A study on transverse mixing in shallow flows within partially vegetated channels.

Increasing instances of river pollution and sewer overflows, coupled with more stringent regulatory requirements for ecological status and flood risk management; demand a deeper understanding of mixing processes and natural solutions to reduce peak pollutant concentrations while minimising flow obstruction.
This thesis presents an experimental study of flow surface velocity and transverse mixing processes in open channel shallow flows with partial vegetation. A technique that simultaneously measures mixing processes and surface velocity fields in shallow flows using low cost cameras and lighting is developed and validated.
The technique is used here to record surface velocity fields and depth averaged concentrations of a solute in a rectangular laboratory flume over a recorded length of 4.48 m. Artificial vegetation is installed at different densities at one bank to simulate partial coverage of emergent vegetation under a range of shallow flow conditions. Using the experimental results, a semi-empirical model for transverse mixing based on the velocity gradients across the shear layer is developed to relate the mixing behaviour to the velocity distribution in vegetated shear layers. The proposed model is validated using surface velocity and depth-averaged concentration of a solute recorded during experiments with two vegetated banks with the same vegetation density and flow depth conditions as the previous experiments.
This thesis provides a semi-empirical predictive model that estimates longitudinal surface velocity and variable transverse mixing coefficient in shallow flows with vegetated banks. The model only requires the vegetation parameters and free flow region data such as velocity and water depth, producing an approach without the requirement for extensive data collection or complex hydrodynamic numerical models. Initial scenario testing of the model suggests that vegetation induced mixing may significantly reduce the impacts of acute pollution events, although further work is required to validate these findings in field conditions.