COASTAL TRAPPED WAVES


Bass Strait Coastal Trapped Waves model (source: ASR)

I was alerted this week by on-the-ball Phil Watson, Principal Coastal Specialist, at NSW Government Office Environment and Heritage, to a “unusual tidal phenomenon”. Phil had spoken to Gary Brassington, Chief Coastal Specialist at the Bureau of Meteorology, about the very large tidal anomaly present along the NSW coast at the moment. This is a positive anomaly of up to a maximum of 40 cm, from Eden into southern Queensland.

At the 1999 NSW Coastal Conference held at Forster, Doug Lord and Colin Nalty presented one of the most memorable papers at these conferences that I have had the privilege of hearing, and later reading (I still have a copy!). 

The title was The contribution of ocean water level anomalies to foreshore erosion and inundation in NSW. In this paper they noted how foreshore erosion, particularly on relatively  sheltered beaches, may be associated with elevated water levels and apparent lower wave conditions. They described such conditions in June 1999, including unexpected flooding of agricultural land. Water levels coincident with king tides exceeded predicted tide levels by up to 0.28m over a period of several days. This event occurred during a period of westerly winds associated with a high pressure system so no storm surge was involved. They related this magnitude and frequency of the positive anomalies to higher sea levels as projected in 1995 by IPCC; this was a forerunner of later work by John Hunter. Their work highlighted the importance of tidal measurements along the coast undertaken by the Manly Hydraulics Lab. They noted earlier observations by Wylie and others published in 1993 in conference publication 93/4 of the 11th Australasian Conference on Coastal and Ocean Engineering. Lord and Nalty conclude that the cause, frequency and magnitude of coastal water anomalies along the NSW coast “are not yet well defined”, but they warn that “these seemingly extraordinary water levels will become the norm and hitherto unseen ocean levels will be achieved during future storms with increasing regularity”.

Phil informed me that anomalous tidal elevations this week can be attributed to an unusual phenomena in Bass Strait creating a COASTAL TRAPPED WAVE along the Victorian coast lasting nearly 8 days. This wave is slowly moving along the NSW coast from south to north coinciding with high spring tides ( predicted 2.0m on Monday at Fort Denison in the evening). If the anomaly reaches 0.40m then the record storm induced elevation of 2.40m at Fort Denison will be reached if not exceeded! Interestingly these high levels are following a period in April of high wind and wave energy that has cut back many beaches from what had been a relatively accreted condition. Again it is worth noting the value of our observational network in understanding the impact of different types of forces that operate on the east coast.

What are coastal trapped waves? Oceanographers have long been interested in such shelf waves. In eastern Australia, a dedicated set of observations were conducted as part of an experiment in the late 1980s on coastal trapped waves (CTWs) as well as more recent studies can be accessed on line. I don’t pretend to understand the complex physics discussed in these papers. However, also online are lecture notes prepared by Mat Tomczak of Flinders University which describe and illustrate in general terms the nature of CTWs. He notes that they propagate towards the equator on the west coast of oceans (NSW coast) and towards the pole on eastern sides of oceans. They have periods of several days to 2-3 weeks and wave lengths of the order of 2000km. CTWs can make a significant contribution to the observed variability of sea level and currents on the continental shelf. It is now evident, according to Tomczak, that the waves are not locally forced by wind fluctuations but continue as free waves even in Queensland where they are clearly not related to local wind conditions. Sea level variations along the Indian Ocean coast can be followed along half the continent as illustrated in his lecture notes.

Gary Brassington of the Bureau has also published on CTWs. It is likely that what we are seeing this week is a good example. This brings me back to the insightful work of Lord and Nalty. Quite clearly these elevated levels are not all that unusual and under certain conditions can lead to inundation and foreshore erosion. In estuaries they may facilitate long term recession compared to many open ocean beaches that may undergo recovery post storm/high water level events.  CTWs are a phenomenon that we must better understand under present day as well as rising sea level and changing wave/wind climate conditions. Hence the imperative to sustain ocean, continental shelf and coastal observation systems which can be used to calibrate models and thus inform management decisions. So I am off tonight to make my own observations at the local Vaucluse reference station, Parsley Bay!

– words by Prof. Bruce Thom