Expansion bar

Position: In-channel

 

Form

Coarse-grained (up to boulder size) mid-channel bar with a fan-shaped planform. Streamlined ridge forms trail behind obstructions in the channel. Foreset beds commonly dip downstream with a very rapid proximal-distal grain size gradation. Often occur downstream of a bedrock constriction that hosts a forced pool. May be colonised and stabilised by vegetation.

 

Process description

As flow expands abruptly at high flood-stage in high-energy depositional environments, it loses competence and induces deposition. Dissection is common at falling stage. These bars remain fairly inactive between large floods, constraining processes at lower flow stages.