|In many coastal seas, wind-generated waves produce a strong bottom orbital velocity which stirs up sand from the bottom. This process strongly influence sediment transport and bottom evolution.
State-of-the-art morphodynamic models traditionally represent the stirring of sand by irregular waves in a highly parametric way. For instance, the bottom orbital velocity is computed on the basis of a single representative wave with the peak period as wave period. This traditional approach does not properly account for the often complex distribution of the wave energy in the spectral domain, as revealed by the wave spectrum. The effects of this parametrisation on the bottom orbital velocity and sediment transport are unclear and motivate the current research.
In this study, a new method is applied to compute the amplitude of the bottom orbital velocity, using spectral information. This method considers a sinusoidal wave for each individual frequency and calculates its contribution to the bottom orbital velocity. The impact of the new method was investigated using a point model, field measurements and a numerical morphodynamic model. The point model computes the bottom orbital velocities for a given wave spectrum for both methods. The field measurements are combined with the point model to validate the new method. The numerical morphodynamic model asses the implications on sand transport and the formation of ebb-tidal deltas, i.e., bodies of sand that are located seaward of a tidal inlet.
The point model showed that the bottom orbital velocity using the new method was between 20% lower and 60% higher relative to the traditional method, depending on the peak period and depth. The difference arises from the frequency-dependent reduction of the bottom orbital velocity by depth. The field measurements suggest that the new method reduces the root-mean-square error between the calculated and measured bottom orbital velocity amplitude reduces by 30% compared to the traditional method. The morphodynamic model reveals that applying the spectral method leads to deeper channels and larger shoals. The computational effort of both methods were similar. The new method is recommended if the peak period is small (< ~6 s), the depth is large (> ~10 m) and/or if the wave spectrum is wide.