Wallabadah Water Gully Rehabilitation

Water Gully starts high in the hills south-east of Wallabadah, 55km south of Tamworth. It runs behind the historic Wallabadah Racecourse, crossing the New England Highway just south of the village. It is a relatively small but very steep catchment which has suffered at the hands of 150 years of stocking, land clearing and, more recently in the 70s and 80s stream straightening and widening.

The result has seen the stream go from a wet, grassy chain-of-ponds system to become a deep, unstable and rapidly eroding channel which is disconnected from its floodplain. The creek is now drier and more prone to erosion. The agricultural productivity of the land is reduced along with the quality of the water and groundwater availability, not to mention that if the erosion continues the potential damage to the racecourse and buildings increases dramatically.

Water Gully was an actively eroding channel with little vegetation.

Water Gully was an actively eroding channel with little vegetation.

To address these issues and to demonstrate actions landowners can take to rehabilitate their own creeks, North West Local Land Services partnered with George Macdonald, the landholder of Jobys Hill, which is directly adjacent to the eastern boundary of the racecourse. The collaboration addresses the 1 km reach of Water Gully immediately upstream of the racecourse.

Stage 1, completed in late 2019 before the rain and Stage 2 which was completed in late March 2020, feature several types of instream erosion control structures designed to restore the stability and ecological function of this section of the creek.

Brad Davies, whose company, adaptiv, specialises in the design and construction of creek and landscape restoration projects assisted North West Local Land Services and George in establishing this creek rehabilitation site. Brad was on-site for both stages working closely with George, North West Local Land Services staff and local contractors to implement the works. He indicated, “the works program is designed to re-establish the nature features and landscape processes, lost from the site since European settlement.”

To ensure the works were appropriate, adaptiv undertook a detailed site assessment recommending the positioning, number and type of structures to be installed. This assessment considered the rehabilitation reach itself, as well as the overall catchment and the reaches of the creek immediately upstream and downstream of the rehabilitation site.

Brad summarised the findings by saying, “the key issues identified in this assessment where that the extensive bed erosion and cutting down of the creek bed, has been caused by a combination of factors including the change in land use following European settlement of the area, land clearing, and channelisation of the creek. At one point, the area was even used as a track for bullocks to bring logs to the highway!”

“The cutting down of the creek bed has resulted in the banks being much higher than they were previously, making them unstable and eroding at an accelerated rate. Another consequence of the bed erosion has been a corresponding drop in the level and capacity of the alluvial aquifer. The lowering of the water table can also result in a reduction in the quality of this water. Groundwater is critical to the viability of agriculture on the property and in the surrounding valley thus re-raising aquifer levels is a key project target. Monitoring of the groundwater quality and quantity will be undertaken to identify if there is any measurable improvement in both factors”, said Brad.

Much of the main features of the instream works are built from a combination of local natural materials and/or redundant farm resources including fallen timber, rock, old fence posts and gates as well as repurposed steel bore casing. These works are designed to arrest the bed and bank erosion and to further raise the bed of the creek. Now installed, sediment will be tapped to further raise the creek bed and to create stable conditions for native vegetation plantings and natural regeneration.

The works are low tech and low cost by design so local landowners can build them with their own resources. Local earthmoving contractors were used to develop community capacity in these types of works. If landowners are considering undertaking similar projects, it is important that they obtain any necessary approvals or permits before commencing any work within a riparian channel. Talk with your local Local Land Services contact for advice on what approvals are required.

Installation of a log sill across the channel.

Installation of a log sill across the channel. The sill will assist in slowing the water thus reducing its erosive power and to create conditions for sediment to deposit and raise the channel bed. Raising the bed of the channel will reconnect the creek with its floodplain. Once the floodplain is reconnected the water will be spread out over the floodplain reducing the destructive impact of larger floods, further improving the stability of the creek.

An example of a simple log jam

An example of a simple log jam that could be installed by landholders on their own farm. All the timber was from the property and arranged in the creek at strategic locations to help slow the flow of water and protect the bank. To ensure the timber stays in place, they can be cabled together, or timber pins used. Approvals or permits may be required for this type of work.

Since the works have been installed there has been significant sediment deposition and extra moisture levels are evident. These conditions have allowed the successful reintroduction and regeneration of native vegetation. There is also evidence that the works focused on intercepting and slowing the movement of the shallow groundwater is working.

Recent deposition in the creek bed

Recent deposition in the creek bed is evident from rainfall events in early 2020 and, in some places, up to a metre in depth. It can be seen from the photo that tree planting has been undertaken to further stabilise the creek bed and banks and that groundcover is increasing.

The landowner, George Macdonald, has been monitoring the creek closely and is impressed how rapidly and substantially the creek has responded. He says, “our vision is to reinstate the natural creek and reconnect it to its floodplain. We figured it was not called Water Gully by chance”.

“Stage 1 was completed in 2019 when the drought was at its most brutal and rain just a distant memory. When the sky opened in January 2020 it generated several large flow events on relatively insignificant falls of 15 to 20 mm. This resulted in the areas between the structures pooling water and catch enough sediment to raise the bed, over about 500 metres in length, up to 1 metre depth in places. It became a totally different creek over a period of 20 days. Subsequent major flows including one which flooded the village saw additional sediment deposited in the rehabilitated section of the creek, while unrehabilitated sections of the creek experienced significant erosion. Since January 1, 2020 to date [June 2020] the sediment has retained significant permanent moisture, allowing us to successfully plant casuarinas and wetland plants in the creek” said George.

The video shows drone footage following the creek bed where one of the structures was installed. It was constructed using large logs cabled in place with geotech fabric on the upper side. The aim of this log sill structure is to stop the creek bed lowering and accumulate sediment to allow revegetation of the channel. Phragmites, cumbungi and casuarinas have been planted in the sediment that was deposited after the first flow.

“Interestingly, the most recent falls of in excess of 25 mm have generated almost no runoff in the creek, pointing to the likely influence of increased pasture cover and increasing water infiltration rates. I am really excited to see how Phase 2 continues this process,” George said.

/Public Release. The material in this public release comes from the originating organization and may be of a point-in-time nature, edited for clarity, style and length. View in full here.