Storm Surge Warning Service (SVSD)

When the wind howls around the houses, people in the Netherlands will often hear warnings on the radio that an extra high tide is expected. "On the basis of this forecast, Rijkswaterstaat recommends a dike watch in the following sectors: ...". This suggests that bad weather is on the way, maybe followed by even worse conditions. But not to worry: the Storm Surge Warning Service is on the alert and keeping a close eye on developments.

Since the great flood disaster of 1953, when a combination of a spring tide and storm force north- westerly winds led to the emersion of large parts of Zeeland and South Holland, the Dutch sea defences have always proved reliable. Even so, extreme weather conditions still pose a danger. We can trust our dikes, but not blindly. Vigilance will always be necessary. This brochure explains how this vigilance is exercised in practice. It describes the arrangements for cooperation between the KNMI weather bureau and the Department of Public Works (" Rijkswaterstaat" in Dutch) and talks about wind rise, warnings and alarms, and the role of the dike and dam authorities. This complex of arrangements is certainly no unnecessary luxury for a country in which half the population lives and works below sea level.

The reliability of our coastal defences is checked regularly and systematically. This is done by the national Department of Public Works (" Rijkswaterstaat"), the provincial public works authorities, the water boards and the municipalities. Supreme control is in the hands of Rijkswaterstaat, which is also responsible for high tide warnings. The provincial governors and the authorities in charge of the dikes and dams must be warned when the tidal rise is expected to reach dangerous levels due to a combination of tidal movements, high river discharges and wind. In the North Sea, high tides occur approximately every twelve hours. The main tidal stream enters the North Sea along the Scottish coast; a less important stream comes in through the Channel. As a result, the difference in level between high tide and low tide is not the same everywhere. Each point on the coast has its own tidal difference. In Vlissingen it averages 3.80 m, and moving northwards along the coast to Den Helder it diminishes to an average of 1.40 m. Thereafter, the tidal difference increases again: at Delfzijl it averages 3 m. But these figures are merely theoretical. The actual tidal difference depends not only on the positions of the sun and the moon. It is also determined by the weather, and primarily by the wind. North- westerly gales are notorious in this respect. They can blow the full length of the North Sea without interruption and sweep the water up against the Dutch coast. The rise on any particular occasion depends on the direction, the force and the duration of the gale.

Our dikes are strong. Since the disastrous storm of 1953, the dikes and dams in the Delta area and along the rest of the coast have been systematically improved. The recent completion of the storm surge barrier in the New Waterway was the last piece in the jigsaw of protective measures. Apart from the Western Scheldt, the entire delta can now be sealed off from the sea by huge storm surge barriers. The dikes and barriers are designed to withstand 'very severe storm surges': that is to say, storm conditions so rare that they will occur on average only once in every 4,000 to 10,000 years. Even though such superstorms are highly unlikely in our own lifetime, we cannot afford to relax our guard. Less extreme weather can also damage the dikes through the impact of breaking waves and strong tidal currents.

Whenever dangerous high tide levels are anticipated, it is the duty of the Storm Surge Warning Service - abbreviated to SVSD in Dutch - to notify the dike and dam authorities and other bodies responsible for public safety. The SVSD is always ready to spring into action, 24 hours a day. During gales, it keeps meticulous watch on developments in coastal tide conditions, particularly if the wind direction is between south- westerly and northerly. It also forecasts critical high tide levels and issues advance warnings to the relevant authorities. The SVSD itself takes no measures to defend the dikes - that is up to the local dike boards.

There are three rise levels that prompt the SVSD to come into action. If the SVSD issues a warning that the first ('pre- warning') level is likely to be reached, the dike and dam authorities take very limited precautions. If the second (warning) level is expected to be reached, they take further measures. And if the SVSD expects the third and highest (alarm) level to be exceeded, more drastic precautions follow. In that case the SVSD will alarm the dike and dam authorities and recommend that the dikes be watched. The pre- warning, warning or alarm (dike watch recommendation) is cancelled as soon as the water subsides below the indicated level after high tide. If the next high tide is also likely to produce dangerously high water levels, the SVSD will issue a new warning or alarm.

sector	Schelde	West-Holland	Dordrecht	Den Helder	Harlingen	Delfzijl
referencestation	Vlissingen	Hook of Holland	Dordrecht	Den Helder	Harlingen	Delfzijl
pre-warning level	310	200				260
warning level	330	220		190	270	300
alarm level	370	280	250	260	330	380

Because the timing of high tides varies from one place to another and because a gale will seldom affect the whole coastline with equal force, the coastal region is divided into sectors. In each sector there is a reference station.

The procedure for issuing warnings and alarms to the various sectors is as follows:

Every day the Hydro Meteo Centre in Hook of Holland (an auxiliary office of the KNMI) produces tidal rise forecasts. The SVSD is notified when the high tide at any reference station is expected to exceed the "information level" (which is as much as 40 to 50 cm below the warning level). This message is generally issued about ten hours before the water is likely actually to reach that level. Based on the information supplied and on his own experience, the SVSD officer on duty a tidal hydrologist - will decide whether or not it is expedient to staff the warning bureau, the SVSD action centre. This will normally be done whenever the warning or alarm level is expected to be reached or exceeded. If the situation is expected to be less acute (perhaps with high tide only around the pre- warning level), the bureau will not usually be manned, but the tidal hydrologist will phone the services on the pre-warning list. Depending on the time of day, he may do this either from home or from his workplace. But, in either case, he can't then relax or go back to bed: he must continue to keep a careful eye on the situation as it develops. If weather conditions deteriorate so far that a decision has to be taken to man the bureau (because the warning level is expected to be exceeded in one or more sectors), the SVSD officer will inform his section heads and issue warnings and/ or alarms. Wherever possible, this will be done at least 6 hours in advance of high tide, so that the dike and dam authorities have time to prepare. These warnings will be issued to a number of bodies concerned with the safety of the coastal provinces, including:
· water boards and dike and dam authorities
· Rijkswaterstaat field services
· the provincial public works authorities
· the Ministry of the Interior (Fire Service and Disaster Response Department).

As soon as an alarm is issued, announcements are broadcast on radio and TV news bulletins. When a dike watch is advised, the following action is usually taken:

The dike authorities call up their personnel and recruit local inhabitants to form dike teams. The central security station is staffed and sandbags are filled and loaded onto lorries. The sea dikes are patrolled by people carrying portophones. Fire brigades and police services are put on full alert. Army and navy commanders confine their personnel to barracks or to their ships. Dike cuttings - places where the dikes are crossed by roads and railway lines - are sealed off. Mayors and chief police officers are notified in the municipalities concerned. They take measures to ensure law and order. The national press agency (ANP) announces on radio and TV where dike wanings have been issued. The sectors affected are also listed on Teletext. People living in these sectors will know that they can expect further information on radio and TV.

This procedure guarantees the maximum state of alert. Of course, it is impossible to predict what will happen next. Usually the damage is confined to minor dune erosion and people breathe a sigh of relief, especially if they have personal memories of the great flood of 1953.