Wave setup

For short, the whole phenomenon is often denoted as wave setup, including both increase and decrease of mean elevation.

[3] Radiation stress is the tensor of excess horizontal-momentum fluxes due to the presence of the waves.

The horizontal momentum equations of the mean flow requires this additional wave-induced flux to be balanced: this causes a decrease in the mean water level before the waves break, called a "setdown".

The radiation stress therefore decreases after the break point, causing a free surface level increase to balance: wave setup.

Incidentally, due to this phenomenon, a small reduction in water level occurs just seaward of the breaker line, in the order of 20% of the wave set-up.

Suppose that the wave setup at this point is only 0.2 m. Then there is a water level difference of 1 m over those 500 m, so a gradient of 0.002.

A harbour with a shape like the one outlined here is usually built this way because the predominant wave direction here comes from the left.

Based on standard longshore transport calculations, no siltation is therefore expected at this port.