Hiroo Ohmori

Changes in the hypsometric curve through mountain building resulting from concurrent tectonics and denudation

Abstract The stages of geomorphic development of mountains have sometimes been explained based on Strahlers diagram of hypsometric curves. Although the Japanese mountains are young, they do not follow his diagram. Hypsometric curves, simulated using empirical laws deduced from the Japanese mountains, indicate that mountain building resulting from concurrent tectonics and denudation does not produce convex curves but concave and/or s-shaped curves. The hypsometric curve of a mountain changes from a concave curve to an s-shaped curve in accordance with the increase in mountain altitude. The succession of the hypsometric curve during concurrent tectonics and denudation is the reverse of Strahlers diagram. The hypsometric integral increases with the advance in stage which is evaluated based on the change in mountain altitude. It has a relatively good correlation with the stage. However, the sequence of change in the hypsometric integral is different according to the extent of the terrain examined, whether the terrain is restricted to the interior of a mountain or pertains to the whole mountain, and whether it includes newly emerged land. The stage should be evaluated based on the course of change in the integral characteristics for the extent of the mountain examined.

Ecological Status of Pinus pumila Scrub and the Lower Boundary of the Japanese Alpine Zone

The Japanese alpine zone, defined as the zone above timberline, is characterized by the presence of Pinus pumila scrub zone. This study shows that Pinus pumila on the Japanese mountains is not an a...

Chemistry of surface water at a volcanic summit area, Norikura, Central Japan: Multivariate statistical approach

Many hydrochemical studies on chemical formation of shallow ground water have been reported as results of water-rock interaction, and contamination of paleo-brine or human activities, whereas the preliminary formation of precipitation source in the recharged region has not been established yet. The purpose of this research work is to clarify the geochemical process of water formation from a water source unpolluted by seawater or human activity. Norikura volcano, located in western part of central Japan provided a suitable source for this research purpose, and hence chemical compositions of water samples from the summit and the mountainside area of Norikura volcano were determined. Most samples in the summit area showed very low electrical conductivity, and lower than 12 microS/cm. On the basis of the chemical compositions, principal component analysis (PCA) and factor analysis (FA), such as kinds of multivariate statistical techniques were used to extract geochemical factors affecting hydrochemical process. As a result, three factors were extracted. The first factor showed high loading on K+, Ca2+, SO2 and SiO2, and this factor was interpreted due to influence of the chemical interaction between acidic precipitated water and rocks. The second factor showed high loading on Na+ and Cl-, and it was assumed to be an influence of seawater salt. The third factor showed loading on NO3-, and it was interpreted to be caused by biochemical effect of vegetation. The proportionate contributions of these factors to the evolution of water chemical composition were found to be 45%, 20%, and 10% for factors 1, 2 and 3, respectively. The same exploration at the mountainside of Norikura volcano revealed that the chemical variances of the non-geothermal water samples were highly influenced by water-rock interactions. The silicate dissolution showed 45% contribution for all chemical variances, while the adsorption of Ca2+ and Mg2+ by precipitation or ion exchange showed 20% contribution. The seawater salt influence or biochemical effect was statistically negligible in this area. The clear differentiation of geochemical process on water formation was found between the summit area and the mountainside area.

Distribution of hazard types in a drainage basin and its relation to geomorphological setting

Abstract Hazards along a river course in a drainage basin are characterized by three types of events: debris flow, turbidity flow and flood. Each has its own channel segment with different sediment transport processes. The sediment transport process is controlled by transportation force: tractive force and slope-direction component of sediment weight, both of which are significantly affected by channel slope. Along large rivers in Japan the boundary between the upstream turbidity flow segment and the downstream flood segment is located at the position with a channel slope of about 1/1000. Along steep, small rivers the boundary between the upstream debris flow segment and the downstream turbidity flow segment is located at the position with a channel slope of about 80/1000. The channel slope depends on the shapes of longitudinal profiles of rivers. The longitudinal profiles of Japanese rivers, main rivers and tributaries, can be described by an exponential, power or linear function. The transportation force of the rivers fitted with exponential functions markedly decreases downstream due to the large curvature of the profiles, causing noticeable sediment deposition in the middle courses. The transportation force of the rivers fitted with power or linear functions maintains its strength through the whole river courses due to the small curvature, causing sediment transportation down to the lower courses. The function types are strongly affected by relief in the drainage basins. The rivers flowing in small relief areas are fitted with exponential functions and those flowing in large relief areas are fitted with power or linear functions. Thus, the distribution of hazard types along a river course in a drainage basin is controlled by the distribution of relief in the drainage basin.