Numerical Solution for Trapped Modes around Inclined Venice Gates

Abstract

For protection against storm tides, four mobile barriers, each of which consists of 20 gates hinged at the bottom axis, have been proposed to span the three inlets of the Venice lagoon. In stormy weather these gates are raised from their housing to an inclination of 50° from the horizon, so as to act as a dam and to keep the water-level difference up to 2 m across the barrier. The gates were originally expected to swing in unison in response to the normally incident waves, but subsequent laboratory experiments revealed that the neighboring gates can oscillate out of phase in a variety of ways and affect the intended efficiency. In this paper we extend the linear theory of Mei, Sammarco, Chan, and Procaccini for trapped waves around vertical rectangular gates and examine the inclined gates by using the hybrid finite-element method to account for the prototype geometry of the gates, the local bathymetry, and the intended sea-level differences. Finite elements are employed only in the immediate neighborhood of the gate, while formal analytical representations are used away from it. The factors affecting the trapped wave period are studied, and the results are compared with existing laboratory experiments by Delft Hydraulics Laboratory.