45 
North Sea 10 km: wave train from the north (H5 m, T1800 s, Fig. 7.5.1, Fig. 6.1.3-5) 
North Sea 10 km: wave train from the north (H 5 m, T 1800 s) interacting with tide and 
extreme storm surge (Fig. 7.5.1-7.5.4) 
7.1 Boundary conditions and input signals 
Each of the wave trains entering the model areas consisted of three successive positive 
single waves. In the computations in sections 7.2 to 7.4, they have been prescribed at the 
appropriate boundary via velocity u ext (cf. section 6.1.6). With respect to the North-East 
Atlantic model, Fig. 7.1.1 (left) shows the temporal evolution of the velocity (light blue curve) 
for a water depth of 2000 m, the selected period of 1800 s, and a wave height of 3 m. That 
corresponds to a positive surface elevation input signal that is independent of depth (Fig. 
7.1.1, right). This is not a realistic formulation, but it allows a straightforward interpretation of 
the propagation and modification of the boundary signal as it travels across the North-East 
Atlantic. 
Fig. 7.1.1: Input signal North Atlantic and North Sea. Left:u = H /h sin 2 (27'/2/r i). right: 
r/ - uh / .yfgh . 
Because of the arbitrarily chosen boundary condition for the North-East Atlantic model and 
its inability to reproduce all features modifying a tsunami in deep water and at the continental 
slope, results from this model were not used as boundary condition for the North Sea model. 
Therefore, an input signal defined in the same manner as that in the North-East Atlantic 
model was prescribed for the North Sea but generally with a standard wave height of 5 m 
(Fig. 7.1.1 in 200 m of water, dark blue curve). In comparative computations, the wave height 
of the input signal was increased to 6 m, 7 m, and 8 m, with an identical period. Fig. 7.1.2 
shows the water level evolution at a point close to the boundary (Bergen), with the time 
related to first entry of the signal into the model area, i.e. Bergen was reached after just a few 
minutes. 
Although higher water levels in the Storegga event were reconstructed for the Shetland 
Islands (Fig. 3.3.2), 5 m appears to be an adequate wave height for the boundary signal 
because such high water levels are not unlikely to result from secondary processes. Using 
the standard signal, water levels exceeding 10 m were simulated, e.g. in Lerwick (Fig. 7.1.3).