Storm Surges, Wind Setup and Tides: Coastal Water Levels

A tide is the regularly recurring rise and fall of sea level, caused mainly by the gravitational forces of the Moon and Sun. A storm can raise water levels further. The observed level results from the combined effects of the astronomical tide and the weather; wave action comes on top of these. A storm surge refers to significantly elevated coastal water levels, not just the additional storm contribution. The astronomical tide itself describes a pattern of water levels; tidal current and tidal range are different measurements.

Storm contribution and total water level

The US National Oceanic and Atmospheric Administration (NOAA) distinguishes between storm surge and storm tide. Storm surge is the weather-related rise above the astronomical tide predicted for that time. Storm tide refers to the water level resulting from the combined storm contribution and tide. Surface waves are not included in either term. These English technical terms therefore describe individual components or the combined water level; they are not German warning categories.

The storm contribution is caused primarily by strong onshore winds pushing water toward the coast. Its height also depends on the storm’s intensity, size, speed and direction of travel, as well as the shape and characteristics of the coast. Shallow coastal waters and bays can affect the rise in water level. Meanwhile, the astronomical tide continues. If a storm coincides with an already high tide, the total water level can therefore be significantly higher than the storm contribution alone. The relevant datum must also be taken into account when interpreting depths on nautical charts.

The reference level used is crucial when interpreting a German warning category. Hamburg explains the BSH categories as starting at 1.50 metres above mean high water (MHW); there, severe and very severe storm surges begin at 2.50 and 3.50 metres above MHW, respectively. Hamburg also uses its own water-level thresholds for local measures. These thresholds should not be equated with NOAA’s term for the rise above the astronomical forecast: MHW is an average reference level, whereas the predicted tide refers to the expected pattern at a particular time. Hamburg explains BSH warning categories and local water-level thresholds.

What a report tells you

A forecast or warning applies to a particular location and period, and specifies a particular measurement or reference level. The stated value may refer to the additional storm contribution, the expected total water level or an official warning category. These figures answer different questions: the storm contribution describes the weather-related increase above the predicted tide, the total water level combines both effects, and a warning category classifies the water level according to the rules of the responsible authority.

Waves are another factor and are not automatically included in the stated storm contribution. The same storm will not produce the same rise everywhere, because coastal shape, water depth and the storm’s path all play a role. Therefore, neither a general term nor a threshold for Hamburg can tell you the water level at every coastal location or the specific impact on a vessel or harbor. For travel or port-related decisions, follow the latest guidance from the relevant authority.