Numerical Modelling of the Tsunami Generated by the Second Storegga Slide

Detailed research on this subject has recently been undertaken by Harbitz (1991, 1992) who has developed a mathematical model for the numerical simulation of water waves generated by the Second Storegga Slide. Harbitz has shown that the likely tsunami run-up values are greatly dependent on the average velocity of the landslide as well as the shear stress at the interface between the water and the slide body. He has shown that a landslide moving at an average velocity of 35 ms-1 would have produced average flood run-up values of between +3 and +5m along the eastern coast of Greenland, Iceland, Scotland and the western coast of Norway. By contrast, a landslide moving with a velocity of 50ms-1 is likely to have produced runup at the coast in excess of +20m. This value for flood runup is considerably higher than the empirically measured values of tsunami runup along the northern coastline of Scotland. Owing to the good correspondence between the measured runup values and the predicted values for a 35ms-1 slide velocity for this area, the latter landslide velocity is considered to be realistic.

Harbitz concluded that there is likely to have been a very marked initial drawdown of water (possibly in excess of -10m) prior to the arrival of the first major tsunami wave (possibly circa 10m high) along the west Norwegian coast. Tsunami runup of up to +10m (equivalent to a water level rise of +20m) follows within the space of two hours. The data of Harbitz suggest that the Second Storegga Slide was associated with two major tsunami waves as well as several minor water level fluctuations. More recently, Henry and Murty (1992) have developed a different numerical model of the Storegga Slide and have derived similar tsunami run-up values to those of Harbitz. These data suggest that there is a broad compatibility between the preliminary results of the landslide-generated tsunami models and the empirical results on tsunami run-up for the Scottish coast as suggested by Dawson et al. (1988). These results should be treated with caution, however, since, the run-up values take no account of the influence of nearshore bathymetry on regional variations in former tsunami run-up. Nor do they include the effect of tidal variations during the period of tsunami propagation. In theory, this effect could add or subtract circa 3m to the tsunami runup above the position of mean sea level on the day that the tsunami took place.

Despite the sophistication of the numerical models there are certain limitations that affect the mathematical calculation of runup. A feature of the tsunami generated in the numerical model of Harbitz (1992) is the high value (circa 200 km) for the tsunami wavelength. This means that, owing to scale differences, the model treats the coastline as almost a vertical wall against which runup takes place. In order to compensate for this inaccuracy, Harbitz defined a series of locations offshore yet close to the coast in circa 70m water depth where the waves were forced to deform over a hypothetically evenly sloping seabed surface (Table 2).

Table 2 Tsunami height offshore, nearshore seabed slope and runup, Second Storegga Slide (after Harbitz, 1992).

Locations	Site Number	Water Depth (m)	Nearshore Seabed Slope (degrees)	Tsunami Height Offshore (m)	Minimum Runup
Northern Scotland	6	61	0.13	1.2	3.7
Eastern Scotland	7	72	0.05	3.3	-
Eastern Scotland	8	76	0.23	2.5	3.3

Thus it was possible to calculate tsunami height offshore (Table 2). The chosen water depths are sufficiently shallow and imply that, owing to the large wavelengths involved, the waves have already been significantly deformed before being forced to move over an inclined seabed surface. The combined effects of these computations means that whereas the calculated values for tsunami height offshore are reasonably accurate, limited reliability can be placed on the computations of runup at the coast.

Time series analysis of tsunami height at the three offshore sites are shown in Figure 2 where one can observe that the average duration of the most dramatic water level oscillations is in the order of 6 hours.

A weakness of the Harbitz model of the Second Storegga tsunami is that it does not make any correction for tidal changes that may have occurred during the progress of the tsunami. The inshore transformation of the tsunami hydrodynamics described by Harbitz is as realistic as present technology allows. To date there have been no nearshore simulations of the Storegga tsunami for any specific coastal area nor for the North Sea region. This is a research priority.