Displacement and transport of marked pebbles, cobbles and boulders during floods in a steep mountain stream

In a small experimental catchment of the Dolomites (Rio Cordon, 5 km(2)) field observations have been carried out on the movement of various sized bed material particles. Displacement length of 860 marked pebbles, cobbles and boulders (0(.)032 < D < 0(.)512 m) has been measured along the river bed during individual snowmelt and flood events in the periods 1993-1994 and 1996-1998. Floods were grouped into two categories. The first includes 'ordinary' events, which are characterized by peak discharges with a return period of 1-5 years and by an hourly bedload rate not exceeding 20 m(3) h(-1). The second refers to 'exceptional' events with a return period of 50-60 years. A flood of this latter type occurred on 14 September 1994, with a peak discharge of 10(.)4 m(3) s(-1) and average hourly bedload rate of 324 m(3) h(-1). The variation according to grain size of total displacement length L-i depends on the degree of mobilization of the individual fractions of the bed surface: L-i is independent of D-i for smaller, fully mobile grain sizes and decreases rapidly with D-i for larger fractions in a state of partial transport. Sustained selective transport without a supply of sediment from upstream leads to the development of a stable coarse armoured surface through progressive winnowing of finer material from the bed surface. With supply unlimited conditions for transport, both the occurrence of extreme events and the duration of a sequences of 'ordinary' floods play an important role in the degree of mobilization of the individual fractions of the bed. Copyright (C) 2004 John Wiley Sons, Ltd.