The Maximum Significant Wave Height in the Southern North Sea

Abstract

The maximum possible wave conditions along the Dutch coast, which seem to be dominated by the limited water depth, have been estimated in the present study with numerical simulations. Discussions with meteorologists suggest that the maximum possible sustained wind speed in North Sea conditions is between 40 and 50 m/s (roughly equal to the wind speed in hurricanes but under different meteorological conditions). The extreme wave conditions in the southern North Sea have consequently been computed for a uniform wind field with a wind speed of 50 m/so The results of a sensitivity analysis show that the results of these computations are not very sensitive for this choice of wind speed. The wave conditions in this uniform wind field (uniform 5 m storm surge assumed) are at a maximum for a wind direction of 330 N (i.e., from NNW). The significant wave height varies from 9.7 m at station BBR (near the Belgian border) and 14.2 m at station EUR. More information is available in tables and maps generated in this study. These values are approximately 25% higher than obtained with the second-generation wave model HISWA (default settings). To verify that the computed extreme wave conditions in the southern North Sea for the uniform wind field are physically realizable (even if the generating uniform wind field is not), a large number of computations has also been carried out with the second-generation wave model DOLPHIN-B for synthetic, extreme but realistic storms (800 storms in which the wind speed does not exceed 50 m/s). It was found with an additional computation with the WAYEWATCH-II model that the maximum significant wave height in the most severe of these storms (a relatively small, intense storm with a slight overshoot in wind speed to 51.8 m/s due to the incremental nature of the search procedures) are almost identical to those computed with WAYEWATCH-II model in the uniform wind field of 50 m/s wind speed. In the southern North sea (water depth less than 40 m), the ratio of significant wave height over local water depth, in the above extreme conditions is fairly constant and about 0.4. This ratio is maintained when a uniform storm surge is increased from 5 m to 6 m in the computations. The insensitivity of this ratio to variations in wind speed, wind field structure and storm surge level supports the notion that the maximum possible wave conditions in the southern North Sea are mainly controlled by the local water depth.