Summary - City of Gosnells

Regional Setting 11due to climate change. In perpetuating the same methods of dealing with run-off as are currently used in theMKSEA and increasing the conveyance capacity in the area, the negative local effects (dewatering anddegradation of valuable plant and animal habitats) and negative downstream effects (pollution of the SwanRiver) will be exacerbated.Central to this issue are the predominantly muddy or clay soils and the low-lying topography of the MKSEA(Figures 6, 7, 11). The opportunities for both development and the environmentally sustainable treatment ofrun-off water are significantly constrained by such landscapes. Three main methods that are used currently tomanage stormwater on the SCP (that could be used in the MKSEA) and problems associated with them aresummarised below.1. In areas of deep sand, the cheapest (and most common) method of artificial drainage that is currentlyused on the SCP to prevent the flooding of property and infrastructure by the increased volume ofsurface flow produced by development, is to gravity feed this water to the lowest points in the landscape(which are usually existing wetlands or constructed swales). To reduce expenses, the run-off from alarge area is concentrated into the least number of basins possible and the receiving basins are excavatedto accommodate the required volumes. If the basins are located in deep sand, the periodic flooding ofthese by large volumes of nutrient-enriched, polluted run-off is a practical and cheap method of disposalof excess water as this water readily infiltrates into the regional groundwater table.However, to disperse the pollution carried in stormwater and to reduce environmental impacts, watersensitive urban design in areas of deep sand dictates that excess run-off is not concentrated into a few,large basins but is infiltrated into the groundwater as close as possible to the source of the run-off inmany small basins (Monk and Chalmers, 2006). Such infiltration basins, according to best practice, alsorequire regular maintenance to remove gross refuse, sediment, nutrient and pollutant build up andnutrient-stripping vegetation. However, the best practice model for drainage is costly as it requires thatvaluable land be devoted to drainage measures.In the MKSEA, the area of the deep sands that could be used for (water sensitive urban design) run-offinfiltration areas is limited to a narrow corridor of dunes adjacent to Tonkin Highway in Precinct 1 and asmaller area in Precinct 2 at the east end of Victoria Rd. There are few, if any, sandy areas suitable forwater sensitive urban design infiltration sites in Precinct 3 of the MKSEA.2. The second option for dealing with run-off from development (and this method is most commonly usedin areas of clay and mud on the eastern SCP where drainage water will not infiltrate the groundwater) isto direct this run-off into a drains or natural creeks. If creeks are used for this purpose it becomesnecessary to regularly excavate their channels, keep the banks clear of natural fringing vegetation andoften straighten these channels and line them with concrete to enable large volumes of nutrient enrichedand polluted run-off to be rapidly exported into major watercourses such as the Swan River. Thesepractices contribute to the degradation of native fringing vegetation, the destruction of native faunahabitat, the spread of weeds, and the deterioration in the water quality of the creeks and downstreampollution and algal blooms in the Swan River.This type of drainage system is currently used throughout Kenwick and Maddington; pollutedstormwater run-off and water from the shallow aquifers intersected by excavated drains are channelledinto Yule Brook and Bickley Brook and hence into the Canning River.3. A third drainage option that has been used on the eastern side of the SCP is the excavation of largebasins in clay soils to retain the stormwater run-off close to the areas where it was produced. In suchretention basins the stored water evaporates slowly or, in peak rainfall events, some of it overflows intothe Swan-Canning River system. Such a drainage basin was excavated in the Woodlupine Brook justwest of the MKSEA to receive the run-off from Roe Highway. In seasons of peak rainfall, the overflowfrom this system flows downstream to the Canning River. In the best examples of such drainageoptions, the water storage basins are vegetated and subsequently provide habitat for water birds andsome natural nutrient stripping by the vegetation.This drainage option does not address the problem of reduced groundwater recharge in such catchmentsthat usually impacts adversely on the water requirements of groundwater dependent ecosystems. In theMKSEA, as most of the land proposed for development lies in the groundwater catchment for BFS 387Tauss, 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