Preliminary modelling of hydrodynamics of purified recycled water inputs to Lake Wivenhoe

Water security has become a major issue within Australia and particularly within South East Queensland (SEQ). In response to projected limits to conventional water supply systems in SEQ a range of innovative projects have been initiated. A key element of this water security response has been the development of the SEQ Water Grid, which includes the capacity for introducing Purified Recycled Water (PRW) into the region's largest water supply reservoir, Lake Wivenhoe. The highly dendritic and thermally stratified nature of Lake Wivenhoe suggests that three-dimensional (3-D) flows are likely to exert a controlling influence on PRW mixing. In this investigation we present the results of initial 3-D simulation of PRW mixing in Lake Wivenhoe with a focus on the boundary condition used to simulate PRW inflows. The proposed Lake Wivenhoe PRW scheme involves the introduction of between 10-232 Ml day-1 of PRW via a sub-surface diffuser located in a side-branch of the Lake. The configuration of the diffuser is expected to result in relatively high port exit velocities that are likely to exhibit jet-like characteristics with length-scales significantly smaller than the scale of the computational grid used for a typically configured far-field numerical model. Further, the numerical model selected for the far-field simulations (ELCOM) does not include a method for representing sub-grid diffuser inflows or sub-grid jet-like inflows of this type. The effects of these sub-grid processes were investigated using two different numerical approaches to simulate the same idealised near-field model domain. The first approach used the OpenFOAM computational fluid dynamics software to simulate flow from the PRW diffuser ports. The second approach used the ELCOM hydrodynamic code to simulate PRW inflow through the bottom boundary of the domain. The aim of these near-field investigations was to assess differences in flow patterns (if any) when using two different modeling approaches. Simulations revealed two distinct modes of PRW flow in the near-field: i) a predominantly surface outflow away from the diffuser; ii) a complex surface outflow away from the diffuser with subsequent vertical mixing due to re-circulating flow resulting in a seemingly well mixed (vertically) flow away from the diffuser. The results of the near-field simulations were then used to develop three representative boundary conditions for PRW inputs to the far-field ELCOM model of the entire Lake. These boundary conditions included : i) a simple mass flux into a bottom cell of the model; ii) a uniform (with depth) lateral mass flux; and iii) a lateral mass flux in the surface layer. Keeping all other aspects of the far-field simulations consistent, the results were then used to investigate the influence of these boundary conditions on the distribution of the PRW plume. Simulation results revealed that the choice of PRW inflow boundary condition significantly affects the dynamics of the PRW plume near (≈ 0-3 km) the inflow boundary. Differences in PRW concentration in the order of 10-30% were predicted depending on the inflow boundary condition. These results suggest that the correct representation of PRW inflow conditions is an important factor for near to mid-field modeling. The influence of the PRW inflow boundary condition on hydrodynamics was found to diminish with distance away from the inflow boundary. This is due to the role of other mixing processes such as wind-driven destabilisation, periodic stratification/de-stratification, catchment inflows and hydro-electric power station flows within the Lake. As such it is more important to accurately simulate the larger scale mixing process when attempting to understand tracer behavior in the far-field (e.g., near downstream offtake points). The model development process has highlighted the value of simulating 3-D processes with the Lake.