Potentials - March/April 2017 - 39

and automated image processing,
which allow the field of view to be focused on the swash zone, even from
a considerable distance (i.e., from a
beach dune or nearby building).

Fig. 7 illustrates this approach applied to the swash zone by Power et al.
(2011). From the original timestack,
a difference timestack is created by
subtracting the pixel intensities of

120

140

t (s)

160

180

200

220

240

850

860

870

880
x (m)
(a)

890

900

910

150
k = 0.64
160

k = 0.92

170

180
k = 1.24
190
t (s)

(x = 0) reverses very quickly (at t ≈ 1),
much earlier than the time of the
maximum runup (t ≈ 2). In the latter
cases [Fig. 6(b)-(d)], this flow reversal occurs progressively later: in the
last instance [Fig. 6(d)], at about the
same time as the maximum runup
occurs (t≈2).
In each panel in Fig. 6, the contour for u = 0 traces the locus of
flow reversal across the swash zone,
which has a distinct gradient (dt/
dx), being large in the first case and
close to zero in the last case. However, verifying this new model with
conventional in-situ acoustic current meters was very difficult, most
notably because the high concentration of bubbles in the surf and swash
zones (as indicated in Fig. 3) and
the very shallow flow depths mean
that acoustic current meters cannot
provide data at the beginning of the
runup or at the end of the rundown.
In addition, every runup commences
at a different cross-shore location
on the beach (Fig. 5), which means
that measurements of flow velocities from an in-situ instrument are
obtained from different relative positions within the runup zone. This
makes collating data for self-similar
comparisons very difficult.
Inspired by an image shown at a
seminar at the Coastal Imaging Lab at
Oregon State University [Fig. 7(a)], we
realized that the locus of flow reversal
already existed in remotely sensed imagery. The locus of flow reversal is visible in the subtle pattern evident in the
streak lines between the bore paths,
which indicate the motion of patches
of foam in the surf and swash zones.
These streak lines trace out arcs
(which are incomplete) as the foam
moves landward and then seaward,
with the apex of each arc representing
the moment at which the flow reverses
direction. The locus of the apexes of
the arcs is then equivalent to the locus
of flow reversal indicated by the u = 0
contour in Fig. 6.
To utilize this remote sensing
approach more effectively to verify
the new swash zone model required
higher resolution data in both space
and time. This is easily achieved with
modern video cameras and optics

200
k = 0.86
210

k = 0.71

220

230

240

845

850
x (m)

855

860

845
(b)

850
x (m)

855

860

Fig7 (a) the timestack from Black's Beach, California. (Image courtesy of Prof. rob
holman.) (b) the left panel is a difference image; the right panel shows the automatically
detected loci of flow reversal (black lines) superimposed onto the original timestack.
(Images courtesy of hannah E. Power, 2011, Ph.D. thesis, University of Queensland.)

IEEE PotEntIals

March/April 2017

■

39



Table of Contents for the Digital Edition of Potentials - March/April 2017

Potentials - March/April 2017 - Cover1
Potentials - March/April 2017 - Cover2
Potentials - March/April 2017 - 1
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Potentials - March/April 2017 - Cover3
Potentials - March/April 2017 - Cover4
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