The temporal and spatial variability of infragravity energy in a barred nearshore zone

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, infragravity 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, suggesting 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.