Surf zone surface retention on a rip‐channeled beach

Abstract

[1] The retention of floating matter within the surf zone on a rip-channeled beach is examined with a combination of detailed field observations obtained during the Rip Current Experiment and a three-dimensional (3-D) wave and flow model. The acoustic Doppler current profiler-observed hourly vertical cross-shore velocity structure variability over a period of 3 days with normally incident swell is well reproduced by the computations, although the strong vertical attenuation of the subsurface rip current velocities at the most offshore location outside the surf zone in 4 m water depth is not well predicted. Corresponding mean alongshore velocities are less well predicted with errors on the order of 10 cm/s for the most offshore sensors. Model calculations of very low frequency motions (VLFs) with O(10) min timescales typically explain over 60% of the observed variability, both inside and outside of the surf zone. The model calculations also match the mean rip-current surface flow field inferred from GPS-equipped drifter trajectories. Seeding the surf zone with a large number of equally spaced virtual drifters, the computed instantaneous surface velocity fields are used to calculate the hourly drifter trajectories. Collecting the hourly drifter exits, good agreement with the observed surf zone retention is obtained provided that both Stokes drift and VLF motions are accounted for in the modeling of the computed drifter trajectories. Without Stokes drift, the estimated number of virtual drifter exits is O(80)%, almost an order of magnitude larger than the O(20)% of observed exits during the drifter deployments. Conversely, when excluding the VLF motions instead, the number of calculated drifter exits is less than 5%, thus significantly underestimating the number of observed exits.

Surfzone hydrodynamics as a key determinant of spatial variation in rocky intertidal communities

“…At the DSZ, simulated zooplankters were concentrated in the surf zone by recirculation in an alongshore system of alternating shoals and rip currents [14][15][16]. Simulated zooplankters were transported into the surf zone over shoals, and they were concentrated in deeper rip channels before some returned seaward in offshore flow episodically with the arrival of wave groups.…”

Vortical VLF Motions Under Shore-Normal Incident Waves

“…This can lead to strengthening of existing rip currents or the generation of transient rip currents (MacMahan et al 2006, Johnson andPattiaratchi 2004). Moreover, the eddy like behavior of vortical VLF motions is found to significantly affect the mixing of material (larvae, nutrients, pollutants) inside the surfzone (Spydell et al 2007(Spydell et al ,2009) as well as the exchange of material with the shelf (Reniers et al 2009.…”

Effects of waves on coastal water dispersion in a small estuarine bay

“…Depending on the system morphology and the local wave climate, ocean waves may strongly impact dispersion processes. Surf zone transport and mixing affect the suspended matter alongshore distribution and nearshore retention [Boehm et al, 2005;Feddersen, 2007;Spydell et al, 2007Spydell et al, , 2009Reniers et al, 2009]. Waves also affect exchanges between the nearshore and the inner shelf [Lentz et al, 2008].…”

“…In the nearshore, this leads to large vertical variations of the total cross-shore current. Also, laboratory and field observations suggested that rip currents might vary from depth-uniform to depth-varying outside the breakers, with higher velocities near the surface [e.g., Haas and Svendsen, 2002;Reniers et al, 2009]. To capture the transport of a tracer in the presence of waves, the vertical structure of the flow may thus be needed.…”

“…Some of these have been further adapted into existing ocean numerical models and applied to reproduce observed circulations [e.g., Uchiyama et al, 2009] and nearshore dispersion [e.g., Reniers et al, 2009]. The main distinctions between the different formulations are (1) the use of an Eulerian or Lagrangian point of view and (2) the separation or not of the momentum into wave and mean flow contributions [Lane et al, 2007;Bennis et al, 2011].…”