Utilisation des traceurs pour les études d'implantation des émissaires en mer

Use of tracers for designing and locating sea outfalls

¹ Chef du Service Scientifique de SOGREAH
² Ingénieur au Service Scientifique de SOGREAH

Abstract

A thorough knowledge of hydrodynamics in the outfall area is necessary in order to estimate within a few hours the extension of polluted water discharged into the sea. Several aspects must be distinguished : turbulent diffusion, convection, differential convection due to variables of diverse origin: topography wind. The sea measurements for assessing these factors are taken with the help of "tracers" which can be radioactive, chemical (Rhoda mine B), or constituted drag floats. Taking into account the problematical character of the phenomena being studied, the measurements must be numerous and simultaneous, implying a well adapted technology. Floats are a simple and economical means of measuring trajectories at shallow depth. They must, however, be precision designed and must not offer any resistance to the wind. Unfortunately, they are not easily seen and a small aero plane is useful for locating them in the sea. The long life of floats is an advantage for long-term measurements. Rhoda mine B is used for measuring turbulent diffusion in the sea, with in particular, the taking of aerial photographs to follow the progress of the colored stain. Rhoda mine B's optical characteristics show that it would not be easily seen at a depth of a few meters. It is demonstrated (equations 1 and 2) that simulation of a steady outfall would be difficult, as it is unrealistic to try to obtain steady flow conditions. On the other hand, instantaneous point injections (Dirac Function) are a rapid and efficient solution. It must be emphasized that propeller-driven boats should not be allowed to pass through the stain, as these would create much greater turbulence than that which has to be measured. Rhoda mine samples are necessary in order to assess vertical diffusion. With aerial photographs, it is possible to assess transverse eddy diffusivity, using formula 4. However, taking account of the fact that the photograph records the Rhoda mine down to a certain depth, but with a weighting due to the absorption of light rays, a correction factor must be taken into consideration (equations 5 and 6). Equation 6 results from an adjustment by the method of least squares. It is not universal and can vary according to the optical properties of the seawater.