Strategic riverbank stabilisation to protect agricultural land

Introduction  

Severe erosion from the 2022 floods tripled the size of a scour pool at the confluence of the Brunswick River and an unnamed creek near Sherrys Bridge, approximately 4km west of Mullumbimby (Bundjalung Country). A scour pool is a deep depression or hollow in a streambed formed by the force of moving water as it removes sediment and rock, often occurring at bends in a river or below obstructions.

The event sent approximately 1,700 tonnes of sediment down the Brunswick River. The flood also increased the scour depth of the pool to 2m below the current bed of the Brunswick River. If not repaired, further floods could promote massive sediment loss both upstream and downstream, and cause irreversible damage to the hydrology, water quality and ecology of the riverine corridor.  

Challenge  

To design and deliver riverbank works that stabilised the site, prevented future flood damage, and improved and restored riparian habitat.

Previously installed rock protection at the culvert upstream of the scour pool was degraded and susceptible to a head cut (a vertical drop in a stream bed that accelerates erosion). This level of degradation could undermine the culvert and cause further damage to the stream and agricultural land upstream for a considerable distance.

If the scour pool was left unchecked, ongoing erosion would continue to degrade the site, causing further loss of agricultural land and riparian habitat, and reducing water quality downstream. 

Works would also need to address the stability of the pool edge, preventing the loss of mature vegetation holding the bank together. 

Most importantly, works would need to ensure a stable transition of flow power from the pool into the Brunswick River, where a distinct 2m change in bed level occurred as a result of flood damage.     

This threatened the environmental and agricultural values of the reach and downstream water quality, and also the adjacent road and bridge. 

Solution  

Local Land Services (LLS), Soil Conservation Service (SCS), Byron Shire Council and Tweed Byron Local Aboriginal Land Council worked together on this high priority site to understand:   

• the degree of further flood impacts on the community's road assets 
• the ongoing environmental impact of taking no action at the site
• the high likelihood of ongoing erosion
• the minimal risk that the proposed works would negatively impact Aboriginal cultural heritage. 

Based on these investigations, LLS and SCS collaborated with the landholder to deliver the following improvements:   

• 155 linear metres of rock revetment was constructed along the scour pool's northern and southern banks to prevent further erosion   
• grade control structures were installed at the upstream and downstream extent of the site to regulate flow and prevent riverbed degradation 
• steep banks were reshaped to a 2:1 slope for improved stability  and revegetation
• soil binder sprayed and jute matting installed to help stabilise the soil
• wildlife friendly fencing was installed to exclude stock, protect native vegetation and improve connectivity 
• 1,200 Lomandras and over 300 native trees were planted to further stabilise banks and restore habitat, with 850 ground cover plants and more trees still to be planted
• restoration efforts improved a further 260m of Brunswick River riparian habitat in the upstream paddock, which resulted in a wider buffer zone, extra trees planted, and weeds managed.

Outcomes  

The riverbank stabilisation works have:  

• stabilised the site
• mitigated the threat to the Brunswick River and the creek
• provided the means for riparian condition to improve
• prevented an increase in the size and depth of the pool during major flood events
• limited future loss of high value agricultural land and the environmental values of the site 
• enhanced riparian habitat and improved connectivity with upstream ecological assets 
• protected the nearby road and bridge.   

These improvements have reduced the cumulative impact of erosion and sedimentation in the Brunswick River.  

Funding and partnerships  

The Riverbank Rehabilitation Project is jointly funded by the Australian Government and NSW under the Disaster Recovery Funding Arrangements.  

Key partners included:

• Local Land Services
• Soil Conservation Service
• Byron Shire Council
• private landholder.