Quantifying Drag on Floating <i>Sargassum</i> Patches Under Combined Air and Water Forcing

Abstract Laboratory experiments were conducted in a unidirectional air‐water flume with glass sidewalls to quantify drag forces acting on pelagic Sargassum spp. patches exposed to steady airflow and water current. Both live and surrogate patches, designed to replicate the morphology and buoyancy of natural sargassum, were tested over Reynolds numbers ranging from to for water and to for air. Drag forces were directly measured using a high‐resolution force sensor, while flow velocities were characterized with ultrasonic anemometry and electromagnetic velocimetry for airflow and water current, respectively. The water‐current‐induced drag coefficient remained approximately constant across the tested range, whereas the airflow‐induced drag coefficient exhibited an inverse relationship with the Reynolds number. Side‐view imaging revealed elongation and deformation of the patches under increasing flow, emerging as a more reliable predictor of drag than the static frontal area in the experiments, while planform area that can be obtained from satellite may be considered as an alternative in natural marine environments. To address the underestimation of drag when using frontal area alone, a correction factor was introduced based on the ratio of the measured drag force to the estimated drag force. This formulation enables parameterization of drag using observable geometric features and flow conditions, with implications for modeling sargassum transport in coastal and oceanic environments.