Understanding upland swamps in the Illawarra - the relationship of swamps and subsidence was presented by BHP Billiton's Gary Brassington, manager - environmental approvals, at a recent subsidence management conference organised by The Mine Subsidence Technological Society (MSTS).
Many of BHP Billiton Illawarra's mining operations in the Illawarra coalfield are overlain with swamps. These upland swamps are located in the Woronora Plateau, covering an area about 975km2. Upland swamps associated with peat accumulations are an important ecosystem within this landscape.
Of the 50 swamps in the area, about 15 overlie the proposed workings of the Dendrobium colliery, Area 3, due to start longwall mining in several years time, and as such a better understanding of the subsidence mechanisms are crucial.
Studies into the swamps focusing on the possible affect of mining subsidence began in September 2002 by BHP Billiton Illawarra. Prior to current studies, baseline information on swamps was negligible, Brassington said. The studies began after an erosion gully formed in one swamp overlying extracted longwall panels.
In two particular swamps (Swamp 18 and 19) lying to the east of Lake Avon, a major erosion gully formed in one of the swamps, the cause of which is unknown. These two swamps are within the sensitive Sydney Water Catchment Area and overlie longwall panels of the Elouera Colliery.
The swamp is located over a series of the Elouera mine longwall panels from which about 3.2m of coal was extracted 350m below the surface, resulting in up to 1.2m of measured surface subsidence.
Prior to the gullying, the swamp had been burnt by an extreme fire event in 2001/02 which reduced the peat layer to a loose ash/sand mixture. This mixture was later eroded as a result of heavy rains in February and April 2002. When the peat is burnt it loses its cohesive structure making the swamp susceptible to erosion in heavy rain.
Prior to the study, the theory around these events suggested gullying and fire may be as a direct result of subsidence. The new study indicates this not to be the case. Both gullying and peat burning as a result of fires were shown to occur in both subsidence areas and areas where no mining had occurred.
However, Brassington said it has not been established if there is any increase in the likelihood of these events, such as gully formation, fire and erosion as a result of subsidence.
Another finding to emerge was the central role played by the surrounding Hawkesbury Sandstone. Brassington said the swamps rely heavily on the surrounding sandstone strata to feed water into the peat, particularly during drought. This means that mine planning has to consider the effect not only of subsidence on the swamps but on the surrounding sandstones which are the swamp's lifeblood.
To date, the outcome of the study is inconclusive regarding any impacts of subsidence on the swamps in question. However, it has provided a strong basis for future studies into swamp and subsidence processes within the area.
