Corindi Wetlands update
Back in 2002 a 50-lot subdivision was proposed for the picturesque village of Corindi, just north of Coffs Harbour on the NSW mid-north coast. Besides the normal issues associated with introducing new housing into a small community, a number of challenges in stormwater management faced the development.
The site soils are clayey, meaning infiltration of water to soil was difficult. A valuable gazetted wetland was situated downstream of the development, and both this wetland system and its creek required a high level of protection from urban stormwater.
Additionally, existing approved subdivision design did not include Water Sensitive Urban Design works in the housing area – only in the flood zone adjoining the protected wetland downstream.
David Pont (now WCG design leader) in partnership with Dr. Keith Bolton accepted the challenge of designing a wetland treatment system of about 1 hectare to treat runoff from the 15 hectare residential catchment. The wetland needed to treat and polish the runoff with its load of sediments and nutrients to a standard where it would not degrade the wetlands and creek downstream.
The wetland also needed to look attractive and integrate with the existing wetland, and be reasonably low maintenance, and not be a mosquito hazard.
Design Approach
Three stormwater wetland design approaches were utilized: NSW DLWC Constructed Wetlands Manual, NSW EPA Treatment Techniques, and the Model for Urban Stormwater Improvement Conceptualisation (MUSIC) developed by the CRC for Catchment Hydrology.
The key features of the wetland system were its relatively large area and shallow depths, and the sequential treatment approach of:
sedimentation zones à macrophyte (wetland plant) cells à gravel bed cells à final cell à overflow to SEPP 14 Wetland
The design task was completed in 2002-03, and the wetlands constructed in 2004. Although some components were not completed according to the design, the wetlands have performed very well in meeting the project objectives, and today form a key aesthetic and functional feature of the site.
Wetland Layout
Figure 1 shows the site layout, with wetland cells, main components, and stormwater flowpaths. The stormwater treatment is accomplished by the plants that are actually in the water – the sedges, rushes and reeds as well as the melaleuca trees with their roots in the water supported by the gravel. This is an innovative system, and there were some initial concerns about the possibility of the gravel clogging with sediments, however it is now evident clogging is not a problem if the system is correctly designed.
Figure 1 Wetlands layout
Monitoring has shown consistently high quality water in the system, with nutrient concentrations usually 'below detection', i.e. too low to record. The wetland has not only functioned successfully in its initial operational phase, but has also provided rich habitat for many species of frogs and birds.
Ecology
Today the large wetland system is a picture of serene beauty. Melaleuca trees have spread upwards from the forest just downstream, and grow around the edges of the treatment wetlands.
The wetlands feature striking colours of blue and green, gold and brown. The varying structure of the different plant species provides visual interest as well as an ecological framework for the water-loving species such as dragonflies, mayflies, crickets and cicadas and the micro-bats that feed on them.
The many frogs deserve special mention. From the earliest time of the wetlands development, frogs such as the Wallum Froglet, Rocket Frog and the Eastern Sign-bearing Froglet moved into the wetlands area in good numbers.
The water in the wetlands is often crystal clear with an organic tinge, reflecting the presence of healthy organic carbon. In the final cell, waterlilies float in the open water zone. An access path winds through the wetlands, offering glimpses of ducks and their ducklings and waterhens through the plants. Song-birds such as Reed Warblers flit through the plant beds catching insects.
The houses creep down the hillside as the subdivision fills in, and residents take their daily walk around and through the wetlands. You hear no mosquitoes, just a pleasant buzz of life from the birds and the frogs.
This wetland system offers a glimpse of the future in stormwater management. As the subdivision fills and the pollutant load in the stormwater increases we plan to undertake intensive research on the system performance, and complete the couple of unfinished sections.