EFFECTS OF TIDES AND STORM SURGES ON NORTH-SEA WIND-WAVES

Effects of tides and storm surges on wind waves in shelf seas are assessed by hindcasting three North Sea storm cases. It is shown that tides and storm surges in shelf seas should be considered as an unsteady medium for wind wave propagation if wave-current interactions are assessed. Tides mainly result in oscillations of mean wave parameters, whereas surges result in systematic variations. Due to accumulation of effects, small wind-induced currents might have a larger impact on wave parameters than larger but oscillating tidal currents. Although these results are obtained from typical North Sea conditions, it is expected that they are fairly representative for tides and surges in shelf seas in general. For the North Sea relatively small tide and surge-induced modulations of mean wave parameters such as the significant wave height or the mean wave period have been found (typically 5% to 10%). Modulations of the spectral density of the wave energy can be of the order of 50% to 100%. Effects of current refraction appear to be negligible, whereas effects of bathymetry-induced refraction is locally distinct in extreme wind and wave conditions. Observed modulations of mean wave parameters in the storm cases considered here appear to be mainly wind induced.