Hydraulic or Seepage Erosion: What Drives Bank Collapse in Tidal Environments?

The collapse of channel banks in tidal environments has typically been interpreted using fluvial concepts that prioritize hydraulic (flow-driven) erosion. Yet daily tidal fluctuations trap pore water in channel banks, potentially producing sustained seepage flows capable of triggering collapse even without strong currents. To assess the relative roles of hydraulic and seepage erosion, we performed scaled laboratory experiments spanning a range of tidal-current and seepage conditions. We identify two distinct failure modes: fast, toppling failures driven by tidal currents and slow, progressive pop-out failures caused by seepage flows. Numerical simulations further show that seepage becomes the dominant driver of bank collapse under ebb-dominant tides as tidal range increases. Integrating experimental and numerical results, we derive dimensionless predictors that unify hydraulic and seepage controls within a common scaling framework. These findings reveal seepage as a critical but previously underappreciated mechanism governing bank collapse in tidal systems.