Spectral signatures of infragravity waves generated by breakpoint forcing

<div> This study introduces a new framework to isolate and characterize breakpoint forcing, a key mechanism of infragravity wave generation in the surf zone. First, we derive an equation-based separation of the total radiation-stress forcing into two distinct components: a breakpoint term directly proportional to local wave dissipation, and a conservative remainder associated with bound-wave dynamics. From this separation, a new physical picture emerges: breakpoint forcing acts as a compact, localized source of long-wave momentum whose behavior can be described by three time-varying descriptors: its total intensity, its centroid location, and its cross-shore width. This compact-source view reveals that the finite width of the surf zone causes waves generated from different parts of the source to undergo destructive interference, acting as a geometric filter on the radiated infragravity waves. The intensity pathway behaves as a monopole source subject to a geometric low-pass filter: it efficiently generates low-frequency energy, but its higher-frequency content is suppressed by phase cancellations inside the source. The centroid and width pathways behave as dipole- and quadrupole-like sources. They carry zero net momentum, making them inefficient at very low frequencies, and act as band-pass filters that suppress the very-low-frequency end of the infragravity band. We validate this framework using the wave-averaged, group-resolving model XBeach-Surfbeat on a range of beach slopes. The results suggest that beach slope is a major control: steeper slopes create a compact source and a broad, energetic infragravity response, while milder slopes broaden the source, strengthening the low-pass filtering and shifting energy to longer periods. </div>