Observations of mixing and transport on a steep beach

Abstract

A B S T R A C TSurfzone mixing and transport on a sandy, steep (∼1/8 slope), reflective beach at Carmel River State Beach, California, are described for a range of wave and alongshore flow conditions. Depth-limited wave breaking occurred close to the shore due to the steepness of the beach, creating a narrow surf/swash zone (∼10 m wide). Fluorescent Rhodamine dye was released as a slug in the surfzone, and the temporal and spatial evolution was measured using in-situ dye sensors. Dye concentration measured as a function of time reveals sharp fronts that quickly decay resulting in narrow peaks near the dye release, which subsequently broaden and decrease in peak concentration with alongshore distance. The measurements indicate two stages of mixing and transport occur inside the surfzone on the steep beach. 1) In the near-field (< 50 m downstream of the dye release location), the dye fully mixed throughout the water column after a few incident waves then continued to disperse in two dimensions, with both advection and diffusion processes being important. 2) In the far-field (> 50 m downstream from the dye release location), the mass transport was dominated by advection. The distance to the farfield is much shorter in the alongshore on a steep beach compared with a dissipative beach. Estimates of crossshore and alongshore diffusion coefficients (κ x , κ y ) were found to be similar in magnitude within the surfzone. Outside the surfzone in the far-field, the results suggest that the mixing processes are independent of those inside the surfzone. The mixing and transport of material observed on this steep beach are found to be analogous to that previously measured on dissipative beaches, however the diffusion coefficients within and outside the surfzone were found to be smaller on this steep beach.

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

“…Under similar wave and surf‐zone conditions, but over larger downstream distance (0.1 ≤ y ≤ 1 km), substantially weaker shoreline D max ( y ) alongshore power‐law decay (non‐Fickian) was observed due to inner‐shelf tracer build‐up and recirculation (Hally‐Rosendahl et al., 2014, 2015). Observations on a reflective beach are qualitatively similar to dissipative beaches, but with the surf‐zone becoming well mixed over shorter length/time scales, for example, within 25 m alongshore or 5 min ( v SZ ≈ 0.8 m s −1 ) of the release, due to the narrower surf‐zone (Brown et al., 2019). Long‐range tracer dispersion is likely non‐Fickian due to differences in surf‐zone and inner‐shelf hydrodynamics and diffusivity.…”

“…For 𝐴𝐴  < 4.5 h , D was on average 65 ppb with a temporal maximum D max = max{D} (t) = 90 ppb (teal diamond) and significant (≈±62 ppb) temporal variability (gray shading). The large variability of 30-min D indicates that surf-zone horizontal tracer gradients were large at 𝐴𝐴 𝐴 𝐴𝐴 = 0.4 km , and we refer to this region as the near-field similar to Brown et al (2019). Just before 𝐴𝐴  = 5 h , the tracer signal rapidly decays, and we define the time of surf-zone plume passage 𝐴𝐴 p (red ▿) as the latest instance of 𝐴𝐴 𝐴𝐴( ) ≥ (1 + 𝐴𝐴max∕4) , where the additional 1 ppb compensates for the surf-zone fluorometer minimum detection level and the higher in situ D max /4 threshold buffers for increased temporal variability.…”

“…Surf‐zone tracer evolution has been studied using either instantaneous shoreline releases (e.g., Brown et al., 2019; Clarke et al., 2007; Harris et al., 1963), or continuous releases (e.g., Clark et al., 2010; Hally‐Rosendahl et al., 2014, 2015). Quantitative analysis of in situ surf‐zone tracer concentration D has been restricted to alongshore ( y ) distances of 10–100 m (Clark et al., 2010; Brown et al., 2019) to $$\approx 1$$ km (Hally‐Rosendahl et al., 2014, 2015), representing advective‐time scales ( t ) from 1 min to 1 hr after release, that is , t ∼ y / v SZ , given quasi‐steady mean alongshore current v SZ .…”

“…Surf‐zone tracer evolution has been studied using either instantaneous shoreline releases (e.g., Brown et al., 2019; Clarke et al., 2007; Harris et al., 1963), or continuous releases (e.g., Clark et al., 2010; Hally‐Rosendahl et al., 2014, 2015). Quantitative analysis of in situ surf‐zone tracer concentration D has been restricted to alongshore ( y ) distances of 10–100 m (Clark et al., 2010; Brown et al., 2019) to $$\approx 1$$ km (Hally‐Rosendahl et al., 2014, 2015), representing advective‐time scales ( t ) from 1 min to 1 hr after release, that is , t ∼ y / v SZ , given quasi‐steady mean alongshore current v SZ . On an alongshore uniform dissipative beach with v SZ ∼ 0.25 m s −1 , continuously released tracer was surf‐zone confined within 200 m downstream ($$\approx 15\enspace \mathrm{min}$$) and the observed surf‐zone cross‐shore ( x ) ensemble‐mean tracer dispersion was Fickian, exhibiting down‐mean‐gradient diffusive flux with constant diffusion coefficient (e.g., Clark et al., 2010).…”

Assessment of larval connectivity in a sandy beach mole crab through a coupled bio-oceanographic model

“…In the latter, the planktonic larval stage is the main dispersal phase that has a pivotal role in connectivity (Cowen & Sponaugle 2009). The larvae of many intertidal species develop in the nearshore and must cross the surf zone to complete their onshore migration to benthic adult habitats (Shanks et al 2015, Brown et al 2019). However, connectivity patterns between sandy beaches related to larval dispersal are still to be explored, and mechanisms that influence the larval distribution and its consequences in the metapopulation dynamics are poorly understood (McLachlan & Defeo 2018).…”