A Warning System For Foam At Sea




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Foam lines at sea (southwest of Den Helder) on 10 May 2020 at 09:50 local time based on a satellite image of Sentinel-2. ©RBINS/REMSEM
A Warning System For Foam At Sea
post by
Kelle Moreau

Foam in Scheveningen due to lots of algae and strong northerly wind

The metre-high foam during the fatal accident of five surfers on 11 May in Scheveningen, the Netherlands, was most probably caused by an exceptional combination of a lot of algal remains and a strong wind from the north-northeast. Dutch and Belgian researchers from various organisations concluded this in a report on the cause of the foam formation. The authors have to confine themselves to conclusions that seem most reasonable at the moment and the available data will be analysed further. The researchers advise to now mainly provide information to water sports enthusiasts and coastguard partners, because the development of an adequate automated warning system will take time.

Reconstruction of the last four days

On Monday the 11th of May, five surfers unfortunately lost their lives off the coast of Scheveningen in the Dutch province of Zuid-Holland. In order to gain insight into the circumstances in which the accident occurred, scientists from all kinds of disciplines joined forces. In their report, they outline the most plausible scenario of the day of the accident.

The reconstruction of the available data shows that a concurrence of weather conditions from the end of April led to the large amount of foam that had accumulated on that day in the corner of the Northern Harbour Head and the beach of Scheveningen. It is most likely that abundant sunlight in the preceding period first caused the growth of exceptionally large amounts of algae in the sea. Around the 10th of May the bloom was decreasing, as a consequence of reduced light due to clouds and of more blending due to the increasing wave action. As a result, algal residues were released into the sea. On Monday 11 May the north-northeast wind blew more or less parallel to the coast and reached force 7 Beaufort at the beginning of the afternoon. The wind then drove the foam that had formed to the south, causing it to accumulate against obstacles that protrude into the sea at right angles to the beach, such as Scheveningen's Northern Harbour Head.

Colony-forming algae

Algae researcher Katja Philippart of the Royal Netherlands Institute for Sea Research (NIOZ) coordinated this research and explained how so many algae could be present in the sea water at the beginning of May. "This species of algae with the scientific name Phaeocystis globosa can live in the sea as solitary cells or in colonies. In colonies, the cells are held together by a mucilaginous protective matrix and the algae can rapidly increase in biomass".

In order to form these colonies, the algae need a lot of light and a large supply of the nutrients nitrogen and phosphate. At the beginning of May, the conditions were ideal to make this happen and the algae colonies reached a very large biomass. A lack of light and a stronger mixing makes the colonies disintegrate again. Philippart: "The clouds of Sunday 10 May probably triggered the disintegration of the colonies into loose, solitary cells. This caused the sugary remains of the matrix to end up in the sea, and due to infections with viruses the proteins from the cells were also released into the water. When proteins and sugars are mixed together by wind and waves, you get foam".

Satellite images reveal algal blooms and foam patterns

The Remote Sensing and Ecosystem Monitoring (REMSEM) team of the Royal Belgian Institute of Natural Sciences (RBINS) has extensive expertise in the use of remote sensing instruments, and analysed and interpreted a combination of images from the Sentinel-2 and Sentinel-3 satellites from the period before the tragic accident. Dimitry Van der Zande concludes that these are useful for the observation of algal biomass and foam in seawater: "A time series of Sentinel-3 images with a spatial accuracy of 300m shows a strong algal bloom in the vicinity of Scheveningen and along the coast of Zuid-Holland at the end of April 2020. High concentrations were still measurable near the coast at the beginning of May. The high resolution Sentinel-2 images, which show a detail of 10m, can then be used to detect the foam".

The detection and combination of such satellite information can contribute to an automatic coastal foam alert system. However, satellites do not provide a continuous stream of images from a fixed location, and, due to cloud cover, they also only provide a partial image. Therefore, as foam accumulation along the coast can be rapid and very localised, satellite imagery is not suitable as the sole source for a foam warning system. The actual observation of foam formation is best done with cameras.

Researchers advise more prevention

Despite the fact that the researchers were able to determine the most likely scenario for the occurrence of the exceptional amount of foam on 11 May, it will be difficult to set up a reliable automated warning system. After all, not only the amount of algae and foam must be accurately monitored, but also the current wind strength and direction must be predictable in real time, in great detail and very locally. That is why the researchers of this study plead for more information to be given to water sports enthusiasts, their clubs and the coastguard partners in the short term, so that they are able to evaluate any risk on the accumulation of foam themselves.

Shortly after the incident in Scheveningen, the Maritime Rescue and Coordination Centre (MRCC) in Ostend contacted the Belgian investigators with the question whether such a foam incident could also occur on our coast. Unfortunately, the answer was that in similar circumstances this is not inconceivable. The MRCC is the first hotline for emergencies at sea and therefore closely follows the development of a warning system. Dries Boodts, acting head of the MRCC: "We also want to follow this advice on our coast. The satellite information provided to us by the RBINS is very useful in this context, to serve as an early warning. It will help to urge users of the sea to increased vigilance and will also be useful in planning Search and Rescue operations. We look forward to the development of further opportunities to provide everyone at sea with the most detailed information possible. Better safe than sorry."

Read the full report on the website of NIOZ.

Ecologists, algae researchers, weather and water experts from the following research institutes, universities, government agencies and consultancy firms participated in the analysis:  Royal Netherlands Institute for Sea Research (NIOZ), Utrecht University (UU), Deltares, University of Amsterdam (UvA), Delft University of Technology (TUD), Water Insight, Groningen University (RUG), Bureau Waardenburg (BuWa), Rijkswaterstaat, Istituto di Scienze del Mare (ISMAR)-CNR (Italy), Royal Belgian Institute of Natural Sciences (RBINS; Belgium), Netherlands Institute of Ecology (NIOO) and Highland Statistics.

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