Abstract: Surfactant is a common pollutant in natural waters. The adsorption of surfactant on sediment changes the surface properties of sediment, thus affecting the rules of sediment's incipient motion. With the fine grain sediment in Wuhan segment of the Yangtze River as research object, the effects of ionic surfactants on sediment's incipient motion were examined both in qualitative and quantitative terms by simulating the starting process of sediments in circulating flumes. Results suggest that the starting process of sediment can be subdivided into seven stages. When flow velocity was 15.0~20.0 cm/s, the water body was obviously turbid and sediments started. The presence of surfactant reduced the suspension of sediments and cut the equilibrium sediment concentration significantly to around 50% of the initial sediment content. A rising concentration of anionic surfactant helped augment the critical starting velocity of sediment from 19.37 cm/s to 23.05 cm/s, while curb the full starting velocity slightly from 37.06 cm/s to 36.15 cm/s. Increment in the concentration of cationic surfactant boosted the critical starting velocity of sediment from 19.37 cm/s to 23.32 cm/s, while lowered the full starting velocity from 37.06 cm/s to 32.12 cm/s. The presence of ionic surfactants promotes the flocculation of sediments, thus evidently affects the incipient motion of sediment. Cationic surfactant has a greater effect than anionic surfactant.

Key words: fine grain sediment, ionic surfactant, sediment's incipient motion, critical starting velocity, full starting velocity, equilibrium sediment concentration