Combined flow-induced bed shear stress and sediment-biologic dynamics in a supratidal flat of the Bahía Blanca Estuary during a positive storm surge

This study examines bed shear stress generated by combined flows ( τ 0 ) in a supratidal flat of the Bahía Blanca Estuary during a spring tide coinciding with a positive storm surge driven by an extratropical storm and evaluates the associated sedimentological and biological responses. Bed shear stress τ 0 was quantified using high-frequency acoustic Doppler velocimeter (ADV) measurements and Reynolds stress estimates. They were analyzed together with cm-scale bedform observations and chlorophyll- a (Chl- a ) and phaeopigment concentrations as proxies for photosynthetic microbial activity. Analyses of synoptic maps revealed an extratropical storm advancing SW–NE generating a positive storm surge of up to ∼1 m. During the event, τ 0 frequently exceeded the critical bed shear stress of sterile sediments ( τ 0 c ), occurring persistently during ebb tide. During slack tide, sediment transport was observed to occur primarily under the influence of wind-induced waves. Despite the stabilizing effect of the biofilm, which increases τ 0 c , observed changes in surficial micro-geomorphology, together with τ 0 estimates, indicate that combined flow forcing was sufficient to induce sediment transport during supratidal inundation. These physical disturbances were accompanied by an approximately 40% reduction in Chl- a concentrations during the storm period and a significant increase in phaeopigments. Therefore, the biofilm covering the supratidal flat sediments did not prevent erosion under the hydrodynamic forces generated during an extratropical storm event coupled with a spring tide. This event-based case study highlights the value of integrating hydrodynamic, sedimentological, and biological indicators in a supratidal setting, providing in situ evidence of the coupled response of supratidal flats to extreme hydrodynamic forcing.