Coastal rivers that build deltas undergo repeated avulsion events—that is, abrupt changes in river course—which we need to understand to predict land building and flood hazards in coastal landscapes. Climate change can impact water discharge, flood frequency, sediment supply, and sea level, all of which could impact avulsion location and frequency. Here we present results from quasi‐2D morphodynamic simulations of repeated delta‐lobe construction and avulsion to explore how avulsion location and frequency are affected by changes in relative sea level, sediment supply, and flood regime. Model results indicate that relative sea‐level rise drives more frequent avulsions that occur at a distance from the shoreline set by backwater hydrodynamics. Reducing the sediment supply relative to transport capacity has little impact on deltaic avulsions, because, despite incision in the upstream trunk channel, deltas can still aggrade as a result of progradation. However, increasing the sediment supply relative to transport capacity can shift avulsions upstream of the backwater zone because aggradation in the trunk channel outpaces progradation‐induced delta aggradation. Increasing frequency of overbank floods causes less frequent avulsions because floods scour the riverbed within the backwater zone, slowing net aggradation rates. Results provide a framework to assess upstream and downstream controls on avulsion patterns over glacial‐interglacial cycles, and the impact of land use and anthropogenic climate change on deltas.
Understanding channel migration is essential in interpreting long‐term evolution of fluvial systems and their deposits. Using data from an experimental delta, we analyzed the kinematics of the upstream channel and assessed the relative dominance of continuous lateral channel migration versus abrupt changes (i.e., avulsions). Detailed investigation of channel centerline location at minute intervals reveals a short‐term correlation between the magnitude of migration rates measured at the same location and a spatial correlation that diminishes with distance between points. The main finding is that the channel migrates across the entire deltaic domain without large and abrupt lateral shifts but through continuous lateral migration at variable rates. Long periods of back and forth small moves are separated by short bursts of rapid lateral migration. This finding contradicts the default expectation that that aggrading systems are characterized by avulsions and suggests that highly mobile rivers tend to avulse less. We contrast this with another experiment conducted under similar conditions but with finer sediment supplied at a lower rate which shows drastically less lateral migration; the kinematics is instead dominated by periodic flow reconfiguration episodes akin to avulsions, an indication that channel migration‐style depends on the sediment load. The characteristics of these two experiments parallel two regions of the Mississippi River, the meandering and highly mobile alluvial plain and the less dynamic deltaic region, suggesting that bedload sediment deposition at the transition into backwater zone plays an important role in re‐shaping the river planform and migration style.
more » « less- Award ID(s):
- 1854452
- NSF-PAR ID:
- 10369114
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Earth Surface
- Volume:
- 127
- Issue:
- 5
- ISSN:
- 2169-9003
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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