Undertow Structure and Stress Partition Beneath Plunging Breakers

Undertow beneath breaking waves controls nearshore mass balance and sediment transport, yet the transition between the lower wave–bottom-influenced region and the overlying flow affected by breaking-generated turbulence lacks a clear dynamical definition. This study investigates the vertical structure of undertow and turbulence beneath two regular laboratory plunging waves propagating over a sloping bottom, based on detailed velocity measurements. Time-averaged and phase-resolved fields, together with turbulent statistics, reveal a vertically differentiated flow, with a lower region more directly influenced by wave–bottom interaction and an upper region increasingly affected by breaking-generated turbulence. To formalize this structure, a classical undertow model is used as a diagnostic framework linking the lower and upper flow regions, allowing the identification of an effective matching level directly from the measurements. This level is not interpreted as a geometric thickness, but as a data-driven interface linking lower-layer wave-induced dynamics to the overlying flow. An independent stress-ratio analysis shows that this level is generally located near a transition in the measured partition between wave-coherent and turbulent shear stresses, supporting its interpretation as a stress-partitioning elevation rather than a purely fitted parameter. The present data exhibit a regime-conditioned relationship between the wave-induced stress evaluated at this level and the forcing acting in the overlying region when combined with wave non-linearity. The organization of the present dataset, together with departures observed for additional data, suggest that undertow dynamics in the surf zone are governed by regime-dependent balances rather than by a single universal law.