The Dynamical Balance of Flow in the Menai Strait

Observations of sea surface slope and along-channel current from two experiments are used to explore the dynamical balance in the tidally energetic Menai Strait. A regression analysis is used to deduce a new value for the sea-bed frictional drag coefficient which achieves a close balance of the forces involved. The regression results are consistent between the two experiments and highly significant (over 90% of the variance is explained), and indicate that the friction coefficient for the Menai Strait is significantly larger than that usually employed for shelf sea flows but consistent with values of the Chézy coefficient for open channel flow. The tidal cycle of along-channel flow is highly distorted as a result of shallow water (frictional) effects and there is a net transport of water to the south-west of the order of 25–30 million tonnes per tidal cycle. The non-linear (advective) acceleration term in the dynamical balance is shown to be small (<5%) compared to the leading (surface slope) term.