After a fun bumpy drive, we arrive on-site at Pine Mountain, and we meet with two of our incredible scientists: Mark Waud (Land Restoration Principal Scientist/Innovative Land Restoration) and Amy George (Land Restoration Senior Scientist).

The property is beautiful, and we are ready to learn more about the day, the project, and what will be done. A laser LiDAR drone is being set up and will be flown by experts along a stretch of the river to map out the area.

This area is of particular interest because the water treatment plant is only 14km downstream and that plant supplies 80% of Brisbane’s drinking water. Erosion and sediment loss along the river carry silt and clay downstream, which in turn can affect the drinking water treatment process.

The project manager Mark Waud says we have been working with landholders in the Mid Brisbane catchment between Wivenhoe Dam and Mount Crosby Water Treatment Plant for about five years.

“This work came out of the Seqwater Source Water Protection Partnerships Program. This program works to build strong ongoing relationships with landholders and identify and deliver source protection projects that will improve the condition of our river systems, protect water quality, and reduce the impact of floods on the river, landholders, and our drinking water,” he says.

“Work we do to prevent sediment loss or other types of pollution entering the river helps to ensure the safety of drinking water, with the added benefit of making it cheaper for Seqwater to treat our drinking water. We work closely with landholders as this work protects their productive land and everyone loves a healthy river.”

Up, up and away

LiDAR drone setup and configuration. 

The LiDAR drone will help the team get a feel of the shape of this specific part of the river after the 2022 floods. Gathering that information will allow them to build a hydraulic model to understand how the water flows and velocities and where the erosion and sediment deposition points are.

This part of the river is what they call an inset floodplain* because the floodplain sits within the river channel, and in this specific case, within a very wide and deep valley. In some areas, we see floodplains where the channel is not so wide or deep, and when the river floods, it spreads out over big flat areas.

With rivers flooding over those floodplains over thousands of years, the deposits of silt create rich, fertile soil and we now grow a lot of vegetables on those floodplains.

Inset floodplains though, tend to be a combination of sand, gravel, and rock. Historically many sites, including this one, were cleared and quarried for sand and gravel to be used in construction. Unfortunately, this can result in the area becoming unstable. This is an ongoing challenge for this site, as when big flood events occur, the area can go under 20 metres of water.

Prevention is better than a cure

One of the most important things to remember is that rivers need room to move and if we straighten them up and narrow them down, they move even faster and produce more erosion. In the area we are looking at, damage from flood events has caused the river to take a shortcut through the inset floodplain. If left unchecked, this will cause it to flow faster permanently and cause further erosion – not just here, but upstream and downstream as well.

When the river causes more erosion, it means more soil is lost and more sediment is brought downstream to the water treatment plant.

The plan is to try and prevent this from happening as much as possible.

This area is one of six reaches that were identified by Healthy Land & Water in different river systems throughout SEQ, where investment could bring a high return for water quality, recreational, agriculture, fishing, infrastructure as well as for wildlife including platypus, koalas, and lungfish. Reducing erosion will also have flow-on benefits to Moreton Bay, helping to protect seagrass habitat, turtles, and dugongs.

Technology gives us a step up

Mapping the area out with the help of a LiDAR drone allows the team to decide, based on hydraulic modelling, what can be done to this section of the river to slow it down and keep it as stable as possible. Restoration can be undertaken through a combination of solutions, depending on the unique site characteristics.

Mark says we that if we had 30-40% tree cover through these areas, this part of the Brisbane River would be a lot more stable.

“If we could combine it with some decent ground cover from grasses and other vegetation, this site should be able to look after itself and the landholder could continue to graze it periodically,” Mark says.

“We need to restore the vegetation on the banks and for it to slowly increase over time. The question is how can the vegetation make the river stable? Wouldn’t the power of the river destroy anything we do?”.

Native vegetation adds resilience

Dr. Amy George explains the effectiveness of native tree species in slowing down flood waters and collecting sediment. 

According to Mark, our native plants are ideal for building stable banks. Species like bottlebrushes and blue gums have adapted over centuries to survive through floods and other extremes.

“A common feature that we see during floods, especially in bottlebrush, is that even big trees can bend over, go underwater for a week, survive, and still grow after the floods,” he says.

“As a result, many species of native vegetation would help slow down the power of the river flow and will capture and retain some of the soil and sediments travelling downstream.”

Vegetation is not the only way we can improve the river’s stability. Mapping can help our scientists to see which other options could be suitable to pair with vegetation restoration. There are many engineering alternatives for bed and bank erosion control techniques, including log jams, pile fields, rock chutes, and rock armouring, however, these techniques are much more expensive and complex to deliver than simple vegetation projects and fencing to protect riparian areas.

Recovery trajectory for the long-term

The team is aiming to find the most efficient ways to achieve restoration outcomes that will give the landholders, our stakeholders, and the environment long-term resilience and the best possible return.

We need to work with nature and focus on the long-term recovery trajectory. In the case of a big flood event, some damage is inevitable, but if the river is stable, the recovery trajectory will be on track to improve over the long term.

This will result in less damage and reduce the cost of flooding over time.

Healthier waterways also reduce the time and cost of water treatment, helping to ensure our drinking water supplies can keep up with the ever-growing demand.

*A floodplain is a flat area of land next to a river or stream that becomes covered in water during flood events. It stretches from the banks of the river to the outer edges of the valley. When it is within the river macrochannel, it is commonly termed an inset floodplain. When it spreads over a large area outside the macrochannel, it is commonly called a floodplain.

About the program

Healthy Land & Water’s Mid Brisbane Partnerships Program is restoring and protecting water quality and building resilience in the Mid Brisbane catchment area.

The program works closely with landholders to deliver a range of projects that involve revegetating riverbanks, installing fencing, removing weeds, and stabilising gullies.

These projects ensure waterways are more resilient to high flow events, reducing erosion and the flow of sediment into the Mid Brisbane River and eventually Moreton Bay.

Read more about it here.

Acknowledgments

Flood damage assessment assistance is being provided through the jointly funded Commonwealth-State Disaster Recovery Funding Arrangements.

This project is delivered by Healthy Land & Water in partnership with Seqwater.