Journal Article

Topography‐Based Particle Image Velocimetry of Braided Channel Initiation

Chadwick, Austin J., Wang, Youwei, Limaye, Ajay B.

2024

Overview

River channels shape landscapes through gradual migration and abrupt avulsion. Measuring the motion of braided rivers, which have multiple channel threads, is particularly challenging, limiting predictions for landscape evolution and fluvial architecture. To address this challenge, we extended the capabilities of image‐based particle image velocimetry (PIV)—a technique for tracking channel threads in images of the surface—by adapting it to analyze topographic change. We applied this method in a laboratory experiment where a straight channel set in non‐cohesive sediment evolved into a braided channel under constant water and sediment fluxes. Topography‐based PIV successfully tracked the motion of channel threads if displacements between observations were less than the channel‐thread width, consistent with earlier results from image‐based PIV. We filtered spurious migration vectors with magnitudes less than the elevation grid spacing, or with high uncertainties in magnitude and/or direction. During braided channel initiation, migration rates varied with the channel planform development, showing an increase as incipient meanders developed, a decrease during the transitional braiding phase, and consistently low values during the established braiding phase. In this experimental setup, migration rates varied quasi‐periodically along stream at the half scale of initial meander bends. Lateral migration with respect to the mean flow direction was much more pronounced than streamwise migration, accounting for approximately 80% of all detected motion. Results demonstrate that topography‐based PIV has the potential to advance predictions for bank erosion and landscape evolution in natural braided rivers as well as bar preservation and stratigraphic architecture in geological records. Plain Language Summary Rivers naturally move across the landscape through gradual migration and abrupt shifts, with direct consequences for riparian ecosystems, fluvial sedimentology, and riverside communities. Current knowledge of river mobility is especially limited for braided rivers, which are composed of multiple interwoven sub‐channels (called channel threads) that can move independently. To address this knowledge gap, we expanded techniques that track channel thread motion from photos by applying them to elevation data. We find that for a laboratory experiment where a braided river is initiated from a straight channel, migration rates first accelerate and then slow down. These rates fluctuate regularly along the river, particularly around incipient meander bends. Channel banks and bars tend to migrate laterally rather than downstream, although this tendency diminishes over time. Findings from this study can help better understand past environments from sedimentary rocks and assess erosion and flood risk from shifting rivers. Key Points Topography‐based particle image velocimetry effectively tracks the migration of braided channel threads in a laboratory experiment During the initiation of a braided channel, channel threads migrate quickly as incipient meanders form and slow down as braiding develops Similar to channel belt growth, migration rates vary quasi‐periodically along stream, at the half scale of initial meander bends

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Avulsion / Bank erosion / braided river / Braided rivers / Braided streams / Braiding / channel thread / Channels / Cohesive sediments / Direction / Elevation / Environmental risk / Evolution / Flood risk / Image filters / laboratory experiment / Laboratory experimentation / Landscape architecture / landscapes / Meanders / migration / Movement / Particle image velocimetry / River channels / River meanders / Rivers / Sediment / Sedimentary rocks / Sedimentology / sediments / Straight channels / Stratigraphy / streams / Topography / Vectors / water