Summary - City of Gosnells

Wetlands Field Survey 776. Wetlands Field SurveyThe current wetlands survey built on the foundation provided by the mapping of Hill et al. (1996). It enabledthe recognition of additional wetlands, geomorphic wetland types and wetland values that were notpreviously known in the area. It also provided a fine scale management category assessment for all of thewetlands that are mapped as CCW and or REW in DEC (2008a). To complete the wetland assessmentrequired by DEC (2007) for recommendations to amend or modify the SCP Wetlands Dataset, further studyis indicated to delineate wetland boundaries more accurately and characterise factors (such as MucheaLimestone aquifers) that maintain wetlands, particularly along the interfaces of the Pinjarra Plain with theBassendean Dunes. As there are complex stratigraphic/hydrological features and threatened ecosystems inthe area, this study should be carried out by specialist wetland scientists with ecological expertise. It shouldalso be at a fine scale that is appropriate to the processes that affect native vegetation and part of an overallplan for determining and restoring the environmental water requirements of the vegetation of the greater areaof the alluvial fan and increasing the quality and quantity of the groundwater in the Yule Brook and BickleyBrook catchments.The MKSEA is part of an alluvial fan complex that forms an almost continuous wetland ecosystem thatextends over most of Kenwick and Maddington and has been mapped as about 70 wetlands (palusplains,sumplands or damplands) that are identified under Unique Function Identifiers (UFIs) in the SCP WetlandsDataset (DEC, 2008a; Table 6.1). Other wetland types (palusplains and floodplains) that are rarer on the SCPhave also been described at a finer scale in the vicinity of the MKSEA (VCSRG, 2001). The system fornumbering individual wetlands using UFIs in DEC (2008a); Figures 9, 10; Table 6.1) is somewhat artificialin the MKSEA (and other areas of the Pinjarra Plain) as the boundaries of wetlands are not clearly evident inlow relief, predominantly wetland landscapes that are underlain by complex stratigraphy. UFI boundaries insuch areas are largely artefacts of the various land uses and cadastral boundaries, whereas the natural wetlandboundaries are, usually, the interfaces between the wetland landscape of the Pinjarra Plain and any uplandareas of low dunes of Bassendean Sands.From the broadscale mapping of surface sediments and landforms in Kenwick and Maddington (Jordan,1986; Figure 6) and the climate of the SCP, it could be predicted that the only parts of the MKSEA area thatare not waterlogged or inundated in winter would be relatively small areas of dunes. In the project area, suchuplands all occur in the broadscale soil type S8 (Figures 6, 11) where deep Bassendean Sands overlie themuddy sediments of the Guildford Formation. Under the Mediterranean climate that is experienced byGosnells (with a mean rainfall of approximately 838 mm per annum) most of the flat or gently undulatinglands of shallow Bassendean Sands over the Guildford Formation (S10), the clayey sands (Sc) and the sandyclays (Cs) of the MKSEA could be expected to be poorly drained. Given appropriate topography, these areason the eastern SCP readily form palusplains (seasonally waterlogged flats), sumplands (seasonally inundatedbasins) or damplands (seasonally waterlogged basins), due to the perching of rainwater by shallow aquitards.To database the plethora of wetlands on the SCP, DEC (2008b) has allotted a Unique Function Identifier(UFI) to each wetland. This number usually applies to the full natural extent of the wetland irrespective ofcadastral boundaries. An exception to this rule (that applies in the MKSEA) concerns the more extensivewetlands in the dataset (those with an area greater than 70 ha) that are not lakes or sumplands and have beendegraded over part of their extent. These are divided into several sections (according to wetland condition)and have a separate UFI for each section. In the MKSEA, although the wetlands in DEC (2008b) are mappedunder a total of about 70 UFI numbers, this large number of wetlands is mainly a consequence of the patchyland use history of this area (and subsequent wetland degradation) and not the natural wetland boundaries. Inthis ecosystem, the individual wetland areas that are mapped under separate UFIs are generallyinterconnected by such factors as groundwater flow, subsurface water flows along impermeable layers andchannelled surface water flows (Perth Groundwater Atlas, 2004; VCSRG, 2001).The natural wetland boundaries of the MKSEA (including the main boundaries between the wetlandlandscape of the Pinjarra Plain and the small upland area of the Bassendean Dunes) particularly in the areasthat have been cleared of native vegetation, are not sharply marked and are difficult to delineate withoutdetailed hydrogeological studies. This was illustrated in BFS 387, where the wetlands were investigated byVCSRG (2001) using the same principles that they had developed in Hill et al. (1996), but at a finer scale.Tauss, C. and Weston, A.S. (2010). The flora, vegetation and wetlands of the Maddington-Kenwick Strategic Employment Area.A survey of the rural lands in the vicinity of the Greater Brixton Street Wetlands. Report to the City of Gosnells, W.A. Version 18.04.10