Dynamical modeling of underwater visibility

Among the various missions assigned to the SHOM and responding to the French Navy and public administration requests (involved in environmental affairs), one of the recent objectives is to ensure better knowledge of the continental maritime area. Among coastal projects, the one we deal in this study consists in evaluating the optical properties of shelf waters. This project called "Turbidity" focuses more particularly on turbidity processes having an impact on submarine visibility. This project is structured around three topics: (1) spatial and in-situ observations of the turbidity, (2) acoustic measurements for detection of particles loads and (3) a modelling approach. The modelling theme, presented here, involves development of simulation tools coupling hydrodynamics (the physical frame is provided by the MARS3D model developed by Ifremer), biological and sedimentary dynamics (provided by the model SiAM3D developed by Ifremer). An optical model completes this tool, in order to convert the turbidity parameters in terms of submarine visibility. The actual configuration of the model simulates the turbid waters above the French Atlantic continental shelf. The project includes also an adaptation of this framework to the Persian gulf area. The model takes into account the mineral and organic (living and non-living) particles which influence the optical properties of water. The biological production follows specific conditions of light, nutrients inputs and ocean dynamics. And the mineral particles are coming from river discharges and exchanges fluxes with the sediment (erosion and deposit). The complexity of the processes requires a modelling approach. At this complexity of particles origin, adds the complexity of the circulation of waters masses and particulate transport in coastal area (dynamic of freshwater plumes, gradients of water's density, influence of mixing by winds, tides and waves, and interactions of these processes with coastline and bathymetry). Consistent with the observations, the model reproduces reasonably well the main algal and mineral seasonal structures. The two mains axes planned to improve quantitative assessment are (1) the modelling of particulate structures and (2) the optical model: (1) modelling presents encouraging results but is actually limited by a coarse spatial resolution of the model, by simplified forcing fluxes (wave, wind and sunshine) and by simplified hydro-sedimentary and biological schemes; (2) the optical module is based on empirical laws coming from literature and is limited to a monochromatic approach; however, in-situ measurements cruises are planned in order to be suited to the specific characteristics of the study areas. The difficulty of this feasibility study is based on the fact that neither in-situ measurements, nor model estimations are directly linked to the visibility parameters. In-situ, visibility is assessed with the measurement of inherent optical properties. With the model, it is deduced from the particles concentrations. Moreover, the biological and sedimentary transport models are not able to reproduce the whole nature and multitude of particles and molecules influencing the optical properties. All this makes the visibility distances difficult to assess.