Floodplain Transition Zone Hydrodynamics: The Role of Riparian and Floodplain Vegetation in Compound Channel Flows

ABSTRACT During floods, many rivers exhibit a compound channel configuration, resulting in significant interactions between the flows in the main channel and the floodplains. Due to its water and nutrient‐rich soils, floodplains are often covered by shallow vegetation and riparian woodland at the interface of the floodplain and the main channel. Owing to the numerous overtopping scenarios, compound channels are viable in both urban and natural watercourses having floodplains with riparian shrubs, in‐line woodland and transplanted trees with different arrangements dictated by shading, species, spacing and occupation of the floodplains. This study investigates the influence of submerged floodplains and combinations of riparian grass, woodlands and shrubs on flow resistance and structure. An experimental campaign was conducted, generating 11 uniform compound channel flows under four different configurations: (i) without vegetation, (ii) with synthetic grass in the floodplains, (iii) with synthetic grass in the floodplains and rods at the floodplain edge and (iv) with synthetic grass in the floodplains and artificial shrubs at the floodplain edge. Primarily, flow mechanisms and turbulence characteristics for three uniform flow depths are analyzed using acoustic Doppler velocity measurements. Then, the increase in flow resistance and reduction of compound channel conveyance capacity were analyzed for individual cases. The synthetic grass in the floodplains minimally influenced the cross‐sectional distribution of streamwise velocity, turbulence intensities and lateral shear. On the other hand, riparian vegetation significantly affected these distributions by impeding lateral shear spreading. The effects of vegetation on flow structure were enhanced by foliage on vegetation elements. Additionally, the study examined coherent two‐dimensional large structures and secondary currents, revealing that riparian vegetation is critical in altering flow patterns. These findings provide valuable insights into the complex interactions between vegetation and flow dynamics in compound channels.