Review on the Bed Scouring in the Constriction Zone of Local River Widenings

Local river widenings serve as a technique for restoring channelized rivers and mitigating river incision. The zone downstream where the widened reach transitions into the channelized river is referred to as the constriction zone. While numerous authors have extensively detailed the hydraulics of contraction in open-channel flow with fixed bed and banks, studies focusing on rivers with movable-beds, particularly gravel-bed, are notably scarce. The geometry of the transition zone is typically selected to prevent scour formation and minimize the backwater effect, which could elevate water levels upstream and pose potential flooding risks. Often, the constriction zone needs artificial stabilization through expensive river engineering structures. This article reviews design criteria and general considerations for downstream transition zones resulting from local river widenings, aiming to facilitate the efficient design of constriction zones.