O’Connell River Pile Fields Project
In 2013 Reef Catchments engaged Alluvium Consulting to undertake a Stability Assessment of the O’Connell River. The Assessment utilised a number of techniques to identified high priority eroding banks and management actions along the main channel. Multi-temporal LiDar analysis between 2013 – 2009 identified the largest eroding bank identified had lost 65 000m3 over the four-year period of the assessment and was expected to continue to lose considerably more due to meander migration. Reef Catchments undertook works to stabilise the bank in December 2015.
The project had a number of objectives including
• Reducing flood related impacts of channel change
• Reducing sediment and nutrient export from the O’Connell River,
Additional outcomes included,
• Contribute to the ecological requirements of the reach and not impact on ecological values
• Reduce the loss of productive agricultural land
• Be cost effective and meet budget constrains
After engaging with the landholder an options report was undertaken to identify the range of options available to stabilise the bank. The options report highlighted that pile fields provided the highest likelihood of success. The aim of the pile fields was to realign the bank closer to its former alignment and provide an environment suitable for both sediment deposition and vegetation establishment.
Seven pile fields were established along the downstream end of the eroding bank starting at the point of inflection of the meander in the zone where meander migration was likely to be most prominent. The total length of the works site in 160m reprofiled to a slope of 1V:3H. After the piles fields were conpleted the site was seeded with grass seed and covered in jute matting. Revegetation was undertaken using appropirate species selected using the Regional Ecosystem mapping.
In total 266 local Ironbark piles 6m long were sourced to construct the pile fields. Of the 6m piles 28 were cut in to 2m and 4m lengths to use towards the top of the bank in each pile field. Piles were driven into the bank two thirds of the length of the pile and extended 5m out into the channel past the toe of the batter slope. Additionally 440 tonnes of rock was brought in to construct a rock tow and line the rows of piles. After completing the pile fields grass seed was spread and the site was lined with jute matting. The site has since been revegetated with native species appropriate for the regional ecosystem.
Description of Project
To date monitoring has included three ground survey. One survey was conducted on completion of the works in January 2016. A follow up survey was undertaken in May 2016 following the wet season. During this period the site lost 376m3. A follow up survey was again undertaken in February 2018 and compared to the initial survey in January 2016. During this timeframe the River was impacted from Severe Tropical Cyclone Debbie (March 2017). This survey resulted in a net erosion of 105m3.
20150921 – Site prior to works. Long sandy bank which prevented vegetation from establishing; 20180202 – Two years following pile fields construction dominated by exotic grasses;
20150921 – Site prior to works
20151221 – Site during construction, jute matting being rolled out after seeding
20162122 – Looking along one pile field. Rock armouring along the piles at bank level prevent scour
20162122a – Looking downstream after completion prior to revegetation
Key Contact – Chris Dench
Coordinator for Water and Waterways Program
P: 4945 2321
M: 0407 538 724
E: Chris Dench