Origin of giant waves explained

Published Mar 23, 2007


The extraordinary waves that wreaked havoc along the KwaZulu-Natal coast on Monday had very little to do with high spring tides, or the much-publicised 18.6-year alignment of the Earth, moon and sun.

Quite simply, it was a very powerful and unusual storm at sea that drove up the height of waves to 8m to 12m, the highest waves recorded along this part of the coast for at least 23 years.

This is according to SA Weather Service principal researcher Ian Hunter, who laid the blame for the flooding and destruction of property on an intense "cut-off" low pressure system - the same type of weather phenomenon that caused the September 1987 floods in Durban and the Laingsberg floods of January 1981.

In an article posted on the weather service's website this week, Hunter said cut-off lows often led to very high coastal wind speeds as well as wave swells that hit the coast at odd angles, causing more erosion.

Hunter noted that the build-up of the bad weather and the point of maximum intensity on Monday morning had been captured by weather prediction models in other countries three to four days in advance.

"Not only was the location correctly predicted at this stage - the forecast intensity was also very close to what actually transpired. Similarly, the global wave models were also foreseeing the 10m waves analysed off the east coast, with a lead time of three to four days."

His comments have raised questions about whether the SA Weather Service warning notices adequately reflected the severity and danger of the impending storm.

Records show that the weather service did in fact issue a warning at 5pm on the Saturday preceding the storm, warning of gale-force winds and predicting "very rough seas with wave heights of 5m or more".

Unlike the weather bureaus of the United Kingdom and the United States, for example, the SA Weather Service did not have a "surge" model that was able to analyse a wider variety of data such as beach slopes and sea depths that contributed to the power of waves at different locations on the coast.

However, in his article analysing the cause of the freak weather event, Hunter noted that the severe storm almost 700km out to sea had coincided with the new moon and vernal equinox - planetary events that result in high spring tides.

Hunter acknowledged that the high spring tides contributed to some extent to the higher water levels - but if there had been no bad weather at the time, the spring high tide level in Durban would have been no more than 50cm above an average high tide.

"Some mention has also been made of the nodal tidal signal (which happens every 18.6 years) implying that this was a major (contributing) factor."

However, in Hunter's opinion, this cyclical alignment of the Earth, sun and moon was not nearly as significant as the wind- and storm-induced "surge" effect that accounted for the monster-sized waves.

It also emerged on Thursday that specialised wave height measuring equipment at Durban Harbour was on the blink on Monday morning, making it impossible to determine the exact height of the waves in Durban. However, average wave heights of 8.5m (as well as occasional wave heights of just over 12m) were measured at Richards Bay on Monday morning by the Council for Scientific and Industrial Research.

Marius Rossouw, a coastal research engineer at the CSIR in Cape Town, said it might be possible to do some "hindcasting" to calculate the Durban wave heights, but it was likely that they were very similar to those measured at Richards Bay.

He noted that waves as high as 8m were recorded about once a year along the Cape south-west coast, but such high waves were extremely rare on the KZN coast.

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