Fundamental Study on Forecasting of Landslide Area (III)
On the distribution of the resistance index from the geological point of view
1981 Volume 34 Issue 1 Pages 42-52
Details
1981 Volume 34 Issue 1 Pages 42-52
It is necessary to estimate the resistance index, that is the shape parameter in the gamma distribution model, in order to forcast area of landslides in the objective zone. Especially geology in various factors that participate in the resistance index is concerned in this study.
Total precipitation and landslide area ratio are measured at the 1km2 circular plots, that are set at the center on aero-photographs of eight regions ex. Fujioka region where many landslides have occured by heavy rain.
The resistance index maps on these regions are drawn by substitution above measured values at each plot into the gamma distribution model. It is obtained by overlay method that the feature of pattern in these maps is closely similar to that in the corresponding geological map.
As a result of this comparison, the resistance index on each rock and stratum in study area is as follows.
Fujioka region: Biotite granite <Hornblende biotite granite <Granodiorite <Shistose hornb. biot. granite <Aplite granite <Seto group.
R. Niyodo region: Suzaki formation <Hayama formation <Zohoin group <Ichinose group <Kamiyakawa formation <Shirakidani formation <Ino formation.
Yamakita region: Conglomerate <Mudstone <Greentuff <Volcanic conglomerate.
The resistance index of biotite granite and new formation are generally less than old formation in consolidification. Then the relation between the resistance index C and absolute year of sedimentary rock is given by the next equation, C=κT0.3 where k is the coefficient dependent on the character of region.
Though an objective area is composed of various rocks and stratums complexly, the relation between total precipitation and landslide area ratio is divided into the next case by the combination of rainfall distribution and resistance index distribution corresponding geological condition.
In the case of the right angle inter section of isohyet and isopleth on the resistance index map, the landslide area ratio is given by a theoretical curve under the mean resistance index at the objective area. But there is some local deviation by geological condition in the same precipitation.
On the contrary, in the case of approximately paralell aligment, landslide area ratio is shown by some theoretical curves under the each resistance index that differs according to geological condition with precipitation class.
