Restoring Australian rivers by understanding and mimicking nature

For the past 15 years we at Alluvium have been building on our collective understanding of river systems and river restoration. This understanding has delivered cost-effective innovations in river restoration and management that benefit communities, industries and natural ecosystems. Recently we gathered these and our emerging innovations under our Research and Development Program to further advance the science in fields of river restoration.

To date a fundamental of our work has been the establishment of functioning geophysical processes as the basis of broader ecological restoration programs. The physical processes become the foundation upon which ecological recovery or rehabilitation can be achieved.

The photo on the left shows Cherwell Creek diversion at Queensland Caval Ridge Mine in 1999. The photo on the right is in 2014. When a group of stakeholders were taken to this site in 2014, they could not determine where the creek diversion started or finished.

We need to see rivers as complex systems rather than water drains

Our innovations rely on knowledge about the shape and form of our river systems. If we mimic the shape and form of our river systems with our restoration projects, then we have the greatest chance of getting the ecological processes right.

In the past, river diversions and restorations were designed as a channel with rock and concrete and were engineered to take water from A to B.  Engineers asked how much water was flowing through a channel, and then designed how much rock was needed to stop erosion. Their instruments of control were very blunt, but despite these investments, river channels kept eroding or filling with sediment, failing to meet expectations and impacting on ecological processes, such as platypuses burrowing and turtle egg laying.

Our work reflects our understanding that rivers have a specific hydrological regime that reflects water flows, sediment transport and vegetation. Our team started accumulating knowledge of our river systems working on Victoria’s rivers in the late 1980s. Team members then worked with Queensland mines modifying poor performing river diversions, and in the past 15 years have been working on rivers across Queensland NSW, ACT Victoria and Tasmania.

Alluvium developed a bank stabilisation design with instream habitat features to help support a range of native fish for a section of degraded stream within Tallebudgera Creek. We developed a design which utilised timber logs and large boulders to stabilise the bank, protect the vegetation establishing on the bank and floodplain and provide instream habitat for the native fish populations. Nearly 7 years after the works were implemented there is a riparian forest with trees over 10 m high and significant increase in instream physical habitat through this reach. Ecological monitoring has observed a significant improvement in macroinvertebrate and fish assemblages.

We bring the sciences together to restore rivers

We use the best available science and develop our own river restoration and management approaches based on that science. Our speciality, as environmental engineers, is being able to model flood events, understand hydraulic forces and how they are distributed along the river network or within a reach, and then model the impacts of flows on sediment transport.

Through that knowledge, we develop an understanding of whether riparian (streambank) vegetation will have sufficient strength to resist channel change during floods.   Our niche and expertise lie in understanding these dynamics, which informs our approach to river restoration.

As an example, in a flood shear stresses increases and instead of shifting sand, will now move gravel and cobbles in the river channel. Eventually, there might be high enough forces that grasses are stripped from the riverbanks, and trees and shrubs are overwhelmed.

Understanding the scale of flood likely helps us to develop restoration strategies that have a high likelihood of success.

Alluvium’s strength is in the linkages of the different disciplines: hydraulic modelling, fluvial geomorphology, sediment transport modelling, and restoration ecology.  We need to know the rates at which plant species grow, suitable plant species to grow in each zone of the channel, and the soil characteristics those plants grow in.

It is the integration of these different science and engineering disciplines that informs our recommended management decisions.

Our innovations mimic natural systems

River restoration starts with establishing native vegetation on riverbanks and not dumping tons of rock.  Riverbanks without vegetation are more fragile and likely to erode than those with vegetation.

But we know that if you just try and plant grass, shrubs and trees on the riverbank, it likely won’t get established in time before the next flood. It can be disheartening for investors and stakeholders to see their plantings wash away. So, we have developed methods for tipping the odds more in our favour.

One method is to insert vertical timber logs into the river system, which we call pile fields. We look to insert them in such a way that slows the velocity of the water as it goes around a river bend. This allows vegetation to establish enough to do the erosion control work.  In the first couple of years, grasses establish.  After two to three years shrubs establish.  And within 15 years, a structurally diverse vegetation community will be established, and the timber piles will have rotted away. The result of this process will be an ever-increasing resistance to erosion and an outcome that reduces the process that  will be a natural, stable riverbank.

We know that large pieces of wood, or ‘snags’, are important in river systems for providing habitat for fish and other animals. So, we’ve looked at how best to install such wood. For example, wood in estuaries is likely to be attacked by marine borers. However, we discovered that the marine borers attack from a particular direction and can be avoided if the wood is placed in a specific way.

