Explanation

According to Emmett O'Loughlin, hydrologist:

Wingecarribee Swamp lies along a narrow valley running E-W between Robertson and Moss Vale and the bulk of the runoff at times of heavy rain comes from the hills on either side. This sinks quickly through the permeable soil to the subsurface water table and is then pushed up under dynamic pressure towards the surface through gaps and voids in the peat matrix where it upwells, breaks the surface and continues downstream by overland flow.

Flow pathways of water in a cross-section of Wingecarribee Swamp and its adjacent hillsides (adapted from O'Loughlin 1997)

The major void in the swamp was the "Dredge Pool" where a pontoon-mounted dredge had dug out peat for many years. Emmett O'Loughlin presented graphic evidence at the Mining Warden's Inquiry that the Dredge Pool was a major point of vulnerability in the Swamp where most of the upwelling at times of heavy rain would concentrate (See diagram above) but his evidence was ridiculed, ignored or kept under wraps.

Some 200mm of rain fell on the catchment On Friday August 7th —about the same as a previous deluge in June 1991, which had no visible effect on the swamp. But this time the swamp collapsed the following day. Why this time? What had changed between 1991 and 1998? One reason was that the extraction rate had escalated in recent years expanding the area of the dredge pool from 4.7ha in 1990 to 20.8ha in 1996. (By 1997 it extended right across the swamp from one shore to the other.)However there was a more direct mining-related reason.

On the night of Saturday August 8th, upwelling in the dredge pool lifted the abandoned pontoon dredge (and crane), from its anchorage and washed it 1500m through the "Buffer Zone" into the (Wingecarribee) Reservoir, tearing a channel through the peat and allowing flood waters to rush through, leading to the collapse of the swamp.

The initial failure of the peatland probably started at a high exposed face of a void in the peat bed, at the edge of a dredge pool in which the peat mining had been in progress. Recent mining had significantly increased the height of the exposed face. The face would have experienced high blowout forces during the storm of August 1998.

Face blowout would have been followed by a collapse in the peatbed that propagated rapidly upslope through the Swamp. Massive volumes of peat and water flows probably then swept into the dredge pool, pushing the dredge into and through the peatland downstream from the dredge pool. The path gouged by the dredge's extended legs then rapidly developed into an incised channel, bordered by a destabilised bed of peat. Total collapse of the lower swamp then ensued.

David Tranter, Robertson Environmental Protection Society (REPS), 1998.

Emmett M. O'Loughlin, Exhibit 75, 1997 Mining Warden's Inquiry

Emmett M. O'Loughlin, Mass Movement in Wingecarribee Swamp on 8-9 August 1998: Likely Causes and Rehabilitation, 25 August 1998.