Bed-load Sorting in an Alluvial River

Abstract

Bed-load transport measurements of different grain-size fractions in a sand-bed river (bankfull discharge = 10 m 3 /s, width = 15 m, depth = 1.5 m, mean grain diameter = 0.5 mm) in southwest Denmark showed that the transport rate in the central grain sizes varies less than in all other fractions in the bed load. This is argued to be related to the role of point bars as sediment reservoirs. During net erosion the lower part of the point bar is eroded, and deposition takes place at the bar top, whereas the situation is reversed during net deposition. Thus, during their growth, point bars undergo repeated episodes of erosion and deposition. During these episodes the bed load is sorted. Measurements of input and output of size fractions from the studied meander bend showed that the overall effect of the size sorting is preferential deposition of fine and coarse fractions, whereas the central size fractions are preferentially transported through the bend. This type of sorting seems to be a general feature in sand-bed river bends and affects the often assumed condition of bed-load continuity for each size range in meander bends. The long-term effect of this is (1) bed load becomes better sorted during its down-valley passage through meander bends and (2) the central size fraction is less abundant in the bulk sediment of point bars than in the average bed load. Thus, during floods when point bars release sediment to the bed load, the size distribution of the bed load is preferentially supplied with sizes on each side of the central fraction.