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This study investigates the influence of tidal straining in the generation of turbidity maximum zones (TMZ), which are observed to extend for tens of kilometers along some shallow, open coastal seas. Idealized numerical simulations are conducted to reproduce the cross-shore dynamics and tidal straining in regions of freshwater influence (ROFIs), where elliptical current patterns are generated by the interaction between stratification and a tidal Kelvin wave. Model results show that tidal straining leads to cross-shore sediment convergence and the formation of a nearshore TMZ that is detached from the coastline. The subtidal landward sediment fluxes are created by asymmetries in vertical mixing between the stratifying and destratifying phases of the tidal cycle. This process is similar to the tidal straining mechanism that is observed in estuaries, except that in this case the convergence zone and TMZ are parallel to the shoreline and perpendicular to both the direction of the freshwater flux and the major axis of the tidal flow. We introduce the term minor axis tidal straining (MITS) to describe the tidal straining in these systems and to differentiate it from the tidal straining that occurs when the major axis of the tidal ellipse is aligned with the density gradient. The occurrence of tidal straining and the coastal TMZ is predicted in terms of the Simpson (Si) and Stokes (Stk) numbers, and top–bottom tidal ellipticity difference (Δε). Based on our results, we find that SiStk2 > 3 and Δε > 0.5 provide a limiting condition for the required density gradients and latitudes for the occurrence of MITS and the generation of a TMZ. [ABSTRACT FROM AUTHOR]

Abstract: In this study we used field data and radar images to investigate the influence of winds and tides on the propagation of tidal plume fronts. The measurements were collected in a shallow shelf region off the Dutch coast, 10 km north of the Rhine River mouth, and they clearly show the passage of distinct freshwater lenses and associated fronts at the surface that propagate all the way to the coastline. These fronts are observed as a sudden drop in near‐surface salinity, accompanied by high cross‐shore shear with onshore velocities at the surface. We determined the arrival time to our measurement site, frontal propagation speed, and structure of the fronts by combining the in situ data and radar images. Frontal Froude numbers show a wide range of values, with an average of 0.44. Our results show that fronts during spring tides are thinner, more mixed, and move faster relative to the ground during calm spring tides when compared to calm neap tides. Downwelling winds during spring tides result in thicker and faster fronts; however, the intrinsic frontal propagation speed indicates that the wind and tide control the frontal propagation mainly due to advection rather than by changing the frontal structure. Strong return currents in the near‐bed layer resulting from fast and thick fronts increase near‐bed turbulence and bed stresses. These high stresses suggest that the passage of fronts in shallow coastal areas can initiate sediment resuspension and contribute to transport processes. [ABSTRACT FROM AUTHOR]

Meirelles, S.; Henriquez, M.; Souza, A.J.; Horner-Devine, A.R.; Pietrzak, J.D.; Rijnsburg, S., and Stive, M.J.F., 2016. Small scale bedform types off the South Holland coast. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 423-426. Coconut Creek (Florida), ISSN 0749-0208. This study presents the small scale bedform states found off the South-Holland coast during a 31+ days field observation of seabed acoustic imagery and near the bed velocities. Six main bed states were encountered: current ripples (C), wave ripples (W), combined wave-current ripples (WC), current ripples with subordinate wave ripples (Cw), wave ripples with subordinate current ripples (Wc) and poorly developed ripples (P). Direct visual detection of the bed state from the images showed good agreement with a simple predictor based on the mobility number. The most frequent type of bedform was C which is governed by the tidal currents. Wave ripple prevailed only during a storm with waves higher than 2 m. The combined Cw, WC and Wc types comprised 22% of the occurrences. Poorly developed ripples were associated with the neap tide during fair weather conditions. [ABSTRACT FROM AUTHOR]