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Large swell waves with a long period breaking on a beach slope tend to have a large swash with water washed well up the beach.  This is often unexpected and takes people by surprise and can cause damage and casualties.  Users should note when large waves with a long period are forecast to run onto an exposed coast.  Extreme forecast index products for waves can alert users to the potential problem.

Example 1: Large swell off South Africa 

Fig2.2-15: Wave energy forecast VT 09UTC 16 Sep 2023, DT 12UTC 15 Sep 2023.   The chart shows extreme wave energy flux (~1300KW/m) being driven towards the exposed southern coast of South Africa.

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Fig2.2-16: Wave energy forecast VT 09UTC 16 Sep 2023, DT 12UTC 15 Sep 2023.  The chart shows the extreme forecast index for significant wave height at 0.9 to 1.0 alerting to the significant nature of the ocean swell.

Fig2.2-1717A: Wave ENSgram DT DT 00UTC 16 Sep 2023. Significant wave heights were forecast to exceed 9 m with wave period above 15 s.  These values were also observed at a nearby buoy.  

Very large waves and swell were induced by a deep depression in southern Atlantic associated with very strong winds. The waves became larger as they approached the coasts and coincided with spring tides. Significant wave heights exceeded 8 m in many places suggesting maximum waves were significantly higher.  There was considerable coastal damage and some loss of life.

Example 2: Large swell off Ecuador and Peru

Giant swell (above 6 meters in height) struck the coasts of Ecuador and Peru causing significant damage to boats and infrastructure.  The swell originated from a storm lying off California several days earlier and was sufficiently powerful to propagate some 5000km while while maintaining a significant amount of energy.  Long period swell was mainly the reason for the damage caused.

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Fig2.2-17B: Snapshot of propagation of long period (21-25sec) swell from Californian coastal area towards Ecuador and Peru coasts.  The significant wave heights are about 1m to 2m but amplification can be expected in shelving coastal waters and swell of some  6m were observed causing destruction.

Sea-surface currents

The interaction of waves with sea-surface currents is not yet included in the operational version of the model.  In particular areas, (e.g. Gulf Stream or Agulhas current), the current effect may give rise to localised changes of up to a metre in the wave height.

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