The Swiss Federal Institute for Forest, Snow and Landscape Research has been measuring bedload transport in a steep mountain torrent in Switzerland with so called hydrophones for more than 12 years. These observations are compared with transport rates predicted by a simple bedload transport formula, which has been established using results of laboratory experiments.
For high flow intensities the predicted relation between runoff an bedload transport shows a trend which is similar to the one observed in the field data, but the prediction generally overestimates observed bedload transport. This discrepancy is mainly due to the fact that irregularities in bed form and grain size distribution in a natural mountain torrent induce additional turbulence. This dissipates energy, which is otherwise available for transport. Therefore the formula represents a maximum capacity of transport. It seems possible to determine bedload transport in a natural stream with a formula established with laboratory data, if modifications are made to account for this energy dissipation.
For lower flow intensities close to the threshold conditions, the increase of bedload transport with increasing runoff is less steep than predicted by the formula. This transition range extends over a wide range of runoff, and it is accompanied by a very high variability. One important factor for this broad transition range is the very wide grain size distribution in the observed mountain torrent, which implies that the assumption of equal mobility is not fulfilled. A further complicating factor is that with these flow intensities bankfull discharge is not yet achieved. As the grain size distribution in the channel changes from place to place, it is possible that the same runoff one time may only flow over big boulders that it cannot mobilise. Another time this runoff may flow mainly over grains it can easily mobilise, producing comparatively important bedload transport. These observations lead the authors to propose a probabilistic approach to describe the behaviour of a steep mountain torrent in this transition range.