Multimodal particle size distributions (PSDs) of fine-grained cohesive sediments are common in a marine and coastal environment. Curve-fitting software in this research decomposed such multimodal PSDs into subordinate log-normal PSDs. The four modal peaks, consisting of four-level ordered structures of primary particles, flocculi, microflocs and macroflocs, were identified and found to alternately rise and sink in a flow-varying tidal cycle due to shear-dependent flocculation. The four modal PSDs could further be simplified to two discrete size groups of flocculi and flocs. This allowed the develoment of a two class population balance equation (TCPBE) model with flocculi and flocs to simulate flocculation involving multimodal PSDs. The 1-dimensional vertical (1-DV) TCPBE model further incorporates the Navier-Stokes equation with the k-ε turbulence closure and the sediment mass balance equation. The model was able to reasonably simulate multimodal flocculation as well as turbulent flow and sediment transport in a flow-varying tidal cycle. The 1-DV TCPBE was concluded to be the simplest model that is capable of simulating multimodal flocculation in a turbulent flow field of a marine and coastal zone.