Land clearing for the subdivision and construction of buildings and infrastructure in Western Australia often results in soil and building sand making its way into nearby gutters and drains.
From here, it is transported through the storm water system and deposited into wetlands, rivers, creeks and streams.
This increased sedimentation can lead to the establishment of isolated surface water pools; increased deposition of sediment in existing pools, making them shallower; and the formation of “alluvial fans” along waterways. This increases the availability of breeding habitat for pest mosquitoes and can lead to an increase in the number and diversity of flood plain mosquito species (SERCUL, 2017).
Sedimentation can fundamentally modify waterway habitats. Issues such as declining water quality (caused by increased water temperatures), decreased oxygen levels, increased incidence of pathogens, and, as a result, a decrease in the number of mosquito predators, are all considered “triggers” for increased mosquito populations (SERCUL, 2017).
The drying climate predicted for South-West Australia will further challenge efforts to maintain water quality and availability in the landscape, and effective erosion and sediment control management will become more important. The increased frequency of intense weather events will increase erosion and may result in greater amounts of sediment being blown or washed off site.
To manage and prevent the increased occurrence of mosquito-borne diseases and mosquito nuisance, it is therefore essential that the management of soil erosion, sediment runoff and sand drift, during the construction of subdivisions, buildings, and public infrastructure, is improved.
The Department of Health encourages and supports local governments in Western Australia to develop a Mosquito Management Plan (MMP). MMPs should consider that all urban development must include preventative measures to mitigate sediment loss – not just in the case of constructed wetlands but also for developments in close proximity to natural wetlands and waterways.
Implementation of MMPs is often the role of Environmental Health Officers (EHOs). EHOs can assist by immediately reporting incidences of sediment loss and/or sedimentation to their Compliance or Environmental Officer. To highlight the impact of sediments, officers should look for: the presence of mosquitoes or mosquito larvae; stagnant water (including water in roadside culverts); flooding; a change in flow and depth of rivers, pools and/or streams over time; sediment deposition, and/or the infilling of lakes and estuaries.
Perth NRM’s Sediment Taskforce has created resources to assist local governments, property developers and builders to manage sediment at construction sites, available for free on the website: www.perthnrm.com/resource/sediment-management/
The Sediment Task Force is supported by the Western Australian Government’s Department of Biodiversity, Conservation and Attractions and Perth NRM.