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)
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.
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.
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.
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.
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
Robertson Environmental Protection Society (REPS), 1998.
M. O'Loughlin, Exhibit 75, 1997 Mining Warden's Inquiry
M. O'Loughlin, Mass Movement in Wingecarribee Swamp on 8-9 August
1998: Likely Causes and Rehabilitation, 25 August 1998.