Joost Brinkkemper, Anouk de Bakker and Gerben Ruessink
Department of Physical Geography
Sand suspension beneath sea-swell waves in the shoaling and surf zone
Motivation Methods
Results
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).
Normalized phase-averaged concentration
at four locations, sorted by relative waveheigth
Conclusions
Non-breaking waves above ripples:
- phase-lag increases upwards
- thus only (onshore) wave-driven sand transport close to bed - cumulative transport is close to zero
Magnitude and direction of short-wave suspended sand transport depends highly on turbulence characteristics and ripple steepness.
Plunging breaker during BARDEXII in the Delta Flume.
Breaking waves:
- small negative phase-lag
- during offshore phase only suspension close to the bed
- onshore wave-driven transport throughout the water column - max onshore transport in outer surf-zone
-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
•
Field-scale laboratory experiment, irregular waves•
One imposed tidal cycle•
Turbulence at three heights above the bed at one location•
Sand concentration at 3-7 heights above the bed at 4 locations•
Coupling with cross-shore wave-orbital motion (uhf) through phase-averagingNon-breaking waves above vortex ripples
• phase-lag between uhf and c positive and increases upward
• c and k peak simultaneously
• wave-driven transport onshore only close to bed
• phase-lag between uhf and c negative
• peak in k preceeds peak in c
• wave-driven transport onshore throughout water column
u hf (m/s) c (kg/m3 )k (m2 /s2 ) k (m2 /s2 )c (kg/m3 )u hf (m/s)
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
0 0.2 0.4 0.6 0.8 1
t/T
0 0.2 0.4 0.6 0.8 1
t/T
z = 0.04 m z = 0.10 m
40 50 60 70 80 90
0 2 4
Cross-shore distance (m)
Elevation (m)
water level range
locations sand concentration measurements
Q hf (kg/m/s)
Ripple steepness
0 0.2 0.4 0.6 0.8 1 1.2
-0.02 0 0.02 0.04 0.06
0 0.05 0.1 0.15
-0.02 0 0.02 0.04 0.06
Relative waveheight
Cumulative transport between z = 0.04 and 0.10 m
• maximum in outer surf-zone (a)
• decreases with ripple steepness (b)
shoaling zone surf zone
a b
timing max uhf trend max c
In creasing H s/h Shoaling zoneSurf zone
