In a natural catchment system it is quite healthy for organic matter in a waterway to decompose slowly and naturally, providing nutrients and habitat for plant and animal life. In a natural system, except during large floods, very little leaf litter, bark, twigs, animal excrement or sediment actually ever reaches the major waterways because it is usually trapped along the way to decompose and provide food or habitat for local flora and fauna.
However, in heavily urbanised catchments, the vast network of pipes and culverts collects a huge quantity of material and directs it quickly and efficiently into the major stormwater drains. The increased quantity of organic matter creates problems for the health of our waterways. The water flows faster through these artificially engineered systems and there are fewer plants and animals to utilise the nutrients. Autumn leaf fall from exotic trees (not native to Australia), heavy metals, toxins, oils, animal excrement and even warm water from road runoff combine to speed-up the decomposition process. This results in a gradual build up in receiving waters like Adelaide’s Torrens Lake or the Patawalonga Lake at Glenelg.
One strategy to reduce this build up of sediment and nutrients in our receiving waters is through the installation of trash racks (also called gross pollutant traps or GPTs) at strategic locations throughout the catchments. Trash racks are designed to trap the larger gross pollutants in stormwater. They can also trap smaller particles but are generally not very effective for capturing very fine particles and dissolved pollutants. Unfortunately the fine particles transport disproportionately large amounts of heavy metals and nutrients and are also a great threat to the health of our waterways. The fine particles can only be effectively trapped in wetlands or at their source before they enter the stormwater system, and so trash racks are only one part of the strategy to reduce stormwater pollution.
In partnership with local councils, the Board has installed a network of trash racks and silt traps in the catchments. A number of different types of trash racks have been installed, ranging from open net type units in open channels, through to ‘state of the art’ units located underground like CDS, Ecosol and Rocla systems. Each of the trash racks has its particular advantages in certain locations.
The majority by volume (over 70%) of material collected from trash racks is organic matter including leaves, twigs and bark. Sediment makes up the next largest component, with litter accounting for around 10% of the total.
It is in the community’s best interests both environmentally and economically to reduce the amount of material that enters our major urban waterways. Despite the high cost of removing this material from our waterways and the visual impact of trash racks, they are a cost-effective and immediate means of removing rubbish and debris from our waterways while other long-term programs are being established to return some of the catchment’s natural filtering features. Local Councils’ street sweeping operations also play a significant, complementary part in preventing litter from entering the stormwater systems. Maintenance and cleaning of trash racks is generally a council responsibility.
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