Seasonal Wave Attenuation in the Muddy Nearshore Environment of the Modern Huanghe Delta from In Situ Observations and SWAN Modeling

Wave energy dissipation has been observed in muddy nearshore environments such as off the coast of the Atchafalaya River, resulting in reduced wave energy during shoaling, essentially protecting the coast from resuspension and sediment dispersion. Fortnight-length time-series wave measurements were recorded at two sites, approximately 13 m and 4 m depth, on the Huanghe (Yellow River) delta front during a summer field campaign in 2016 and a winter field campaign in 2018. Significant wave height (H_s) during summer increased from deep to shallow water (0.8 to 1.1 m), consistent with Linear Wave Theory; however, H_s decreased during winter from deep to shallow water (4.2 to 2.2 m), suggesting different bed conditions, e.g., an unconsolidated seabed or fluid mud. Wave-current shear velocity exceeded the threshold to move very fine sand 90% of the measurement period during winter compared to 42% during the summer, demonstrating a seasonal disparity in sediment transport capacity. However, nearbed suspended-sediment concentration varied with alongshore tidal currents during summer, but not in winter, despite higher current velocities than those observed during the summer. This observation, combined with measured suspended-sediment concentrations of 3 g/L at 55 cmab suggests the presence of a thin muddy layer with the lutocline inhibiting the suspended-sediment mixing above it. SWAN modeling will be used to quantify the impacts of wave refraction, bed friction or the presence of mud on waves approaching the Huanghe delta using summer and winter wave observations.