On 7 June 2017, an extreme storm associated with high winds and record-high waves, passed the south-west coast of South Africa, near Cape Town. The unusually large storm resulted in wind speeds of up to 120 km/h, with ocean waves of up to 20.5 m recorded offshore of Cape Point (34 12.1414° S, 18 17.6580° E). This has been the biggest storm recorded since wave measurements started at this location in 1978, 40 years ago.
The Council for Scientific and Industrial Research (CSIR), under contract by Transnet National Ports Authority (TNPA), monitors real-time ocean wave conditions near the main ports in South Africa, i.e. at Saldanha Bay, Mossel Bay, Cape Town, Ngqura, East London, Durban and Richards Bay. The wave data are displayed at the Ports for operational purposes and stored by the CSIR using an Integrated Port Operations Support System (IPOSS). The CSIR also monitors long-wave conditions inside the ports of Ngqura and Cape Town.
During the storm of 7 June 2017 fortunately no maritime or marine incidents occurred, but extreme long waves with heights of up to 0.7 m were recorded at the Container Terminal in Schoeman Dock, in the Port of Cape Town. To date, this is the most severe long-wave condition recorded within the Port. Moored ship motions (surge) of up to 10 m range occurred during this event, with the breaking of mooring lines. The moored ship motions were recorded by video. Due to the strong wind, container (un)loading had already stopped and fortunately no accidents took place in the Port.
Long-term recording reveals that meteorological conditions around the south and south-west coast of South Africa are seasonal, primarily occurring during the (austral) winter months of June to August. Strong winds and waves are associated with frontal systems, approaching the Cape coast primarily from the west and south-west directions (between 200 and 265 deg. TN). Annual mean significant heights at Cape Point during the 40-year recording period indicate no specific trend or distinct change in the number of storm events per season. However, there appears to be an increase in storm intensity (pressure gradients, wind speeds and wave heights).
Monitoring, analyses and prediction of these adverse conditions and continuous improvements in accuracy and quality of data lead to a better understanding of the extreme characteristics and will lead to improved forecasting and maritime safety, both for maritime activities and for port operations and safety.
The paper outlines and illustrates the details of this extraordinarily large storm event, which was generated by exceptional atmospheric conditions. The paper contributes to insight into coastal and port engineering in terms of weather, sea state forecasting and associated long-wave generation. This provides important data for the calibration of wave models and is also used by the maritime and marine industry, to provide warning against imminent adverse meteorological and sea state conditions.