Evidence for spatially variable friction from tidal amplification and asymmetry in the Pertuis Breton (France)

The semi-diurnal tides are amplified and distorted as they propagate into the Pertuis Breton, a semi-enclosed shallow basin in the north-eastern part of Bay of Biscay, in France. This paper investigates the influence of bottom friction on amplification and phase lag of the tidal constituent M2 and its overtide M4 in the Pertuis Breton. A fine resolution two-dimensional (2D) numerical model is implemented to simulate tidal propagation. The model solves the depth-averaged shallow-water equations on a finite element grid using the TELEMAC 2D software. A two-zone parameterisation of friction coefficient is introduced to evaluate the impact of smooth mud flats on the tidal asymmetry and amplification in the Pertuis. Fitting the model to observed amplitudes and phases of M2 and M4 evaluates the decrease of Chezy friction coefficient from the mud flats to the rest of the Pertuis as 100:60. This conclusion is supported by the direct estimation based on morphology and composition of sea bed in the Pertuis Breton.