Turbulent viscosity in natural surf zones

Abstract

[1] Waves breaking in the shallow surf zone near the shoreline inject turbulence into the water column that may reach the bed to suspend sediment. Breaking-wave turbulence in the surf zone is, however, poorly understood, which is one of the reasons why many process-based coastal-evolution models predict coastal change during severe storms inadequately. Here, we use data collected in two natural surf zones to derive a new parameterization for the stability function C m that determines the magnitude of the eddy viscosity n t in two-equation turbulent-viscosity models, n t = C m k 2 /ɛ, where k is turbulent kinetic energy and ɛ is the turbulence dissipation rate. In both data sets, the ratio of turbulence production to dissipation is small (≈0.15), while vertical turbulence diffusion is significant. This differs from assumptions underlying existing C m parameterizations, which we show to severely overpredict observed C m for most conditions. Additionally, we rewrite our new C m parameterization into a formulation that accurately reproduces our Reynoldsstress based estimates of turbulence production. This formulation is linear with strain, consistent with earlier theoritical work for large strain rates. Also, it does not depend on ɛ and can, therefore, also be applied in one-equation turbulentviscosity models. We anticipate our work to improve turbulence modeling in natural surf zones and to eventually lead to more reliable predictions of coastal evolution in response to severe storms.

Field Observations of Turbulence, Sand Suspension, and Cross‐Shore Transport Under Spilling and Plunging Breakers

“…Intra-wave suspended sediment load depends on sediment settling velocity, which again depends on grain size, and sediment diffusivity, which represents a variety of vertical mixing mechanisms. In the surf zone, vertical mixing is caused by both turbulence injection from breaking waves and from fluid interaction with the seabed (e.g., Grasso & Ruessink, 2012). Recent field (Aagaard & Hughes, 2010;Ruessink, 2010;Scott et al, 2009) and laboratory (Brinkkemper et al, 2016;Shin & Cox, 2006; work has shown that surface-injected breaker turbulence may impinge on the seabed and stir the sediment.…”

Shortwave Sand Transport in the Shallow Surf Zone

“…It is thus not necessarily the velocity asymmetry that is driving the onshore wave‐induced transport but rather a combination of processes that are connected to and scale with A u . To illustrate that, for example, the turbulent kinetic energy k in the water column scales with , k was estimated from velocity measurements collected during BD (Brinkkemper et al, ), AM (Grasso & Ruessink, ), and a field campaign at Truc Vert beach (France) in 2008 (Grasso & Ruessink, ; Ruessink, ). As mentioned in section 2, these measurements could not be collected at SE.…”

Observations of Nearbed Turbulence over Mobile Bedforms in Combined, Collinear Wave-Current Flows

“…The length scales, λ, are thereby inferred using λ = 2πz/k * with k * taken at the peak of the spectrum. The mean vertical turbulence flux is defined as k E w m 3 /s 3 , and the vertical gradient can therefore be inferred given the vertical and horizontal offsets between the two ADVs were close enough to allow estimates of structure functions (statistical moments) of the flow field following Kolmogorov (1941) theory [23,61].…”

“…In addition, such models also impose a zero-flux condition limiting turbulent quantities; and by consequence sediment concentrations, at the interface of the wave boundary layer with the core flow [22], despite documented evidence of vertical momentum exchanges extending beyond the oscillating layer in the nearshore [23][24][25]. Holmedal et al contend that such simplification is problematic given production and dissipation of turbulent kinetic energy are taking place within the current boundary layer at the interface with the wave boundary layer [22].…”