The downstream fining of gravel-bed sediments in the Alpine Rhine River

Abstract

The field study of the downstream changes of size, shape and petrographic composition of the Alpine Rhine sediment concerned the reach between the confluences with the Landquart River and the Ill River. The results show the relative importance of sorting effects at the beginning and the end of the study reach. In the middle part of the study reach, where sorting effects seem to be neglibile, the weight reduction coefficient a _w reaches values definitely lower than 0.010/km. Surprisingly, only one lithology is dominant, namely limestone with approximately 5 to 50% quartz or silica. Differences between carbonate and non-carbonate lithology groups is only of lesser significance.

The laboratory abrasion experiments were performed in a specially adjusted tumbling mill of 690 liter volume. Sediment mixtures with grain sizes ranging from 2 to 128 mm were used and the initial mass was usually about 400 kg. The test mixtures were sieved from the bulk samples taken from the alternate bars of the Alpine Rhine. The abrasion distance travelled in the laboratory abrasion set up corresponded to a range of 40 to 80 kilometers. The mean weight reduction coefficient a _w of each mixture was then determined from the observed weight changes. Their values a _w =0.005–0.018/km confirm the weight reduction coefficients resulting from the field study.

(formerly Institute for Water Management, Hajdrihova 28 SLO-61000 Ljubljana Slovenia)