This paper investigates the 1-D+ modelling of overbank flows in case where Floodplains (FP) feature a streamwise roughness transition from bed-roughness to emergent rigid vegetation, or vice versa (representing e.g. a change from highly submerged dense meadows to emergent trees areas). The simulations are compared with experimental data collected in a compound channel flume. The meadow is physically modelled by artificial plastic grass and the trees by rigid stems. The upstream discharge distribution between the Main Channel (MC) and the FP is varied in order to alter the lateral mean flow between MC and FP. To account for the upstream discharge distribution, the 1D+ model developed by Proust et al. (2009), the Independent Sub-sections Method (ISM), is used. The volume drag force exerted by the stems is modelled by a formula established by Nepf (1999). The ability of ISM to predict flow depth, mean flow in the FP, and the depth-averaged Reynolds-stress at the junction MC/FP is assessed.