Event Summary: Better understanding floodplain deposition with modelling: implications for management responses

Background

Over the past few years Misko and the team at Alluvium have been chipping away at an exploration into stream bank erosion and how that’s modelled, particularly within the Great Barrier Reef catchments. This has led to looking at floodplain deposition modelling processes and river classification systems.

During this discussion we heard about the background steps followed in previous QWMN research development and innovation project work funded by DESI which assessed issues associated with sediment modelling in the Great Barrier Reef catchments (ie Prosser (2018) and Alluvium (2022)).

As part of the Paddock to Reef modelling programme the team needed to identify all sources of sediments and nutrients within the catchments that drain out to the Great Barrier Reef (GBR). As a result of this body of work Misko became curious about how modelling was being used to prioritise sub-catchments for stream rehabilitation and erosion stabilisation interventions. He became interested in drilling down into the modelling and understanding how it all worked ie – what assumptions were being made, where were there potential limitations in the modelling, and how that could be improved.

One of the key aspects of the modelling he unpacked was the ‘sediment budget’ within a catchment. He explained that the models account for primary sources of sediment in a catchment and all the sinks as well.

Due to extensive monitoring at the end of the catchments there is great confidence in the sediment export numbers to the Reef. However, there’s less confidence in the rest of the numbers. Misko’s concern was that this may lead to certain areas in the catchment not being invested in for stream rehabilitation and erosion stabilisation interventions.

2022 QWMN RDI Project

This is where the previous 2022 RDI project focused their efforts. They’ve been working to build more confidence in some of the estimates around stream bank erosion and floodplain deposition, with the aim to refine some of those equations and processes further to improve those estimates.

To reach this point, Misko and team explored sediment modelling options out there and came across the following barriers:

• data availability – LIDAR data is limited in the GBR catchments – particularly as you move away from the coastal fringe
• finding a model that works uniformly across the whole catchment – one model will work for one particular channel type with one data source, but it’s not going to be applicable to another channel where you don’t have good data.

This led to the conclusion to stick with the SEDNET model for the GBR catchment. However, they still recognised it needed improving so they conducted high resolution hydrodynamic modelling with a variety of different river case studies including: the Mary, the Raglan, the Fitzroy, the Murray and the O’Connell Rivers.

One of the key outcomes from this work was highlighting the issue with the reach length being modelled. Typically, reach lengths are defined based on catchment areas and quite often they’re on average about 10 to 15 kilometres long. But across many of Queensland’s coastal streams, there can be a huge variation in the river form and condition and bank height along the 10 to 15 kilometre reach. You might have areas with really expansive flood plains and then other areas that are really confined by bedrock or terraces.

This body of work led to recommended changes being taken up by the Paddock to Reef modelling team to adjust reach lengths.

2023 RDI Project Findings and What’s next 

Piet and Misko’s conversation followed with a combination of findings from the 2023 RDI project and steps being taken for the 2024 RDI project, which has recently commenced.

This work brings the detailed modeling back to the GBR catchment to:

• inform river classification
• better calibrate the models against the monitoring data to determine the sediment budget of the catchment
• break up the river network into the geomorphic reaches for the catchment modelling
• parameterize the different variables that are required in the model.

Further Resources

To watch the full conversation, go here.

To listen to a snapshot of the 2023 project, go here.

And to check out the 2024 Tender outcomes for the RDI projects, go here.