Rock toe

Streambank Rehabilitation Case Study of the O’Connell

The O’Connell River is one of the four major river systems in the Mackay Whitsunday Region, forming at the base of the Clark Conner Range.

This river provides areas of important aquatic and terrestrial habitat for native species and is an important water source for irrigating productive grazing and sugarcane land.

The O’Connell River has been identified as a significant contributor of sediment to the Great Barrier Reef with a large proportion derived from channel erosion.
In an effort to reduce sediment and nutrient exports and improve aquatic health of the river, Reef Catchments commissioned a stability assessment to understand the location, cause, and extent of channel erosion through the O’Connell River.

Case Study of the O’Connell

Project Name O’Connell River High Priority Systems Repair
Short Project Name O’Donnell Rock Toe Stabilisation project
Managing Organisation Reef Catchments
Project Start Date Nov-14
Project Finish Date May-15
Project Location Bloomsbury
Coordinates 663652.41, 7714327.60
Funding Source Australian Governments Reef Programme
Cost of Works $50 000
Description of Project The O’Connell River is one of the four major river systems in the Mackay Whitsunday Region, forming at the base of the Clark Conner Range.

This river provides areas of important aquatic and terrestrial habitat for native species and is an important water source for irrigating productive grazing and sugarcane land.The O’Connell River has been identified as a significant contributor of sediment to the Great Barrier Reef with a large proportion derived from channel erosion.

In an effort to reduce sediment and nutrient exports and improve aquatic health of the river, Reef Catchments commissioned a stability assessment to understand the location, cause and extent of channel erosion through the O’Connell River.With Funding provided through the Australian Government’s Reef Programme, a LiDar capture of the river was taken to be able to develop Digital Elevation Models of difference by overlaying LiDar from 2013 with Lidar from 2009. The digital elevation model of difference was used to assess sediment loss over the four year period. Combined with an analysis of stream power and the use of hydraulic modeling Reef Catchments was able to identify and prioritise on-ground actions to reduce the supply of sediment and nutrients to the Great Barrier Reef.This site lost 9 000m3 of river bank between 2009 – 2013 and is expected to continue to lose considerably more over the coming years ahead. Because of this loss, it was identified as a high priority site.  To be able to stabilise the bank and minimise any future erosion, Reef Catchments has worked with the landholder to implement bank stabilisation works.

A range of potential options were identified to stabilise the eroding bank. These options ranged from low cost with a high risk of failure; to high cost with a low risk of failure. Working with the landholder to identify potential resources, the selected option was a 2m rock toe which ranked as having a high likelihood of success.

This design includes a 1m toe below the bed with 1m exposed above. This will stabilise the toe and prevent the bottom of the bank from being scoured.  The near vertical bank was then reprofiled down to the toe at a slope of 1V:2.5H (1m vertical: 2.5m horizontal).  This will allow the energy of the river to dissipate as it moves higher up the bank.

The bank was then revegetated with native species, which will ultimately stabilise the bank by providing a mix of ground cover, understory and canopy species. Over time as silt is deposited on the rock toe further revegetation will occur providing long term stabilisation of the bank.

Finally, large woody debris was pinned into the bed to encourage scour of the bed. This will create refuge for aquatic species including fish during the dry season and also aid in promoting revegetation by trapping seeds.

In March 2017 the site was subjected to the impact and significant flooding event associated to Severe Tropical Cyclone Debbie. The site largely withstoof the event and associated flooding, however existing vegetation at the downstream end was brought down by the flood exposiong the root ball resulting in erosion at the downstream end.

Monitoring Data Site Ground surveys have been undertaken in January 2016, May 2016 and February 2018. Vegetation survival assessment has also been undertaken in 2018