Experimental study of the density influence on the incipient motion and erosion modes of muds in unidirectional flows: the case of Huangmaohai Estuary

Abstract

The incipient motion and erosion behavior of bottom muds in coastal areas have wide applications in coastal engineering, water environment management, and also the protection of marine benthic communities. Due to the strong inter-particle cohesion of fine sediments and the diversity in its physical properties, the mechanism of the coastal mud erosion is not well understood and the determination of the threshold for the incipient motion remains a challenge. In order to investigate the influence of the mud density on the incipient motion of fully disturbed coastal mud, experiments were carried out using a 22-m-long laboratory flume and mud samples from Huangmaohai Estuary, South China Sea. Muds with densities ranging from 1100 to 1550 kg/m3 were tested under unidirectional open channel flows, which yielded threshold velocities ranging from 0.11 to 1.67 m/s, corresponding to bed shear stresses ranging from 0.029 to 4.191 N/m2. Based on the experimental results, an empirical formula for the threshold velocity at the incipient motion of coastal muds with different densities is presented. The computed results together with the proposed formula were also compared with other theories. Besides, four different erosion patterns of coastal muds at incipient motion were identified according to experimental observations, namely erosion in the pattern of fluid muds, strips, pieces, and blocks. The physical mechanisms for different erosion patterns were also analyzed and interpreted. The presented experimental achievements well enrich our knowledge of coastal mud behavior and lead to a deeper understanding of the mechanisms of the incipient motion of bottom muds.