Reconstruction of magnitude and frequency of debris flows in the rebaixader torrent (eastern pyrenees, spain) by dendrogeomorphological analysis

Abstract

Occurrence of debris flows has received little attention in the Pyrenees, probably due to the small risk shown by most of the debris- flow prone sites in this mountain range. Nevertheless, the event of Biescas (which occurred in August 1996 and that caused 87 casualties) demonstrates the existence of high risk spots in the region. Historical data on debris flow occurrence are usually scarce or lacking in the Pyrenees, as in many mountain ranges. This makes dif cult the determination of their frequency. Dendrogeomorphological techniques have shown their ef ciency in reconstructing temporal series of debris ow events from which frequency can be assessed. The determination of magnitude of an historic event can be done by distinguishing its deposits, but this is not a trivial task in debris fans that accumulate deposits corresponding to consecutive debris flows, especially if only a conventional geomorphological analysis is carried out. In catchments where debris- flow frequency is moderate to high (i.e. return period of several years) deposits corresponding to each event (event deposits) are identifiable using a set of relative dating criteria, like cross-cutting relationship, size and cover percentage of lichens on blocks, forest density and size of trees colonizing the deposits. The event deposits can be mapped and, subsequently, trees damaged by the flows sampled for dating of events. This approach was tested in a Pyrenean catchment where debris flows develop in periglacial deposits and under a submediterranean climate (Tordò creek). Now the method has successfully been extended to assess the frequency-magnitude relationship of debris flows in a different setting, the Rebaixader creek, in which the mobilizedmaterial is a glacial till and the local climate is a high mountain one but with mediterranean in uence. Seven event deposits have been mapped and dated in the Rebaixader creek, corresponding to debris-flows events occurred during the last eighty years, involving a depositional area 90,000 m2. A magnitude-frequency relationship was prepared for the site and is compared to that of the Tordó creek.