Abstract
Abstract. Storm tides are a major hazard for the German North Sea coasts. For coastal protection and economic activities, planning information on the 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 while still remaining within physical plausibility. In the face of a limited number 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 are 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 is 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 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 influences of an extreme river runoff and of sea level rise are studied. The extreme river runoff of 1200 m3 s−1 increases the highest water levels by several decimetres 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 towards implications for coastal zone management in times of climate change.
Highlights
The North Sea (Fig. 1), lying at temperate latitudes (51 to 62◦ N), is exposed to the impact of storms mainly occurring from September to April
3.1 Extreme storm tides at the coasts of the German Bight
Following the procedure described in the previous section, events were selected for Borkum and ranked with respect to their water levels and their durations
Summary
The North Sea (Fig. 1), lying at temperate latitudes (51 to 62◦ N), is exposed to the impact of storms mainly occurring from September to April. Storm tides accompanied by severe winds represent a major natural hazard for low-lying coastal areas. Two major storm tide disasters that caused large damage at the North Sea coasts occurred in the years 1953 and 1962. The main dikes were reinforced, secondary dikes were introduced and storm surge barriers were constructed to protect coasts of adjoined rivers. Due to these measures more recent storms, e.g. in 1976 or 2013, caused no severe damage water levels higher than those of 1962 have been observed at various coastal sections (e.g. NLWKNa, 2010; NLWKNb, 2007).
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