Observations and analyses of floc size and floc settling velocity in coastal salt marsh of Luoyuan Bay, Fujian Province, China

Abstract

In coastal environments, fine-grain sediments often aggregate into large and porous flocs. Electro-Magnetic Current Meters (EMCM) and Laser In Situ Scattering and Transmissometry (LISST-ST) have been deployed within a Spartina alterniflora marsh of the Luoyuan Bay in Fujian Province, China, to measure the current velocity, the floc size and the settling velocity between 15 and 22 January 2008. During the observations, the near-bed water was collected in order to obtain the suspended sediment concentration (SSC) and constituent grain size. Data show that: (1) the nearbed current velocities vary from 0.1 to 5.6 cm/s in the central Spartina alterniflora marsh and 0.1–12.5 cm/s at the edge; (2) the SSCs vary from 47 to 188 mg/dm3. The mean grain size of constituent grains varies from 7.0 to 9.6 µm, and the mean floc sizes (MFS) vary from 30.4 to 69.4 µm. The relationship between the mean floc size and settling velocity can be described as: ws=adb, in which ws is the floc settling velocity (mm/s), a and b are coefficients. The floc settling velocity varies from 0.17 to 0.32 mm/s, with a mean value of 0.26 mm/s, and the floc settling velocity during the flood tide is higher than that during the ebb tide. The current velocity and the SSC are the main factors controlling the flocculation processes and the floc settling velocity.