coastal upland swamps and longwall mining · (longwall) mining. this has implications for the...

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Nathan Garvey – Senior Consultant Ecologist

© Copyright Biosis 2017

Coastal Upland Swamps and Longwall Mining


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Overview

Over the past decade, we have seen an increased scrutiny of impacts arising from subsidence associated with underground (longwall) mining. This has implications for the viability of mining projects in the Southern Coalfield. Assessments now demand better data.

This has led to the development of new and innovative techniques for assessing these impacts.


Key issue



Overview

Tonight we will review:

>  Upland swamps – what are they?

>  New methods for mapping of upland swamps

>  The risk assessment process

>  Impact assessment process

>  A better understanding of how mining may impact upland swamps

>  Future assessment and monitoring


Topics



Upland swamps – what are they?

>  Range of definitions - depends on if you are an ecologist, geomorphologist or a hydrologist (or a regulator).

>  Hydrology versus floristics

>  Coastal Upland Swamps in the Sydney Basin Bioregion endangered ecological community (EEC):

>  “A range of vegetation and fauna associated with periodically waterlogged soils on the Hawkesbury sandstone plateaux” (TSSC 2014)


Well, that depends!




>  Poorly permeable Hawkesbury sandstone

>  Impeded drainage, resulting in varying levels of soil moisture

>  Acidic soils, ranging from mineral sandy loams to highly organic soils

>  Vegetation (floristics)


Key elements


Case study (cont.)

23 July 2015 © Copyright Biosis 2015

>  Conceptual model of upland swamp hydrology

>  Inputs

>  Throughputs

>  Outputs

>  Impacts from mining

Assessment of indirect impacts (cont.)




Vegetation communities occur across the hydrological gradient (Keith et al. 2006)

>  Five vegetation sub-communities:

>  Banskia Thicket

>  Sedgeland heath

>  Restioid heath

>  Cyperoid heath

>  Tea-tree Thicket


Vegetation communities and floristics



Filling a gap

Mapping upland swamps

>  Initial set up of monitoring sites identified inadequacies in regional vegetation mapping

>  Concerns leading to errors in impact assessment process

>  Need for a better method, applied over large areas (1000s of hectares)

>  Built on ACARP work completed by Jenkins & Frazier (2010)




Step 1 – Canopy Height Model (CHM)

Mapping upland swamps

>  Develop Digital Elevation Model (DEM) for ground and non-ground returns.

>  Subtract ground from non-ground to get the CHM


Step 2 – Modeling upland swamps

>  Using the CHM we look for areas with CHM < 8 m in height and an area > 0.05 ha

>  These are mapped as ‘potential wetlands’



Step 3 – Ground-truthing

Mapping of upland swamps

>  Confirm that ‘potential wetland’ is an upland swamp.

>  Map the vegetation sub-communities within each upland swamp.

>  Data is collected using a GIS-capable tablet computer


Step 4 – Mapping >  Data collected in the field is mapped in a GIS and quality

assured.



Guidelines

The risk assessment process

>  Risk assessment process first developed during the Metropolitan Coal Project Planning Assessment Commission report (PAC 2009)

>  Further developed in the Bulli Seam Operations (BSO) PAC report (PAC 2010)

>  Draft Upland Swamp Environmental Assessment Guidelines (OEH 2012)




Initial risk assessment


>  Identify “swamps at risk of negative environmental consequences”

>  systematic tensile strains >0.5 mm/m

>  systematic compressive strains >2 mm/m

>  depth of cover less than 1.5 times longwall panel width

>  tilt (transient or final) >4 mm/m

>  predicted valley closure of >200 mm

>  ‘maximum observed closure strain’ >7.0 mm/m.




Initial risk assessment


>  PAC (2010) determined that these were a “threshold for investigation – not a conclusion that the swamp will be impacted or suffer consequences“

>  Thus, any swamp identified during the initial risk assessment requires further assessment.

