Long‐Distance/Time Surf‐Zone Tracer Evolution Affected by Inner‐Shelf Tracer Retention and Recirculation

The evolution of a surf‐zone released tracer (≈100 $\approx 100$ Liter over 4 hr) was observed for ≈30 $\approx 30$ h. Surf‐zone tracer was transported alongshore (y) with relatively steady mean speed vSZ ≈ 0.18 m s−1, consistent with obliquely incident wave forcing. Maximum in situ surf‐zone tracer concentration decayed exponentially with 1.6 km alongshore e‐folding length scale, that is, 2.5 hr advective time scale. Surf‐zone tracer time‐series evolved downstream of the release from a top‐hat structure for y ≤ 1 km to increasingly skewed farther downstream. Within ≈1.5 $\approx 1.5$ km of the northward propagating tracer front, inner‐shelf tracer was confined to onshore of ≈4LSZ $\approx 4{L}_{\mathrm{S}\mathrm{Z}}$ (surf‐zone width LSZ ≈ 100 m) and was alongshore patchy. A coupled surf‐zone/inner‐shelf tracer advection‐diffusion‐exchange box model reproduces the observed surf‐zone downstream max concentration decay and temporal skewness, with surf‐zone flushing time kSZ−1≈2.3 ${k}_{\mathrm{S}\mathrm{Z}}^{-1}\approx 2.3$ h. A weaker inner‐shelf unidirectional‐exchange rate kIS ≈ kSZ/2 indicates reduced horizontal mixing outside the surf‐zone. Surf‐zone temporal skewness is linked to inner‐shelf tracer storage, differential surf‐zone/inner‐shelf advection, and recirculation, that is, non‐asymptotic shear dispersion. On the inner‐shelf (≈3LSZ $\approx 3{L}_{\mathrm{S}\mathrm{Z}}$), tracer vertical structure differed in the morning versus afternoon suggesting internal tide and solar forced thermal modulation. Model parameters representing surf‐zone processes are well constrained by existing observations and scales. However, the many overlapping inner‐shelf processes make a single process based generalization of inner‐shelf cross‐shore exchange rate (i.e., kIS) and alongshore transport difficult. Plain Language Summary Transport and mixing impact nearshore systems, such as larval recruitment in intertidal ecosystems and water quality impacts from coastal pollution, and can be studied using shoreline released tracers, like fluorescent dye. In the region of depth‐limited wave breaking, the surf‐zone, alongshore directed currents driven by oblique breaking waves transport tracers over long distances. Tracer is also mixed across the surf‐zone by eddying currents and exported onto the inner‐shelf (region offshore of the surf‐zone) by rip currents, which decreases shoreline tracer concentration. Horizontal mixing also increases tracer plume length‐scales, known as