Model Description

Model Domain

The model domain includes three components: the estuary itself, the major rivers flowing into the estuary, and the adjacent coastal ocean which may significantly influence the temperature and salinity characteristics of the estuary. The estuary model extends from north to south, with the major Preston and Collie Rivers discharging into the southern end. The connection with the open ocean, “The Cut”, is also located at the southern end. In the Collie River, the salt wedge extends approximately 4 km upstream (McKenna & DoW, 2007). The model domain extends westwards into the Indian Ocean far enough such that the open boundary can be considered to be essentially unaffected by the waters exiting from the harbour entrance i.e. to a point where boundary values of temperature and salinity can be assumed completely oceanic.

Model Configuration

The pilot model grid contains 404 x 215 grid cells in the horizontal. In the estuary, the grid spacing is typically about 80 m, but decreases considerably in the region through “The Cut”. In the outer, oceanic, part of the domain, the grid spacing increases to over 2 km. The lower reaches of the Collie River, up to the Collie River junction, are resolved by up to three grid cells across the river. Further upstream, the river channel is defined by a single row of cells. There are 18 layers in the vertical, with vertical grid spacing increasing from 0.5 m near the surface to 5 m at the bottom. Plots of the model grid are given for the estuary region, the Bunbury area and the Collie river junction.

Model Forcing

Data required to force the model consists of meteorological data, oceanic sea-level, oceanic salinity and temperature, and river flow and river temperature (river salinity at the upstream boundaries being assumed zero).

Meteorological data (wind, atmospheric pressure, air temperature, dew point and cloud cover) are available from the Bureau of Meteorology weather station at Bunbury. However, in order to force the model with spatially-varying meteorological parameters, and to enable a forecasting capability, the hydrodynamic model is forced by outputs from the BoM atmospheric model ACCESS. These outputs are available as hourly records in both near real-time and as a forecast, and have a spatial resolution of approximately 10km.

Sea-level data for the region are available from the Bunbury tide gauge, which lies within the model domain. Tides along the ocean open boundary are determined from global model harmonics, but measured low-frequency sea-level data are not available. Instead, the global tide sea levels are supplemented with low-frequency sea level taken from the BoM OceanMAPS modelling product. OceanMaps provides near-real time data and also a short-term forecast, allowing us to run the estuary model in similar modes.

Oceanic salinity and temperature, as a function of position and depth, are also available from OceanMAPS as daily records in both near real-time and as a forecast. This data also has a spatial resolution of approximately 10km in the region.

River flow and temperature data are required at 3 river boundary points of the model. Gauged flow data for the Brunswick, Collie and Preston rivers are used, with gauges located at Cross Farm, Rose Road and Boyanup respectively. River level data is obtained through the Bureau of Meteorology and converted to flow rates. In the absence of river temperature data, temperature is specified by low-pass filtering modeled air temperature from the ACCESS model.

Model Calibration

The hydrodynamic model will be calibrated against data collected by the Department of Water during a year-long sampling programme. The observations include frequent measurements of surface and bottom water temperature and salinity by moored instruments at three locations in the estuary. These data will be used to fine-tune the model parameterisation, and allow on-going verification of the model performance.