<p>Queenscliff Bombie and perfect A-frames at North Steyne. - Photo: Tom Shand</p>

Queenscliff Bombie and perfect A-frames at North Steyne. - Photo: Tom Shand

Manly – bathymetry (it matters!)

January 02 2012 | Posted in Blog, 

Courtesy of Ben Macartney and www.coastalwatch.com

Manly Beach holds a unique place in Australia's surfing history and geography. It is the birthplace of Australian surfing - Hawaiian Duke Kahanamoku gave the first-ever surfing demonstration at nearby Freshwater beach in 1915 - and to this day, the area ranks among the country’s best and most unique beach breaks. As any well-salted Sydney surfer knows, a combination of east-northeast swell and westerly winds can transform Manly from a user-friendly beachie to Sydney’s version of the Mexican Pipeline, Puerto Escondido.

Facing straight east, Manly is renown as one of a handful Sydney locations that draw in the full brunt of easterly swells; generating wave heights a foot or two larger than surrounding beaches – and it rarely maxes out.

To understand why, we need to investigate Manly’s bathymetry – the shape of its ocean floor. The major feature is the offshore reef known as Queenscliff Bombie, which plays a central role in re-organising any easterly swell as it approaches North Steyne and the adjacent zones, South Manly and Queenscliff.

Dr Tom Shand, Coastal Engineer at the Water Research Laboratory and local surfer, explains: “The Bombie is essentially a continuation of the Queenscliff Headland. It rises from about 15-metres water depth to five-metres water depth over about 100 metres. The steep profile really makes the waves stand up. At low tide a big swell breaks top to bottom out there.”

The Bombie bends the straight swell lines, creating A-Frame peaks.  Unlike an island, it doesn’t break the swell up. Instead, as waves move over the Bombie they slow down, while the swell on either side refracts and diffracts, focusing energy into the centre as they move back into deeper water.

Adds Shand: “The shape of the Bombie means it’s really sensitive to direction. On a south swell, you get waves refracting, but it will be focusing up into Queenscliff headland, rather than directing waves into Manly. Its influence on the surf is completely dependent on swell direction and period. Any swell from southeast through to northeast focuses on the bombora and then continues travelling primarily straight into North Steyne, leading to large, good-shaped A-frames with secondary A frames also directed to the north and south into Manly and Queenscliff.”

Direction and period are key – northeasterly swell and longer wave periods are more effected by the Bombie and subsequently deliver better waves along North Steyne.

The photo (above) clearly illustrates the warping affect the Bombie has on waves arriving at North Steyne; modifying swells into highly symmetrical A-frames.

 

The long range forecast: Not too shabby…
Courtesy of Ben Macartney and www.coastalwatch.com

Forecasting conditions two months out is about assessing probabilities more than calculating details, but the good new is that there are several factors hinting at good surf over the waiting period for the Australian Open of Surfing.

First, February is traditionally a consistent month for East Coast surf.

Early February 2010 saw a week-long run of easterly swell topping out at four-to-six feet – an episode backed up by two more three-to-four foot swells through the second half of the month.

February 2009 saw a solid southerly swell breaking a long flat spell on the 12th, followed by an extended run of easterly swell that peaked at four-to-six feet on the 16th and again at solid six foot plus on the 18th. February 2008 was also good, starting with an extended run of two-to-four foot easterly swell through the first week, followed by another solid east-north-east swell from the 21st to the 23rd that went back-to-back with a three-to-four foot south-southeast swell.

It’s a good track record, backed by the another element suggesting good surf in 2012; a well-established La Niña currently affecting the Pacific Ocean. As discussed in the Tropical Cyclone outlook for 2012, stronger trade-winds and more cyclones in the Australian tropics equals more swell.

We’re already off to a promising start, with the first East Coast tropical cyclone of the season, Tropical Cyclone Fina having tracked southward across the Coral Sea – in all likelihood contributing to the optimal conditions described above for Manly over the Christmas to New Year period.

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