84-9 Variable delta-plain channel mobility during coastal transgression

Abstract:

River-dominated delta plains are continually shaped by movement of distributary channels (i.e., channel migration and avulsion). The long-term sustainability of these plains therefore depends largely on channel regulation of the distributed water and sediment. On prograding, fan-shaped delta plains, this regulation is expressed through gradual channel migration and repeated autogenic channel re-routing (avulsion), resulting in the construction of delta lobes beyond the shoreline as the river delta builds basinward. However, there has been less attention paid to the details of variable channel migration rates during the systematic landward retreat of river deltas. Here we present experimental observations detailing seven successive avulsions and channel migration associated with overall landward shoreline retreat. By utilizing an adapted Particle Image Velocimetry (PIV) to map channel bank migration vector fields, we quantify channel migration rates during aggradation between these avulsion events. Physical modelling was conducted in a 3.8 ´ 3.8 m basin at the Queen’s University Coastal and Deepwater Sediment Laboratory using a cohesive sediment mixture. Over the 300-hour experiment, base level was raised by a total of 150 mm while sediment and water discharge were held constant. Under these conditions, the upstream migration of in-channel deposition dominated the delta plain, modulating channel mobility and causing nodal avulsions at the delta apex. Between successive nodal avulsions, delta plains evolved through a recurring four-phase sequence with variable duration: (1) sediment bypass through the main distributary channel, driving mouth-bar growth and shoreline progradation; (2) landward migration of in-channel deposition within the main distributary; (3) extensive delta-plain aggradation through high channel mobility driven by in-channel deposition; and (4) reorganization of smaller delta-plain channels prior to establishment of a new single main channel. Our preliminary results suggest that: 1) periods of elevated lateral channel migration rates occur during punctuated shoreline transgression, whereas low migration rates occur during regressive shoreline pulses, and 2) spatio-temporal variability in channel mobility directly correlates with aggradation rates across the delta plain.

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