2.1.6.2 Translating surface water mine footprint areas into time series of hydrological changes

A number of assumptions need to be made in the surface water modelling to represent the hydrological impacts of mining developments on water-dependent assets. These assumptions are consistent with the policy and legislative framework governing the operation of mines (see Section 2.3.4 of Dawes et al. (2018)). This section discusses the approach for defining surface water footprint time series and characterising their hydrological responses pre- and post-development.

Table 20 lists the assumptions made in generating the time series of footprint areas to represent the areas within AWRA-L where hydrological changes must be applied. It also includes the assumptions made for representing hydrological changes due to mines. More detail about the implementation of these assumptions can be found in companion product 2.6.1 for the Hunter subregion (Zhang et al., 2018).

Table 20 Assumptions made in surface water modelling for representing hydrological impacts of mines and generation of time series data

	Characteristic	Assumption for modelling	Basis for assumption
Generating time series	Start date for baseline mines	As specified in development consent that was current at December 2012	Many mines have been operating in the Hunter subregion for decades. Over that time, the mining companies have changed, operations have been consolidated, some operations have ceased and new developments have commenced. The Assessment team does not have the information to model impacts of all historical mines. For modelling, the Assessment team use the timelines of the operation that was current at the date selected for defining baseline and additional coal resource developments.
	Start date for additional coal resource development (ACRD) mines	as specified in development consent or as indicated by a mining company report or representative, if different from what was in development consent or if not commenced and uncertain when it will commence, assume 2018.	ACRD activities do not always commence when the proposal indicates. This can be because of delays in the approval process, or following approval delays related to the mines operations. Start dates have been determined from current mine reports or mine company contacts. A start date has been estimated for mines that were proposing to start by a date that has passed without them having done so.
	End date for baseline mines	As specified in development consent (includes a change in end date from an approved modification that does not involve an expansion of mining area). Does not include extensions in time to accommodate expansion of mining areas that commenced after December 2012. These become part of coal resource development pathway (CRDP) as ACRD.
	End date for ACRD mines	Either: end date in development consent or end date estimated from mine plan life.
	Maximum footprint	Assumed to occur at end of mining operations (i.e. rehabilitation is assumed to commence at end of mine life). Except, where data have been obtained that indicates a footprint has contracted and expanded over time (applies to baseline only).	Rehabilitation is typically undertaken progressively, but details of this and also hydrological recovery times are not known. Approach is conservative.
	Interpolation between data points	Linear	Use only data available.
	Post-mining longwall footprints	Sustained at maximum footprint to 2102	Subsidence is permanent.
	Post-mining open-cut footprints	Sustained at maximum footprint for 10 years. Scaled back to final void area (if known) or to 0.16 times the maximum footprint area for a further 10 years	Backfilling of pit is likely to occur more quickly than this; infiltration properties of infilled mine unknown.
	Final void areas	If there are polygon(s) from environment impact statement (EIS), then use these If final voids not known, then use 0.16 of maximum footprint area	For mines for which data were available, final void area was compared to the maximum footprint area. The median of the ratio of final void area to the maximum footprint area was 16%, with a range of 4% to 32%.
	Open-cut mines above longwall panels	Open-cut disturbances over-ride longwall disturbances – longwall footprints must be clipped using open-cut footprint to avoid double accounting. Open-cut hydrological impact (i.e. 100% reduction in runoff) in areas of intersection; longwall hydrological impact in unique areas	Open-cut mining impacts are at the surface and assumed to have direct and complete impact on surface drainage. Where longwall panels go under existing open-cut excavations, they will have no additional impact on drainage at surface.
	Longwall mine site facilities over longwall panels	Underground mining site facilities override longwall mine panel hydrological impacts. To avoid double accounting, clip the site facilities area from longwall panel polygon.	As above. Site facilities associated with any mining operation involve disturbances at the surface, which have direct impact on runoff processes.
	Longwall panels over longwall panels	Union of the polygons to obtain maximum footprint. Adding polygons will result in double accounting of hydrological impacts.	Extractions from deeper seams are assumed to not increase the impact at the surface caused by the first level of excavation.
Surface water hydrological impact	Longwall mine	Some minor subsidence ensues resulting in a long-term (permanent) 5% reduction in surface water runoff from the affected area.	Subsidence is not modelled, but is inevitable where longwall mining occurs. Impacts on surface runoff can vary from very little to more than 50% interception, although the latter is unlikely. A 5% reduction in runoff is assumed as mines are required to minimise this impact and make good on streamflow reductions. The Assessment team does not have any basis for varying this by mine location, longwall panel depth or other factors.
	Bord-and-pillar mines	Zero impact on surface runoff.	There is negligible subsidence because bord-and-pillar mining method generally involves minimal collapsing of the mine access shaft once extraction is completed.
	Longwall mine site facilities	100% reduction in surface runoff (from affected areas) for duration of mining and subsequent 10 years, then linearly reduced to zero over the next ten years.	Mines comply with NSW Water Act 1912, NSW Mining Act 1992, NSW Protection of the Environment Operations Act 1997 and NSW Water Management Act 2000. Runoff generated from disturbed areas is retained on-site. Site drainage is designed to ensure that dirty water is not discharged to river network. Assume that site facilities are not abandoned immediately when mining ceases, allowing for rehabilitation of site. Rehabilitation is assumed to return disturbed areas to pre-disturbance conditions over 10 years, following completion of rehabilitation. Anecdotal evidence from environmental officers at Glencore suggested 5-10 years for return to undisturbed conditions.
	Open-cut mine, site facilities, runoff contributing areas to water management storages	100% reduction in runoff from affected areas for duration of mining; sustained for 10 years following cessation of mining; reduced over a further 10 years to 0.16 of maximum footprint area (see final voids).	Mines comply with NSW’s Water Act 1912, NSW’s Mining Act 1992, NSW’s Protection of the Environment Operations Act 1997 and NSW’s Water Management Act 2000. Runoff generated from disturbed areas and intercepted catchments is retained on site. Site water management is designed to ensure that dirty water is not discharged to river network. Assume 10 years to complete rehabilitation and abandon site following cessation of mining. Rehabilitation is assumed to return disturbed areas to pre-disturbance conditions over 10 years, following completion of rehabilitation.

