Assessing the temporal stability of satellite-derived bathymetry in intertidal flats using Sentinel-2A/B/C

Intertidal environments exhibit shallow depths, low relief, and strong optical variability, which limit the use of conventional bathymetric surveys and complicate the interpretation of satellite-derived bathymetry (SDB). This study evaluates a multi-temporal SDB approach based on Sentinel-2A/B/C imagery in a bayhead delta characterized by extensive intertidal flats under mesotidal regime. To address the interpretation of apparent bathymetric variability in low-relief intertidal systems, we introduce the Intertidal Bathymetric Stability Index (IBSI), a dimensionless diagnostic that relates temporal variability in SDB to its validated uncertainty. The method is applied to three campaigns in 2025 using multi-scene compositing within restricted temporal windows. Validation against in situ bathymetric data shows that composite SDB products achieve median absolute errors of 0.51-0.58 m with negligible bias for depths up to ∼10 m, outperforming single-scene retrievals under variable optical conditions. IBSI values identify spatial contrasts in retrieval stability, separating shallow intertidal flats with consistent SDB estimates from river-influenced channels, vegetated areas, and the intertidal-subtidal transition, where variability exceeds method detectability. The proposed SDB-IBSI framework does not aim to detect bathymetric change directly. Instead, it defines where multi-temporal SDB can be interpreted within detection limits. This diagnostic approach supports process-based analysis, modelling, and monitoring design in shallow, turbid environments.