Vertical profiles of suspended sediment concentration estimated from acoustic backscatter are used to re-examine the relation between sediment stratification and floc size in a cohesive sedimentary environment. Concentration measurements by optical backscatter sensors at two vertical levels are used to calibrate the acoustic backscatter intensity. In spite of the complexities due to the rapidly changing flow and cohesive sediment properties, the estimates of sediment concentration reproduce well the observations. Together with the vertical profiles of suspended sediment concentration, measured current profiles are used to calibrate a one-dimensional-vertical (1DV) boundary layer numerical model for combined wave-current flow on muddy beds. The numerical simulations are used to investigate the effect of the floc size on the vertical structure of the suspended sediment concentration profile. For similar flow conditions, smaller flocs result in more mixed profiles with higher concentration in the upper water column and lower near-bed concentration. This is the first time this effect is seen in direct field observations, and confirms previously published numerical results. (C) 2012 Elsevier B.V. All rights reserved.