Transverse mixing and pollutant dispersion in open-channel flow with interacting vegetated shear layers

Pollutant discharges from combined sewer overflows and wastewater treatment works can degrade river water quality, and their impact is strongly controlled by transverse mixing. Vegetated channel margins generate turbulent shear layers that can enhance this mixing, but the role of interacting shear layers produced by vegetation on both banks remains poorly quantified. This paper presents an extensive high-resolution laboratory dataset for single- and dual-bank emergent vegetation in shallow open-channel flow. Surface velocities and depth-averaged solute mixing were measured using large-scale particle image velocimetry and planar concentration analysis. A finite-difference solution of the advection-diffusion equation was used to infer transverse mixing coefficients. The datasets are used to develop bulk parameterisations linking vegetation projected area and the velocity contrast between vegetated and free flow regions to shear-layer structure and transverse mixing. For single-bank flows, the displacement of the velocity inflection point and the inner and outer shear-layer thicknesses collapse to simple functions of vegetation density. Additionally, the maximum transverse mixing coefficient scales with the product of water depth and velocity difference between the vegetated region and the free flow. For dual-bank configurations, new empirical relationships quantify the reduction of centreline velocity and the enhancement of transverse mixing as shear layers interact across the free flow gap. The resulting model reproduces measured velocity and concentration fields with coefficients of determination typically exceeding 0.9 using only bulk flow and vegetation properties. These parameterisations provide a practical tool for representing transverse mixing in numerical models and for assessing how vegetated banks influence pollutant dispersion in rivers.