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Subsidence - a threat to our water

Alteration of habitat following subsidence due to longwall mining - key threatening process.

Conservation status in NSW: Key Threatening Process

Alteration of habitat following subsidence due to longwall mining was listed as a KEY THREATENING PROCESS on Schedule 3 of the Threatened Species Conservation Act 1995 [15 July 2005]. Longwall mining is an underground coal mining technique that involves removing a panel of coal. Longwall mining can cause the land above the mined-out coal to destabilise and collapse this is known as subsidence. Subsidence due to longwall mining has been recognised as causing habitat alteration, such as causing cracks beneath a stream or other water body. This may lead to a temporary or permanent loss of water flows and could cause permanent changes to riparian community structure and composition.

Mine subsidence and the environment

"Mining under watercourses on floodplains, or in other relatively flat-lying areas, may result in localised diversion of water flows and possible increases in the incidence of flooding, erosion and other impacts.

The rock or soil mass within the influence of mining may move three-dimensionally at any given point. In other words, the extraction of coal may result in vertical as well as horizontal subsidence movements.


(NSW Department of Primary Industries, Primefact 21, February 2006)


The Wallarah 2 Coal Project will be directly beneath a major flood plain - Dooralong and Yarramalong Valleys. This area is the major water flow-through of the underground aquifers, which supply 68% of the water to Wyong Creek/River and Jilliby Jilliby Creek.

Jilliby Jilliby Creek will be situated directly above the proposed coal mine (vertical subsidence zone) and Wyong Creek will run parallel to it. The Wyong River and Creek, Wyong catchment weir, the ‘pump pool’ for Mardi Dam, and the proposed Porter's Creek weir, are all located within the horizontal subsidence zone of the proposed coal mine project. This horizontal subsidence zone also encroaches on the northern boundary of Mardi Dam and a portion of the dam itself, which was proclaimed water catchment in 1987.

The mining company cannot predict when, where or the extent of damage caused from horizontal subsidence. It can extend for up to three (3) kilometres. The Wallarah 2 coal project puts at risk the major water facilities of the Central Coast.

At the June 2006 Wallarah 2 Coal Project community liaison meeting, Mr Graham Cowan, a senior engineer with the Department of Primary Industries, said this about subsidence predications and subsequent damage: “Until it (the longwall coal mine) is mined you won’t know, things will change and they will be dealt with.

Longwall coal mining not only poses a threat to the water supply, both surface and subsurface, it also poses a threat to the habitat of the various endangered and threatened species of flora and fauna. There are nineteen (19) international waders, recorded under the Australian Government agreements with China and Japan, whose fragile habitat is entirely dependent upon the health of the water catchment river systems, and thirty-three (33) endangered or threatened species of flora and fauna within the catchment valleys.

In 2004 a report by River Care, in association with Hunter-Central Rivers Catchment Management Authority, National Heritage Trust and the Department of Infrastructure, Planning and Natural Resources, declared the Jilliby Jilliby Creek water system as one of the most pristine in New South Wales. This report also condemns the damage that will be caused by the impact of longwall coal mining.

Longwall coal mining has an appalling record of environmental destruction, and history clearly shows that remedial measures in respect of lost and damaged water systems have been unsuccessful.


Subsidence - Residual and Active

Gas Flows and Methane Oxidation

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Subsidence Map

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Waratah Rivulet - destroyed by environmental vandalism


The impact of longwall coal mining subsidence on a river system.

Waratah Rivulet is a stream that is located just to the west of Helensburgh and flows into the Woronora Dam from the south. Along with its tributaries, it makes up about 29% of the Dam catchment. The Dam provides both the Sutherland Shire and Helensburgh with drinking water.


Cataract River - longwall coal mining victim


In the mid 90's the Cataract River flow totally disappeared into the cracking in the river bed to a depth of some 500 metres into the mine workings and methane and ethane gasses were expelled from the ground measuring up to 20 litres per second in some areas. TV reports showed at the time that the gas was sufficient to BBQ sausages on.


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Related Links


Mining Impacts on Metropolitan catchments and water supply areas

Mine Subsidence - Legislative Council Hansard (Extract)

Mining muddies waters - National Parks Association

NSW Rivers of Shame

Metropolitan water supply under threat - Total Environment Centre

Wallarah 2 underground coal project - Legislative Assembly Hansard (Extract)

Lee Rhiannon, MLC, Longwall mining debate gagged

Diega Creek, Lake Macquarie - Death of a river system

Rivers SOS - NSW Rivers in a State of Shock