Toward understanding tidal flat regime shifts: Contrasting hydro-sediment dynamics revealed by paired observations of two tidal flats

Global muddy tidal flats are increasingly at risk of shifting from accretion-dominated to erosion-dominated states under the combined pressures of climate change and intensified human activities. Understanding the process basis of tidal flat regime shifts hinges on clarifying the fundamental differences in coupled hydrodynamics, sediment transport, and morphological processes that sustain erosional versus accretional states and their diagnostic implications. However, process-based studies that directly compare these contrasting states through paired observations within the same tidal flat system remain limited. Here, we conducted coordinated, high-frequency measurements of hydro-sediment processes at two morphologically contrasting sites—one eroding and one accreting—within the Jiangsu coast, China, which is experiencing a system-wide regime shift and exhibits strong spatial heterogeneity, with coexisting erosional and accretional sectors. Results show that the eroding flat experiences stronger wave forcing and higher bed shear stresses, which drive persistent seaward sediment export and ongoing bed erosion. In contrast, the wide and gently sloping morphology of the accreting flat enhances wave energy dissipation, promotes tidal current development, and generates early-flood velocity pulses that trigger SSC surges, creating critical sedimentation windows that sustain long-term accretion. Notably, SSC was higher at the eroding tidal flat than at the accreting site, underscoring that tidal flat regime shifts are not governed by sediment availability alone but rather emerge from complex interactions among morphology, hydrodynamics, and sediment transport processes. These findings advance process-based understanding of tidal flat evolution and provide mechanistic insights to support the management of coastal erosion risks under multiple stressors.