Introduction

In European countries the term “torrent” refers typically to steep channels in Alpine headwater catchments, with channels steep enough that debris flows can occur apart from fluvial sediment transport. According to this definition, such catchments are associated with drainage areas of less than about 25 km2 and, typically, with mean channel gradients steeper than 10% (Rickenmann & Koschni 2010). This term has not often been used in the recent English scientific literature. As pointed out by Slaymaker (1988), the English expression “debris torrent” (originally used in the US Pacific Northwest) was associated with a debris-flow event, which is in contrast to the European meaning of the term referring to a steep stream channel.