March 2006 Forum
The evolving science of constructed wetlands
The guest speakers were Dr Allin Hodson, Coordinator of the Urrbrae Wetlands Complex, and Dr Paul Dalby, a Director of Wellington Wetlands Developments.
In a lively session, Allin Hodson described the teething problems of a constructed wetland and the difficulties of matching the varying objectives of stakeholders with the evolving dynamics of a working wetland. Key issues revolve around the major impacts of first flushes each winter season, with the accumulated debris from the catchment hitting the wetland in the first pulse of inflowing water. Allin gave a thought-provoking presentation inter-weaving the science, practical management and education potential of the Urrbrae Wetland. Some examples included the research to isolate the source of detergent-like foams during first flushes (saponins generated by native eucalypts to repel insect attack, not human over-applications of pesticides as originally thought), the ineffectiveness of trash racks to capture pulverized eucalypt leaves and fine organic sediments from road surfaces, and the difficulty of isolating the source of oil pollutants reaching the wetland.
This working constructed wetland example clearly demonstrates the need for ecological input as well as engineering know-how into the design of constructed wetlands. The objectives for ongoing management of the wetland also need to be very clear, to ensure that these objectives can be met over time, and that changing perceptions and objectives are transparent if management objectives need to be changed.
Paul Dalby presented an innovative new development proposal near Wellington on the Lower Murray which aims to incorporate wetland repair into a housing development. The housing development would fund rehabilitation of a former dairy swamp to provide a significant area of wetland habitat in a region with less than 6% of natural wetlands remaining. The current design includes proposals for temporary and permanent wetlands, islands, protected areas to encourage turtle breeding, management of water regimes to reduce the impact of carp, gradual replacement of European willows with river red gums and other native plants, and protection of aquatic plants. Incoming residents would be encouraged to become involved in monitoring and management of the wetland, with extensive walking trails through the area, a conservation zone and bird hides. This is an example of a new development approach which seeks to combine economic and environmental benefits in one project.
Postgraduate tour of Urrbrae wetlands complex with Allin Hodson
(Photo: Anne Jensen)