Linear response of Venice storm gates to incident waves

Abstract

In laboratory experiments, it is known that the mobile storm barriers designed for Venice inlets can be excited subharmonically by incident waves to oscillate in modes that can reduce the efficiency of protection against storm tides. Based on the idealization that a barrier is composed of many gates across the middle of an infinitely long channel, resonance of the articulated structure has been shown to originate from the existence of trapped modes, and is possible only via a nonlinear mechanism. In reality, the inlet geometry is different in that two parallel jetties are present only on the sea side; the barrier faces a large open space in the lagoon. Here we give a linear scattering theory for such a geometry. An energy identity is derived to provide a global check for numerical computations. Gate responses are first carried out for a wide range of incident wave frequencies. Resonances are found at frequencies close to the natural frequencies predicted recently by Li and Mei (Li & Mei 2003 Appl. Ocean Res. 25 , 115–126). Responses to random incident waves described by JONSWAP spectrum are discussed.