Bottom shear stress in the surf zone

Abstract

To investigate the bottom shear stress in the surf zone, detailed laboratory measurements were made of the free surface elevations and velocities for the case of regular waves spilling on a rough, impermeable 1:35 slope. The velocity profiles were measured at several vertical lines in the cross-shore direction to include the shoaling region seaward of breaking, the break point, the transition region, and the inner surf zone. Each vertical line included measuring points at a fraction of the grain height above the rough, fixed bottom. A logarithmic layer was found to exist in the bottom boundary layer for most of the phases over a wave period seaward of the break point and in the surf zone. A regression analysis was used at each phase to estimate the shear velocity and bottom roughness from the phase-averaged horizontal velocities in the lower portion of the bottom boundary layer. The bottom friction factor was estimated from a quadratic friction equation based on the measured horizontal velocity above the bottom boundary layer together with the estimated shear velocity. The quadratic friction equation with the fitted friction factor was shown to predict the temporal variation of the bottom shear stress within a factor of 2. The bottom roughness estimated from the grain size assuming rough turbulent flow was shown to agree qualitatively with the measured values. The cross-shore variation of the friction factor estimated from an empirical formula developed for nonbreaking waves was shown to agree within a factor of 2 of the measured values.

Suspended and Bedload Transport in the Surf Zone: Implications for Sand Transport Models

“…The positive correlation between both parameters suggests that wave breaking turbulence effectively enhances sediment entrainment from the bed. This is consistent with previous observations of (intermittent) suspension following breakinggenerated eddy arrival at the bed (Nielsen, 1984;Nadaoka et al, 1988;Scott et al, 2009), which has been physically explained by high instantaneous turbulent bed shear stresses (Cox et al, 1996;Zhou et al, 2017) and by upward-directed pressure gradients in the bed that occur directly under large-scale rotational vortices (Sumer et al, 2013). Additional analysis was done to assess to what extent the concentration at ζ = z a is controlled by local pick-up.…”

Experimental study of the turbulent boundary layer in acceleration-skewed oscillatory flow

“…Fitting (6.1) to measured instantaneous profiles, with d , z 0 and u * all varying to optimise the fit, leads to temporal variation in bottom roughness, which is not realistic since roughness should be constant if the boundary layer is fully developed and rough turbulent (e.g. Cox et al 1996). Therefore, the representative values of z 0 and d for each of the sand-rough and gravel-rough beds were established as follows: the log profile was fitted to the profile of the amplitude of the first harmonic, u 1 (z), for each experiment and the resulting z 0 and d for the same bed type were averaged.…”

“…Present understanding of oscillatory boundary layers is based on laboratory experiments, including experiments in which beds are oscillated in otherwise still water (Keiller & Sleath 1976;Krstic & Fernando 2001) and experiments in smallscale wave flumes (Sleath 1970;Kemp & Simons 1982, 1983Cox, Kobayashi & Okayasu 1996;Mirfenderesk & Young 2003;Dixen et al 2008) or oscillatory open channel flumes (Chen et al 2007). However, the physical limitations of these facilities mean that the amplitude Reynolds number, Re = u 0 max a/ν (where u 0 max is the amplitude of the free-stream horizontal velocity, a the amplitude of the freestream water particle excursion and ν is the kinematic viscosity), and the relative roughness a/k s (where k s is the equivalent bed roughness) are low and are not representative of conditions under full-scale waves.…”

Sand transport processes in the surf and swash zones

“…Under breaking waves, higher entrainment and mixing rates lead to an increase in concentrations at outer flow elevations compared to shoaling locations (Nielsen, 1984;Nadaoka et al, 1988;Aagaard and Jensen, 2013). The increased pick up rates under breaking waves can be attributed to high instantaneous bed shear stresses (Cox et al, 1996) and to upward directed pressure gradients in the bed under the breaking point (Sumer et al, 2013).…”

“…Breaking induced vortices may invade the WBL (Cox and Kobayashi, 2000;Huang et al, 2010;Chassagneux and Hurther, 2014) and can substantially enhance near bed TKE (Scott et al, 2005) and bed shear stresses (Deigaard et al, 1991;Cox et al, 1996;Sumer et al, 2013). Most of these studies were conducted in small scale wave flumes (mostly over rigid, planar sloping beds), which may not fully reproduce the properties of breaking induced turbulence and of the WBL hydrodynamics under full scale waves.…”