Layout: C&M • Faculty of Geosciences • ©2011 (8024)
10−3 10−2
−3.5
−3
−2.5
−2
−1.5
−1
−0.5 0
ε
(m2 s−3)10−3 10−2
ε
(m2 s−3)10−3 10−2
ε
(m2 s−3)10−3 10−2
ε
(m2 s−3)10−3 10−2
ε
(m2 s−3)ξ
(m)3.5 > h ≥ 3 m 3 > h ≥ 2.5 m 2.5 > h ≥ 2 m 2 > h ≥ 1.5 m 1.5 > h ≥ 1 m
Sea surface
Sea bed
Hs/h =[0.38 0.48]
Hs/h =[0.48 0.53]
Hs/h =[0.53 0.58]
Hs/h =[0.58 0.7]
H s (m) h (m)H s/h
70 75 80 85
Yearday
ε (m2 s−3 )
0 1 2
0 2 4
0.4 0.6 0.8
10−4 10−3 10−2
High significant wave height (0.5 - 2 m)
Shallow-water depth (< 3 m)
High breaking-wave conditions
Strong turbulence disposition rates
ADVO 1 (bottom) ADVO 2 (middle) ADVO 3 (top)
N
Truc Vert beach Arcachon lagoon
Bordeaux
Biarritz
Cap Ferret sand spit
Capbreton canyon
-1000m -50m
-400m
Gironde estuary
Introduction
High-energy breaking waves induce
major morphological changes of sandy beaches which are not well predicted yet. In particular, it is not known how far breaking-induced turbulence can penetrate through the water column and stir sediment. Hence, we used a recently collected fi eld dataset (1) to determine the vertical structure of the turbulence dissipation in the shallow- water surf zone beneath high-energy breaking waves; and (2) to investigate how this vertical structure is affected by breaking-wave intensity. We see this as the fi rst step to determine the infl uence of surface-generated turbulence on surf
zone sediment transport.
Methodology
The fi eld data were collected at the macrotidal Truc Vert beach, France (Figure 1) during a 18-day period in 1–3 m water depth with strong
cross-shore and alongshore currents under high-energy wave conditions (offshore signifi cant wave heights reaching 8 m).
Based on a turbulent inertial subrange analysis, the turbulence
dissipation rates ε are estimated from 24-min long, 10 Hz, 3-component fl uid velocities measured at
three elevations between the sea bed and the wave trough level (Figure 2).
In total, 501 dissipation rate observations were available for our study.
Vertical structure of the turbulence dissipation rate in the surf zone
Figure 1. Field site location
Florent Grasso and Gerben Ruessink
Department of Physical Geography, Utrecht University (f.grasso@geo.uu.nl)
Figure 3. Average dissipation rates ε measured at ADVO 1-3 for different water depths h and breaking-wave conditions Hs/h. At each point, horizontal and
vertical brackets are ± ½ standard deviation. An increase of ε close to the sea surface characterizes an increasing importance of the surface-generated
turbulence induced by the breaking waves, whereas an increase of ε close to the sea bed characterizes an increasing importance of the bed-generated turbulence induced by the bottom boundary layer.
Figure 2. Instrumented rig at low tide. Wave velocities are measured by three ADVOs during high tides.
Conclusions
The vertical structure of the turbulence dissipation rate of our data demonstrates surface-generated turbulence as a dominant source of turbulence, especially for shallow-water conditions.
• The turbulence generated in the bottom boundary layer is not negligible, especially for the lower breaking-wave intensities Hs/h.
• The turbulence dissipation increases with the breaking-wave intensity and then saturates for Hs/h > 0.58.
• This saturation may be due to less vortex injection as waves modify from breakers into bores.
Figure 4. Hydrodynamic
conditions, and turbulence dissipation rates measured
at the rig during the 18 days. Hs/h = 0.38 and
0.48 represent the
approximate boundaries between non-, weakly,
and fully breaking
conditions. The strong turbulence dissipation
rates confi rm the
energetic conditions of our study.
