The flushing time in Mururoa lagoon, a coral island in Polynesia previously used for nuclear tests, determined from a numerical model. Mururoa is a coral reef that surrounds a lagoon. Access to the lagoon is through a very narrow channel. Water also enters the lagoon at high tide across depressions in the reef known as hoa. The first figure in this frame shows the lagoon topography. |
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The model takes into account tides and wind driven water movement and includes flow over the coral reef as well as through the access channel. The figure on the left shows the circulation in the form of streamlines along which the water circulates. The zero streamline separates anti-clockwise circulation near the channel from clockwise circulation in the lagoon. Notice that most of the lagoon circulation is closed, so exchange with the ocean can only occur through turbulent diffusion across streamlines. The figure on the right shows the water residence time or flushing time in days. Most of the lagoon is flushed within less than 100 days, but there is a less well flushed region in the east where the flushing time exceeds 140 days. Adapted from Tartinville et al. (1997). |
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