Abstract:
This fellowship aimed at studying and modeling the tridimensional hydrodynamic circulation in the case of a wavedominated
dissipative beach. The studied site corresponds to Saint-Trojan Beach, located to the South-West of
Oléron Island in the central part of the French Atlantic.
Two instruments were deployed to characterize hydrodynamics in the surf zone: with an ADCP courantometer and a
RBR pressure gauge.
The modeling system used in this study is based on the coupling of the circulation model SELFE and the spectral
wave model WWM-II.
Model/data comparison revealed that the modeling system was capable of reproducing water levels and waves with a
high accuracy compared to previously published studies.
Different theories for vertical radiation stress were compared in terms of their capacity to reproduce the circulation in
the surfzone.
The comparison with the measurements showed that only the Mellor (2003) theory was able to reproduce the
undertow in the surf zone. Besides, setup predictions in 2DH and 3D were compared with measured setup. This
comparison comparisons revealed indicated that the setup predicted with the 3D approach was locally twice as large
as the 2DH setup while fitting much better with the data. This interesting behavior is explained by the presence of
strong undertows, which induce shoreward oriented bottom stress.
Furthermore, the comparison of the relevant terms in the momentum equations showed that the bottom stress
represents about 50 % of the wave-induced radiation stress gradients, which supports our hypothesis.
This study confirmed the necessity of using fully coupled wave and current 3D models to properly account for the
vertical current structure of currents, including undertows, which has implications on water levels along the
shorelines during storms.
