Spatiotemporal Influences of Open-Coastal Forcing Dynamics on a Dense Multiport Diffuser Outfall

Detailed spatiotemporal analyses of near-field outfall dynamics are reported for an inclined brine multiport diffuser discharging into a dynamic open-coastal embayment. Three-dimensional variations in near-field discharge dynamics were captured using near-continuous in-situ monitoring of physicochemical properties in parallel with measurements of dissolved oxygen and ambient hydrodynamic conditions. Temporal analyses were conducted using principal component analysis and concentration–duration–frequency methods to show near-field salinity variations are generally localized to within 30 m of the diffuser and are highly sensitive to ambient crossflow dynamics. Periods of low near-bed velocities and high-velocity shear corresponded to the lowest return and boundary brine dilutions with instantaneous near-bed salinity increases of up to $1.5\mathrm{g}/\mathrm{kg}$ and dissolved oxygen (DO) reductions up to $1.9\mathrm{mg}/\mathrm{L}$ measured immediately downstream of the diffuser. Both trajectory and dilution demonstrated strong correlation with near-bed crossflow magnitude and were assessed against laboratory-based empirical models. Measured dilutions were well approximated by models that accommodate crossflow; however, trajectory properties were generally overpredicted. The quantitative characterization of temporal plume dynamics within an unsteady coastal setting provides valuable insights into the applicability of existing modeling approaches and regulatory assessment. Improvements to monitoring strategies are also proposed.