This story from Sorell Council shows an example of how councils maintain, repair and upgrade parts of the stormwater system and directly help protect the environment. Sewerage is piped from people’s houses for special treatment. Rain water and surface water goes into a separate system of ‘stormwater’ drains provided by councils. Stormwater drains downhill and ends up in our inland waters and oceans. Although stormwater is relatively clean, it can still present some risk to the environment.
Orielton Lagoon, shown in Figure 1, is an unnatural lagoon, created by the Sorell causeway, which was built in 1872. Orielton Lagoon is part of the larger Pitt Water Orielton Lagoon which is one of ten RAMSAR Wetland sites recognised in Tasmania. The RAMSAR Convention is an international treaty which identifies wetlands of world significance and promotes good management.
Figure 1: Orielton Lagoon is located between Midway Point and Sorell
The Pittwater Orielton Lagoon area provides a temporary home to some of our annual international bird visitors. Some fly for more than 10,000 kilometres non-stop from Siberia and Alaska straight to Sorell! The lagoon area is a critical feeding site for weary travellers to replenish energy reserves for their long journey home to their breeding grounds. Some of the water found its way into another swale and drained normally. However, because of the unusual ‘sodic’ soil in the area, most of the water actually went underground and, along with water from the catchment generally, was causing ‘tunnel erosion’ shown in Figure 3.
Figure 2: Some of the species of bird that can be seen at Orielton Lagoon,
from left to right, the Pied Oyster Catcher, Spoon Bill, and Bar-tailed Godwits
So the Council had a situation which was leading to erosion, and when erosion occurs near water, the fine particles can build up in the water and prevent sunlight from reaching the water plants. This limits the plant’s ability to make oxygen which can radically change the environment and harm life through suffocation and by other chemical reactions. Silt also carries a lot of nutrient, which is great for growing vegies, but in water bodies can speed up the loss of oxygen in water and damage to aquatic life.
Figure 3: Because of the 'sodic' soils in the area, the stormwater went under the topsoil and began to cause 'tunnel erosion'
Council wanted to tackle the problem but the cost, particularly of a traditional hard engineered solution using concrete, was going to be a barrier, and there was the silt issue to tackle somehow. The solution hit upon was quite innovative and illustrates the complexity of environmental management work councils do in their engineering, civil construction and environmental management work. Sorell Council’s solution also shows how important the coordinating role of councils is in complex projects.
Sorell Council initiated discussions with a range of stakeholders and a project was developed. The project partners and their roles are described in the table below.
Sorell Council’s new drainage design avoided the need for expensive and obtrusive concrete forms, instead rebuilding a gently sloped swale, removing the weeds and planting those with native grasses and other species. By allowing the stormwater to travel slowly along the swale, much of the water soaks into the ground. The native plants trap the silt, and the nutrients in the silt fertilise the land plants. The silt is largely prevented from entering the water and causing problems. Used in this way, plants can also trap and even detoxify heavy metals and other contaminants.
The outfall design also avoided concrete, instead using ‘gabions’ as shown in Figure 4. Gabions are wire cages filled with rock, designed to temporarily stabilise land. Plant roots and soil work their way into the rocks and stabilise the structure while the wire breaks down and disappears.
Figure 4: 'Gabions' were used to create the new outfall
Costs were able to be kept to a minimum in part, by using the Green Army - a work for the dole program. Also through the great work of volunteers from the Pitt Water Orielton Landcare and Conservation Volunteers Australia Groups.
In total, the project was completed for about $60,000 which was affordable from the existing rates and Council budget.
The area in which the trial tunnel erosion works were carried out has stabilised under several significant rain events. Planted areas will be monitored and watered through the first summer, with more planting on top of the several thousand plants already planted. Weed control will continue throughout the foreshore, with trial autumn cool burns being investigated.
The construction of the outfall is the first stage in a long term project to protect Orielton Lagoon from the impacts of stormwater and to re-establish native vegetation across this section of the foreshore. Further tunnel erosion remediation works will be carried out across the Orielton Foreshore area with the Green Army, Pitt Water Orielton Landcare, and Conservation Volunteers Australia, and an appropriate maintenance regime for the remediated areas will be put in place.
Figure 5: Volunteers and work for the dole participants worked in all sorts of weather
The project was managed by Paul Gray, Sorell Council’s Natural Resource Management Facilitator, which is a shared position, with Natural Resource Management South. Pied Oyster Catchers and a Spoon Bill have been recorded feeding close to the outfall. Bar-tailed Godwits migrate to Orielton Lagoon and are regularly seen near the site. There are also land animals enjoying the new site including Bandicoots! It seems everybody is happy with this solution to a tricky environmental problem.