A novel modeling approach was used to investigate the residence times of Oyster Cove, an artificial canal system connected to adjacent water bodies by unidirectional and bidirectional flow structures. A field program was carried out to evaluate and quantify the exchange of water through the system of flow structures and to gain an understanding of the mixing dynamics within the artificial canal. Results from the field program were also used to validate a three-dimensional circulation model and a flushing model used to quantify the existing residence time of the canal system. Finally, the model was used to compare several hypothetical design alternatives, to identify the effect on the canal’s residence time, by changing the positions of the flow structures and using different combinations of structures. The comparison showed the significant improvements in residence times that could be achieved.

A modelling study was carried out using separate 3-dimensional circulation model and flushing model components of WQMAP to investigate the residence times of Oyster Cove, an artificial canal system connected to the adjacent Saltwater Creek estuary, Gold Coast, Australia. The flow between the canal system and estuary is controlled via a system of uni-directional and bi-directional structures. To validate the circulation model and the influence of the flow structures, a detailed field experiment was carried out using flow meters, tide gauges and fluorescent dye. Following the validation process, the model was used to examine the existing mixing dynamics and residence time of the canal estate. Finally, the model was used to compare a hypothetical design alternative to the existing layout and quantify the effect on the canal's residence time by changing the position of the flow structures and using different combinations of flow structures.

A method to model the influence of a hydraulic structure connecting two water bodies is presented. This method was incorporated into an existing two-dimensional (depth-averaged) hydrodynamic model. Specifically, the flow in and out of a cell (used to represent the hydraulic structure) is calculated using a broad crested weir formula and is determined from the time-varying head difference between the two systems. An example application of the method is also presented. In this example the hydraulic structure cell was used to model the flow through an automated bi-directional hydraulic structure connecting an estuary to an artificial lake system. The gates of this hydraulic structure are programmed to open four times each day (once during each semi-diurnal tidal phase) and remain open for a period of 2 hours, allowing alternative and partial exchange between the two water bodies. Hence, the model setup involved the specification of the opening and closing times of the gates and the calibration of the discharge coefficient. Tests indicated that these were the most sensitive parameters which ensured the correct volume of water exchange between the two systems. Finally, the model-predicted results were compared with available surface elevation observations at two sites within the lake. The comparison showed a good agreement (RMS error <0.09), quantifying the ability of the hydraulic structure cells to simulate the flux between the estuary and lake for each opening.

Research was undertaken to examine the mixing and exchange of saltwater between a microtidal estuary and a lake connected by an automated box culvert bi-directional gated structure. The gates open four times each day and remain open for a period of 2h. They are programmed to open once during each semi-diurnal tidal phase, thus allowing alternative and partial exchange between the two water bodies. To determine the mixing dynamics, vertical profiles of temperature and salinity were collected as water flowed from the lake into the estuary and vice versa. The profiles were taken over a number of days and during various tidal ranges to examine the effect of tidal amplitude on the mixing dynamics of the plume. The results showed that the structure permits the exchange of saline and freshwater between the two systems, maintaining the lake system as a saline (brackish) environment. It perpetuated a permanently stratified environment on both sides of the structure under all tidal ranges examined.