Observations of infragravity period oscillations in a small marina Darshani T. Thotagamuwage n , 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 article info Article history: Received 8 October 2013 Accepted 2 July 2014 Keywords: Harbours Oscillations Australia Water levels Wave groups Seiches abstract We measured water levels in Two Rocks Marina, Western Australia, to investigate infragravity-period (25–300 s) oscillations and their forcing mechanisms. Spectral analyses identified four dominant oscillations in the infragravity band, which were generated through excitation of the marina’s natural periods. The oscillations were present at all times, independent of the offshore conditions, indicating that they were forced by a continuous external energy source. The spectral energy of the oscillations increased by 50 times during storm events (higher incident wave heights), in comparison to calm events (lower incident wave heights). Wave heights of oscillations within the marina were strongly correlated with offshore incident swell wave heights and reached maximum of 0.5 m. The groupiness factor of swell waves around the marina was 0.6–0.85. Bound infragravity waves associated with swell wave groups were identified as potential forcing mechanism of infragravity-period oscillations within the marina. The bound infragravity waves have broad frequency spectrum without dominant periods matched the marina’s natural periods however, bound infragravity waves of periods in the proximity of the marina NOPs were adequate to generate oscillations at the NOPs of the marina. Frequencies of the oscillations were independent of the forcing frequency, and determined by the marina's geometry. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Infragravity period waves are surface gravity waves with periods between 25 and 300 s (frequencies between 0.003– 0.04 Hz) and wave lengths between 100 m and 10 km (Rabinovich, 2009). Infragravity waves are generated, mainly through nonlinear interactions of wind generated waves. The propagation of infragravity waves towards coastal areas, which contain, for example, harbours and lagoons, can excite oscillations within these water bodies. Resonance, when the period of incident infragravity waves is close to natural oscillation periods (NOPs) of the water basin, generates higher amplitude oscillations causing undesirable water motions. Such conditions interrupt berthing operations, further resulting in harbour downtime followed by economic losses (McComb et al., 2005; Van der Molen et al., 2006; Rabinovich, 2009; Uzaki et al., 2010). Very long-period incident waves such as tsunamis (Gilmour, 1990; Hinwood and McLean, 2013), waves originated from atmo- spheric pressure disturbances (Vilibic and Mihanovic´, 2003; De Jong and Battjes, 2004; Uzaki et al., 2005; Pattiaratchi and Wijeratne, 2014), and internal waves (Rabinovich, 2009), can cause significantly high amplitude oscillations followed by extensive damage to harbour operations. Very long-waves can affect only large harbours because their NOPs are generally longer than 10 min, which matches the very long-period wave band. In contrast, very long-period waves cannot excite NOPs of small harbours (where the surface water area is about 1 km 2 and the depth is about 5–10 m) because, their NOPs are shorter than the very long- period wave band (Okihiro and Guza, 1996). On the other hand, NOPs of small harbours cannot be directly excited by short waves either because short wave periods are typically less than 25 s. However, infragravity waves can excite NOPs of small harbours because their NOPs are in the similar range of the infragravity wave periods (Wu and Liu, 1990). Various harbours and ports around the world, such as Port of Sines Portugal (Gierlevsen et al., 2001), Port of Long Beach California (Kofoed- Hansen et al., 2005), Hualian harbour Taiwan (Chen et al., 2004) experience frequent oscillations in the infragravity period band, excited by short waves (Rabinovich, 2009). Bowers (1977); Mei and Agnon (1989), and Wu and Liu (1990) carried out theoretical and laboratory experiments to study the influence of incident short waves on infragravity period harbour oscillations. These studies found that bound infragravity waves (associated with regular swell wave groups), and free infragravity waves (generated by breaking of swell wave groups) can excite NOPs of harbours in the infragravity period band. Field observa- tions at few harbours, [Esperance harbour in Australia (Morison and Imberger, 1992), Barbers Point harbour in Hawaii (Okihiro et Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/oceaneng Ocean Engineering http://dx.doi.org/10.1016/j.oceaneng.2014.07.003 0029-8018/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ61 8 6488 8121; fax: þ61 8 6488 1015. E-mail address: 20699555@student.uwa.edu.au (D.T. Thotagamuwage). Ocean Engineering 88 (2014) 435–445