Flow and grainsize pattern in a sharply curved river bend

Abstract

The main characteristics of river flow and grainsize in a bend of the sand bedded meandering river Dommel, The Netherlands, are presented. Measurements were carried out at a relatively low discharge in a sharply curved bend following a long straight reach. In the studied bend, secondary circulation is restricted to the thalweg area; only in the downstream part of the bend it exists over the entire cross-section. Therefore, on the entire pointbar platform, which comprises the larger part of the bend, the median sedimentation diameter of the bedload material is governed by the distribution of the longitudinal components of the bed shear stress only. KEY WORDS River flow Secondary circulation Grainsize Bed shear stress Meander bend *Present address: Rijkwaterstaat-North Sea Directorate,

Flow in meander bends with recirculation at the inner bank

“…Only very recently have 3‐D velocity measurements been used [e.g., Rhoads and Sukhodolov , 2001; Frothingham and Rhoads , 2003]. Some authors have also [ Ashmore et al , 1992], or instead [ Van Alphen et al , 1984], mapped the difference in flow direction between surface and bed.…”

“…In laboratory bends of moderate curvature and symmetric cross section the helix occupies the full width. In natural bends it may be restricted to the outer part of the channel if a shallowly submerged point bar steers the inner‐bank flow entirely outward [e.g., Dietrich and Smith , 1983; Van Alphen et al , 1984; Thorne and Rais , 1985].…”

On secondary circulation, helical motion and­Rozovskii-based analysis of time-averaged­two-dimensional velocity fields at confluences

“…Because the depth-averaged velocity vectors are oriented tangentially to the depth-averaged streamlines, centrifugal forces associated with any curvature of these streamlines will be oriented perpendicular to the depth-averaged vector (Rozovskii, 1957, p. 201). Differences in the strength of centrifugal forces over depth in a curving flow subject to a transverse pressure gradient can induce helical motion, as documented rather conclusively in studies of meandering rivers (Jackson, 1975;Hooke, 1975;Hickin, 1978;Alphen et al, 1984). This helical motion will be expressed as systematic deviations of individual velocity vectors at several adjacent verticals from the orientation of the depth-averaged vectors.…”

“…Prus-Chacinski, 1954;Yen, 1965;Jackson, 1975;Hooke, 1975;Hickin, 1978;Alphen et al, 1984;Thorne and Furbish, 1995). This approach is not restricted to the analysis of flow in meandering rivers, but also can be used to infer the presence and strength of helical motion in straight channels with curved flow streamlines (Leopold, 1982;Nelson and Smith, 1989).…”

Grain-Size Sorting in Meander Bends Containing Large Woody Debris

“…In meander bends containing mixed bed material, coarse particles are expected to shift outward through the bend along with the shifting zone of maximum boundary shear stress, while finer particles shift inward, resulting in a "crossover" pattern (e.g., Dietrich and Smith, 1984;Van Alphen et al, 1984;Dietrich, 1987). While coarse particles are present along the inner bank at the bend entrance and shift toward the outer bank near the bend apex, coarse particles do not remain dominant along the outer bank through the bend exit either before or after LWD removal.…”