**Luigia Riefolo 1, Pasquale Contestabile 2,\*, Fabio Dentale <sup>3</sup> and Guido Benassai <sup>4</sup>**


Received: 26 January 2018; Accepted: 19 February 2018; Published: 23 February 2018

**Abstract:** This paper describes a set of experiments undertaken at Universitat Politècnica de Catalunya in the large wave flume of the Maritime Engineering Laboratory. The purpose of this study is to highlight the effects of wave grouping and long-wave short-wave combinations regimes on low frequency generations. An eigen-value decomposition has been performed to discriminate low frequencies. In particular, measured eigen modes, determined through the spectral analysis, have been compared with calculated modes by means of eigen analysis. The low frequencies detection appears to confirm the dependence on groupiness of the modal amplitudes generated in the wave flume. Some evidence of the influence of low frequency waves on runup and transport patterns are shown. In particular, the generation and evolution of secondary bedforms are consistent with energy transferred between the standing wave modes.

**Keywords:** spectral analysis; low frequency; wave grouping; eigen analysis; eigenmode; random waves; combination waves
