Assessment of floodplain tree density variation on discharge conveyance capacity and flow velocity in meandering compound channels

Abstract Three different mangrove tree densities (sparse, average, and dense) on a meandering compound channel floodplains were used to assess the effects of rigid trees on the conveyance capacity, streamwise velocity variation, and distribution at the main channel. The numerical simulations were carried out using a Computational fluid dynamics (CFD) model under non-vegetated (reference) and vegetated conditions, considering three relative depths of 0.26, 0.35, and 0.45 along with their corresponding flow discharges. The model was validated against existing experimental data, yielding an average relative error of 4.81% a PBIAS value of 3.8% in depth-averaged velocity predictions, affirming the model’s reliability. Results indicate that the presence of rigid vegetation on floodplains leads to increased overall flow depth for a given discharge. Specifically, dense vegetation raised the relative depth by approximately 24% compared to the smooth-bed condition. Additionally, vegetated floodplains enhanced the conveyance efficiency of the main channel, with improvements in discharge capacity ranging between 30.6% and 36.8% across the tested depths. Although the average velocity across the compound channel decreased due to tree presence, localized acceleration was observed in the main channel. In particular, velocity increases of 45.21% and 43.02% were recorded at apex cross-sections CS1 and CS7, respectively, while mid-channel section CS4 showed a 22.98% rise. These velocity patterns, driven by enhanced main channel flow (U mc ), intensified centrifugal forces and promoted the development of stronger secondary flow cells in the channel bends.