>  In the past, these criteria have been used to determine the risk of negative environmental consequences

>  There is now mounting evidence to indicate that the maintenance and persistence of upland swamps in areas subject to subsidence is much more complex than has been previously recognised (Biosis, 2014c).




Potential impact mechanisms

Impacts to upland swamps

PAC (2009) identified three potential impact mechanisms to upland swamps:

>  Fracturing of bedrock due to tensile strains and increased porosity of underlying strata

>  Tilting, resulting in re-concentration of run-off or diversion of flows around swamps

>  Buckling and shearing, resulting in enhanced fracture connectivity, resulting in vertical then lateral drainage




Impact assessment – Mechanism 1


>  Potential for fracturing of bedrock assessed using subsidence predictions

>  However, basic measures fail to take into consideration the pre-existing hydrology of these swamps






>  Assessed using measures of compressive strain and valley closure

>  Largely restricted to larger valleys where closure and compressive strains concentrated

>  However, these are often in areas were there is substantial flow, potentially “masking” any impacts.






>  This impact mechanism has been largely unassessed in the past

>  Biosis used a flow accumulation model to assess changes in flow pre- and post-mining

>  Develop digital elevation model

>  Model flow accumulation

>  Deduct predicted subsidence

>  Re-model flow accumulation



Case study (cont.)

23 July 2015 © Copyright Biosis 2015

>  Flow accumulation modelling

>  Changes in flow pre – versus post-mining

Assessment of indirect impacts (cont.)



Impact assessment


>  These individual assessments are use to provide an integrated risk assessment for upland swamps

>  Face the challenge that impacts may take decades to manifest

>  How do we provide regulators with confidence?

>  Regulators are looking at past impacts




Past impacts


There have been a number of impacts to upland swamps attributed to longwall mining:

>  Swamp 18

>  Drillhole Swamp

>  Flatrock Swamp

>  Swamps on Lizard Creek

But not all swamps that are mined beneath show impacts – are they just not observable?

And what of their resilience?




Past impacts


It is almost certain that drying of upland swamps increases their susceptibility to impacts:

>  Fire

>  Rainfall and scouring

>  Climate change

But are these impacts directly attributable to mining?




Historical impact assessment


>  Impacts to upland swamps to date have been:

>  Stark

>  Readily observable

>  Limited to a small number of the swamps mined beneath

>  What about more subtle changes?

>  Hindered by a lack of long-term monitoring data

To address this gap we undertook a historical assessment to look at changes in extent of upland swamps in area of subsidence.






>  Aerial photography from 1951, 1963, 1983 and 1998

>  Extent of upland swamps mapped by two observers

>  Checked by third observer

>  Compared with data from more detailed mapping done in 2012

>  Statistical analysis:

>  Rainfall analysis

>  Regional spatial analysis: centroid of the swamps, geographic uniformity and GLM (change versus subsidence)






>  Initial assessment showed little change 1951 to 1984, but a marked decline 1984 - 2012






>  This coincided with a dramatic decline in rainfall since the 1980s






>  There is no statistical association or trend between areas where swamp has been converted to woodland. The conversion appears to occur randomly over the study area

>  There was also no significant association between the amount of subsidence or distance from the subsidence edge and swamp changes

>  Interpretation: there are a number of factors driving changes in extent of upland swamps:

>  Anthropogenic factors?

>  Fire history?




Where do we go from here

Future assessment

>  Remote sensing

>  LiDAR data and processing

>  Use of multispectral and hyperspectral data – ACARP project

>  Allows for more targeted quantitative monitoring

>  Collaboration within industry to provide more robust data

>  Better confidence to regulators and public




Conclusion

>  We have certainly moved forward in our assessment of impacts to upland swamps

>  Still have a way to go in providing regulators and public with confidence

>  We need to educate on these amazing, dynamic and complex systems


coastal upland swamps and longwall mining · (longwall) mining. this has implications for the...

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