*Corresponding author. Tel.: (31) 302533864; Fax: (31) 302540604; E-mail: b.ruessink@frw.ruu.nl Continental Shelf Research 18 (1998) 585 — 605 The temporal and spatial variability of infragravity energy in a barred nearshore zone B.G. Ruessink* Department of Physical Geography, Institute for Marine and Atmospheric Research Utrecht, Utrecht University, P.O. Box 80.115, 3508 TC Utrecht, Netherlands. Received 23 February 1996; in revised form 27 August 1997; accepted 22 September 1997 Abstract Recordings of near-bottom pressure at six cross-shore positions in a gently sloping multiple bar system were analysed to study the temporal and spatial variability of infragravity (0.004—0.04 Hz) energy. The temporal variations in infragravity levels at each position were, as expected, strongly related to those in the offshore incident wave energy. Furthermore, infra- gravity energy was better correlated to swell than to sea energy and, under non-breaking conditions, was tidally modulated with larger values during low water than during high water. During low-energy conditions infragravity energy was almost constant over the profile, sugges- ting a dominance of free long-wave motions; however, it rapidly grew in the onshore direction during more energetic, though non-breaking conditions, caused by significant bound-wave contributions to the total long-wave field. Infragravity energy was significantly damped under surf-zone conditions, even causing a decline in the onshore direction. The data indicates that the release of bound long waves during the breaking process of the incident sea and swell may be the main source of free infragravity motions.  1998 Elsevier Science Ltd. All rights reserved 1. Introduction The height of waves in the infragravity band (typically, 0.004—0.04 Hz) is, in general, only of the order of a few millimeters in the deep ocean (Webb et al., 1991), but may increase to a few decimetres, or even to more than a metre, close to the shore (e.g. Guza and Thornton, 1982; Holman and Sallenger, 1985; among others). Numerous field experiments have shown that infragravity energy is strongly correlated with that in the short-wave band (typically, 0.04—0.4 Hz), indicating some kind of local forcing by the incident sea and swell (e.g. Guza and Thornton, 1982; Nelson et al., 1988; Elgar 0278—4343/98/$19.00  1998 Elsevier Science Ltd. All rights reserved PII S0278 — 4343(97)00055-1