Grain sorting process and scour in a curved channel with coarse gravel bed

In a curved channel with a movable bed, systematic laboratory tests were carried out by using wide grain size distribution and rather high longitudinal slopes as found in pre-alpine rivers in Europe. Scour formations at the outer side of the bend and the transversal grain sorting process were studied in detail. The scour hole is armored by grain sorting. At equilibrium condition of the scour, this armoring layer has a mean grain size which is typically almost the double of the initial mixture. At the inner side of the bend, a bank of fine material is formed with a mean diameter of about 60% of the initial grain size distribution. Grain sorting process seems to highly influence the maximum scour depth. A comparison with some empirical scour formulas revealed that in general these underestimate transversal bed slopes and maximum scour for such conditions.