Key concepts

• In this book, coastal waters are waters that are shallow enough to affect the waves, adjacent to a coast, possibly with (small) islands, headlands, tidal flats, reefs, estuaries, harbours or other features, with time-varying water levels and ambient currents (induced by tides, storm surges or river discharge).

• Under certain idealised conditions (constant wind blowing perpendicularly off a long and straight coastline, over shallow water with a constant depth), the significant wave height is determined by the wind speed, the distance to the upwind coastline (fetch), the time elapsed since the wind started to blow (duration) and the depth. So are the significant wave period and the energy density spectrum.

• Under these idealised conditions, the spectrum has a universal shape: the TMA spectrum, which is a generalised version of the JONSWAP spectrum (see Chapter 6). The directional width of this spectrum seems to be the same as in deep water (30°, one-sided width).

• Under more realistic, arbitrary coastal-water conditions, the spectral energy balance of the waves is used to compute the wave conditions. This shallow-water version of the energy balance is conceptually a straightforward extension of the energy balance in oceanic waters (see Chapter 6). It represents the time evolution of the wave spectrum, based on the propagation, generation, wave-wave interactions and dissipation of all spectral wave components individually.

• As in oceanic waters, an Eulerian representation (based on a computational grid projected onto the coastal region) should be used for computations with the spectral energy balance.

• Ambient currents can be accounted for by replacing the energy density with the action density (i.e., the energy density divided by the relative frequency) in the energy-balance equation and taking some other relatively simple (conceptually) measures.

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