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that strong interactions are not to be expected. Nevertheless, simple addition of both 
influences is not adequate to deal with the problem. That applies similarly to the 
simultaneous occurrence of a storm surge and a tsunami. The following figure gives an 
impression of the mutual interactions of a tide (Fig. 6.1.6 top, black) and the Helgoland signal 
from Fig. 6.1.1 (Fig. 6.1.1, bottom, blue). Mutual interactions are weak but the residual signal 
(Fig. 6.1.6 bottom, green) differs clearly from the signal that is unaffected by the tidal wave 
(Fig. 6.1.6 bottom, blue). 
An example of a coinciding tide, storm surge, and standard signal entering the North Sea 
from the north (3 positive single signals, period 1800 s, wave height 5 m) is described in 
section 7.5. 
Fig. 6.1.6: Superposition of tide and input signal (1 positive signal, period 1800 s, wave 
height 3 m, at the northern boundary of the North Sea, MAFITIN model, Lehfeldt et 
al. 2007). 
6.1.6 Boundary conditions 
If the tsunami generation process itself has not been modelled, a way to include it is by 
prescribing initial conditions for the surface elevation (Buch et al. 2005 and Kerridge 2005, 
run H). If the source of the tsunami is outside the model area, the tsunami has to be defined 
as a signal entering the model area. This involves two types of problem. The signal must be 
physically plausible, i.e. it must adequately represent the signal arriving from a potential 
source region, and developments at the boundary must be modelled in a numerically 
adequate way. 
In simulations of hypothetical tsunami different kind of boundary conditions have been 
cohosen. The BAW simulations (MARTIN model, 2D, finite elements) in section 6.1.5 are 
based on a positive analytical signal entering the North Sea. In the simulations described in 
the DMI report (Buch et al. 2005, model MOG2D, finite element method), the problem has 
been solved with respect to the North Sea by locally using a finer grid resolution. Winter 
(Winter 2005) used the simulation of a tsunami triggered by the Storegga slide (Bondevik et 
al. 2005) to obtain boundary values for computations of wave propagation into the North Sea. 
Similarly, in the UK report (Kerridge 2005), a tsunami simulation assuming a potential 
submarine slide (run H) using the fine grid (N10, 2D, horizontal resolution 3.5 km) is driven 
by boundary values from a coarser model run (NEA, 2D, horizontal resolution 35 km). In 
further simulations, a wave from southerly direction is prescribed at the boundary of NEA,