The relationship between eroded flocs and cohesive beds was investigated from different aspects using an erosion rig with kaolinite as a material model. The erosion product sizes and densities were analysed in situ using macro-photography technique. Separate experiments were also performed to analyse the cohesive bed density. Cohesive bed density and age effects on the erosion product size and critical yield stress were investigated and correlated to the process conditions. The results indicate that the eroded flocs represent well the material in the bed surface layer. The study also provides an insight into the structure of the bed surface which reveals a significant amount of voids between the flocs in the surface layer. The mean maximum floc diameter was correlated to the critical stress for erosion. It is concluded that the mean maximum floc size can be used to characterise the properties of the cohesive sediment bed. (c) 2012 Elsevier Ltd. All rights reserved.