A field survey was organized on the French Mediterranean coasts to investigate the effects of the tsunami induced by the 21 May 2003 Boumerdès-Zemmouri (Algeria) earthquake (Mw=6.9). The results show that eight harbours were affected by important sea level disturbances that caused material loss. Unfortunately, the low sampling rate of the French tide gage records (10 min) does not allow for a proper evaluation of the tsunami wave amplitudes since these amplitudes were probably underestimated in the harbours where these sensors are installed. The survey brings to light regional and local contrasts among the harbours' hydrological responses to the tsunami. To better understand these contrasts, a numerical simulation of the sea level elevations induced by the tsunami was conducted. The simulation showed a certain correlation between the field results and the wave amplification along the coast; however it underestimated the observed phenomena. Another simulation was then conducted using high resolution bathymetric grids (space step of 3 m) centred more specifically on 3 neighbouring harbours, however, again the simulation results did not match the amplitudes recorded through the observations. In order to better understand the wave amplification mechanisms inside each grid, a Gaussian signal was virtually broadcasted from the source to the harbours. Virtual sensors identified the periods which are stimulated – or not – by the arrival of the signal in each grid. Comparing these periods with those previously recorded emphasizes the proper period of each waterbody. This paper evaluates the limitations of such a study, focusing specifically on (1) the importance of having accurate and precise data about the source (the lack of information about the signal amplitude leads to an underestimation of the tsunami, thus reproducing only a fourth to a third of the observed phenomenon), (2) the need for networked tide gages with high resolution records and short sampling rates, and (3) the importance of conducting field studies immediately after a tsunami occurs.