SWAN’s underestimation of long wave penetration into coastal systems

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Jacco Groeneweg, Joana van Nieuwkoop

Thursday 2 july 2015

14:55 - 15:15h at Antarctica (level 0)

Themes: IAHR/COPRI Symposium on Long Waves and Relevant Extremes

Parallel session: 12D. COPRI Symposium: Long waves and relevant extremes

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One of the unresolved issues in the spectral wave model SWAN is that wave penetration of swell waves into complex tidal inlets or estuaries is often underestimated by SWAN. Previous studies into this problem revealed that two-dimensional nonlinear interactions, and especially the sub-harmonic interactions could play a major role in the transmission of energy from flats into navigation channels. Since this process is missing in SWAN, this process can explain SWAN’s underprediction of wave energy penetration into a complex tidal inlet system. With this mechanism in mind, it was examined in this study under which conditions SWAN underestimates the wave energy when propagating into complex tidal inlet systems. For this purpose, various existing hindcasts in complex tidal areas have been reanalysed. It was concluded that a mismatch between measured and computed swell energy is often already observed in the tidal inlet for conditions where the ebb-tidal delta is of influence. Whether or not the low-frequency wave energy is underestimated by SWAN is to a large extent determined by the local geometry. For a number of cases it was clearly seen that 2D nonlinear interaction plays an important role. In those cases the waves first propagated over the ebb-tidal delta or tidal flats and subsequently came across a channel. From 2D wave spectra it was seen that the observed waves could enter or cross the channel, whereas the computed waves were refractively trapped to the channel edges.