Bottom friction and wind drag for wave models

Waves propagating in shallow water dissipate energy in a thin, turbulent boundary layer near the bottom. This friction can be estimated with a simple quadratic friction law scaled with an empirical coefficient. Two values of this coefficient have been recommended by previous studies (for sandy bottoms): a high value for waves in a storm and a low value for swell. We show here that, in contrast to current practise, the lower value should be used for both applications. The reason is that the high value, dating from the early 1980s, was inferred from observations in a severe storm using a relatively high wind drag. Our review of a large number of more recent observations, gives a new wind drag parameterization with lower values. With this new parameterization we infer from the same storm the lower value of the bottom friction coefficient. Using this lower value also improves the estimates of wave growth in shallow water and of low-frequency wave decay in a tidal inlet, independent of the wind drag.

► The JONSWAP formulation for the bottom friction has been re-assessed.
► An alternative parameterization of the wind drag coefficient has been introduced.
► The Texel storm of 1976 has been re-examined.
► Using the new wind drag parameterization requires using a lower bottom drag coefficient.
► The low value for bottom drag is recommended for both locally generated waves and swell.