Wetlands Australia 32: Building stable estuarine foreshores for an ecological diverse future
Author: Marjolein Oram, City of Gold Coast, Queensland
The Coomera River, situated in the northern Gold Coast - Queensland, experienced significant bed and bank erosion in the upper estuary. City of Gold Coast engaged a geomorphic engineer to undertake an erosion assessment and develop an innovative holistic design using hardwood trees.
In 2015 the Damian Leeding Memorial Park adjacent to the upper estuary of the Coomera River was undergoing bank scour and bed erosion causing a risk to infrastructure and people’s safety. There was also a longer term threat that the river could break into the recreational lake within the park.
Significant bank scour causing trees to fall down the bank 2015. Photo: City of Gold Coast.
This unique project stabilises and protects a stretch of 450 metres of the park foreshore from erosion. Hardwood instream structures were established by placing 96 hardwood tree logs with root balls on the riverbed together with systematic revegetation of the riparian bank. The logs are tethered to 106 marine hardwood piles to reduce risk of logs washing downstream during a major flood event. The instream structures are designed to reduce velocities on the foreshore by manipulating the hydraulic forces during tidal and flood flows. Two rows of hardwood logs and root balls are placed parallel to the bank. The logs closest to the bank provide beach protection for mangrove seedlings to establish and grow, while the second row of logs will restore bed erosion by controlling the bedload sediment deposition and erosion (caused by flow separation when flow hits the object that then forms eddies and vortices).
Excavator on barge placing hardwood trees on the river bed – February 2018. Photo: City of Gold Coast.
Importantly the vegetation rehabilitation, consisting of 7100 native plants with deep roots, was used to tie the whole project together. The works were finished in May 2018 and now the river tidal dynamics and plant growth are starting to create a stable foreshore with multiple benefits. The project is stabilising the bank, keeping the park safe from erosion, allowing mangrove seedling to re-establish and sediment to build up behind the logs. Extra benefits are improvements to water quality, fisheries habitat and overall aquatic ecological values.
A 5-year Monitoring Program is measuring the success of the project. Outcomes include:
- Mangrove seedlings are growing behind the logs
- No more fallen trees
- Sediment is filling in around the logs
- Fish, crustaceans and wader birds are using the site
This unique project is a best-practice showcase of a natural bio-engineering alternative to stabilise a foreshore riverbank in an estuarine environment. The City of Gold Coast is rolling out more natural erosion protection works throughout our estuaries.
The final Coomera River Estuary Stabilisation Works – May 2018. Photo: Remi Oram.