Influence of offshore topography on infragravity period oscillations in Two Rocks Marina, Western Australia Darshani T. Thotagamuwage ⁎, Charitha B. Pattiaratchi School of Civil, Environmental and Mining Engineering & UWA Oceans Institute, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia abstract article info Article history: Received 18 November 2013 Received in revised form 7 May 2014 Accepted 8 May 2014 Available online xxxx Keywords: Infragravity waves Seiching Topography Modal structure Two Rocks Marina Western Australia Infragravity (IG) period oscillations in harbours and marinas can often lead to interruption in harbour operations due to excessive vessel movements. Field measurements in Two Rocks Marina in south‐west Australia have shown that IG period oscillations were always present and the amplitude of the oscillations was related to inci- dent swell climate and was enhanced during storm events. The marina is fronted by two shallow, shore-parallel, reef systems located ~3.2 and ~4.7 km from the shoreline. The area experiences continuous swell and frequent storm systems, particularly during winter months. This paper describes the application of a Boussinesq wave model, validated using field data, to examine: (1) source of the IG waves incident on the marina; and (2) modal characteristics of the IG period oscillations inside the marina. The cross-shore evolution of the IG wave en- ergy was examined using simulations with different contrasting incident wave conditions, which included mea- sured and idealised wave spectra. The model results indicated that free IG waves were generated as the wind/ swell waves propagated over the offshore reef systems independent of the external forcing. During stormy sea condition, the IG energy over the primary and secondary reefs increased by a factor ~ 10 and ~ 8 respectively, com- pared to the IG energy at offshore. The IG wave spectrum near the marina entrance did not contain any major energy peaks, and has an almost constant energy distribution across the IG wave frequencies. However, the fre- quencies similar to the marina’s natural oscillation periods were excited within the marina. The predicted energy distribution maps and water level snapshots inside the marina identified different oscillation modes, which in- cluded mode 1 and mode 2 oscillations corresponding to a partially enclosed water body and, zeroth mode cor- responding to an open-ended water body. This study showed that in coastal regions characterised by complex offshore topography, IG waves are generated independent of offshore wave conditions, and harbours located in such environment are at risk of IG period oscillations, depending on their geometry. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Many ports and harbours have been designed for protection against wind-generated short period waves with periods between 3 and 25 s. Long breakwaters are able to prevent these shorter period waves from entering a port or a harbour (Demirbilek, 2007; Van der Molen et al., 2004). However, long waves with periods of 25 to 300 s, also defined as infragravity (IG) waves, can cause disturbances in harbours and ma- rinas because of their diffraction through entrance and resonance prop- erties inside harbours (Mei and Agnon, 1989; Rabinovich, 2009). Natural oscillation period (NOP), or natural frequency, is a funda- mental property of a basin which depends on the basin’s geometry (Pugh, 1987). When the period of incident long waves is close to one of the natural frequencies of oscillation in the harbour, higher amplitude oscillations can be generated inside the harbour through resonance phenomenon, even if the incident long wave amplitude is small. In such conditions, berthing operations can become unsafe and be interrupted due to excessive vessel movements causing damage to mooring lines and fenders, resulting in harbour downtimes and eco- nomic losses (McComb et al., 2005; Rabinovich, 2009; Uzaki et al., 2010). Several studies have been undertaken to determine forcing mecha- nisms responsible for inducing long period oscillations in harbours. Wind waves propagate as well-defined groups, from deeper water to water depths less than a few metres deep (Van Rijn, 1990). Longuet- Higgins and Stewart (1964) described a mechanism of ‘set-down be- neath wave groups’ which produce ‘bound infragravity waves’ associat- ed with wave groups. As waves approach shallow water, the quadratic nonlinear interactions approach resonance, and in water depths of the order of few metres, significant amount of wave energy can be trans- ferred from the wind waves to the IG waves (Bowers, 1977; De Girolamo, 1996; Elgar and Guza, 1985; Mei and Agnon, 1989). This im- plies that IG wave energy is generally low in deep water and increases where the depth decreases such as near offshore reefs and at the shoreline. Hydrodynamic studies, using both field and numerical approaches, in the nearshore region have provided information on spectral Coastal Engineering 91 (2014) 220–230 ⁎ Corresponding author. Tel.: +61 8 6488 8121; fax: +61 8 6488 1015. E-mail address: 20699555@student.uwa.edu.au (D.T. Thotagamuwage). http://dx.doi.org/10.1016/j.coastaleng.2014.05.011 0378-3839/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Coastal Engineering journal homepage: www.elsevier.com/locate/coastaleng