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Rock fall or rock-fall refers to quantities of rock falling freely from a cliff face. A rockfall is a fragment of rock (a block) detached by sliding, toppling, or falling, that falls along a vertical or sub-vertical cliff, proceeds down slope by bouncing and flying along ballistic trajectories or by rolling on talus or debris slopes,” (Varnes, 1978). Alternatively, a "rockfall is the natural downward motion of a detached block or series of blocks with a small volume involving free falling, bouncing, rolling, and sliding".
Favourable geology and climate are the principal causal mechanisms of rockfall, factors that include intact condition of the rock mass, discontinuities within the rockmass, weathering susceptibility, ground and surface water, freeze-thaw, root-wedging, and external stresses. A tree may be blown by the wind, and this causes a pressure at the root level and this loosens rocks and can trigger a fall. The pieces of rock collect at the bottom creating a talus or scree. Rocks falling from the cliff may dislodge other rocks and serve to create another mass wasting process, for example an avalanche.
A cliff that has favorable geology to a rockfall may be said to be incompetent. One that is not favorable to a rockfall, which is better consolidated, may be said to be competent.
Typically, rockfall events are mitigated in one of two ways: either by passive mitigation or active mitigation. Passive mitigation is where only the effects of the rockfall event are mitigated and are generally employed in the deposition or run-out zones, such as through the use of drape nets, rockfall catchment fences, diversion dams, etc. The rockfall still takes place but an attempt is made to control the outcome. In contrast, active mitigation is carried out in the initiation zone and prevents the rockfall event from ever occurring. Some examples of these measures are rock bolting, slope retention systems, shotcrete, etc. Other active measures might be by changing the geographic or climatic characteristics in the initiation zone, e.g. altering slope geometry, dewatering the slope, revegetation, etc.
The picture shows a road, which stops big rocks.
Effects on a tree
Effect on a tree. The tree roots may rotate - rotational energy. The tree may move - translational energy. And lastly deformation - either elastic or plastic. Dendrochronology can reveal a past impact, with missing tree rings, as the tree rings grow around and close over a gap. If you look microscopically then you see a difference with a callus tissue. A macroscopic section can be used for dating of avalanche and rock fall events.
Assessment of risk
Simulation – need a lot of data. Rapid Assessment - based on Mean Tree Free Distance. Gsteiger (1993), the probable mean distance between 2 tree impacts in a forest stand. Should not be longer than about 30m and or steeper than 30 degrees. A rock will just not stop and keeps on going if it is.
- Whittow, John (1984). Dictionary of Physical Geography. London: Penguin, 1984. ISBN 0-14-051094-X.
- http://books.google.at/books?id=PyocAQAAMAAJ&pg=PA88&lpg=PA88&dq=Eisbacher+%26+Clague,+1984,+p.48&source=bl&ots=J2KXCPiyC-&sig=RvynhQTY_WzoWr3kKrHDd-4c9TU&hl=en&sa=X&ei=1CYKUbaCFYXItAb084CwDg&redir_esc=y#v=onepage&q=Eisbacher%20%26%20Clague%2C%201984%2C%20p.48&f=false U.S. Geological Survey Professional Paper, Issue 1606 Debris Flows from Failures of Neoglacial-Age Moraine Dams in the Three Sisters and Mount Jefferson Wilderness Areas, Oregon Eisbacher & Clague, 1984, p.48
- Ritchie, A.M. (1963). Evaluation of rockfall and its control. Highway Research Record, No. 17, pp. 13-28.
- Pierson, L.A., Gullixson C.F., Chassie R.G. (2001) Rockfall Area Design Guide. Final Report SPR-3(032), Oregon Department of Transportation and Federal Highway Administration, FHWA-OR-RD-02-04.
- Pantelidis, L. (2010). Rock catchment area design charts. In Proceedings of GeoFlorida 2010 (ASCE) Conference on Advances in Analysis, Modelling and Design (pp. 224-233). doi 10.1061/41095(365)19
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