Beach Rotation; How & Why Sand Moves On Our Beaches In Storms
COASTALWATCH | Environmental News
By Senior Coastal Scientist, Andrew Short
Beach rotation - Why some beaches have gone, while others are wider than ever or what happens when you have a super El Nino and a winter wave drought.
You may have noticed that last spring and winter there was a wave drought – no big swell, no cyclones or lows off the coast, just generally low south swell. Not good for surf and not good for some beaches. This summer, for the east coast of Australia there has been an incredible cycle of swell pumping for all the major point breaks, something certainly, that southeast Queenslanders have been rejoicing.
If you have been to Terrigal or Collaroy beaches in NSW, in December you would have noticed the beach has been eroded and rocks and old seawalls exposed at Collaroy (Figures 1 to 3). Walk to the northern end of the same beaches and you will find plenty of sand (Figure 4). In fact a lot of beaches along the NSW coast at the end of 2015 have very eroded southern ends while the northern ends are wide, some wider than ever.
So what’s been happening?
Since August, we have had generally, low south swell, and not any substantial big easterly waves, usually generated by the east coast lows or cyclones that form just off the coast. As a result the southerly waves have been moving sand northward along the beaches. In embayed beaches typical of the central and south NSW coast, this means the sand is moving out of the southern end and accumulating at the northern end. As a result the south end erodes while the north end builds out – a phenomena call ‘beach rotation’. This is to distinguish it from beach ‘oscillation’ when the whole beach is eroded during big wave events, then recovers during the following period of lower waves.
Beach rotation was first identified and documented on Narrabeen-Collaroy beach in the early 1990’s, and its now being identified up all around the world. On the NSW coast it is most apparent on the headland bound ‘embayed’ beaches that typify the coast, with the headlands acting as natural barriers and keeping the sand trapped in the embayments.
What drives the waves and the rotation is the Southern Oscillation Index (SOI), more commonly known as El Nino (negative SOI) and La Nina (positive SOI), and also called ENSO (El Nino-Southern Oscillation). This index records the air pressure difference between Tahiti and Darwin (Figure 5). When its positive (La Nina conditions) there is a strong pressure gradient, which drives stronger trade winds which pushes warm water into the western Pacific (i.e. east coast Australia) which in turn breeds more and more intense tropical and east coast cyclones. These conditions produce the higher northeast and east swell and great surf. The same surf pushes the sand south, as beaches rotate to the south, with the northern ends eroding while the southern end build out.
When the SOI is negative (as at present) the pressure differential is weak, the trade winds are light and the warmer Equatorial Current that usually flows across the Pacific Ocean water relaxes and, in fact flows backwards towards South America, resulting in cooler water off eastern Australia and fewer if any tropical and east coast cyclones. All the while, the mid-latitudes cyclones, located well south of Tasmania, continue to produce low to moderate southerly swell, which moves sand northwards and the beaches rotate to the north, as at present.
Figures 5 and 6 plot the SOI and Moruya beach width since 2008. Note that when the SOI was positive (La Nina conditions) between 2008 and 2014, the southern end of Moruya beach was wider than normal (red line). Since the negative SOI started in 2014 the southern end has eroded severely and is in fact, the narrowest it has ever been (Figure 7), while the northern end is the widest it's ever been (Figure 8). This is a manifestation of strong northerly beach rotation, as a result of the strong negative SOI, or ‘super’ El Nino as some are calling it. If you look more closely you will see that whenever the northern end widens the southern end erodes and vice versa, all part of the beach rotation that occurs whenever we have prolonged changes in wave direction. This occurs not just at Moruya and Narrabeen, but on all embayed beaches along the central-southern NSW coast (Figure 9).
While El Nino means low waves for eastern Australia, in the north, the warmer than average water has also caused the breeding of big storms off the western Americas with multiple hurricanes generating their biggest waves. It’s the reason why there has been barely three days between swells in Hawaii and Western America, the super big wave season this northern winter has not been disappointing for surfers.
Likewise, during La Nina the northern Pacific get lower waves, while we get the cyclones and east coast lows and bigger easterly waves. This circum-Pacific relationship was recently confirmed comparing waves and beach data from around the Pacific (USA, Canada, Japan, Hawaii, NZ and eastern Australia) including our long-term Narrabeen beach measurements, as well as a beach time series from the south coast’s Moruya beach shown in Figure 2.
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