Sedimentation of Reservoirs

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SACKUNG Michael J. Bovis University of British Columbia, Vancouver, BC, Canada

Synonyms Deep-seated gravitational deformation; Gravitational spreading; Slope sagging Definition Sackung is a German term denoting slow, deep-seated gravitational deformation of slopes. Discussion Sackung-type movements have typical surface expression as uphill-facing (antislope) scarps, tension cracks, grabens, and anomalous ridge-top depressions running roughly parallel to the contours in steep mountain topography. Individual scarps, grabens, and cracks (collectively referred to here as “linears”) have a typical relief of 1–10 m and may be traced over distances of 100 m to more than 3 km. Linears may be arranged subparallel to one another, or en echelon, and comprise slope-movement complexes covering areas of 1–10 km2 that are clearly visible on air photographs (Figure 1). Material properties and movement mechanisms Sackung features were first recognized in the European Alps, but have since been recognized in many other mountain areas. Hürlimann et al. (2006) provide a comprehensive citation of papers from the past 40 years. Linears occur over a wide range of rock types, ranging from ductile sedimentary rocks to hard jointed intrusive and metamorphic rocks, but often are best displayed and preserved in hard rocks with closely spaced discontinuities dipping steeply into a slope. This structure favors

flexural toppling, likely an important mechanism for development of many antislope scarps. The feasibility of flexural toppling may be assessed kinematically for joint poles, a, falling outside a friction circle drawn f degrees flatter than the slope angle b, where f is the friction angle along the through-going discontinuities, i.e., a < (b  f) (Bovis and Evans, 1996, Nicol et al., 2002). Numerical modeling of large-scale sackung movements is now fairly common, using both distinctiveelement and continuum methods (Nicol et al., 2002; Eberhardt, 2008). Modeled ground-motion vectors agree fairly well with long-term measured ground movements. Such 2-D modeling also shows that sackung movements are sensitive to water pressure fluctuations and to slope steepening by either Neoglacial or fluvial undercutting of slopes (Bovis and Stewart, 1998; Ambrosi and Crosta, 2006).

Hazard implications Sackung linears have been reported adjacent to many Rock Avalanche detachment zones, which has provoked discussion of a probable connection between slow, ductile sackung-type movements and sudden rock avalanche detachments. Some linears probably developed in response to an abrupt change in longitudinal stress as the avalanche detached, but in many cases, the morphology and degree of weathering of linears indicate a far greater age than the rock avalanche. This implies that sackung linears are an expression of premonitory movements in stressed slopes, some of which fail catastrophically, though Hewitt et al. (2008) caution that sackung movements are not a necessary precursor for all catastrophic detachments. Where minimal erosion of linears has occurred sin

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