Classical vs. holistic timescales: the Mururoa atoll lagoon case study

Deleersnijder, Eric [UCL]

A priori timescale estimates are commonly used to build dimensionless parameters allowing one to compare the order of magnitude of the terms of the governing equations of eco-hydrodynamics without having detailed information on their solutions. By contrast, over the past two decades, timescales obtained at every time and position as the solutions of partial differential problems have been developed to diagnose geophysical and environmental fluid flows and the related reactive processes. Such timescales are radically different from the previous ones. They are holistic in that they take into account all the processes under consideration. By revisiting the results of previous numerical studies of the Mururoa atoll lagoon it is seen that timescales of the previous type cannot adequately predict the rate at which the water of this semi-enclosed domain is renewed and, hence, are of little use to evaluate the flux of radionuclides toward the Pacific. On the other hand, the lagoon averaged-residence time, if evaluated in an appropriate manner (i.e. from the results of a three-dimensional model rather than by means of the tidal prism), is instrumental in building a reduced-dimension model capable of predicting the fate of radionuclides in the aforementioned domain of interest. These findings are complemented by the results of a one-dimensional transport model admitting analytical solutions, supporting the relevance of holistic timescales for understanding transport processes.