Movement characteristics of a creeping slope influenced by river erosion and aggradation: Study of Xinwulü River in southeastern Taiwan

Abstract

The average surface displacement rate of 0.1 m/year of the studied slope adjacent to the Xinwulü River has been measured over the past three decades. Damage to the retaining structures of a highway and tunnel through the slope has frequently been reported. Maintenance and reconstruction have been required at intervals of several to ten years. In this study, multi-temporal remote sensing images, site investigations, and ground and tunnel monitoring are interpreted to clarify the movement characteristics of the slope and the factors that affect them. The results of this study reveal that riverbed erosion and aggradation of the adjacent river dominate the movement characteristics of the slope. When the riverbed is low, the direction of slope movement ranges between that of the maximum slope gradient and the dip direction of foliation. Gravitational deformation, which is associated with a deep slip surface, is the main cause of movement; it is facilitated by gullies that develop upward from the sides of the Xinwulü River and the surrounding sliding masses. When the riverbed is high as a result of the aggradation of debris from upstream, the slope moves in the direction of the maximum gradient. Water washes out the slope toe, which has a relatively low strength because it has previously been displaced. Slope collapses occur frequently along the river bank, and gullies actively grow upward, stimulating the movement of the sliding masses with a shallow slip surface. Furthermore, the cyclic erosion and aggradation of the river account for the formation of shallow and deep slip surfaces of 33 m and 90 m, respectively. The results of this study demonstrate the effects of riverbed erosion and aggradation on the creeping deformation and stability of the adjacent slope in the Xinwulü River region; this is valuable for formulating countermeasures to protect the study site and maintain the existing tunnel.