Joost Brinkkemper and Gerben Ruessink
Department of Physical Geography
Sand suspension beneath sea-swell waves on a field-scale laboratory beach
Motivation Methods
Conclusions Next
Results
• Non-breaking waves above ripples:
- phase-lag increases upwards
- Thus only (onshore) wave-driven sand transport close to bed
• Breaking waves:
- small negative phase-lag
- during offshore phase only suspension close to the bed
- onshore wave-driven transport throughout the water column Sand suspension is phase-coupled to the short-wave orbital motion and highly depends on the wave conditions.
The study will be extended with field measurements to include transport;
• beneath spilling breakers
• in the inner surf-zone.
The ultimate aim will be to develop a parameterization that quantifies the effect of surface-induced turbulence on sand transport beneath broken waves and can be em-
bedded in operational models.
Breaking waves and bores inject large amounts of turbulence into the water column as vortices, which can travel downward and entrain sand from the bed. The timing of sand entrainment with respect to the wave orbital motion determines the magnitude and direc- tion of sand transport by sea-swell waves. Coastal evolution models rarely include the effect of this surface-induced turbulence on sand suspension and subsequent transport to predict surf-zone mor- phodynamics.
Here, we compare sand stirring by breaking waves to non-breaking waves above ripples by using laboratory measurements collected during the Barrier Dynamic Experiment II (BARDEXII).
Sand suspension is phase-coupled for low (non-breaking) and high (breaking) rela- tive waveheights.
-1 -0.5
0 0.5
1 1.5
0 0.2 0.4 0.6 0.8 1
0 2 4 6 8
0 0.2 0.4 0.6 0.8 1
0 2 4 6 8
0 0.2 0.4 0.6 0.8 1
t/ T 0
0.005 0.01 0.015
0 0.2 0.4 0.6 0.8 1
t/ T
0 0.05
0.1 0.15 -1 -0.5
0 0.5
1 1.5
0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1
−100
−50 0 50 100
0 5 10 15 20
−0.5 0 0.5
1 1.5
Runs phase -lag c with u
hf(
o) Tr anspor t (kg/m/s) Rela tiv e w av eheigh t
Onshore
Offshore
u
hf(m/s) c (kg/m
3) k (m
2/s
2) k (m
2/s
2) c (kg/m
3) u
hf(m/s)
0 0.2 0.4 0.6 0.8 1
Non-breaking Breaking
Schematic representation of the lowest five sand concentration sensors between 4 and 17 cm above the bed. Three are used for the analyses here, indicated with a red circle, yellow square and green triangle.
The measurement rig in the flume.
The sand concentration instruments are encircled. Turbulence was meas- ured using the three ADVs.
17 cm
• Field-scale laboratory experiment, irregular waves
• One imposed tidal cycle
• Turbulence at three heights above the bed
• Sand concentration at seven (three shown here) heights above the bed
• Coupling with cross-shore wave-orbital motion (u
hf) through phase-averaging
Non-breaking waves above vortex ripples
The measurement rig deployed in the field.
Plunging breaker during BARDEXII in the Delta Flume.
• phase-lag positive
• phase-lag increases upward
• c and k peak simultaneously
• wave-driven transport onshore close to bed
• phase-lag negative
• phase-lag decreases upward
• peak in k preceeds peak in c
• wave-driven transport onshore throughout water column
Sand concentration
Turbulence Wave-orbital motion
Sand concentration
Turbulence Wave-orbital motion
Breaking waves
above subdued ripples
0.04 m
0.10 m
0.17 m
0.04 m
0.10 m 0.17 m
0.17 m
0.17 m
