Defining the Outfall Mixing Zone

The numerical modelling will employ a 2D depth averaged stream tube approach to describe transverse mixing from a side-channel point source. This method allows for the inclusion of the primary features such as longitudinal channel shape changes; non-uniform velocity distributions and spatially variable local mixing, which affect the distance downstream to create cross-sectionally well-mixed conditions.

Other effects, such as the input flow conditions (momentum); vegetation distribution and seasonal variations in flow and vegetation will be included and the numerical model used to explore uncertainty in predictions.

Field validation will take the form of transverse mixing studies at a few contrasting sites. This involves combining channel bathymetric and flow studies downstream of an outfall, with detailed tracing, to quantify the transverse mixing coefficients and hence define the mixing zone. This will also provide an opportunity to evaluate the use of aerial drones to describe the receiving flow field and alternative approaches to rapidly assess the mixing zone.