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8 Conclusions 
The population living on the German North Sea coast is familiar with the changing tides and 
the occurrence of storm surges with high water levels (4-5 m above NN, German ordnance 
datum). Observing the weather, they know when to expect a storm surge. External surges 
generated outside the North Sea are either insignificant or, in the majority of cases, they are 
part of a storm surge event. Storm surges can be predicted with high accuracy. The 
phenomenon of waves "coming out of the blue", like the tsunami of December 2004, has 
raised concerns among the German population. The BSH, as the authority issuing the official 
German water level predictions, felt compelled to investigate the occurrence of historical 
tsunami in the North Atlantic Ocean, and to study the special characteristics of tsunami 
propagating in wide, shallow shelf seas like the North Sea. 
Different events might conceivably cause a tsunami in the North Sea. The risk of a volcanic 
eruption and its consequences is low or non-existent in the North Sea. Meteoritic impacts in 
the North Sea are relatively unlikely. Tsunami research has seriously considered the 
consequences of minor meteoritic impacts in the North-East Atlantic. Near-earth asteroids 
are subject to continuous monitoring, however, and are planned to be either deflected or 
destroyed if they pose a real threat. 
There exists historical and geological evidence for the impacts of an earthquake off Lisbon 
and of a slope failure at Storegga off the Norwegian coast. Tsunami whose primary cause is 
an earthquake pose only a low threat to the German North Sea coast. A more realistic 
scenario is an earthquake of low magnitude triggering a major slope failure in the North-East 
Atlantic. It is not possible to estimate the probability of occurrence of slope failures 
attributable either to this or to another cause. The most recent slope failure that had an 
impact on the North Sea was an event off Newfoundland in 1929. However, no evidence for 
a tsunami has been found in historical records of water levels at Cuxhaven. 
The BSH is operating a model system consisting of non-linear, hydrostatic models. It 
comprises two three-dimensional baroclinic models and one two-dimensional barotropic 
model for the North and Baltic Seas, as well as a two-dimensional barotropic model for the 
North-East Atlantic Ocean. The question as to whether this modelling system, which is used 
operationally for storm surge predictions at the BSH, is suitable for simulating waves 
comparable to a tsunami has been discussed. Tsunami have frequencies and wave lengths 
ranging between those of sea/swell and tides/storm surges. There is no model that is equally 
suited to all uses, but each model is based on reasonable assumptions for the particular 
process to be simulated. Wave models and tide/storm surge models differ fundamentally in 
this respect. Neither of these models is suitable for tsunami simulation without prior 
modification. Nevertheless, only minor modifications were made to existing models at the 
BSH and other institutions before running simulations of the propagation of tsunami-type 
signals in the North-East Atlantic and North Sea. Two-dimensional barotropic models were 
used almost exclusively. The North Sea model (North Sea 2 km) used for the simulations in 
section 7 is a barotropic non-linear hydrostatic model and is thus suitable for computing the 
propagation, modification, and attenuation of tsunami with periods of 1800 s or more on the 
shelf. The North-East Atlantic model of the BSH was found to have limited suitability for 
simulating the propagation of medium-length waves in the deep ocean and their modification 
on the continental shelf. The BSH models in their current configuration are not suitable to 
simulate near-shore processes. 
According to the model simulations, the prescribed signal travels about seven hours from the 
northern boundary of the North Sea (e.g. Shetland Islands) to Esbjerg, and about nine hours 
to Cuxhaven. A secondary signal that forms in the North Sea, mainly due to diffraction and 
reflection on coastal features, is higher in Cuxhaven than the primary signal and arrives a 
few hours later. 
The standard signal used in the simulations (3 positive single waves, T 1,800 s, H5m, from 
the north) leads to a water level of 0.5 m in Cuxhaven for the primary signal, and 1 m for the