One important of coal mines is the interception of surface that would otherwise flow to the stream network. It is important, therefore, to determine the areas where surface runoff will be intercepted. This area is termed the footprint of the mine, and it can differ from the footprint. For the purposes of , the surface water footprint covers the entire area disturbed by mine operations, including pits, roads, spoil dumps, water storages and infrastructure. It may also include otherwise undisturbed parts of the landscape from which natural runoff is retained in reservoirs. The footprint does not include rehabilitated areas from which surface runoff can enter the stream network, or catchment areas upstream of drainage channels that divert water around a mine site but do not retain it.
Mine footprint areas change over the lifetime of a mine’s operations. As new parts of the lease are opened up for active use, the footprint increases. As mined parts of the lease are rehabilitated and their runoff returned to natural drainage, the footprint decreases although not necessarily to pre-mining condition. As well as the area of any final voids, the final mine footprint may also include the area covered by any infrastructure (e.g. dams, levee banks, roads) that are intended to remain on the site after final rehabilitation.
Time series of mine footprints for and mines were compiled from spatial data supplied by mining companies and the NSW Department of Trade and Investment, or extracted by the Assessment team from environmental impact statements and related documents, Landsat TM and Google Earth imagery. Details of data sources and methods used to define mine footprint time series, including the assumptions made, are provided in Section 2.1.6 of companion product 2.1-2.2 for the ().
Figure 7 provides an example of temporal evolution of the mine footprint area derived for the Ashton Coal Mine's open-cut and underground mines. The open-cut mine is included in the baseline and CRDP, whereas the underground mining is a baseline only. For modelling purposes, it is assumed that the for the open-cut mine starts in 2018, with the maximum footprint area occurring in 2022, before reducing to a final void area in 2035. The underground mine is assumed to commence operation in 2006 and to reach its final extent by 2023.
The green line shows the baseline; the blue line shows the coal resource development pathway (CRDP).
The full set of surface water mine footprint time series (for the 30 modelled baseline mines and 17 modelled additional coal resource developments) can be found in Section 2.1.6 of companion product 2.1-2.2 for the Hunter subregion (). The maximum extents of the mine footprints are shown in Figure 4.
Table 4 provides details of the maximum mine footprint areas for open-cut and underground for the baseline and additional coal resource development by contributing area for each AWRA-R . Node 1, which is the most downstream node in the Hunter River network, has the largest contributing area and its mine footprint areas represent the totals from all mines upstream of that point. Note that there can be instances where the combined maximum footprint areas of the baseline and additional coal resource development exceed the contributing area because of assumed changes in the timing of baseline rehabilitation due to the additional coal resource development.
The primary ways in which coal mining affects streamflow are through interception of direct runoff and groundwater-mediated changes in baseflow. For an open-cut mine, interception of runoff is assumed to occur in the area covered by the mine’s surface water footprint. Within this area, 100% of the streamflow that would have been generated in the absence of the mine is assumed to be retained on site and does not contribute to predicted streamflow.
For an underground mine, surface associated with the collapse of the longwall panels is expected to lead to increased ponding at the surface. This increased ponding is likely to result in a decrease in natural flow to the streams. A 5% reduction in runoff in areas covered by an underground mine footprint is conservatively (i.e. impact is likely to be smaller) assumed, which factors in regulatory requirements on mining companies to minimise the impacts from mine subsidence through such steps as appropriate longwall orientation and drainage management.
The hydrological change to baseflow is estimated using the groundwater model, which is described in detail in Section 220.127.116.11 of companion product 2.6.2 for the Hunter subregion (). The groundwater model estimates monthly baseflow for each surface water model node under the baseline and CRDP. The difference between CRDP and baseline simulations is taken as the monthly hydrological change in baseflow, and is then equally partitioned to obtain the daily changes.
Additional coal resource developments that were not modelled due to insufficient data (see Section 2.3.4 of companion product 2.3 for the Hunter subregion ()) are considered further in the impact and analysis (see companion product 3-4 for the Hunter subregion, as listed in Table 2).
Table 4 Contributing area and maximum mine footprint area to each model node in AWRA-R
ACRD = additional coal resource development
Product Finalisation date
- 18.104.22.168 Methods
- 22.214.171.124 Review of existing models
- 126.96.36.199 Model development
- 188.8.131.52.1 Spatial and temporal dimensions
- 184.108.40.206.2 Location of model nodes
- 220.127.116.11.3 Choice of seasonal scaling factors for climate trend
- 18.104.22.168.4 Representing the hydrological changes from mining
- 22.214.171.124.5 Modelling river management
- 126.96.36.199.6 Rules to simulate industry water discharge
- 188.8.131.52 Calibration
- 184.108.40.206 Uncertainty
- 220.127.116.11 Prediction
- Currency of scientific results
- Contributors to the Technical Programme
- About this technical product