Flow processes near smooth and rough (concave) outer banks in curved open channels

Flow processes near the (concave) outer bank in curved river reaches are investigated in a laboratory flume, with focus on the influence of the bank roughness. An outer-bank cell of reversed secondary flow occurs for all the investigated roughness configurations of the outer bank. The cell widens the outer-bank boundary layer, which reduces the flow forcing on the bank, but also advects high-momentum fluid toward the lower part of the bank, which enhances the flow forcing on the bank. Increasing the roughness of the outer bank causes a considerable widening and strengthening of the outer-bank cell, which amplifies both effects on the flow forcing. The widening of the outer-bank boundary layer induced by the outer-bank cell reduces the effective width of the channel, i.e., the width where most of the discharge is conveyed, which increases significantly the sediment transport capacity in the central part of the channel. The near-bank processes are qualitatively similar but show considerable quantitative variations around the bend. The outer-bank cell reaches its maximum strength in the zone that is most vulnerable to bed scour and bank erosion, which corroborates its morphological relevance. The outer cell is mainly generated by reversed near-surface gradients in the profile of the streamwise velocity. The anisotropy of the cross-stream turbulence is the major mechanism opposed to the outer-bank cell. Increasing roughness of the outer bank amplifies all dominant mechanisms but does not modify their relative importance. The relevance of the experimental results for natural curved river configurations is discussed.