Bench and point bench (oblique-accretion bench)

Position: In-channel

 

Form

Bench

A distinctly stepped, elongate, straight to gently curved feature that is inset along one or both banks. These significant in-channel sediment storage units are often situated atop bar deposits. They may comprise obliquely attached mud-rich drapes with a convex geometry in suspended load systems, or obliquely and vertically accreted sand deposits in bedload systems. Sedimentary structures tend to be quite distinct from the floodplain.

Point bench

Point bench: Distinctly-stepped, bank-attached unit developed along the convex bank of a channel. Typically has an arcuate planform with a planar surface that is elevated above the point bar.

 

Process description

Bench

Bench: Formed by oblique-and vertical-accretion of bedload and suspended load materials during small to moderate floods within widened channels. During the rising stage of flood events, bedload materials are deposited atop step-like features. During the waning stages, suspended load materials are deposited as flood couplets atop bedload materials. Oblique accretion benches represent low energy falling-stage suspended load deposition in sand-bed and mud-rich streams. Deposition is often promoted by riparian vegetation. Benches are a major agent of channel contraction in over-widened channels.

Point bench

Point bench: Deposition along the convex bank via vertical and/or oblique accretion of interbedded sands and mud indicates slow lateral migration or lateral accretion within an overwidened bend.