COMPARISON OF BEDLOAD TRANSPORT IN A STEEP MOUNTAIN TORRENT WITH A BEDLOAD TRANSPORT FORMULA

摘要

The Swiss Federal Institute for Forest, Snow and Landscape Research has beenmeasuring bedload transport in a steep mountain torrent in Switzerland with socalled hydrophones for more than 12 years. These observations are compared withtransport rates predicted by a simple bedload transport formula, which has beenestablished using results of laboratory experiments.For high flow intensities the predicted relation between runoff an bedload transportshows a trend which is similar to the one observed in the field data, but theprediction generally overestimates observed bedload transport. This discrepancy ismainly due to the fact that irregularities in bed form and grain size distribution in anatural mountain torrent induce additional turbulence. This dissipates energy, whichis otherwise available for transport. Therefore the formula represents a maximumcapacity of transport. It seems possible to determine bedload transport in a naturalstream with a formula established with laboratory data, if modifications are made toaccount for this energy dissipation.For lower flow intensities close to the threshold conditions, the increase of bedloadtransport with increasing runoff is less steep than predicted by the formula. Thistransition range extends over a wide range of runoff, and it is accompanied by a veryhigh variability. One important factor for this broad transition range is the very widegrain size distribution in the observed mountain torrent, which implies that theassumption of equal mobility is not fulfilled. A further complicating factor is that withthese flow intensities bankfull discharge is not yet achieved. As the grain sizedistribution in the channel changes from place to place, it is possible that the samerunoff one time may only flow over big boulders that it cannot mobilise. Another timethis runoff may flow mainly over grains it can easily mobilise, producingcomparatively important bedload transport. These observations lead the authors topropose a probabilistic approach to describe the behaviour of a steep mountaintorrent in this transition range.