In all our projects, our aim is to provide the minimal amount of structural intervention to get the vegetation established to provide the ongoing stability.

Erosion from the South Pine River in southeast Queensland was a concern for the Moreton Bay Council as it reduced parkland and destroyed a road. Our river restoration work here was unique as we needed to establish marine plants in this estuarine reach of the river. This meant we needed to create a flat area for mangroves to grow and another gentler slope up to the upper banks of the river. Even just a few weeks after construction, dozens of mangrove seeds had settled in the flat bench area. The photo on the left shows the reach in 2018. The  one on the right was taken in 2020 and shows strong mangrove and salt couch grass growth.

We help decision makers to choose the best options for restoring their rivers

We use what we call ‘cumulative probabilistic failure analysis’ as a way of determining the chances of success of a river restoration project. This analysis looks at the probability of a flood event occurring, within the vegetation establishment phase, that has the capacity to wash the establishing vegetation away. The analysis combines the increasing resistance of the vegetation through time and the increasing likelihood of large events over increasing periods of time.

Through time the vegetation becomes increasingly resistant to floods, but over the same period of time there is an increasing likelihood of a big flood. Our analysis of these competing trajectories has helped clients decide how best to invest in stream restoration including identification of an preferred level of confidence in restoration efforts.

We can tell a client, for example, that if they just plant vegetation without any structural works, they might have a 50% chance of success. But if they put in a some limited complementary structural interventions to support the vegetation efforts, their chance of success might increase to 95%. In this way, clients can make decisions around the level of investment necessary to establish a stable, healthy and resilient river system.

Objective based decision making

The decisions our clients make is based very much on their objectives. Some are purely interested in reducing the sediment load reaching the coast. Others want to create fish habitat or recreate mangrove ecosystems. And others want a combination of outcomes.

Each project is different. Not only are there different aims and objectives, but you’ve got different physical, ecological, social and cultural systems interacting. Each location has its own river dynamics, ecology, communities, landholders and stakeholders.

Over the years we have seen many river engineering projects fail because someone has used a cookie cutter approach and expect to get the same outcomes.   There are so many pictures of post completion works which look great when done, but fail in the next major rain event.  Those as the pics that never make to the webpages.

To tackle a river restoration project properly the designer needs to understand the history of a waterway and how it has changed. We need to understand its current condition and trajectory as well as the needs of the community and stakeholders. Then we can design river restoration outcomes to meet their needs.

We are making our innovations more cost effective

Very high on our agenda in our work is a focus on constantly improving our technical innovations for river restoration.  Our people have for over 30 years been involved in advancing the science and engineering. For example, a new project funded by the Great Barrier Reef Foundation is looking at the flow dynamics around the timber pile structures. Our aim is to get a detailed understanding of how those structures impact local water velocity and sediment transport.

Such research will mean we can be more precise in our design of these pile field. How far up the bank do they need to go? Where do we best place them to reduce hydraulic forces? How can we ensure the best conditions for vegetation to establish to protect the lower and upper riverbanks?

This knowledge will help our clients to make sure they are not over investing in river restoration works or having to redo works a few years down the track after the first major flow event.

The timber piles used to help establish vegetation at Graham Park in Kenilworth, southeast Queensland in 2015 have almost disappeared.  Now, vigorous vegetation growth is ensuring the stability of the Mary River banks at this site.

Our future challenges are social and economic

Creating robust river systems in rural areas does need the engagement and buy in from local landholders to support the associated revegetation works and maintain the system beyond the establishment phase. However, we know some landholders can be reluctant to put vegetation back into the landscape as it may impact on agricultural productivity, and the maintenance is just another task.

Why would a landholder give up 30 to 40 metres of prime agricultural land to deliver a river restoration project’s environmental outcomes?

On top of the physical and ecological processes, to be effective, our work needs to consider the social and economic challenges around creating productive and resilient landscapes. We might be now at stage of fine tuning the technical elements of river restoration. Our collaboration between engineers and physical scientists now must extend to social scientists, economists, and policy analysists and include opportunities for Traditional Owners in river restoration.

It will only be through such broader collaboration that we can achieve the full benefits of river restoration projects: resilience to floods, reducing loss of land and infrastructure, improving ecological habitats and biodiversity, sequestering carbon, and creating cooler, shaded areas for the amenity of everyone.