The model is a three-dimensional model with vertical sides. Its base is defined by the(BA) geological model (see companion product 2.1-2.2 ( ) for more details) for the Surat and Gunnedah basins in the Namoi subregion (Bioregional Assessment Programme, ). Its top is defined by the land topography (Caltech/JPL, ).
The model’s outer boundary was chosen to extend beyond the outer boundary of the subregion and coincide with the(PAE) in the northern direction. The limits of the model boundary have been selected to go beyond the expected hydraulic of coal seam gas (CSG) development and coal mining in the Namoi subregion. The exception to this is in the eastern boundary where a natural boundary condition exists due to the presence of the Hunter-Mooki Thrust Fault.
220.127.116.11.1.1 East boundary
The eastern boundary of the Figure 11). There might be some groundwater flow across this boundary, particularly in the alluvium, but the flux and timing are uncertain. Assumption of a no-flow boundary is conservative and follows the groundwater modelling methodology (see companion submethodology M07 (as listed in Table 1) for groundwater modelling ( )). The implication of this assumption in the predictive modelling is that groundwater to the east of the Hunter-Mooki Thrust Fault cannot contribute to the mining and CSG extractions, which if it was occurring would result in overprediction of at the model boundary.model along the Hunter-Mooki Thrust Fault is assumed to be a no-flow boundary (
18.104.22.168.1.2 North boundary
No-flow conditions are assumed for all model layers along the northern boundary of the model (Figure 11). The Gunnedah Basin units continue with the corresponding Bowen Basin units along this boundary. For the model layers corresponding to the Great Artesian Basin (GAB) and other Surat Basin units, this boundary is approximately aligned with the regional flow direction within the Coonamble Embayment of the GAB ( ).
Assignment of a no-flow boundary condition to the north of the model implies that water sources north of this boundary cannot contribute to the CSG and mine water extractions within the. Given that this boundary is far away from any expected hydraulic head changes induced by the mining and CSG operations this boundary condition best reflects the current understanding of the flow in this region. This also follows the precautionary principle as effects in the Namoi subregion will be overstated as flow from the Surat and Bowen basins in the north would be prevented because of the no-flow assumption. More research is needed to improve the understanding of flow across this boundary.
22.214.171.124.1.3 North-west and west and south-east boundaries
The north-west boundary of the model is aligned along the extent of the Figure 11). This boundary slightly extends beyond the western extent of the Lower Namoi alluvium. The head-dependent flow boundary condition is used for the model layers that exist along the north-west boundary. Similarly, a general-head boundary was used for the model layers representing the units of the Surat Basin to characterise the flow boundary in the straight western edge of the model. The general-head boundary condition was also used to characterise the flow across the south-east boundary of the model. These model boundaries are expected to have no influence on the simulation of changes caused by the coal mining and CSG development because of the large distance (>100 km) of these boundaries from the coal resource development locations. The head-dependent boundary condition was simulated using MODFLOW’s general-head boundary (GHB) package. However, influence of the hydraulic characteristics of these boundaries are further tested and evaluated in a stress-test of the model, reported in Section 126.96.36.199.3.1, to explore the prediction caused by the uncertainty in the parameters of the GHB package.in this direction (
Data: Bioregional Assessment Programme ()
Product Finalisation date
- 188.8.131.52 Methods
- 184.108.40.206 Review of existing models
- 220.127.116.11 Model development
- 18.104.22.168 Boundary and initial conditions
- 22.214.171.124 Implementation of the coal resource development pathway
- 126.96.36.199 Parameterisation
- 188.8.131.52 Observations and predictions
- 184.108.40.206 Uncertainty analysis
- 220.127.116.11 Limitations
- Currency of scientific results
- Contributors to the Technical Programme
- About this technical product