Channel Migration in Experimental River Networks Mapped by Particle Image Velocimetry

Deltaic river networks naturally reorganize as interconnected channels move to redistribute water, sediment, and nutrients across the delta plain. Network change is documented in decades of satellite imagery and laboratory experiments, but our ability to measure and understand channel movements is limited: existing methods are difficult to employ efficiently and struggle to distinguish between gradual movements (channel migration) and abrupt shifts in river course (channel avulsions). Here, we present a method to extract channel migration from plan-view imagery using particle image velocimetry (PIV). Although originally designed to track particles moving in a fluid, PIV can be adapted to track channels moving on the delta surface, based on input estimates of channel width, migration timescale, and maps of the wet-dry interface. Results for a delta experiment show that PIV-derived vector fields accurately capture channel-bank movements, as compared to manually drawn maps and an independent image-registration technique. Unlike other methods, PIV targets the process of channel migration, excluding changes associated with channel avulsions and overbank flow. PIV-derived migration rates from the experiment span an order of magnitude and are reduced under lower sediment supply and during sea-level rise, supporting recent models. Together, results indicate that PIV offers a fast and reliable way to measure channel migration in river networks, that channel migration rates under non-cohesive conditions can displace channels a distance comparable to their width in the time needed to aggrade similar to 10% of the channel depth, and that migration direction is similar to 60% orthogonal to mean flow direction and similar to 40% flow-parallel overall.