Cross‐shore tracer exchange between the surfzone and inner‐shelf

Abstract

Cross-shore tracer exchange between the surfzone and inner-shelf is examined using temperature and dye measurements at an approximately alongshore-uniform beach. An alongshore-oriented plume is created by releasing dye continuously for 4.5 h in a surfzone alongshore current. The plume is sampled for 13 h from the release point to 700 m downstream, between the shoreline and 250 m offshore (6 m water depth). Within the surfzone ( 2 m depth), water is relatively warm, and dye is vertically well mixed. On the inner-shelf (3-6 m depth), alongshore currents are weak, and elevated temperature and dye co-occur in 25-50 m wide alongshore patches. Within the patches, dye is approximately depth-uniform in the warm upper 3 m where thermal stratification is weak, but decreases rapidly below 3 m with a strong thermocline. Dye and temperature vertical gradients are correlated, and dye is not observed below 18 C. The observations and a model indicate that, just seaward of the surfzone, thermal stratification inhibits vertical mixing to magnitudes similar to those in the ocean interior. Similar surfzone and inner-shelf mean alongshore dye dilution rates are consistent with inner-shelf dye properties being determined by local cross-shore advection. The alongshorepatchy and warm inner-shelf dye is ejected from the surfzone by transient rip currents. Estimated Stokes drift driven cross-shore exchange is small. The transient rip current driven depth-normalized heat flux out of the surfzone has magnitude similar to those of larger-scale shelf processes. Dye recycling, from the inner-shelf back to the surfzone, is suggested by relatively long surfzone dye residence times.

Depth Dependence of Nearshore Currents and Eddies

“…This paper emphasizes the transition to depth‐uniformity with increasing wave breaking, but outside the surfzone stratification likely played an important role. Richardson numbers suggested substantial inhibition of vertical mixing by stratification outside the surfzone, whereas previous work suggests that breaker‐induced mixing if often sufficient to eliminate most stratification inside the surfzone (Hally‐Rosendahl et al, ; Kumar & Feddersen, ). Observations outside the surfzone were further complicated by the presence of significant divergent motions at eddy frequencies (i.e., eddy kinetic energy only 1.2–2.2 times greater than potential).…”

“…Inside the surfzone, Richardson number, which does not account for breaker‐induced mixing, may become less relevant. Previous studies suggest that breaker‐induced mixing is often sufficient to eliminate stratification in the surfzone (Hally‐Rosendahl et al, ; Kumar & Feddersen, ).…”

“…We estimated at X1 by assuming (i) that near‐surface density at X1 equaled near‐surface density measured in 8 m depth and (ii) that density near the seabed elevation at X1 equaled the density at the same elevation in the 8 m profile (similarly for other instrument locations). This likely overestimates Ri , since likely decreases onshore of 8 m (Hally‐Rosendahl et al, ; Kumar & Feddersen, ). Both outside and inside the surfzone, bin‐averaged Ri exceeded 0.05 in all but one case (not shown; the exception was X1 for ).…”

A Conceptual Model of a River Plume in the Surf Zone

“…Transport of freshwater from a river entering the surf zone could be reasonably expected to be influenced by surf zone circulation processes, as has been shown for passive tracers (Brown et al, ; Clark et al, ; Hally‐Rosendahl et al, ). The strongest surf zone circulation is driven in the alongshore direction by obliquely directed breaking waves (Feddersen et al, ; Longuet‐Higgins, ).…”

“…Small river mouths are often unengineered, allowing surf zone wave breaking to occur near or at such outflows. In regions with higher population densities, engineered structures such as jetties are present at most river mouths, preventing wave breaking and the influence of surf zone dynamics on buoyancy and momentum‐driven river plume processes (Clark et al, ; Hally‐Rosendahl et al, ; Horner‐Devine et al, ).…”