Wave and sediment modelling for the worlds third largest sand island
JBP staff recently took a ferry from Brisbane to Moreton Island to round off our work in the Moreton Island Coastal Hazard Adaptation Strategy and the Cowan Cowan Shoreline Erosion Management Plan with Alluvium Consulting. JBPacific’s work was to undertake a coastal processes study for the island and to develop concept options for its long-term management.
This project was really all about sand. Moreton is the third largest sand island in the world, and our approach included a lot of numerical modelling and field investigations to understand the complex tide, wave and sediment interactions. The local communities face west into Moreton Bay, and are most affected by wind-generated waves and currents. Our first step was to develop a SWAN spectral wave model to understand the local conditions, which we forced with years of recorded wind data.
A question we get asked a lot is why do we use SWAN? I think that whilst there are a range of wave models available, as a spectral wave model SWAN is very fast, it includes a lot of processes (wind-wave interactions, shoaling, wave breaking and dissipation), and its open source and free to use - so it's great for collaboration between organisations (before you ask, you can download it via Source Forge).
We then fed the nearshore wave and tide conditions into our JBP Beach Evolution Model (JBEM) to track long-term sediment movement. We developed this over the last few years to combine several different wave and sediment transport equations in Python, which gives us a lot of room to calibrate the model. For those interested, we also have several blogs dedicated to the benefits of using python for our work that you can check out on our webpage.
Using JBEM we were able to identify the main wave-driven sediment pathways running south along the inner island. However it was during our work to validate the model trends against historic imagery that we stumbled across a more impressive sight along the northern coastline. As visualised through a Google Earth Engine timelapse, there are sand slugs bigger than a football stadium being moved around through currents and wave action. I’m not sure I will see anything of that magnitude again unless we get a change to work on the Dutch Sand Engine (Holland, I am still waiting for my invitation). A very impressive end to our project!