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2.6.1 Surface water numerical modelling for the Hunter subregion

Executive summary

Google Earth image of the Hunter River west of Muswellbrook

Coal resource development can potentially affect water-dependent assets (either negatively or positively) through impacts on surface water hydrology. This product presents the modelled hydrological changes in response to likely coal resource development in the Hunter subregion after December 2012.

To quantify impacts of coal resource development in the Hunter subregion, two potential futures are considered in a bioregional assessment (BA):

  • baseline coal resource development (baseline): a future that includes all coal mines and coal seam gas (CSG) fields that are commercially producing as at December 2012
  • coal resource development pathway (CRDP): a future that includes all coal mines and CSG fields that are in the baseline as well as those that are expected to begin commercial production after December 2012.

The difference in results between CRDP and baseline is the change that is primarily reported in a BA. This change is due to the additional coal resource development – all coal mines and CSG fields in the Hunter subregion, including expansions of baseline operations, that are expected to begin commercial production after December 2012.

In the Hunter subregion, coal mining has been occurring for over 100 years. The BA for the Hunter subregion includes 42 baseline mines and 22 additional coal resource developments. Twelve baseline mines were not modelled because they were within the tidal zone of the river system or under Lake Macquarie and adjacent urban areas, did not have associated additional coal resource developments and/or due to lack of data. Five additional coal resource developments were not modelled due to lack of data, low likelihood of impact at the surface or being under Lake Macquarie. Therefore, the surface water numerical modelling in the Hunter subregion includes 47 mines comprising 30 baseline mines and 17 additional coal resource developments. In the Hunter subregion, there are no CSG fields in the CRDP.

Surface water modelling of the Hunter subregion follows the companion submethodology M06 for surface water modelling. In particular, the modelling is undertaken as follows:

  • The model includes rainfall-runoff modelling and river modelling.
  • Streamflow inputs are obtained by accumulating output from the Australian Water Resources Assessment (AWRA) landscape model (AWRA-L) for input into the AWRA river model (AWRA-R).
  • Changes in baseflow from the Hunter subregion groundwater model are also fed into the AWRA-R model at points along the river network.
  • The river model integrates the potential baseflow and runoff changes due to the modelled coal resource developments.
  • The modelling domain includes part of the Hunter river basin and part of the Macquarie-Tuggerah lakes basin.
  • Daily streamflow predictions are produced at 65 model nodes.
  • The model simulation period is from 2013 to 2102.

Evaluation of the model assumptions on predictions shows that most assumptions are unlikely to have a significant effect on predictions. However, predictions are sensitive to the implementation of the CRDP – particularly in catchments where the mine footprint is a large fraction of total catchment area. Predictions may also be affected by the criteria for choosing the most appropriate parameter combinations and representation of river regulation in the river model.

The surface water modelling results show that the additional coal resource development in the Hunter subregion has the potential to cause large changes in the flow regime of some streams. This is particularly evident for the hydrological response variables that characterise high-streamflow conditions at model nodes where the additional coal resource developments cover a large proportion of the contributing area.

In general, the hydrological effects attributable to the additional coal resource development are greater in the small tributaries of the Hunter River than along the river itself. The biggest impacts (flow reductions of up to 80%) occur at nodes 7 to 9 (Loders Creek, including Doctors Creek), which enter the Hunter River just upstream of Singleton, and at nodes 52 (Dry Creek) and 55 (unnamed creek) in the vicinity of Muswellbrook. The catchments of nodes 7 to 9 include the Bulga and Mount Thorley–Warkworth mines, while the catchments of nodes 52 and 55 include the Bengalla and Mount Pleasant mines. Other nodes with substantial percentage changes in the high-streamflow hydrological response variables are nodes 26, 27, 29 and 35. The first three of these nodes are all located in the vicinity of the Glendell, Integra, Liddell and Mount Owen mines, while the catchment of node 35 includes parts of the Drayton South and Mount Arthur mines. All these nodes have relatively small catchment areas. Although there are bigger predicted changes in maximum raw change (amax) at nodes further downstream, the proportional impacts of these changes are diluted by relatively unaffected inflows. The prediction that the biggest changes occur downstream of multiple mine developments highlights the cumulative nature of potential hydrological changes.

The changes to the low-streamflow hydrological response variables attributable to the additional coal resource development appear to be slightly larger than those to the high-streamflow hydrological response variables. However, the uncertainty in the predicted change and the timing of the maximum change are greater for the low-streamflow variables.

There is a substantial change in the low-streamflow hydrological response variables in the two nodes of the Wyong river basin. These nodes are located near the proposed Wallarah 2 and Mandalong underground mines. In the most heavily affected year, these reductions in baseflow are predicted to turn a perennial stream into one that flows on only about 40% of days. Although this is a large reduction, it must be remembered that the projections presented in Section 2.6.1.6 are for the worst-case year during the entire simulation period (2013 to 2102). There is no implication, particularly for the low-flow variables, that the changes will be this severe in every year. When local hydrogeological and geological data are used to constrain groundwater model results in the Wyong river basin, groundwater drawdowns are predicted to be smaller and less extensive, resulting in little to no changes in baseflow (product 2.6.2). Where results from the regional scale modelling flag a risk of large hydrological changes from additional coal resource development, a more locally relevant assessment of potential hydrological changes should be made using local information to constrain the set of regional simulations to those representative of the local conditions.

The results suggest that changes to low-streamflow characteristics are caused by a combination of the instantaneous impact of interception from the additional mine footprints and the cumulative impact on baseflow over time caused by watertable drawdown. The changes to high-streamflow characteristics are dominated by direct interception of runoff.

The surface water numerical modelling described in this product needs to be considered in conjunction with the groundwater numerical modelling (product 2.6.2). Together they provide key inputs to the receptor impact modelling (product 2.7) and underpin the analysis of impacts on landscape classes and assets in product 3-4 (impact and risk analysis).


Last updated:
15 June 2018
Thumbnail of the Hunter subregion
PRODUCT FINALISATION DATE
2018
ASSESSMENT