ABSTRACT
In mountain rivers and torrents, hydrologic and hydraulic processes are characterized by extreme variability, both in the spatial and the temporal domains. The main causes of this lie in the strong interaction of geological constraints, earth surface processes, and the channel network in alpine catchments (Hassan et al., 2005a; Comiti & Mao 2012; Church 2013). This results in a high variability of the following parameters:
• sediment supply or availability and sediment transport • composition of the grain size distribution of the stream bed and of the source
areas • channel geometry along the stream and in the lateral direction • highly variable (but generally low) runoff depths • flow behavior in the transition regions subcritical-supercritical-subcritical flow
Typical torrent channels are greatly influenced by these parameters so that a consideration of the geological and morphological conditions in and along the channel is of great importance. Sediment is often supplied to the channel by colluvial processes, while the channel bed may consist partly of bedrock and, thus, may have only a semialluvial character. Mountain rivers, in contrast, typically have an alluvial streambed that reflects a single dominant formation process. In European countries, torrents refer typically to Alpine catchments with channels steep enough that debris flows can occur in addition to fluvial sediment transport. According to this definition, such catchments are associated with drainage areas of less than about 25 km2 (Rickenmann & Koschni 2010; Marchi & Brochot 2000; Marchi & D’agostino 2004). Typical differences between torrents and mountain rivers are summarized in Fig. 3.1.
