29th September

'Surf beat' recorded in Dingle Harbour

John Rapaglia, Luca Zaggia, Kevin Flannery

The first week of 2014 was marked by two large storms whose impacts were felt along the full length of the Atlantic Coast of Ireland. The extreme pressure gradients from these storms led to strong winds and subsequently great (> 30 m waves) offshore of Ireland’s west coast.

Although somewhat tempered by the time they reached Dingle Bay, wave heights during the late night of Jan 3 and early morning of January 6 reached 15 m. The department of Biology at Sacred Heart University, CT USA, in collaboration with Dingle Oceanworld Aquarium deployed two CERA Diver pressure sensors in Dingle Harbor during these storms.

10.1 Surf beat

One pressure sensor was located at the mouth of Milltown Creek and was set to read the water elevation at a frequency of 0.07 Hz in order to capture the tidal oscillation throughout the entire study period (January 1-January 8, 2014). The second pressure sensor was deployed 100 m N of the Dingle Harbor Lighthouse and set to record water elevation at a frequency of 2 Hz in order to capture waver height during the storm.

 Astronomical high tide Both storms were marked by a ~1 m storm surge at the mouth of Milltown Creek. The amplitude of the surge was most likely attenuated by bottom friction this far from the Harbor Inlet, yet it was still great enough to top the seawall protecting the ‘Wild Atlantic Way’ from the water.

The maximum surge from the January 3 storm hit at astronomical high tide during a new moon; therefore water levels reached about 4.5 m above mean low water. Anecdotal evidence suggests that water levels have not been this high in more than 20 years.

Individual wave amplitude during the storm on January 3 reached as high as 0.8 m inside Dingle Harbour, with significant wave height being 0.6 m. A 0.8 m wave has the energy of 800 J/m2. This energy can cause significant erosion along the coast of Dingle Harbour.

Immediately before and after the January 3 storm, very strong currents were witnessed periodically entering and exiting Milltown Creek. These currents were accompanied by a 0.3 m fluctuation in sea level at the low frequency pressure sensor site.

During the height of the storms on both January 3 and 6, these elevation fluctuations reached 2 m with a period of about 9 minutes. In other words, the entire water column was forced into and out of the creek within a 9 minute period! It is believed that this nine-minute period wave is known in the scientific literature as an infragravity wave or ‘surf beat’ and these waves were forced by the open Dingle Bay swell.

In other studies it has been determined that the open swell is correlated to the magnitude of these waves in coastal areas, and this same relationship was clearly seen in Dingle Harbour. What is fascinating, however, is that the magnitude of infragravity waves is usually in the order of tens of centimeters; in Dingle Harbour the pressure sensors recorded waves with a height of 2.

These infragravity waves, given that they involve the movement of an entire water column, may have significant impact on the benthic ecosystem of Dingle Harbour, the estuary of Milltown Creek and on navigation within the Harbor during storms. It is likely that this same phenomenon was occurring in other semi-enclosed embayments around the coast of Ireland during these storms.

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