2.6.1 Surface water numerical modelling for the Gloucester subregion

Executive summary

View of the Gloucester valley NSW with the Barrington River and associated riparian vegetation in the foreground and the township Gloucester in the distance looking south from the Kia Ora Lookout, 2013 Credit: Heinz Buettikofer, CSIRO

Coal and coal seam gas (CSG) development can potentially affect water-dependent assets (either negatively or positively) through a direct impact on surface water hydrology. This product provides modelled estimates of the potential surface water impacts of likely coal resource developments in the Gloucester subregion.

First, the methods are summarised and existing models are reviewed, followed by details regarding the development of the model. The product concludes with predictions of the hydrological characteristics of the system that may change due to coal resource development (referred to as hydrological response variables) also taking into account uncertainty.

Surface water modelling in the Gloucester subregion follows the companion submethodology M06 (as listed in Table 1) (Viney, 2016). No river modelling was carried out because the rivers in the subregion are unregulated and their catchments are relatively small. Instead, predicted streamflow is obtained by accumulating output from a spatially-explicit streamflow model (the Australia Water Resource Assessment Landscape model, AWRA-L).

The modelling domain comprises the Gloucester and Karuah river basins and includes 34 modelling nodes at which daily streamflow predictions are produced. The model simulation period is from 2013 to 2102. The conceptual model for the Gloucester subregion (in companion product 2.3 for the Gloucester subregion), indicates that CSG and large coal mining development have the potential to directly affect the regional groundwater system and that this direct effect can propagate through to the alluvia of the Gloucester and Karuah river systems. Any impact on the groundwater in the alluvium of those rivers in turn has the potential to affect streamflow and therefore surface water resources in the stream networks of the Gloucester and Karuah rivers. CSG development may impact streamflow if aquifer depressurisation reduces baseflow, while open-cut coal mines, in addition to reducing baseflow through groundwater drawdown, will intercept and retain surface runoff which has the potential to affect streamflow directly.

Surface water modelling results estimate hydrological changes arising from coal resource development by comparing the difference in predicted water levels between two possible futures – the baseline and the coal resource development pathway (CRDP) – to provide an estimate of changes that are attributable to the additional coal resource development (ACRD). Results are reported at 30 receptors, which are points in the landscape where water-related impacts on assets are estimated.

There are three open-cut coal mining operations in the Gloucester CRDP, as well as one coal seam gas (CSG) field. The Stratford Mining Complex and the Duralie Coal Mine are both baseline mines (i.e. in commercial production as of December 2012) that also have future expansion projects. The proposed open-cut mine at the Rocky Hill Coal Project is also in the CRDP, although it is not part of the baseline. The AGL Gloucester Gas Project is the proposed CSG field in the CRDP. Importantly, at the time the CRDP was finalised for the Gloucester subregion (October 2015), the proposed Rocky Hill Coal Mine was assumed to begin operations in 2015 and that starting time has been adopted for surface water modelling. Likewise, the AGL Gloucester Gas Project was included in the finalised CRDP, even though the development of this proposed CSG field was later abandoned by the proponent in late 2015.

The prediction results show that the ACRD in the Gloucester subregion has more noticeable impacts on hydrological response variables in northern receptors than in the southern receptors. They are particularly apparent in streamflows along the Avon River, a tributary of the Gloucester River, and where two of the three coal mines and most of the proposed Gloucester CSG field are located. Despite there being one coal mine with an ACRD footprint in the Karuah river basin, there is comparatively little hydrological impact on any response variables in the southern part of the subregion.

The comparison among the 30 receptors shows that the relative hydrological changes are larger for the receptors where the maximum ACRD percentage is larger. For every hydrological response variable, the biggest impacts are predicted to occur at node 14 on the Avon River. This node is located downstream of the ACRD expansions to the Stratford Mining Complex and within the proposed AGL Gloucester CSG field.

The ACRD impacts on the low streamflow hydrological response variables do not appear to be more noticeable than those on 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 flow variables.

These results suggest that changes to low flow 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 groundwater table drawdown, while the changes to high flow characteristics are dominated by direct interception of runoff.

Testing of the model provided confidence in predicting the impacts of coal resource development for each hydrological response variable in each receptor location in the Gloucester subregion. The model assumption that has the largest effect on predictions is the implementation of the CRDP. The numerical predictions are only valid for the mine footprints and CSG wells implemented in the model sequence.

Outputs from the surface water modelling are used for the receptor impact modelling (product 2.7) and in the impact and risk analysis (product 3-4).

Last updated:
1 June 2018