Hydrologie du Grand Etang de Miquelon. Estimation des flux et renouvellements

This study aims to assess the hydrological fluxes between the ‘Grand Etang’ of Miquelon and the oceanic part. Bathymetric surveys and Doppler current meter data acquisitions were carried out from August 14th, to November 5th, 2014 and from April 1st, to end of July 2015 to allow the following estimates : Total area (227 Ha) and characteristics volumes (total : 6,7 106 m3; oscillating 2,8 106 m3 (40 %); half-tide 5,2 106 m3) Velocity Index of channel to evaluate the average current speed at cross-section channel Fluxes, on flood and ebb tide respectively : Instantaneous flow: maxima at 94,2 et 65 m3/s mean 37.2 et 24.5 m3/s. in 2014 and maxima 85,03 et 73,62 m3/s mean 26,22 et 28,20 m3/s in 2015. Circulating fluxes by tide : maxima 1,06 and 1,09 106 m3, mean 0,66 and 0,71 106 m3 in 2014 maxima 0,93 and 1,13 106 m3, mean 0,44 à 0,75 106 m3 in 2015 Renewal rate per tide is decreasing: the main channel providing less inputs from ocean waters (from 13 to 8%). This is balanced by water inputs probably dripping under the sandbank (from 1 to 22%). They seemed to be compatible with a sustainable aquaculture activity. Meteorology has an effect by the way of atmospheric pressure (negatively correlated to flood fluxes) and by wind effect (negatively correlated in 2014, and less in 2015, more on ebb fluxes). Tidal asymetry duration was reported: flood time was lower than the ebb one (4,9 h. vs 7,4 h.), which results in outflow fluxes significantly greater than inflow through the channel. Hypothesis of secondary inflow by percolation from ocean is formulated. The coastal channel seems to be in a filling phase. Grand Etang’s renewal is more and more carried out by another origin, maybe with a filtration effect, not favorable to filter feeders. Hydrodynamic modelling has been carried out, facilitating the renewal time estimates (about one day) and flushing time from 1 to 7 days.