Bedload transport and flow resistance in steep channels :introduction to the issues in the context of mountain basins of the Central European region

Abstract

Although sediment transport in the uppermost parts of mountain basins may be supply- limited, bedload transport in small torrents represents a potential hazard during extraordinary floods. The authors attempt to review recent worldwide research of bedload and hydraulics the results of which could potentially be applied in local steep channels. Approaches to the calculation of flow resistance (Manning relationship, logarithmic and power-law relationships) and critical conditions of the incipient motion of a bed particle (critical shear stress, stream power, flow velocity and discharge) are closely discussed in order to describe hydraulic and bedload transport conditions in steep headwater streams. Difficulties are pointed out in connection with the estimation of critical conditions of bedload transport in headwater streams with poorly sorted bed material and more or less developed bedforms. In the second part of the paper the authors present two pilot studies from the Western Carpathians and test potential applicability of specific formulas in this region. Results of the formulas point to relatively nonselective character of bedload transport during high magnitude floods in Beskydian headwater streams. An important role is played by the intensity of sediment delivery to active channels in limited sediment-supply conditions. The authors also point out that a majority of bedload transport and flow resistance equations were originally created on the basis of flume experiments or they were tested in Alpine torrents and therefore it is supposed that they might work well in local torrents after being calibrated to local conditions.