Storm surge warning for the German North Sea coast

Abstract

Storm surges along the German North Sea coast are tidal surges that reach a peak of 1.5 metres or more above mean high water (MHW). Severe and very severe storm surges exceed 2.5 and 3.5 metres above mean high water respectively. Storm surges along the German North Sea coast are triggered by westerly winds from approx. 7 to 8 Beaufort. At the Hamburg, St. Pauli gauge, the long-term average is 5 to 6 storm surges per year; less frequent are severe (2 in 3 years) and very severe storm surges (every 5 years). The period of occurrence is essentially limited to the winter half-year. The Federal Maritime and Hydrographic Agency (BSH) is responsible for warning of storm surges along the Baltic and North Sea coasts and the tidal river sections of the Ems, Weser, Jade and Elbe. Warnings of storm surges are distributed in very different ways. Warnings have been broadcast on the radio for many decades. In Hamburg, firecrackers are set off by the police. In recent years, dissemination via the BSH website and a customisable telephone distribution system has become established. Since the 2021/22 storm surge season, warnings have been fed into warning apps such as NINA via the Modular Warning System (MoWaS) of the Federal Office of Civil Protection and Disaster Assistance (BBK) and thus reach more directly affected citizens. The possibilities offered by new media make it necessary to further develop warning strategies. For example, we are currently working closely with the BBK on the automated provision of warnings via the NINA warning app. This leads to faster and more precise distribution of storm surge forecasts. The warnings and forecasts described take place against the background of the mean sea level rise and the associated rise in mean high water level. It is also the responsibility of the Federal Maritime and Hydrographic Agency to monitor this and to provide and analyse it at tide gauges such as Cuxhaven Steubenhöft, where measurements have been taken for around 180 years. When creating the forecasts, we set up the Flood Early Warning System (FEWS), which pools and helps to process the data and creates and publishes reports. With the help of our developed Model Output Statistics System (BSH-MOS), precise and individualised forecasts up to one week into the future are possible for up to 40 gauges. Among other things, MOS evaluates water level measurements, wind forecasts from the German Weather Service (DWD) and area-based modelled water level forecasts from the BSH model system.