It Takes 2 Swells to Fix a Sandbank

Every few years the waves at Noosa change, for better or for worse. off. First Point doesn’t break like it used to, and the general talk in the town is that the spot is "just not what it was." Then a solid swell finally rolls through, the forecast apps light up, and somehow the wave still doesn't break like it did. People blame the climate, cyclones, rising sea level, wind & tide even the moon.

But the biggest factor is the sand…

A University of the Sunshine Coast study published in the journal Geomorphology in 2021 tracked exactly this, using more than 60 years of aerial imagery of the beaches around Noosa Headland. Daniel Wishaw and Dr Javier Leon, working with Helen Fairweather and Allen Crampton, wanted to know how sand actually travels around a headland and into the bays on the other side: Alexandria Bay, Granite Bay, Tea Tree Bay, Little Cove, and eventually Main Beach. What they found changes how you should read a swell forecast.

Headland bypassing, the technical term for sand moving around a point like Noosa's, doesn't happen just because a big swell turns up. It needs a sequence. First, a moderate swell has to hit from a direction that pushes sand up against the headland itself, loading it at the apex. Then a second swell, from a different angle, has to arrive to carry that sand around and release it into the bays. Skip step one and step two has nothing to move. Skip step two and the sand just sits there, banked against the point, going nowhere.

That's not an abstract finding. It's the exact story of Noosa's Main Beach for the better part of three years. A major swell event in 2019 delivered a huge load of sand into the system, more than 60,000 cubic metres by some estimates. Wishaw later described it as sand sitting "in the bank" for Main Beach. But 2020 and 2021 were quiet summers. No follow-up swell arrived at the angle needed to flush it on, so the sand just stayed there, stacked up near First Point, flattening the wave without most surfers understanding why.

Then, on New Year's Day 2022, Cyclone Seth sent three days of serious easterly swell straight into the coast. On paper, that's the kind of event that should have sorted everything out. It didn't. The swell direction wasn't the one the system needed. The sand barely shifted. First Point stayed stuck in the same holding pattern it had been in for two years, despite a cyclone swell most surfers would have bet their board on fixing it.

A 2023 follow-up study by the same USC team, presented at the Australasian Coasts and Ports Conference (fittingly, held right here on the Sunshine Coast), dug further into the mechanics. Storms do most of the heavy lifting, responsible for 60 to 70 percent of all sand movement around the headland despite only occurring on about 6 percent of days. But that movement depends entirely on wave direction sitting in a precise balance between southeast and east-to-northeast. Shift that balance by even 10 to 15 degrees, roughly what current climate models project for this coastline, and the researchers found the system starts starving beaches of sand instead of feeding them.

The two-swell rule

The practical version of all this: a good-looking swell forecast tells you almost nothing about whether a particular sandbank is going to be any good, unless you also know what came before it. One big swell from the right direction can load a system with sand for years. A different swell, from a different angle, is what's needed to actually release it onto the beach you're paddling out at. If that second swell never arrives, you can get cyclone-strength surf and still paddle out to a flat, soft, lifeless bank. That's exactly what happened at First Point in January 2022.

It's also why we don't book one spot for the week and hope. Peregian, Sunshine Beach, Noosa, Castaways and Marcus Beach are a twenty-minute drive from each other, but they don't run on the same sandbank clock. An open beach break like Peregian answers to local wind swell on a timescale of days. A point like Noosa, tucked behind its headland, is working through a multi-year sand budget that a single good forecast can't override. Reading that difference, not just the swell height on an app, decides where we take a lesson on any given morning.

Learning to read a coastline like that, rather than just taking our word for it, is a big part of what time in the water with a coach is for. Get in touch or check the website for current availability.

Sources:

- Wishaw, D., Leon, J., Fairweather, H., & Crampton, A. (2021). "Influence of wave direction sequencing and regional climate drivers on sediment headland bypassing." Geomorphology, 383, 107708. https://doi.org/10.1016/j.geomorph.2021.107708

- Wishaw, D., & Leon, J. (2023). "Mechanisms of Headland Bypassing During Modal and Storm Wave Conditions." Proceedings of the 2023 Australasian Coasts & Ports Conference, 144-149. https://research.usc.edu.au/esploro/outputs/conferencePaper/Mechanisms-of-Headland-Bypassing-During-Modal/99992998002621

- Jarratt, P. (2022). "Big seas can't shift sand." Noosa Today. https://noosatoday.com.au/news/21-01-2022/big-seas-cant-shift-sand