Holding firm: building a rock chute to stop erosion on the Avon

The severity of the major rainfall event across Gippsland in 2021 led to a rapidly eroding gully posing a threat for the landholder, public and natural environment on a farm near Stratford. The solution was a team effort to engineer and construct a large rock chute to protect the Avon River.

During the rainfall event, water rushed down an existing drainage line into the Avon River, damaging vegetation, dumping a huge load of sediment into the waterway and severely eroding the bank.

The resulting erosion was also rapidly progressing back up the gully and heading towards a nearby public road. This ‘upslope erosion’, known as a ’headcut‘, is often caused when water has to drop over a vertical edge to enter the river.

“In rivers with historic bed deepening (otherwise known as incision) water entering the river has to get from the level of the floodplain to the level of the river. In doing this the water expends all its kinetic (movement) energy, causing rapid erosion of the underlying soil. If you picture the power a waterfall has when it hits the bottom of its fall, it’s easy to imagine how much damage water can do when it’s falling over a vertical or near-vertical edge,” explained Elsa Burnell, West Gippsland CMA’s Project Officer.

In the Avon River system this process is exacerbated by the extremely erosive subsoils and large amount of historic deepening of the River channel that occurred in the 80s and 90s.

”The system is still adjusting to this,” said Elsa.

Without intervention the erosion would worsen with each rainfall event, causing further damage to the waterway, the surrounding property and potentially impacting the road.

“A headcut like this can easily travel 100m in one rainfall event,” said Elsa.

Elsa worked with a team from West Gippsland Catchment Management Authority and the landholders to assess the damage and develop a plan to restabilise the bank and gully to prevent further erosion.
Because of the steep slope and constrained site, specialist engineers were engaged to help design a solution.

Over 1000 cubic meters of large rock was delivered to the site and placed in the eroding gully to create an armoured rock ‘chute’ The rock chute guides the water into the river and allows it to dispel its energy on a surface that is resistant to erosion.

“We needed to get the water from the higher floodplain level into the river without doing so much damage,“ Elsa said. Essentially, a rock chute allows the water to expend its energy on a surface that won’t erode.”

Protecting this gully from erosion will not only protect the road and surrounding paddocks, it will also prevent a huge amount of sediment from entering the river system and flowing on into the Gippsland Lakes.

After constructing the rock chute with machinery, the team then got to work on revegetating the surrounding area. The plants will help to protect the structure, slow future flows and further stabilise the riverbank providing resilience in future rain events.

A visit to the site two years later revealed that the rock chute and planting were holding strong and in good shape despite a number of flood events since construction. “I was really pleased to see that there was no new erosion and no sediment getting into the river,“ concluded Elsa.

Flood Recovery Program

In June 2021, widespread storm and flood damage across Gippsland impacted in many catchments.

Since then, a comprehensive flood recovery program has been rolled out across more than 150 sites in partnership with local government, agencies and landholders supported by funding from Department of Energy, Environment and Climate Action (DEECA).

Since the event, the flood recovery program delivered:

  • 150 waterway structures.
  • Just under 37 kilometres of fencing.
  • Over 27 hectares of vegetation.
  • Earthworks at 165 sites.
  • 155 management agreements with landholders.

New life for Noojee Reserve

In June 2021, widespread flooding heavily impacted  Gippsland catchments including the famous town of Noojee located high in the headwaters of the Durt’Yowan (Latrobe River).  

The river is a much loved feature running through the town, providing shade and recreation at many spots along its banks.

The Noojee Reserve is usually a tranquil spot with shelter, picnic tables, winding tracks and toilet facilities. In the June 2021 storm event, the public reserve was completely inundated as the river broke its banks and roared through the park taking out banks, trees and paths.  

“This public space gets a large flow over it when the river is high,” said Dan Cook, Project Coordinator for West Gippsland CMA. 

When the initial emergency was over and the area declared safe, the team at West Gippsland CMA moved in to assess the damage. Working in partnership with Baw Baw Shire Council, a large amount of debris covering the reserve was removed and  the debris taken away.

“The park was inundated and the high flow had exposed part of the banks that we had repaired in an earlier event,” said Dan.  

The fast moving  event had moved large logs that had previously been installed to support the river bank causing it to slump and wash out.  

The team used rock to repair and reinforce the banks. Local contractors worked under the guidance of the CMA to expertly stabilise the banks, armouring it to withstand future events.

“We put the rock in, shaped the bank back up and stabilised it – job done!” Dan said.  

Now the reserve is open again, the river has slowed and locals and visitors are once again enjoying fishing, walking and picnicking at this tranquil spot in the heart of Noojee.

Noojee Reserve after the June 2021 floods
Large sections of the river bank were washed away
Rocks were used to repair and reinforce the banks