Moods of Sydney Harbour

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Much has been written about “our harbour”: its geological setting, geomorphic evolution, First Nation settlement, the arrival and impact of the British First Fleet, its natural systems and their degradation, wartime conflict, the ships and yachts, and so on. As one who grew up on its shores and now lives with a view of its waters, here is a personal reflection on some of its moods.

I use the term mood deliberately because it is so dynamic, so full of action and surprises. Each morning at breakfast I am greeted with an array of colours changing slowly as the sun rises. But the presence of clouds invokes different sights depending on just how much of the sky is covered. Gloomy dark days makes the water look black and forbidding. And it darkens as rain sweeps towards the Heads.  Patchy cloud cover casts shadows that flicker from shore to shore forming different hues of blue. When cloudless, the Harbour shimmers under a summer sea breeze calling for us locals to plunge in. At night ferries twinkle by, and the cruise liners creep along ablaze with light.

It is the wind that drives the moods. Still one minute, then roaring from the south as a front moves through. Southerlies are one thing, but big offshore storms create crashing waves around Middle Head, and even onto rocky outcrops like Sow and Pigs and Bottle and Glass. Over what time period has these processes operated to carve the rock shelves? White water on high tides rolling into the bays during the 2016 storm forced me off walkways at high tide. And during winter, the westerlies carry flotsam and jetsam of Mosman residents across the Harbour to cover the sands of Vaucluse; it is a little cross we have to bear.

The Harbour has its own sounds and smells that add to the moods. On occasions you are woken by fog horns and hear that ferries are cancelled. Larger ships at times blast at yachts and small boats crossing their paths. Our Navy offers many exciting visions, especially when the submarines were based in the Harbour. To see those vessels sleeking into port in the early mornings was a reminder of the history and activity of Australian and visiting fleets (welcome or not!). Seaweed washed up on beaches creates its own atmosphere, but to the relief of current Rose Bay residents the massive loss of kelp and sea grass from the shallow flats has removed that wonderful rotting smell, a signal of coming home on the tram in the 1950s along New South Head Road.

Besides my fascination to learn more about natural history of the Harbour, especially the growing number of different fish species we see in snorkelling at places like Parsley Bay, there is a curiosity about slicks. On still mornings, and sometimes later in the day depending on winds, the outer harbour is streaked with patches of seemingly flat water. The patterns of these ripple-free surfaces are variable. They can run parallel like a braided river intersecting then diverging. Weirdly they may swing at angles to the shore or sit as outlier patches. I am tempted to get a photo record of the changes but think this is beyond me to achieve any really useful record.

I have spoken to a few colleagues and checked some of the literature on slicks, but have not found a satisfactory explanation for their presence in Sydney Harbour. Winds destroy them but may stimulate them. A paper by Romano and Marquet (1991, Estuarine, Coastal and Shelf Science,v.33) states that the highest probability of slick formation corresponds with wind speeds between 2-4m/sec, but they do not exist above 7m/sec. Slicks are seen as surface micro-films often associated with pollutants (such as oil) or organic matter, or variations in salinity. They can be attached to headlands and rocky reefs, or linked to sewer outfalls, or to massive occurrences of plankton. Some researchers point to oceanic processes such as convergence zones, deltaic plumes where there are fronts, and internal waves. However, I cannot see how many of these explanations are appropriate in the Harbour, so I must continue to ponder and seek help from those who know more.

Bruce Thom

Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2018, posted 16 September 2018, for correspondence about this blog post please email austcoastsoc@gmail.com 

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