Low-frequency waves generated on a submerged curvilinear sill by bichromatic wave groups with and without an opposing current are studied experimentally in a laboratory flume. The second-order wave maker theory is used to generate the primary waves and the spurious long waves were significantly suppressed. The generation of harmonics, wave blocking of the short waves and especially the penetration of long waves through the blocking point have been observed. The evolution of long waves before and after the blocking point is described in detail.
Surf beat was first observed and termed by Munk (1949) and Tucker (Tucker, 1950) outside the surf zone. It is a wave motion at the period of 0.5 ~ 5.0 min, which is also referred as infragravity wave later in the literature. The observed long waves were believed to be generated in the surf zone as a result of a beat phenomenon induced by the primary incident wave groups. The incidence of surf beat into the harbor may have a negative effect on the tranquility of the harbor (Bowers, 1977; Mei and Agnon, 1989; Smallman and Cooper, 1989) and on the cargo work on the moored ships (Nagai et al., 1994). Surf beat may also be responsible for the change of the coastal morphology and some complex topography (Holman and Bowen, 1982; Osborne and Greenwood, 1992; O" Hare and Huntley, 1994; Ciriano et al., 2005), and influence the sand transport in the nearshore (Smith and Mocke, 2002). Wave current interaction occurs often in the real costal region. However, the low-frequency waves generated in the waves with current is rarely studied to the authors' knowledge. The low-frequency waves generated on a submerged curvilinear bar in the bichromatic waves with an opposing current are observed and analyzed in this paper.