Short-term morphodynamics of a submarine dune bank in a macrotidal environment: observations from the Creïzic bank (South Brittany, France)

International audienceMarine sand dunes and sand banks can exhibit rapid morphological changes in coastal areas, which may posea hazard for navigation in shallow water or generally in areas near marine infrastructures. Understanding theirdynamics is still a scientific challenge, especially their migration processes and its consequences on the seabedtopography and on sediment transport. In this study, the morphodynamics of a submarine sandbank in a macrotidalenvironment is investigated at short time scale. The aim is to quantify the morphological changes and migrationof the bank and overlying dunes (primary and secondary sedimentary structures) in order to estimate sedimenttransport corresponding to bedform migration. The study site is the Creïzic submarine dune bank, located in theGulf of Morbihan (South Brittany, France). The Creïzic bank is about 1200 m long, 600 m wide with dune heightsfrom 0.1 m to 3.5 m. It is situated in water depths between 5 m and 20 m and is mainly composed of shellyand coarse sand. The main hydrodynamic forcing factor at play on this sand bank is a strong and unsteady tidalcurrent. During storm events, waves, mostly from fetch, also have an impact as the bank is shallow. The bankfeatures three main sedimentary structures: a central dune field at the middle of the bank and two dune fields oneither side of the bank. Former studies at yearly time scales have shown that the central area is highly mobilewith significant net migration of the dunes. To investigate the morphodynamics of the Creïzic bank, a 37-day fieldcampaign has been carried out in the fall of 2017. A high-resolution (50 cm grid size) bathymetric dataset has beenacquired using a mulitibeam echosounder (MBES), with repeated surveys at the time scales of a month (entirebank), a tidal cycle and hour scales (portions and patches on the bank). From this data, velocities and directionsof migration are estimated by computing differential Digital Terrain Models (DTM), and quantified with a spatialcross-correlation algorithm. The preliminary analysis of the data set confirms the high mobility of the primaryand secondary sedimentary structures at short time scales, with migration distances up to 1 m over one hour forbedforms with metric heights and wavelengths of the order of 30 m in the central zone, and reaching 50 cm overtwo hours for dunes with heights between 0.2 m and 0. 5 m and wavelengths between 2 m and 4 m on the south-eastside of the bank