Hydrological observation was carried out at two basins occupied mainly by evergreen coniferous forest of Chamaecyparis obtusa and deciduous broad-leaved forest of Quercus crispula in order to clarify the influences of different forest type on water balance. The analysis of short-time period water-budget method showed that the evapotranspiration of coniferous forest was more than broad-leaved one. By calculating transpiration from Priestley-Taylor equation, intercepted evaporation was separated from evapotranspiration. The result of comparison of intercepted evaporation between two basin was that intercepted evaporation of coniferous basin was more than that of broad-leaved basin. From separating total runoff into direct and base flow components; it was shown that there was no difference in direct runoff of two basins, so the differences of total runoff of two basins were caused due to the difference of base flow. From the above analysis of water balance components, it is clarified that the different forest type influences the amount of intercepted evaporation and base flow.

View full abstract

Selection of installation sites for fish-ways often adapts previous experience of locating fish-ways. Therefore, with the goal of developing a fish population dynamics model designed for networking of water areas, we investigated applicable values of the important parameter r which represents the growth rate of fish populations on newly connected areas. The target species was field gudgeon, Gnathopogon elongatus elongatus. We calculated survival rates using cohort life tables obtained by assuming a stable age distribution, based on the results of field surveys of their total length, and past studies about reproduction. To consider the uncertainty of these survival rates, we performed Monte Carlo simulations on a newly connected area under density-independent condition, giving survival rates some random fluctuation. The range of introduced fluctuation was derived from past studies about other fish species. A value for r of 0.36 is thought to be an applicable and conservative value on average that avoids overestimating the effects of networking of water areas. The range of fluctuation of r should be determined depending on the length of time over which computations are performed.

View full abstract

Localized scouring downstream of weirs caused by frequent riverbed dredging often results in river flood disasters, such as the destruction of weirs and the washing away of bed protection works. Downstream bed protection works are usually undertaken to prevent such scouring, but there is a great deal of uncertainty about their effectiveness. The authors therefore decided to conduct a study in which hydraulic experiments were performed to investigate how effective five types of bed protection works, including step-type and slope-type works, would be in reducing scouring. In order to re-create the riverbed scouring conditions that occur downstream of weirs, the authors conducted these experiments using a meandering channel under conditions where alternating bars would be created. The experiments yielded the following conclusions: (1) all five types of bed protection works were found to be effective in reducing riverbed scouring to a certain degree; (2) scouring is expected to be reduced by the use of bed protection works that horizontally alter the direction of the water flow passing over the weir and maintain the water flow spread; and (3) the depth and length of bed protection works are important factors in achieving greater effectiveness, and scouring can be most effectively reduced by using step-type bed protection works.

View full abstract

The survey of interview, resource acquisition, photographic operation, and questionnaire were carried out in the “n” Community in the “y” District in Hakusan City in Ishikawa Prefecture to investigate the actual condition of paddy field levee maintenance in the area where land-renting market was proceeding, large-scale farming was dominant, and the problems of geographically scattered farm-land existed. In the study zone, 1) an agricultural production legal person rent-cultivated some of the paddy fields and maintained the levees, 2) another agricultural production legal person rent-cultivated some of the soy bean fields for crop changeover and land owners maintained the levees. The results indicated that sufficient maintenance was executed on the levees of the paddy fields cultivated by the agricultural production legal person, the soy bean fields for crop changeover, and the paddy fields cultivated by the land owners. Each reason is considered to be the managerial strategy, the economic incentive, the mutual monitoring and cross-regulatory mechanism, etc.

View full abstract

According to the results of the centrifuge model test to determine seepage failure phenomenon of the upper seepage flow, a critical hydraulic gradient at a boiling state value 25% higher than Taylor's critical hydraulic gradient, and a small scale effect of the hydraulic gradient with a stress level at the boiling state was observed. Also, displacement behavior of a upper ground layer was observed close to Taylor's critical hydraulic gradient. In particular, the influence of non-uniformity of the ground was small and behavior of the overall seepage failure into soil ground could be confirmed. The centrifuge model test is considered to be effective in the experimental study of seepage failure.

View full abstract