Valley infill swamps on sandstone Formed where peat material several metres thick has been deposited on a sandstone base and where the hydraulic gradient within the swamp is controlled by the gentle dip of the valley floor and the slow migration of water through the swamp. Valley infill swamps on alluvium Formed where a layer of peat material has been deposited in a broad valley that is not directly on top of sandstone but is instead on sand, talus or other alluvial material so that the watertable in the swamp is integral to the groundwater system in the surrounding terrain rather than perched above it on a sandstone rock base. Headwater swamps—formed where a typically thinner layer of peat and other organic material has been deposited in the base of the broad valley upstream of a waterfall or other rock feature that substantially influences the hydraulic gradient through the swamp; Hanging swamps or valley side swamps—formed where sandstone rock strata is exposed at or near the surface on the side of a valley and the swamp relies on the integrity of the underlying rock strata and a combination of rainfall and groundwater infiltration for recharge. Valley infill swamps Valley infill swamps on sandstone The key characteristics of a valley infill swamp on sandstone are illustrated in Figure 2.2. Sandstone strata form the base of the swamp, much as it does elsewhere along the stream channel. However, the stream gradient is low enough in the area of the peat swamp for peat to accumulate sufficiently thick to support an ecological community that is distinctly different to elsewhere along the stream channel, thereby defining the extent of the peat swamp. Water flowing in the stream channel upstream and downstream of the swamp typically flows in one or more channels across the surface of the peat so that the watertable within the peat swamp is near the surface and well above the level of the sandstone base. Flat Rock Swamp on the Waratah Rivulet was an example of this type of swamp before the bushfire in 2001. Figure 2.2 Illustration of key characteristics of a valley infill peat swamp. Figure 2.3 is a photograph of this type of swamp in a tributary of Waratah Rivulet, west of Helensburgh on the Woronora Plateau. The extent of the swamp is evident in the base of the valley by the change in vegetation. © Copyright, Ken Mills
© Copyright, Moffitt 2000 Scheibner & Basden 1986 Figure 3.2 Schematic structure map of the Sydney © Copyright, Geoscience Australia 2013: www.ga.gov.au/oceans/ea_ofs_sdny_Strat.jsp Figure 3.3 Stratigraphy of the Sydney Basin. Southern Coalfield The general stratigraphy and depositional environment of the Southern Coalfield above the Shoalhaven Group comprises: Wianamatta Group—formed in a shallow marine to alluvial floodplain that is today exposed across much of the Cumberland Basin Hawkesbury Sandstone—formed in an alluvial environment, the outcrop of which dominates the sandstone formations around Sydney Narrabeen Group—formed in estuarine and alluvial environments, with lithic and quartzose sandstones, shales and mudstones Illawarra Coal Measures—formed in an alluvial and deltas, with the economic coal seams. The current economic coal resources of the Southern Coalfield are located within the upper half of the Illawarra Coal Measures. These economic resources consist of four seams, which are shown in Table 3.1. Table 3.1 Major coal seams of the Southern Coalfield. Seam Thickness (m) Mining Use Interburden thickness Bulli 2–5 Room-and-pillar and longwall mining Thermal and coking Typically now m, but historically 30–500 m Accessible resource to a depth of about 800 m Balgownie 1.5–2 Limited mining in the past Thermal and coking 5–30 m Bulli Seam Wongawilli 6–18 Room-and-pillar and longwall mining Thermal and coking 30–50 m below Bulli Seam Tongarra ~3 Limited mining in the past Thermal 70–80 m below Wongawilli Seam
the mined seam 2 From the mined seam to about 1w above the mined seam Large downward movement. Characterised by extensive conjugate shear fracturing with numerous open fractures, particularly around the margins of the zone, and numerous inclined fractures throughout. Complete drainage of groundwater. The interface between this zone and the less disturbed strata above and to the sides of this zone accommodates some relatively large differential movements for rock strata within a short distance. This interface zone is characterised by open shear fractures and fractures between rotated blocks of intact material. 3 From 1 to 1.6w above the mined seam Transitional ground movement. Vertical opening of horizontal bedding planes with horizontal fractures being dominant in
Differential Global Positioning System (DGPS) Inexpensive over small study areas. Accurate to 20 mm in the vertical direction and 5 mm in the horizontal direction (USGS 2000b). Restricted resolution and accuracy. Error increases with distance from reference station. Accuracy reduced due to tree cover. Airborne Laser Survey (ALS) Measures the return time of pulsed light to provide high-resolution topographic data High-resolution data over a defined area. Can be supplied with corresponding aerial photographs. Over flights can be commissioned whenever the client requires (subject to weather conditions). Requires independent survey over flights. Expensive Prone to localised distortion if effective algorithms to remove vegetation are not applied.
