WAVE PHYSICS IN A TIDAL INLET Leo H. Holthuijsen 1 , Marcel Zijlema 1 and Paul J. van der Ham 1 Time scales of all processes, except diffraction, that affect waves penetrating a tidal inlet under flood conditions and propagating over the tidal flats have been computed with the third-generation spectral wave model SWAN. The processes are those of propagation (shoaling, bunching, refraction and frequency-shifting), generation (wind), dissipation (whitecapping, depth-induced breaking and bottom friction), wave-wave interactions (quadruplet and triad interactions) and work done by the radiation stresses against the currents. These time scales are of the order 100 s - 1000 s, except the time scale of the work done by the radiation stresses, which is of the order of 10,000 s and except in deeper water where the time scales are much longer (channels and the open sea side of the islands). Introduction Waves entering a tidal inlet and subsequently propagating over tidal flats, will shoal, refract and eventually break on a beach. In addition, they may receive some energy from the wind and from wave-current interactions (through work done by the radiation stresses) and they will also loose energy due to white- capping and bottom friction. These processes may be affected by tidal currents, which will also induce frequency shifting of the waves. All these processes can be represented as terms in the spectral energy (or action) balance equation of the waves. In a generic sense these terms are fairly well understood. However, their relative importance in a real situation is not well known, e.g. the time scales of the various processes have not been computed earlier. We compute these time scales, including those of propagation in geographic space (shoaling) and spectral space (bottom- and current-induced refraction and frequency-shifting), and the work done by radiation stresses against the current for a tidal inlet in the Netherlands in a flood case. The wave model The wave model that we use for the computations is the SWAN model (Booij et al, 1999). This model is formulated in terms of the action balance: dN(a,0;x,y,t) dc gx N(a,0;x,y,t) dc gy N(a,0;x,y,t) dt dx 8y dc e N(a,0;x,y,t) dc a N(a,0;x,y,t) _ S{a,6;x,y,t) 89 da a 1 Faculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, 2628CN, Delft, the Netherlands 437 Coastal Engineering 2008 Downloaded from www.worldscientific.com by KAINAN UNIVERSITY on 11/06/16. For personal use only.