O.4 Settling

Unattached pathogens able to be settled within the WQ Module, at a user specified constant settling velocity, \(V_{settle}^{pth}\). This is applied equally to both unattached alive and unattached dead pathogens, and a negative velocity is downwards settling. The relevant fluxes are then calculated as follows.

\[\begin{equation} \left.\begin{aligned} \href{AppDiags.html#WQDiagAliveSedmtn}{F_{sedmtn\langle computed\rangle}^{pth_a}} =& \frac{V_{settle}^{pth}}{dz} \times \left[ PTH_a \right] \\ \\ \href{AppDiags.html#WQDiagDeadSedmtn}{F_{sedmtn\langle computed\rangle}^{pth_d}} =& \frac{V_{settle}^{pth}}{dz} \times \left[ PTH_d \right] \\ \\ \end{aligned}\right\} \tag{O.6} \end{equation}\] \(V_{settle}^{pth}\) is the user specified settling velocity and \(dz\) is the relevant cell thickness. The latter divides the flux to produce a per volume result for consistency with other corresponding diagnostics. \(\left[PTH_a\right]\) and \(\left[PTH_d\right]\) are the concentrations of unattached alive and dead pathogens, respectively. Settled pathogens are not permitted to resuspend.

If attached pathogens are simulated, then their settling velocity \(V_{settle}^{pth_t}\) is also specified as a constant (with a negative value again being downwards). This specification should be comparable to the settling velocity/ies specified in the TUFLOW FV Sediment Transport Module configuration. The relevant flux is then calculated as follows.

\[\begin{equation} \href{AppDiags.html#WQDiagAttachSedmtn}{F_{sedmtn\langle computed\rangle}^{pth_t}} =\frac{V_{settle}^{pth_t}}{dz} \times \left[ PTH_t \right] \tag{O.7} \end{equation}\]

The limitations of the above approaches to modelling pathogenic settling are recognised. Future releases of the WQ Module will offer direct linkages between the TUFLOW FV ST Module and WQ Module, and treat pathogens as sediment fractions that are transported using the advanced techniques and models offered by the ST Module, including resuspension.