Impact of vegetation in open channels on flow resistance and solute mixing.

This thesis has investigated the impacts of vegetation on flow resistance and mixing in open channel flow. Existing methods and models proposed by previous research which predict flow and mixing in vegetated channels have been presented and discussed. The most pressing issues have been identified as a lack in understanding of how vegetation affects solute mixing, and a lack of verification of existing flow resistance models in situations involving real rather than simulated vegetation. To address these issues, a detailed laboratory study has been undertaken. This involved growing real vegetation in the laboratory environment and conducting tests whilst the vegetation grew in size, density and stiffness. Two vegetation types (Carex and Phragmites Australis) were used to provide an indication of how different plant species affect flow and mixing. Experiments involved the collection of flow resistance, velocity, turbulence and transverse and longitudinal mixing data at different stages of plant growth and whilst the vegetation was in both emergent and submerged states. This involved the use of an acoustic Doppler velocity probe to measure velocity and turbulence. Measurements of mixing were made using CYCLOPES-7 fluorometers with fluorescent tracer used as solute. The presence of vegetation increased the channels flow resistance. As the vegetation grew the resistance increased. In emergent conditions direct measurements of velocity and Reynolds stress were retarded compared to non vegetated experiments and reduced longitudinal mixing was observed. In submerged conditions more complex profiles of velocity and Reynolds stress were measured and longitudinal mixing was dependant on the canopies submergence ratio and the rate of vertical mass transport between the flow above and within the canopy. Results were compared with predictions made by existing vegetated flow models. New models and methodologies for predicting flow and mixing in vegetated canopies have been presented and tested against the data with good results.