ABSTRACT BOOK Sandstone Landscapes ... - Sandstones.org

PJiří ZvelebilInstitute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, V Holešovičkách41, CZ-182 09 Praha 8, Czech Republic (zvelebil@irsm.cas.cz)Nature friendly management of rock fall danger: 22 years of safety monitoring on sandstonerockslopes in Labe canyonPicturesque landscape evolved on Cretaceous Sandstones in Bohemian Switzerland National Park and inLabe Sandstone Preserved Natural Landscape brings us - besides an aesthetic joy, also a constant thread ofrepeating rock falls from steep, from 40 to 100 m high sandstone walls. A historical statistics for the LaberiverCanyon, which has started in the year 1780, revealed that recurrence intervals of catastrophic rockfalls with volumes in order 10 2 and 10 3 m 3 lay mainly within the interval from three to ten years. Smallerfalls occur practically every year.Already in 1980, a systematic monitoring of rock walls and stability interpretation of data gained werechosen as the main tool to control actual probability degree of rock fall occurrence, and hence to minimizerockfall danger for society, and, in the same time, to satisfy strong demands for nature protection in theareas of the Labe–river Canyon and of Hřensko village. Sources of risk – unstable and potentially unstablerock walls and objects, were localized in a sequence of rock fall risk zoning maps.The idea backing the application of Safety Monitoring is simple. A catastrophic rock fall is the ultimate butno the inevitable end of much more longer (tens or even hundreds of years) lasting process of activity ofslope movements within an unstable slope. Therefore, if there is any probability of further long-timedevelopment of the failure in the case in question, the monitoring can be used as a full-worth equivalent totechnical remedial works. The inevitable cases requiring such technical remedy are only the ones, for whichthe danger of rock fall, declared according the results of some type of numeric stability analysis, could havebeen proved by monitoring data. That is, which exhibits precursors of danger of rock fall occurrence withinthe short time intervals from months to 2 years.At present time, the regional net consists of more then 600 monitoring sites in 3 sub-nets along 16 km ofsandstone rock walls in the area of Labe-river canyon, on the main rock walls just above block of flats inDěčín town, and on the walls of valley sides of Kamenice-river canyon in the area of Hřensko village.During 21 years of service of the net, 16 cases of immediate rock fall danger have been indicated insufficient advance to take inevitable precautions – four-times for rock objects with volumes in the order of10 3 m 3 , eleven-times for volumes of 10 2 m 3 . The last 5 cases took their place in Hřensko village just inspring of 2002.The monitoring know-how is now being passed to the Group for Rock Walls Monitoring & Maintainingestablished in the frame of the Bohemian Switzerland National Park, and five new sub-nets are underconstruction along the routes with highest tourist traffic in the area of rock walls and rock towns of thisNatural Park.