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Discussion papers
https://doi.org/10.5194/nhess-2019-351
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/nhess-2019-351
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 29 Nov 2019

Submitted as: research article | 29 Nov 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Natural Hazards and Earth System Sciences (NHESS).

Very severe storm tides in the German Bight (North Sea) and their potential for enhancement

Iris Grabemann1, Lidia Gaslikova1, Tabea Brodhagen2, and Elisabeth Rudolph2 Iris Grabemann et al.
  • 1Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, 21502 Geesthacht, Germany
  • 2Bundesanstalt für Wasserbau, Wedeler Landstraße 157, 22559 Hamburg, Germany

Abstract. Storm tides are an essential hazard for the German North Sea coasts. For coastal protection and economic activities, planning information on probability and magnitude of extreme storm tides and their possible future changes is important. This study focuses on the most extreme events and examines whether they could have become more severe under slightly different conditions still remaining within the physical plausibility.

In the face of limited amount of observational data on very severe events, an extensive set of model data is used to extract most extreme storm tide events for locations in the German Bight, in particular Borkum and the Ems estuary. The data set includes water levels and respective atmospheric conditions from a hindcast and future climate realizations without sea level rise describing today's and possible future conditions.

A number of very severe events with water levels exceeding those measured near Borkum since 1906 has been identified in the data set. A possible further amplification of the highest events is investigated by simulating these events for the North Sea with different phase lags between the astronomical tide given at the open model boundaries and the wind forcing. It was found that superposition of spring tide conditions, different timing of the astronomical high water and atmospheric conditions during the highest storm event would cause an enhancement of the highest water level up to about 50 cm.

The amplified water levels of the two highest events from the data set are used to analyse the effects in the Ems estuary using a high-resolution model of the German Bight. Additionally, the influence of an extreme river runoff and of sea level rise is studied. The extreme river runoff of 1200 m3 s−1 increases the highest water levels by several decimeters in the narrow upstream part of the Ems estuary. This effect diminishes downstream. The sea level rise increases the water level in the downstream part of the Ems estuary by the amount applied at the model boundary to the North Sea. In the upstream part, its influence on the water level decreases.

This study may serve as a first step towards an impact assessment for severe storm tides and their implications for coastal areas and activities.

Iris Grabemann et al.
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Short summary
Storm tides threaten the low-lying regions of the North Sea protected by dikes. Very severe storm tides with very low probabilities of occurrence could be important for coastal risk management due to their potential high impact. We searched an extensive data set of simulations and identified extreme storm tides higher than observed since 1900. We investigated how these events evolved in the near-shore areas of the Ems estuary and assessed their potential for physically plausible amplification.
Storm tides threaten the low-lying regions of the North Sea protected by dikes. Very severe...
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