East Coast braces for erosion from sustained swell event
If you’ve been reading the Swellnet forecasts, you’d be aware that the East Coast, and particularly the northern half of the East Coast, is on the cusp of a sustained swell event.
From this weekend through to at least Christmas, parts of the coast - again, mainly in the north - will see wave size fluctuating between four and ten feet.
It’s the result of a classic La Niña pattern as atmospheric moisture above the Coral Sea spawns tropical disturbances that in turn create swell. However, waves aren’t the only thing this coming event will generate. There’ll also be a higher than average threat of coastal erosion.
You’ll recall that back in July, a number of beaches suffered severe erosion with people ordered from their homes at Wamberal, Old Bar and Stockton both losing fifty metres of sand, and trees falling into the sea at Clarkes Beach, Byron Bay.
It should be noted that, at the same time, many north coast pointbreaks had the best sand in recent memory - the distribution of sand was far from equal.
La Niña is especially worrying for erosion hotspots as the increase in tropical activity causes the mean swell direction to swing more towards the east, compounding the threat of erosion.
The East Coast mostly receives south swells, and their predominance has shaped the coastline. The general pattern is large rocky headlands at the southern end of broad, sandy bays. The headlands protect the lee shores from the southerly energy.
However, this leaves those parts of the coast - i.e those lee shores - vulnerable to swells from the east or northeast, as waves from that direction reach the coast with their energy undiminished. Historically, most of the large erosion events have been during east and northeast swells.
Yet swell direction is only one aspect. The other is the nature of swells - meaning their size and consistency - that arrive from that direction, especially under the scenario currently unfolding.
Unlike a classic winter southerly swell, that glances the coast as it moves perpendicular to the shoreline, we can expect the swell to hit straight on so there’ll be less bathymetric interference and more consistency in arrival. The scouring of sand will be incessant, especially during the high tide phases (more on those later).
As for size, though local seabreezes can kick up small windswells from the east and northeast, this isn’t currently the case. Instead, what we’ll see is a high pressure ridge traversing the Tasman Sea while being squeezed by tropical activity to its north, setting up vast areas of trade-winds aimed at Queensland and northern NSW.
This alone will generate a consistent, moderate-to-large easterly trade-swell, but because of the warmer than normal sea surface temperatures in the Coral Sea and Western Pacific (driven by La Niña), there's an increased chance of a Tropical Cyclone.
When Tropical Cyclones drift south into an established trade-flow they turbocharge an already active sea state, producing larger swells and increasingly this looks to be the case during this weather event.
Through the weekend a tropical depression just west of New Caledonia looks to deepen, further squeezing the northern flank of the high. The depression will form into a low, with a significant fetch of easterly gales due to ramp up on top of an already active sea state. We'll keep a close eye on these developments when or if they unfold.
At present, the largest waves are forecast on Monday afternoon, though if the aforementioned cyclone eventuates all bets are off. Early next week, homeowners and coastal councils will be watching properties through the mornings as the high tides are some of the largest of the year. High water is approximately 9am on Monday for the areas in question, peaking at 1.9 metres, then moving back by around an hour each day though with similarly high levels.
// CRAIG BROKENSHA and STU NETTLE