Suspended sediments over silt-sand mixtures under waves.

Peng Yao, Min Su, Zhengbing Wang, Changkuan Zhang, Yongping Chen, Marcel J.F. Stive

Monday 29 june 2015

17:15 - 17:30h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 3A. Sediment - Erosion

Deposits on tidal flat area are generally the mixtures of clay, silt and sand. The different types of sediment bed (clay-silt-sand mixtures) will result in different sediment transport processes, affecting the morphological changes. As these fine materials are mainly transported in suspension, it is important to understand the mechanisms of suspended sediment transport. The sediment in a silty coast behaves different from that of a sandy coast and muddy coast. One of its distinctive characteristics is the formation of the high concentration layer (HCL) near bottom during the stormy weathers. Although this phenomenon is well known, the formation and transport mechanism is poorly understood. Historically, many theories have been proposed to describe the near bed sediment concentration for non-cohesive sediments, whether these formulations are suitable for the silt-enriched HCL is unclear. We conducted a series of flume experiments to explore this knowledge gap on the HCL of the silty coast. Two types of bed materials with different silt content are investigated. The present study mainly concerns the suspended sediment concentration under various wave conditions. Quasi-steady high concentration layer has been observed in a wide range of wave conditions for both sediment beds. Suspended sediment concentrations are measured using a transverse suction system. The concentration inside the HCL (lowest measuring point) varies from 3 g/l to 60 g/l. Then, several existing near bed concentration predictors are tested against the experimental data. The results show that the latest formula of Van Rijn (2007a; 2007b) can give the best overall fitting results and can be used as a predictor for silt classes after coefficient re-calibration. This study is the first step in a systematic investigation of morphological changes over silty tidal flats.