Streams What was impacted? What were the impacts? What are the potential causes or contributing factors? History/Description Monitoring Remediation Current situation Waratah Rivulet, Woronora Catchment (upstream of Flat Rock Swamp) Cracking of bedrock Loss of surface water flows (appears to return approximately 500 m downstream) Presumed subsidence from Darkes Forest Colliery The Darkes Forest Mine was created in 1971; very few details available No monitoring details found No known remediation Cracks still visible and surface water can be seen to disappear down cracks. Given the passage of time this is indicative of an area that has not naturally remediated. Waratah Rivulet, Woronora Catchment Cracking and tilting of bedrock. Loss of surface flows and loss of water from permanent pools. Reduced water quality and growth of iron oxidising bacterial mats. Loss of habitat. Subsidence related to longwall mine extraction directly beneath Waratah Rivulet. Tilting to the east resulting in alteration of flow Longwall mining commenced in 1995. Longwall widths began 125 m wide and were increased up to 162 m wide for Longwall 11. Initial cracks were noticed when Longwall 10 was undermined. This was followed by cracking, failure of rockbar WRS3 and loss of surface water flows (4-4.5 Ml/d) associated with the extraction of Longwall 11 (Galvin & Associates 2005). Rehabilitation was attempted in 2006 using drill holes and sand curtains in the fractured rockbar and some preliminary improvements in water retention behind the rockbar have been noted. Risk assessment for Longwall 12 suggested further impacts were likely. Monitoring by mining company including subsidence, hydrology, riparian vegetation, macroinvertebrate and fish surveys Monitoring instrumentation was installed at rockbar WRS1 prior to commencement of Longwall 9. Subsidence movements that occurred as Longwall 10 approached caused instrument failure (Mills & Huuskes 2004). Monitoring at rockbar Remediation attempted using drill holes and sand curtains. These have since been damaged by further mining and fracturing. Still assessing latest attempt at remediation with sand and colloidal silica binding agent. Creek still ceases to flow during low flow conditions.
Accelerated bank erosion. Despite the damage to the Rivulet, approval was given to continue with adjacent panels, which subsequently caused further fracturing of the rockbar and riverbed (Krogh, pers. comm., 2007). WRS3 commenced in October 2003. Recent roundwater monitoring by SCA Monitoring of remediation impacts including macroinvertebrate and macrophyte surveys. Cataract River below Broughtons Pass Weir Loss of surface flow including water loss from permanent pools. Reduced quality. Growth of iron oxidising bacterial mats. Gas emissions and vegetation dieback. Fisk kills. Rock Subsidence related to longwall extraction. Longwall mining began at Tower Colliery in the vicinity of the Cataract River in 1988. Longwall 3 was extracted during 1990 and was the first of 10 to pass directly beneath the river. Panels started 110 m wide, and were increased in to 155 m in 1992. The adverse impacts on the river were reported in 1994, when residents reported the loss of surface water from some of the more permanent pools. There was also a decrease in water quality, release of gas, vegetation dieback, fish kills and bacterial mats. In 1998, a Mining Wardens Court Hearing concluded that 80% of the drying of the Cataract River was due to longwall mining operations, with the balance attributed to reduced flows regulated by Sydney Water. During 1999 grouting was carried out to Study by DIPNR (2003) found it difficult to determine loss of flow and effectiveness of remediation due to lack of sufficient pre-mining data. Mining-induced fractures were first observed and mapped in the Cataract River in 1996 (Coffey 1996 cited by MSEC 2006), prior to the commencement of Tower Longwall 14. Additional fractures were observed between Weirs 4 and 5 during and after the extraction of Tower Longwall 14, and these were mapped in 1997 (Coffey 1997 cited by Grouting to prevent surface water losses. Use of environmental flows to compensate for surface flow losses. A return to natural flow levels has not occurred. Environmental flow releases were not considered by DIPNR (2003) adequate to keep the river flowing or to maintain water quality.
