Fortnightly Tides in a Lagoon With Variable Choking

Modulation over the spring-neap cycle of the frictional forces in a constricted (choked) channel connecting a lagoon to the open sea can induce a fortnightly-period rise and fall of sea level in the lagoon. A similar effect is also observed in the upper reaches of tidal rivers. Intuitively it might be expected that the fortnightly lagoon tide would be exactly in phase with the open-sea spring-neap cycle, with maximum frictional set-up of lagoon sea-levels at springs. A nonlinear model is used to demonstrate that this need not be the case, however, and that the fortnightly lagoon tide can lag the open-sea spring-neap cycle by up to 3 days. The model is supported by observations from an artificial lagoon in North Wales, which is subject to two widely differing choking regimes as a result of the opening or closing of one inlet by a sluice gate.