Towards a general model of the impact of urban development on vegetation communities in wetlands
The impacts of urbanisation on catchment hydrology are well established: increased peak flows and hydrograph flashiness, decreased infiltration and baseflow. However, the hydrological effect of urbanisation on receiving wetlands downstream of areas undergoing urbanisation is less well understood.
The diversity and abundance of wetland vegetation communities is dependent on variations in key hydrological characteristics, including depth, inundated area, inundation duration, frequency and seasonality. Changes in the flow regime from the upstream catchment are likely to alter these characteristics and consequently impact on the diversity and condition of vegetation communities. The value of wetlands as faunal habitat is largely dependent on wetland vegetation, so changes in vegetation communities are likely to impact wetland fauna.
An innovative approach to assessing the impact of urbanisation on wetland hydrology is presented in this paper. Laverton wetland is an important waterbody on the site of the former Laverton RAAF base south-west of Melbourne that provides habitat for a number of threatened bird species including: Lewin’s Rail, Baillon’s Crake, Australian Bitten, Latham’s Snipe, Great Egret and the Intermediate Egret. The wetland has been designated under the Environmental Protection and Biodiversity Conservation Act 1999 on the basis of its faunal significance.
A considerable area of the upstream catchment has been identified as available for development under the Melbourne 2030 planning framework. To assess the impact of urbanisation, catchment-scale rainfall runoff models of pre- and post-development conditions were developed and linked to a detailed digital terrain model of the wetland to generate long-term time series of variations in water depth and inundation area.
Analysis indicates urbanisation will have a significant impact on wetland hydrology at an annual and seasonal time scale. Changes in the frequency and duration of inundation will disadvantage emergent plant species such as Carex tereticaulis and Juncus spp.) and advantage other species such as Phragmites australis and Typha spp. leading to long-term changes in vegetation community structure and habitat availability.
To mitigate impacts on wetland hydrology, a range of scenarios have been developed to determine the volume and timing of water that must be diverted from or into the wetland to preserve key hydrological characteristics and vegetation communities. The method developed in this study can be applied to other high value wetlands affected by changed hydrology.