knickpoint. Subsequent heavy rainfall during 1978 led to gully erosion. The bursting of the small dam and dry swamp sediments (as a result of subsidence) are thought to have enhanced this erosion (Young 1982). evident in the 25 years since impacts occurred. 18 (Native Dog Creek, Avon Catchment) Extensive gullying and erosion. Severely burnt in 2001 bushfires. Subsidence related impacts are likely to have made the swamp more susceptible to burning and erosion. Longwall mining beneath swamp between 1995 and 1997. Consultant Biosis noticed cracking of swamp sediment, desiccated vegetation and loss of water from pools in 2001 (Harrington 2001). The 2001 bushfires severely the swamp to its peat layer indicating swamp sediments were dry prior to burning (Tompkins & Humphreys 2006). Major erosion gully discovered in 2002 following a series of storm events. Evidence found that bedrock beneath the swamp had cracked and water had drained to within the bedrock layer (Gibbins 2003). No baseline monitoring data prior to mining Subsequent studies and monitoring following impacts. Most studies recognise that natural erosion was occurring prior to mining (EarthTech 2003 and Tompkins & Humphreys 2006). However, the rate of erosion was enhanced following the bushfires (Tompkins & Humphreys 2006). No known remediation. Gully still exists. Some vegetation has regenerated. Native Dog Creek still ceases to flow during low flow conditions. Swamp 19 (Native Dog Creek, Avon Catchment) Cracking of exposed bedrock in centre of swamp. Subsidence has increased the gradient of the swamp. It Longwall mining history similar to 18. The 2001 bushfires severely burnt the swamp. Longwall mining has locally increased the No baseline monitoring data prior to mining. Assessment of by mining consultants (EarthTech 2003) did not No known remediation. Gully still exists. Some vegetation has regenerated; however, moisture levels in
Dry and scoured pools. Severely burnt during 2001 bushfires. is likely to have caused the cracking observed and contributed to drying of the swamp making it more susceptible to gradient of 19 (EarthTech 2003). Significant subsidence cracking noted below the swamp in Native Dog Creek (EarthTech 2003). Cracking visible in exposed rock in middle of prove or disprove the relative contribution of subsidence compared to drought and natural erosive processes. the swamp are still affected. Flat Rock Swamp (Waratah Rivulet, Woronora Catchment) Extensive gullying erosion. Subsidence and tilting of swamp are likely to have made the swamp more susceptible to natural erosive processes. History of bord and pillar and longwall mining. The 2001 bushfires the swamp, but not to the same high intensity as Swamps 18 and 19. Mining of Longwall 8 commenced during October 2001. Erosion gullying was first noticed in February 2002. In 2003 subsidence cracks were noticed downstream in Waratah Rivulet. Mining reports suggest fracturing below lower end of Flatrock Swamp and possible changes to swamp bed gradients and flow paths (Mills & Huuskes 2004). Evidence of natural erosive processes acting in swamp prior to mining (Tompkins Insufficient baseline monitoring data prior to mining (swamp vegetation survey 2001 to 2003). Monitoring and assessment of causes of collapse by mining companies and others following erosion Difficulties proving/disproving relative contributions of natural erosion, drought, flood, fire and subsidence due to lack of baseline data (EarthTech 2003; Krogh 2004, Krogh, submitted; Tompkins & Humphreys No known remediation. Gully still exists. Some vegetation has regenerated.