Author: Nuno Sergio Marques Antunes
The periodical rise and fall of oceanic waters in coastal areas - the tides - is a phenomenon that has been observed for centuries. However, their practical effects concerned only seamen and those whose life was somehow directly related to the sea. The relevance of the tidal phenomenon in the international law of the sea only emerged recently, alongside other developments during the twentieth century, namely the appropriation of vast oceanic areas by states and the need to define the spatial limits of their jurisdiction at sea.
The existence of tides and their effects were addressed in both the 1958 Convention on the Territorial Sea/Contiguous Zone and the 1982 United Nations Convention on the Law of the Sea (UNCLOS). Both conventions refer to concepts such as "low-water line", "low-tide elevations" and "high tide". These concepts play an important part in establishing the limits of state jurisdiction over maritime zones. In turn, the accurate definition of these lines and features is dependent upon the tidal datum adopted in a particular area.
The main aim of this Briefing is to discuss the significance of the tidal datum in the unilateral definition of maritime limits and in bilateral maritime boundary delimitation.The author first provides some elementary notions related to tides. The concept of tidal datum is then introduced, and its relationship with nautical charts examined. The relevant provisions of UNCLOS are analysed, with special emphasis placed upon the importance of tidal datums to the interpretation of these provisions. Finally, examples of state practice and jurisprudence are examined in order to determine whether there is a "more suitable" or legally binding tidal datum.
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