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

Abstract

AbstractThe evolution of a surf‐zone released tracer ( Liter over 4 hr) was observed for 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 km of the northward propagating tracer front, inner‐shelf tracer was confined to onshore of (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 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 (), 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.