Introduction
Nearshore sandbars vary in shape from alongshore ridges to remarkably periodic alongshore undulations in cross-shore
position (crescentic sandbars). In a double sandbar system, the shape of the inner
bar may be coupled to that of the outer bar, implying that the inner bar features do not evolve independently from those in the outer bar. This coupling is similar to the correspondence between crescentic sandbars and shoreline perturbations, resulting in an alongshore alternation in beach width.
Our aim is to characterise the temporal and
spatial variability involved in the coupling within a double sandbar system.
Melbourne Northern
Territory Western
Australia
South Australia
Queensland
New South Wales Victoria Perth
Tasmania CanberraSydney Brisbane
5 km South Stardbroke Island Currigee
Nerang River
Broadbeach
Burleigh Heads
Tweed Heads Surfers
Paradise
Pacific Ocean
Currumbin Queensland
New South Wales
N
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@
Figure 7: In-phase coupling (left) generally coincides with a negative spatial lag (mode of -40 m), which means the inner barline shape is positioned to the north of the corresponding
outer bar shape. Out-of-phase coupling (right), on the other hand, exhibits a bimodal distribution of lags and either coincides with a zero-lag (mode of 0 m, exactly 180° out-
of-phase) or a positive spatial lag (mode of 100 m). The observed lags are apparent from the examples above (Figures 4, 5 and 6) and correspond to the dominant south-easterly
angle of wave incidence (30° shore-normal).
Coupled behaviour within a double sandbar system
Timothy Price, Jantien Rutten and Gerben Ruessink Utrecht University, The Netherlands
Conclusions
• The inner and outer barlines are coupled frequently (at least 40% of the time).
• The inner bar predominantly (80%) couples in-phase with a slight northward displacement with respect to the outer bar.
• Alongshore migration of crescentic patterns may either cause a phase-shift of the coupled pattern or result in the uncoupling of the composite bar system.
• Understanding the evolution of a double sandbar system requires insight into the interaction between the inner and outer bar.
Distance (m)
Distance (m)
-3000 -2500
-2000 -1500
-1000 -500
0 500
1000 1500
2000 2500
3000 0 200 400 600
Methodology
We use a 9.3-year data set of daily time-exposure images, collected at the Gold Coast, Australia (Figure 1). The high-
intensity, alongshore continuous bands in these images reflect the bar crest lines (Figure 2). Using cross-correlation, we
determine coupling between the inner and outer bar crest lines.
Results
Figure 3: Cross-correlograms of the inner and outer-bar crest lines versus time. The black contours are the 95% significance level for nonzero correlation.
The barlines couple for at least 5 consecutive days during 40% of the total period.
t.price@geo.uu.nl
Figure 2: The four oblique, 10-min time-exposure images are merged and
rectified to yield a plan view image. The two alongshore white lines represent wave breaking on the sandbars.
Figure 1: Location of the study site.
S N S N S N
A B C
10/01/06 0
250 500 750
17/01/06 0
250 500 750
26/01/06 0
250 500 750
alongshore distance (m) 30/01/06
−2500 −2000 −1500 −1000 −500 0 0
250 500 750
cross−shore distance (m)
28/08/02
05/09/02
01/10/02
alongshore distance (m) 08/10/02
cross−shore distance (m)
0 250 500 750 0 250 500 750 0 250 500 750
−2500 −2000 −1500 −1000 −500 0 0
250 500 750
22/12/02
31/12/02
04/01/03
alongshore distance (m) 08/01/03
cross−shore distance (m)
0 250 500 750 0 250 500 750 0 250 500 750
−2500 −2000 −1500 −1000 −500 0 0
250 500 750
=
=
=
Figure 4: In-phase coupling between the inner and
outer barlines.
During 80% of the coupling events the barlines are coupled in-phase (a positive correlation), that is, an outer bar horn facing a shoreward perturbation of the inner barline.
Figure 5: Out-of- phase coupling
between the inner and outer barlines.
During 20% of the coupling events the barlines are coupled out-of- phase (a negative correlation),
where the onshore welding of the
crescent horn leads to a seaward bulge in the barline.
Figure 6: Obliquely incident waves
deform the inner and outer barline features and
cause them to
migrate differently alongshore.
Coupling
disappears due to oblique wave incidence (80%
in time) or due to morphological resets during heavy storms (20% in
time).
−200 −100 0 100 200 0
20 40 60 80 100 120 140
Spatial lag (m)
Frequency (days)
In−phase coupling
−200 −100 0 100 200 0
5 10 15 20 25 30 35 40
Spatial lag (m)
Frequency (days)
Out−of−phase coupling
2000 500
0
-500
lag (m)
2001 2002 2003
B C A
2004 2005 2006 2007 2008
0 1
-1 0.5
-0.5
