Tidal Flushing and Vertical Diffusion in South West Arm, Port Hacking

Summary

South West Arm (SWA), a small Australian estuary, is hydrodynamically a small fjord with highly intermittent river discharge; tidal inflow sinks into it in a thin turbulent sheet. An existing water quality model is adapted to the situation in SWA. It assumes horizontal homogeneity and allows for entrainment and interleaving of the tidal inflow, passive convective cooling, and vertical eddy diffusion, and it predicts running-mean values over a tidal cycle. Both the kinetic energy of the tidal inflow and the potential energy released by the turbulent sheet of sinking water are considered as possible energy sources for the diffusing eddies.

Application to the response of SWA to a rainstorm results in energy conversion efficiencies of 0.025 – 0.05, comparable to those found in a reservoir and in a Norwegian fjord. However, reasonable simulations of flood response can be obtained for a rather wide range of parameter values. Application to spring warm-up in SWA needed slightly lower conversion efficiencies — around 0.025 — to get satisfactory results; but these efficiencies are in any case uncertain to within a factor of 3, due to lack of knowledge of the kinetic energy of the inflow. The fact that a marked spring-neap cycle is observed in the rate of temperature increase at 16 m during spring warm-up in SWA suggests that kinetic energy influx is the major contributor to eddy diffusion there.

Order-of-magnitude estimates for dissolved oxygen show that (a) during spring warm-up, dissolved oxygen concentration at the bottom of SWA is principally a balance between eddy diffusion and biological consumption; and (b) estimates of the rate of diffusion through 13 m depth, using diffusivities calculated from observed temperature structure, agree well with measured consumption rates. Oxygen response to a rainstorm is modelled reasonably well.