INFRAGRAVITY PERIOD OSCILLATIONS IN A CHANNEL HARBOR NEAR A RIVER MOUTH

Florian Bellafont,Gaã«L André,Stéphane Abadie,Volker Roeber,Denis Morichon

doi:10.9753/icce.v36.papers.8

Abstract

Port of Bayonne, located in SW France, is a channel harbor situated near the river mouth of the Adour. Long-period oscillations have repeatedly caused snapping of mooring lines of berthed ships and have led to wave resonances in an adjacent marina (seiche). To investigate mechanisms for generation of theses oscillations, a field campaign was carried out during a one-year return-period storm (Hs = 6 m and Tp = 15 s): four pressure sensors were deployed inside the port. To complement the data and to better understand the governing processes that lead to the wave transformations in Port of Bayonne, the storm event was computed with the Boussinesq-type model, BOSZ. The data confirm the model results, which show generation of long infragravity (IG) waves by the incident swell around the harbor entrance and free propagation of these waves without amplification over far distances inside Port of Bayonne. Excited by these long waves, resonance oscillations are only noticeable in a small enclosed marina. Though the IG-waves are not causing substantial changes to the water level along the harbor channel, they are suspected to excite the shipsâ€™ eigen modes, which consequently results in mooring problems.

Highlights

Harbor entrances are usually designed for protection against wind and swell waves with periods ranging between 5 s and 20 s
To study the waves transformation during the propagation in the river, power spectral densities (PSD) at the pressure sensors are presented in figure 4 (18h of data)
A significant part of the short waves energy is dissipated by the breakwaters: despite an Hs greater than 5 m at the wave buoy, HsS W is equal to 1 m at the Port mouth (PS0)

Summary

Introduction

Harbor entrances are usually designed for protection against wind and swell waves with periods ranging between 5 s and 20 s. Many harbors experience agitation problems caused by long waves with periods longer than 30 s. A harbor is a semi-enclosed water basin located on the coast and an oscillating system. With energetic external forcing, a standing wave, associated with vertical oscillations and strong currents, develops: this phenomenon is called coastal seiche (Rabinovich, 2009). A stationary system of coastal seiche can hardly develop in river ports, because there is no upstream boundary on which a wave can be reflected. Infragravity waves (IG), when entering the river, can excite the system constituted by the ship at dock and its mooring lines (Van Der Molen et al, 2006). The temporal variation of the breaking zone causes a time-variation of the wave set-up and generates free long waves (Symonds et al, 1982)

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