Buoys Follow One Direction
How much does the swell direction influence the surf at your local?
For surfers on the East Coast of Australia, swells arrive from a wide range of directions depending on their source; anywhere from the north-northeast to the south-southwest.
These directional changes can result in large waves at one location and tiny waves at another - on the same day - depending on each beach's exposure to the swell energy.
But along the southern coastline of Australia there is a much smaller range of swell directions, owing to the source and track of swell producing weather systems - usually anywhere from the west to the south. Additionally, the extremes of this boundary (west swells or south swells) are less common than prevailing south-westerly swells.
Victoria's Surf Coast is uniquely positioned with a broad swell window to the southwest, however incoming swells are refracted as they round Cape Otway. This narrows the apparent swell direction at the coast, so when viewed from shore the majority of swells appear to arrive from the south (give or take a few degrees).
So how can you tell if there's a westerly swell or a southerly swell running through Bass Strait if they all appear to be arriving from a similar direction at the coast?
On the East Coast of Australia, you can check the live data from any one of a number of directional wave buoys between Mooloolaba and Eden.
However, buoy data is unfortunately quite limited along the southern Australian coastline, with one buoy each for South Australia, Victoria and Tasmania. And of these three buoys, only one - Victoria's Point Nepean buoy - records swell direction. This buoy is located off Portsea Back Beach, near the western end of the Mornington Peninsula and is a useful tool for remotely confirming the arrival of Southern Ocean swells, and monitoring their trends along central Victorian beaches.
What's unusual about the Point Nepean buoy is that its directional readings are somewhat misleading. And there's an interesting reason why this is the case.
See the image to the left. This is a snapshot of the wave direction data from Point Nepean, taken on the final day of this year's Rip Curl Pro. Interestingly, it shows the swell direction swinging between 205 degrees (SSW) and 190 degrees (S) approximately every six hours.
At any other coastal location, a periodical oscillation in swell direction such as this would be considered abnormal.
But at Point Nepean there are a number of environmental factors that have an influence on incoming swells. After (somewhat crudely) superimposing tidal data from Port Phillip Heads over the direction data, it became clear that the two datasets are almost perfectly in sync with each other (see the second attached image).
This poses several questions. Are the strong tidal currents of Port Phillip Heads interfering with the instrumentation on the wave buoy? Or, are the regional tides of Bass Strait influencing the direction of incoming swells?
By isolating the swell direction data at the turn of the high and low tides – commonly referred to as slack water – it would be reasonable to expect very little current at this time, and therefore minimal influence on the directional data.
But, this is not the case. The observations show that the swell direction is always more southerly at the turn of the low, and more westerly at the turn of the high.
Upon further research, it turns out that the unique physical characteristics of Port Phillip Heads delays the timing of slack water by up to three hours, falling almost mid-way between each tidal phase. Therefore, if we look at the directional data during the middle of the tidal cycle, we can observe a consistent reading from approximately 198 degrees (S/SW). This would appear to be the 'true' swell direction in northern Bass Strait.
We've spoken to the Port of Melbourne (who own the buoy) as well as a number of coastal engineers, but no-one was able to validate this rough theory before the article went to print.
However, the general consensus was that the buoy instrumentation is accurately recording the environmental swell direction, so any unusual changes in swell direction are probably related to currents, rips and eddies associated with large daily tidal movements through Bass Strait and Port Phillip Heads.
There are other factors that can result in erroneous buoy direction data, but we'll leave them for another article. In the meantime, if you regularly use the Point Nepean buoy data, it's always worth looking for abnormal trends before making a decision on where to surf. //CRAIG BROKENSHA with BEN MATSON Link: Point Nepean wave buoy