ACRD = additional coal resource development, CRDP = coal resource development pathway

2.1.6.2.1 Calculating final void areas for surface water modelling

The final void in an open-cut mining operation refers to the pit or pits that are left following the completion of landscape rehabilitation. For some open-cut mines, final void areas were specified in environmental management plans and these have been used directly in defining the final area of disturbance in the footprint time series. For open-cut mines, where the final void area was not provided, it has been assumed that (i) there is a final void and (ii) a reasonable estimate can be made based on final void area to maximum footprint area from other mines where data are available. The ratio of final void area to maximum footprint area for each of the mines for which these data were available was quantified (Table 21). The median proportion was estimated to be 16% of the maximum footprint area (mean 20%), with a range of 4% to 52%. The median value was adopted for calculating the final void area (A_{final_void}) from maximum footprint area (A_{max_footprint}) at other open-cut mine sites as follows:

(3)

Table 21 Open-cut mines used to define the ratio of final void area to maximum footprint area

Mine	Maximum area (km2)	Final void area (km2)	Ratio of final void area to max area (%)
Ashton	5.07	0.34	6.7%
Bulga	44.24	7.09	16%
Drayton	7.69	2.42	31.5%
Drayton South	17.01	2.62	15.4%
Liddell	10.67	2.88	27.0%
Mangoola	19.93	0.86	4.3%
Mount Pleasant	32.58	4.15	12.7%
Mount Thorley	27.33	14.13	51.7%
Ravensworth	26.85	1.34	5.0%
Wilpinjong	12.76	3.63	28.5%
Median	18.47	2.75	15.7%

Refer to Table 18 for references and datasets for each mine

Data: Bioregional Assessment Programme (Dataset 4, Dataset 21)

Last updated:

18 January 2019

2.1.6.2 Translating surface water mine footprint areas into time series of hydrological changes

2.1.6.2.1 Calculating final void areas for surface water modelling

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