, to evaluate river runoff induced by snow melt, a numerical model was established. In the estimation of distribution of precipitation in the basin, Rader data was used considering the effects of snow flake drift during falling onto the ground. As a runoff model, a tank model was employed. The validity of the proposed model for estimation of river runoff induced by snow melt was examined by comparing with measured inflow into a dam.

View full abstract

Long period waves are sometimes significantly large and cannot be neglected even for engineering purposes in the nearshore zone. The growth of amplitude of long waves and phase shift between the short wave envelope and incoming long waves on a slope have been studied but not clearly understood.
We present a theoretical model for the long wave evolution on the basis of the linear long wave equation with forced term. Time-series of incoming long waves is calculated from the incoming short waves at the deeper edge of a slope. Transfer function and phase shift between squared of short wave envelope and incoming long waves are also predicteable.

View full abstract

The erosion process of cohesive sediment in the estuarine channel of the Chikugogawa River was studied using thermometers. Sixteen self-logging thermometers were attached to a fishing rod at intervals of 0.1 m; fifteen were immersed in a muddy bed and one was placed in the river water. The progress of bed erosion was successfully estimated from the difference between the water and mud temperatures. The bottom shear stress was calculated from the velocity profile of an ADCP; it was found that bed erosion occurs when the shear stress exceeds 1.8 N/m². The erosion rate was explained by the frictional flow velocity and the water content of the bed sediment. The coefficient of erodibility in the erosion rate equation depended on the adhesion of the particles; the adhesion of the river bed sediment was greater than the silt clay filled in the experimental channel because of the thixotropy developed during the sedimentation process.
The suspended load discharged from the basin was 243,000 t and the load passing through a 10-km cross section was 900,000 t. This result shows that the suspended sediments that discharged from the Chikugogawa River into the Ariake Sea was composed of products from the river basin and eroded bed sediments from the estuarine channel; the quantity of the latter was several times larger than that of the former.

View full abstract

This study aims to develop a new technology, which uses an UAV (unmanned aerial vehicle) and digital photography, in order to evaluate temporospatial geomorphology in intertidal flats. The UAV can make an autonomous flight. Because the water surface of the intertidal flats is tranquil, the waterline (water's edge), evaluated by aerial photography from the UAV, corresponds to a very precise contour of the intertidal flats. High resolution photographs are also useful to study the interrelation between morphological information and bioactivity in the intertidal flats. The method is applicable to not only natural intertidal flats but also artificial intertidal flats to measure morphological dynamics.

View full abstract