Wave setup at river entrances has been re-examined, making use of numerical, analytical and physical models, in an attempt to explain the field data presented by Hanslow et al. (1996) and Hanslow et al. (1992). In these two studies, where manometer tubes were used to measure the mean water levels, the data from the Brunswick River entrance showed little or no measurable wave setup, despite the fact that the waves were often larger than 3m HRMS and the average depth of the entrance is less than 4m. These conclusions have also been supported by an analysis of tide gauge data from the same entrance, as well as from three neighbouring river entrances.
This lack of wave setup has been partly explained by considering the various mechanisms of wave height decay in river entrances and how each contributes to the variation in the mean water level. For example, dissipation from wave breaking is significantly different to that of bottom friction. Even for the same amount of energy dissipation, wave breaking will result in much more setup than bottom friction. This is due to the fact that during breaking the wave motion can no longer be considered irrotational, whereas, apart from the bottom boundary layer, the wave motion over a rough bed is irrotational.
Bottom friction is not the only dissipative process in river entrances that leaves the wave motion irrotational. Wave propagation over a porous bed and more significantly, wave propagation between rubble-mound breakwaters also fit into this category. In narrow, jettied entrances, wave decay due to energy dissipation in the side walls is often as significant as that due to breaking. As the wave propagates up the entrance, the side wall dissipation induces diffraction (wave crest curves) which in turn tends to reduce the wave setup. Again, this is a consequence of the irrotationality of the wave motion away from the rock walls. Physical model experiments were conducted to verify this conclusion.
Of all the aforementioned processes, only wave breaking is capable of raising the mean water level above the still water level (i.e. in the absence of waves). Therefore wave setup at a river entrance is dependent on the relative importance of wave breaking compared with other mechanisms such as bottom and side wall dissipation.