Weathering-related slope movements in sedimentary formations
Published:January 01, 2010
The interrelationship between slope deformation and fault-induced weathering as a predisposing factor for the development of sliding is analysed through several case studies from the Western Carpathians in the Czech Republic. The study area comprises flysch nappes with alternating sandstone and shale of different permeability. These lithological structures are affected by systems of faults. Recurring slope instability is found associated with zones of deep weathering in tectonically weakened areas. Climatic variability of landslide activity can be identified during the Holocene by means of radiocarbon dating and pollen analysis. Areas affected by recurring landsliding suggest gradual and cyclic landslide frequency.
To clarify the erosion processes on a marly bare slope in the southern Alps, the erosion processes on a steep and erodible slope composed of Black Marls formations were observed using a time-lapse video camera. The observations revealed that miniature debris flows (MDFs) occurred at the time of the rainfall–runoff event during which the most severe erosion took place over the 3 month observation period. Analysing the camera images, we show some characteristics of the MDFs and discuss them in the context of real rainfall–runoff phenomena observed at the outlet of the small experimental basin that contained the visually observed slope. The following results were obtained. (1) It is roughly estimated that the total amount of sediment discharged by the MDFs was not quantitatively negligible in comparison with the total sediment discharge from the entire experimental basin. (2) The MDFs occurred only during the rising limb of the hydrograph, which lasted 6 min. The visual observation also provided information on triggering conditions. Compared with the precipitation data, it was found that a rainfall intensity of 1 mm min−1 triggered MDFs and an antecedent cumulative rainfall of 10.4 mm was also enough to trigger an MDF. This is consistent with a previous study in Draix. (3) Although the Darcy–Weisbach friction factor and Manning's n of the MDFs observed differ considerably, they are consistent with results in the literature reporting erosion phenomena on very steep slopes up to 36°. (4) Based on this observation and a review of the literature, on very steep and highly erodible slopes, MDFs or similar phenomena might play an important role in erosion and transport processes. It is important to consider this possibility when examining erosion models for catchments composed of steep and erodible slopes.
Figures & Tables
Weathering as a Predisposing Factor to Slope Movements
This volume is intended to provide an up-to-date overview of the approaches, methodologies and techniques devoted to better understanding of the weathering conditions of rock masses on slopes. According to the local conditions, a variety of slope movements may take place and involve weathered rock masses. Shallow and rapid soil slips evolving to debris flows are probably the most common type of slope movement. At the same time, deep-seated, intermittent landslides can also affect large volumes of weathered rocks and soils. Despite the high frequency of landslides in weathered materials, and the damage and casualties they repeatedly cause, little is known about the relationship between weathering and slope movements. This book presents worldwide case studies, where a variety of geological and geomorphological settings are discussed. The content is divided into three sections: the first is devoted to broad aspects of the weathering/landslide processes; the second and third sections include papers dealing with igneous/metamorphic and sedimentary weathered rocks, respectively.