Deposits

•Lateral accretion deposits laid down by migrating rivers, eroding into floodplain & laying down sediment in their wake

•Accretion of point bar deposits from lateral accretion can be seen as series of concentric ridges on inside of bends: meander scrolls (Willis, 1989)

•Points bars subjected to slow helical flows & aren’t prone to erosion, allowing expansion towards outside meader bend - deposition occurs due to slow flows 

POINT BAR DEVELOPMENT - BANK ATTACHED 

•Super-elevation of flow on outside of bend produces high pressure

•High-pressure forces flow down then back up, creating shear layer with flow near the outer bank (outer bank cell)

•Deposition can occur on inner bank as flows are slower à allows formation of point bar – first flows are pushed into outer bend due to centrifugal force

•Transverse current combined with downstream current produces helicoidal flow

•Zones of flow separation can cause deposition on inner bank (Nanson, 1980)

•Vertical deposits composed of fine material (typical of meandering channels) that settles out of suspension when overbank flows inundate the floodplain

•Increased area of contact with roughness of floodplain surface reduces flow velocities & layer of sediment draped across floodplain

•Fining upwards sequence - coarser particles settle out first but then progressively overlain by finer material

•Fining of sediment away from channel - only finest material carried to the edge of inundated area – lightest

•Over number of years, cumulative effect of overbank flow leads to development of a vertical sequence of thin layers

Chute bar and Plug bar (Dijk et al., 2012)

•During overbank flow, thalweg changes to shortest route

•Flooding flow traverse existing point bar - chute bar forms - extreme scour

•Period of bifurcation à river flows around old meander bend and new channel •Curvature at chute bifurcation between former active channel and chute channel leads to such a diversion of water & sediment that former channel closed with a plug bar

•Dijk et al. (2012) used flume to understand meander/chute bar formation

•Huge quantities of sed released from cutoffs over 2 years à equivalent to volume mobilised over 60 years of lateral accretion (USA, Zinger et al., 2011)

Neck cut-off (Zinger et al., 2011)

•Prorgessive migration of an elongated bend onto itself

•Neck cut-off takes place to avoid self-intersection of 2 reaches that come into contact à important shortening mechanism (Camporeale et al., 2008)

Ultimately forms/deposits oxbow lake

Counter-point accretion: associated wth deposition of concave bank benches at confined meander bends

•As over-tightened meander bend migrates, bench deposits become incorporated into the floodplain

•Braiding developed by systematic sorting as the stream deposits load sizes it is no longer able to carry

•This bedload deposition will result in a number of different types of bars and -  bedforms -in turn, impede/divert flow

•Banks which are easily eroded will facilitate this process (Miall, 1982)

•Braid bar accretion occurs when bars are abandoned and gradually become incorporated into the floodplain deposits

•Can happen when a large flood lays down extensive bar deposits

•Also occurs when bars are abandoned because the main braid channels shift to another part of the valley

•Brice (1964) defined mid-channel bars are being un-vegetated and submerged at bankfull 

Involves shift in position of a channel, commonplace in braided reaches where flow frequently abandons & reoccupies sub-channels

•Can involve diversion of flow into newly eroded channel cut into floodplain

•Leaves behind the previous deposits of braided rivers

•Floodplains of braided rivers typically higher energy and have different sediment composition to meandering floodplains (larger sediments)

•High energy fluvial envs w/steep gradients, large & variable dischargers, dominant bed-load & non-cohesive banks •Very little silt and clay = increases erosion rates

•Positive feedback increases prominence of bar form and channel widening à acts as nucleus for further deposition (traps sediment)

•Accretion through shifting of primary channels (avulsion)

•Local deposition and channel incision lead to formation of abandoned braid-bars à mosaic of deposits form along braided rivers

•Floodplains of braided rivers have huge quantities of sediment moving through

Longitudinal

•Mid-channel - not attached to banks (different from meander point bars)

•Typically formed during high flood flows

•Coarse sediments deposited during high flow become a barrier at low flow

•Flow diverges around tear-drop shaped structure, causing further bank erosion

Transverse

•Cross-channel - can be attached to one bank (but not always!)

•Grow in the downstream direction diagonally across the channel, with bar head accretion