Center for Research in Radiation, Isotopes, and Earth System Sciences, University of TSUKUBA: Radionuclides Environmental Transfer Section

Hydrogeomorphology has been my field since I studied it as my master's thesis.
So far, I have been studying especially the water cycle and sediment production, primarily on the natural environment.

The first research is about the mechanism of rainfall-runoff.
In Hillslope Hydrology, the major analysis before the 1960s was based on numerical model. However, since the field observations in the 1970s, it had been revealed that the model in forest mountain (which must be outflow because the infiltration capacity was low) was basically an error.
Field studies have been still carried out actively.
In recent years, the effect of rainfall runoff from bedrock has emerged as a new research findings and our research has been watched with interest.

The second research issue is about slope failure and the behavior of water associated with sediment discharge.
Detailed field observations revealed that the slope failure occurs and the frequency of it depends on the difference of the behavior of water in the soil of the slope and the water of the foundation rocks. In addition to this subject, we put to practical use of warning and evacuation whether the slope failure occurs during the heavy rainfall or after rain has stopped. Furthermore, we have been performing field observation and studying the sediment discharge by biological activity.

The third case is the mechanism of failure.
So far, we have studied the failure mechanism in the slope of dry sand-like particles by laboratory experiments and numerical modeling. As a result, we found out that the stability of slopes composed of granular materials is not represented by the angle of shear resistance, is due to the rotation of the particles.

The fourth research issue is the mechanism of surface soil erosion. In plantation, especially single storied cypress forest, understory vegetation may be lost and surface soil erosion occurs when the crown closure proceeds. In order to elucidate this mechanism, we made laboratory experiments and field experiments. We also clarified the mechanism for decreased infiltration capacity in the region of volcanic ash fall. In addition to the elucidation of these mechanisms, the amount of erosion in the past and sediment movement in the entire basin can be estimated by the evaluation using environmental isotopes Cs-137.

Those researches of land use, water, and terrain is closely related to each other and considered to be a very basic issue regarding the use of sustainable environmental future. For example, in the study of soil erosion, overland flow occurs easily when it rain in cypress forest plantations. Soil erosion not only occurs, the flooding of downstream areas occurs as peak flow rate is increased. The flow rate of the dry season will decrease on the other hand. Valuable guidance on how to appropriate forest management on water resources can be expected by the accumulation and modeling of the future outflow data and the tracer data.

At the same time, we are willing to continue to carry out the research on the mechanism of rainfall-runoff and the elucidation of generation mechanism of collapse. It will be a significant impact on evacuation warning and valid land use in the region by clarifying the region, and how, or what type of collapse occurs.