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The prism of the Lignano tidal inlet was approximately constant over the last forty years, although the section width has halved. This has led to questions concerning the factors that most influence the tidal prism, and on the applicability of the well-known A–P relationship. A conceptual scheme of the sea–channel–lagoon system has been used to perform a sensitivity analysis of different parameters that characterize both the basin and the inlet cross-section. A 2D hydrodynamic model has been applied to evaluate the prism and compare it to the one derived by a static method, which is the basis of the analytical derivation of the A–P linkage. Three regimes have been found in the prism variability as a function of the basin extension: a linear static regime between prism and basin area; an asymptotic regime in which the prism depends only on the basin bottom friction; and an intermediate one. In addition, the roles of the inlet and channel sizes on the prism value have been investigated. The results, compared to the empirical relationships between the prism and the inlet cross-section, show that a variation in the cross-sectional area does not always corresponds to a change in tidal prism. [ABSTRACT FROM AUTHOR]