The bay of Brest is a complex estuarine system subject to strong tidal environment. This vast tidal system has a surface area of 181 km2 and drains a catchment area of 2800 km2. It is open to the ocean through a narrow gully one mile wide, through which no less than 700 million m3 of seawater, i.e. nearly one third of the total volume of the harbour, passes during each tidal cycle.

The request for a VIRAB mission is driven by the questions related to axis 1 of the LGS concerning the sedimentary record and the Source to Sink balance at the scale of the margins. In the study of the Continent-Ocean continuum, platforms and coastal areas are privileged sites for the storage and redistribution of sedimentary material towards deeper domains. Between 2013 and 2016, the Rade de Brest was the subject of numerous studies to understand the evolution of its physical environment since the last rise in sea level. The geometry of the sedimentary deposits and their chronology (Gregoire et al, 2017) was clarified (SERABEQ surveys) as well as the sedimentary dynamics of current deposits (Gregoire et al, 2016) and sedimentation rates (Ehrhold et al, 2016). Although they have been described in the past as mixed silted sedimentary zones (Fig. 1, Gregoire et al., 2016), their often heterogeneous nature and the proportions of the different classes is difficult to understand, due to a lack of continuous observations of these environments.

More recent work (PEPITE project) shows that these shallow waters have historically been subjected to hydrodynamic conditions that completely transform the installed muddy sedimentary sequences into drapes of coarse sediments (Ehrhold et al., 2021).

The objectives of the survey :

1. The aim is to finalise the development of a new video camera system towed over the bottom (MICASED). A test day was devoted to this from the Albert Lucas on 5 November 2020 (fig. 2). The first day of acquisition of VIRAB1 will be used to make the final adjustments and the first acquisitions on the desired profiles. MICASED ((MIni CAmera System for SEDimentology, fig. 2) is a small, light video frame (< 15 kg) with 2 HD cameras (front with on-board image return and vertical), a combination of lasers to size objects on the bottom and a compass to orientate the sedimentary and biological figures.
   
2. The nature of the seabed on the terraces which occupy 40% of the surface area of the harbour (less than 10 m) is poorly known. The work carried out a few years ago (Gregoire et al., 2016) to arrive at the most realistic representation of the diversity of the shallow waters, in terms of bio-sedimentary heterogeneity and the sedimentary dynamics processes in relation to the colonisation of the maerl and crepidula biocenoses, needs to be consolidated by acquiring a denser and updated data set. They will allow the synthesis carried out in 2016 to be revisited in order to establish a more robust knowledge base of the sedimentary diversity of the Rade and to better characterise certain habitats. High-quality video observation will also make it possible to distinguish the presence of pebbles or small boulders that are difficult or impossible to remove by the skips and which are elements sought in the framework of the MaSCoET project, for example.