土砂災害と対策の時代変化
ハゲ山モデルによる小起伏山地の災害分析
2002 年 54 巻 6 号 p. 43-50
詳細
2002 年 54 巻 6 号 p. 43-50
Two types of devastation of a mountain, man-made and natural, occur in Japan. Most sediment disasters for the past hundred years have been given rise to by man-made forest devastations. Historical changes of sediment disasters has analysied with the conceptual model in the previous paper. The past hundred years have divided into the following four periods. The characteristics of erosion types, sediment discharge types and sediment disasters are summerized as follows.
I Predominant period of surface erosion.
1) Age of the active front zone (1600-1900 A.D.)
The most sivere surface soil loss, hillslope denudation and torrent devastations occurred. River beds rose up and formed dry rivers by heavy sediment discharges. Most countermeasures using today started during this period such as prohibitation of forest cutting, check dams and hillside works.
2) Age of the bareland (1900-1950 A.D.)
The widest bareland covered Japan. Recovery of barelands by hillside works and devastated torrent started during this period as the work of the central and the state government.
II Predominant period of landslides.
3) Age of the active preparatory zone (1950-1990)
Shallow landslides predominate in sediment disasters during heavy storms. Warning and temporary evacuations started as a countermeasure for sediment disasters.
4) Age of the sound forest(1990- )
Mature forests have recovered on most mountains. The influence of human impacts on erosion and sediment discharge disappeared. All erosion process are natural and deep landslides predominate. Prediction and forcast of landslide occurrances are essential to prevent disasters.
