This paper presents the method to improve the stability of model parameters and accuracy of the tank model. The tank model includes so many parameters and high flexibility that it is difficult to identify the unique and best parameter for a certain watershed. This instability of model parameters causes instability of accuracy in long-term analysis. To decrease the flexibility and increase the physical fidelity of the model, new restriction conditions are introduced into the parameter identification process. This new condition could improve stability of the model considerably. The result suggests that stable parameters can be identified even from short-term data set by using appropriate restriction conditions.

View full abstract

Upon the recent progress of numerical modeling techniques, prediction of DNAPL transport and its application to field sites have posed the possibility of remediation and measures against groundwater contamination. In the study, a coupled TVD finite difference with the random walk model is applied for analyzing migration of dissolved and undissolved DNAPL with entrapment. The mechanism of DNAPL fingering and dispersion processes is illuminated and simulated on numerical analyses. As a resulte, the validity of the proposed model is well confirmed based on laboratory experiments and filed site data.

View full abstract

An open channel flow with submerged rubble mound groin was analyzed by developing a two-dimensional numerical model. The computational domain in the analysis was divided into two regions, i.e. (1) a single-phase open channel flow and (2) a liquid-solid-phase flow consisting of an upper layer over a rubble mound groin and a porous media flow in the groin. In both the regions the system was modeled to be a two-layer system. Momentumn and mass exhange through the interface was considered. The numerical model was tested through comparison with a laboratory data and good agreement between them was obtained.

View full abstract

A falling water is characterized by the appearance of water fragmentation and splash. The sub-particle-scale (SPS) texture model to give a sub-particle-scale texture to the solution of the particle method, which is superior in expression of water fragmentation, is proposed, for a production of physical CG. The particle method is applied as the physical model in the particle scale; while, the random-walk particle-tracking model is applied to the diffusion of mist which is a sub-particle-scale physical process. Processes to produce the CG, which expressed both of a texture of large-scale jet breakup in a particle scale and a texture of mist in a sub-particle scale, by the ray tracing are explained in detail.

View full abstract

Tall and flexible plants such as common reeds are deformed by drag force of flood flow. This deformation, linked to flow resistance, affects flow velocity and depth. Drawing on this physical link, a model is developed to describe the deformation of plants and vertical profile of mean velocity and Reynolds stress of flow above and through flexible vegetation communities. The model is supported by real scale flume experiment using a common reed community. In addition, this work extends an estimation method of Manning's roughness coefficient for practical use by including the measured values that represent shape and strength characteristics of flexible plants.

View full abstract

The unexpected migrations of some living things are caused by the intake and withdrawal from river and lakes. Particularly serious are problems with fish straying from their normal migratory routes.
It is expected that a method in which small pieces of colored cloth are allowed to drift in a flow will be effective in preventing fish from straying. In this paper, some prevention methods are examined through experiments. The authors also seek to clarify the behavior of these fishes, especially Ayu by combining the numerical analysis for the flow, and the random walk method for the behavior of fishes. As a result, the prediction of the behavior of fishes is now possible. And using the numerical methods the authors present some methods for preventing Ayu from straying.

View full abstract

The density change and the water particle motion in a two-layered water is investigated in a 2D wind flume with sloping bottom. In this study, spatial density profiles were obtained using multi-channel conductivity probes, and compared with the luminance distributions measured by digital video cameras for imaging techniques. In addition, water particle velocities were measured using particle image velocimetry (PIV) technique. The aspects of diffusion and advection of the mixture of the upper-layer and lower-layer water were well illustrated from the measured results obtained by the conductivity sensors arranged along the flume. The measurements using PIV were compared with the predictions of k-ε turbulent model. Although the flow and density fields were strongly affected by the internal seiche, the numerical results show good agreement with these experimental results.

View full abstract

Laboratory experiments were made to parameterize the gas transfer velocity of CO₂ at the surface of wind waves. The local flux of CO₂ was estimated at several fetches in a wind-wave tank by using the profile method. The dissolved CO₂ concentration on the water side was measured by using a gas-liquid equilibrator made of hydrophobic porous tube. The local gas transfer velocity was obtained from the flux and dissolved CO₂ measurements. On the basis of the experimental results and the dimensional analysis, we proposed a new type of empirical expression for the gas transfer velocity of CO₂. With the fetch relations for wind waves, the expression can be described in terms of local dimensionless wind-wave parameters.

View full abstract

In order to investigate the formation mechanism of oxygen-deficient water mass in western area of Ariake Sea, field observations were made in September 2002 and July 2003. A high turbidity layer, in which the concentrations were 20∼40 ppm, was formed near the bottom by strong currents during the observation periods. When DO concentration was high, turbidity and DO concentrations near the bottom have a negative correlation. The oxygen consumption rate by SS took 7 times the value of SOD, and it suggests that the oxygen consumption by SS plays an important role for formation of oxygen-deficient water mass in the western area of Ariake Sea.

View full abstract

In recent years, the importance of total sediment control from mountains to estuaries has been realized. Also, under greater consideration are reservoir sedimentation problems in river basins, as well as estimation of sediment discharge and its transportation properties. Observation of reservoir deposition provides useful input data for these estimations. An examination of this data, collected from reservoirs throughout Japan, is the focus of this paper. Based on the analysis of the relation between annual variations and other factors, this thesis identifies characteristics of sediment deposition and yield properties from watersheds. This research concludes that there are three general types of annual variations of sediment deposition. These types are originated by variation of sediment yield in mountain areas and are possibly related to backsand accumulation at the upstream of reservoirs.

View full abstract

To continuously monitor the concentration of periphyton detaching from river bed, we attempt to confirm the applicability of a memory-type fluorometer to measure a periphyton concentration in river water with laboratory experiments and field observations. By using the experimental data and observed results, the concentrations of chlorophyll-a measured with the fluorometer, Chl.a (F), are compared with those analyzed with a spectrophotometric method, Chl.a (A). The results of the experiments and observations indicate that the results of Chl.a (F) have a good correlation with those of Chl.a (A), demonstrating the fundamental applicability of the fluorometer to the continuous monitoring of the periphyton concentration in river water.

View full abstract