高知県繁藤地区の大規模崩壊について (その2)-崩壊に対する地下水の効果-

Abstract

Hydrological properties of the groundwater system which might have affected the occur-rence of the large scale landslide near the Shigeto Station on the Dosan Line of the JapaneseNational Railways on July 5, 1972 was investigated through the continuous observation ofthe groundwater level and the discharge of a spring. The observation was started inNovember 1973 and continued until December 1976 although some interruption was involved.On this basis, it is revealed that unconfined groundwater occurs in fractured rock at thegentle ridge-type slope behind the landslide area. It stands to reason that a part of thisgroundwater was led to the spring in the landslide area through a permeable channel formedby the fracturing of rocks in the fault plane which crosses the area in question.Simulation of the groundwater system is carried out using a tank model based on thisinterpretation and is verified by comparing the result with the observed values. It thenyieds the estimation of the hydrograph of the spring at the time of the occurrence of thelandslide. It is concluded that the rainfall on July 5, 1972 whose duration was of the sameorder of magnitude with the time constant of the recharge of the groundwater system was, unfortunately, effective as a trigger for landslide though the total amount of rainfall was notso great as to be called "record-breaking".