Field-Scale Numerical Modeling of a Dense Multiport Diffuser Outfall in Crossflow

A numerical investigation of near-field brine discharge dynamics is reported for the Gold Coast Desalination Plant offshore inclined multiport brine diffuser. Quasi-steady computational fluid dynamics simulations were performed using the Reynolds Averaged Navier Stokes equations with a $k\text{-}\omega$ Shear Stress Transport turbulence closure scheme. Simulations used an iterative mesh domain with length of 400 m, width of 200 m, and average depth of 24.2 m. Longshore crossflow conditions were examined with a current velocities range of $0.03–0.26\mathrm{m}/\mathrm{s}$. The alternating port orientation of the diffuser resulted in simultaneous co- and counterflowing discharges. Impact distance, impact dilution, and terminal rise locations were compared against the existing literature, and dimensionless empirical equations were fitted as functions of the current speed. Transverse spread and resulting salinity increases were also assessed against field measurements. For the first time, the areal extent of seafloor salinity increase is examined, with the quasi-quiescent regime holistically presenting the worst-case conditions. Plume trajectory, dilution, areal salinity intensity, and plume dispersion after impact each reflect distinct variations between jet- and crossflow-dictated regimes at a threshold value of $u_{r}F\approx 0.8$ ($u_r$ = ambient to jet velocity ratio; $F$ = jet densimetric Froude number). This behavior depends on the presence of the arrested upstream sublayer that, in turn, has consequences for the application of empirical models to multiport discharges under low-crossflow regimes. This study demonstrates significant advancements over existing empirical and integral modeling methods, with strong application potential for designers, plant operators, and regulators.