Decadal‐Scale Bathymetric Change in a Shallow Coastal Bay

Abstract

Coastal marshes and bays must accumulate sediment to maintain their position relative to mean sea level during periods of sea level rise (SLR). Resuspension of bay‐bottom sediment, which together with tidal inundation promotes marsh deposition, is sensitive to water depth, but quantitative measurements of bathymetric change across shallow bays are rare. This study quantified bathymetric change over a 24‐year period in a shallow coastal bay lacking fluvial sediment input by repeating a high‐resolution survey. On average, shallow tidal flats (0–2 m) deepened 1.9 ± 0.3 cm yr −1 , due to a combination of relative SLR and erosion, while a large channel bisecting the bay accreted 6.6 ± 0.7 cm yr −1 . The net sediment volume eroded from shallow tidal flats far exceeded that estimated for marshes to keep pace with relative SLR. Month‐long model simulations were consistent with observations, reproducing erosion of shallow subtidal flats dominated by storm‐wave‐driven resuspension and deposition in the channel. Tide‐induced resuspension of channel bed sediment during fairweather conditions resulted in net sediment export to the coastal ocean over the simulation period despite import during a northeasterly storm. Modeled sediment fluxes to adjacent marshes based on the two bathymetric surveys were approximately the same despite the increase in depth, consistent with nearly constant rates of marsh accretion in nearby marshes over the same two decades. These findings highlight the value of decadal‐scale bathymetric surveys, the potential deepening of shallow coastal bays without large external sediment sources, and the importance of fairweather conditions when sediment budgeting.