Journal
Journalof
ofCoastal
CoastalResearch
Research
SI 64
pg -- pg
2012
2016
ICS2011
ICS2011 (Proceedings)
Poland
ISSN 0749-0208
Beach Morphodynamics and Characterization of Sand Bars in
Mesotidal Environments
M. G. Albuquerque†, L. J. Calliari‡, R. M. C. Guedes∞ and D. P. Paula§
†Departamento de
Geoprocessamento
IFRS, Rio Grande
96201-900, Brazil
[email protected]
‡Instituto de
Oceanografia
FURG, Rio Grande
96201-900, Brazil
[email protected]
∞Department of Earth and Ocean
Sciences.
Waikato University
3240-Hamilton
New Zealand
[email protected]
§ Centro de Investigação Marinha e
Ambiental
UALG, Algarve
8005-139, Portugal
[email protected]
ABSTRACT
Albuquerque, M. G., Calliari, L. J., Guedes, R. M. C. and Paula, D. P., 2011. Beach Morphodynamics and
Characterization of Sand Bars in Mesotidal Environments. Journal of Coastal Research, SI 64 (Proceedings of
the 11th International Coastal Symposium), – . Szczecin, Poland, ISSN 0749-0208.
High frequency monitoring aims at characterizing the changes that occur in beach environments and the mobility
of sand bars near the coast. This study was carried out on the coast of Futuro Beach, Fortaleza (CE), in the
northeast of Brazil. From June to July 2007, daily profiles of high frequency were carried out for 30 days in a
row in order to characterize the position and migration pattern of the sand bars in an environment of sediments
with different modal classes. The morphodynamics of the beach was characterized according to the conceptual
model proposed by Masselink and Short (1993) to classify the effects of tides in beach morphology. For this
study, video images were also collected to assess sand bars in the area. The video-imagery data showed that there
are two sand bars on that beach: one of them, which emerges at the low tide, is located near the coast in the
intertidal zone and the other one, which is constantly submerged, is located farther from the coast. The width of
the beach profiles varied between 80 m and 137 m with average migration of sand bars of 4.08 m/day. The
surveys also enabled to verify that the average value of the sand bar migration was 4.64 m/day at three
monitoring points. Taking into account that the causes of the migration processes and the behavior of sand bars
are not fully understood yet, studies of changes registered in beach profiles, such as the migration rates of sand
bars in the surf zone, are important to comprehend these phenomena and to select the best monitoring frequency.
ADDITIONAL INDEX WORDS: beach morphodynamics, profile of high frequency, bar migration.
INTRODUCTION
Sand bars are morphological features predominantly parallel
to the shoreline, formed in shallow waters as a response to the
action of hydrodynamic variables. They are typical of many sandy
beaches around the world, particularly those characterized by
unconsolidated sediments of sand fractions with average grain size
between 0.1 and 0.5 mm (Lau and Travis, 1973). Given the
dynamic and morphological characteristics exhibited by highly
variable sandy bars, they have been the subject of many studies
(Guedes et al., 2009). The understanding of physical processes
related to the interaction between the bar system and the surf zone
is very important to the management of coastal areas since the bar
systems are significant reserves of sand which protect the beach
from the action of storm waves (Lippmann and Holman, 1990).
Changes in the height and the position of these features are the
first hint of the variability of the beach profile (Sallenger and
Holman, 1993). Therefore, a better understanding of the migration
processes of sand bars and of high frequency monitoring is
necessary not only to promote a reliable characterization of the
changes that occur in the beach environment but also to
characterize the mobility of banks near the coast. Besides
information on the morphodynamics of the environment, prior
knowledge of the behavior of sandy bars and of their positions is
important since they have a significant effect on the coast with
respect to the evolution and stability of the beach profile, both in
the short and in the long term. It is also essential to understand the
mechanisms of previous cross-shore sediment transport. Studies
by Davis (1973) discuss the formation of two types of bars
associated with wave actions: swash bars and break point bars.
The former are formed under conditions of well-defined ripples
and low wave steepness, where the movement of sediment
shoreward can eventually be welded to the submerged part of the
beach being periodically exposed during tides. The latter are
associated with storm waves, with high steepness they are always
below the water level with an exchange of sediment between the
submerged and subaerial parts of the beach. The morphology of
the bars indicates the temporal and spatial variability of sediments
along the coast. There are several discussions about the formation
of bars and the processes associated with their migration, which is
not well understood yet (Hoefel and Elgar, 2003). Therefore, this
study aims to characterize the morphodynamic behavior and the
mobility of sandy bars on Futuro beach, a mesotidal beach located
in the northeastern coast of Brazil and composed of variable sized
grains.
STUDY AREA
Futuro Beach (Figure 1) is located in the metropolitan area
close to Fortaleza and is one of the most touristic beaches in
Brazil, specially for swimming and leisure. It is also characterized
Journal of Coastal Research, Special Issue 64, 2011
2012
Morphodynamic in mesotidal environments
Ruessink, 2001) since these images reduce temporal variability in
the breaking waves that naturally occur as a function of the pattern
in the random wave field. Thus, hourly averaged images were
created by video recordings using a digital camera (Sony)
positioned on the top of a 5m tower. The recordings were made
during the transition between the low and the high tide, on July
7th, 2007, between 08:24 am and 01:24 pm. The collection of
hourly samples lasted 10 min, totalizing a 1-hour sample. Since
the beach morphodynamic classification model proposed by
Wright and Short (1984) is limited to microtidal beaches, we used
the conceptual model presented by Masselink and Short (1993)
(Equation 1) to characterize the morphodynamic behavior of
Futuro beach.
(1)
This beach model classifies beaches according to wave, tide
and sediment characteristics, parameterized by the dimensionless
fall velocity which is also known as Dean’s parameter (Equation
2) and by the relative tide range (RTR). Dean’s parameter (1973)
expresses the difference in the type of morphodynamics and the
RTR by taking into account variations in the mean spring-tide
range (TR) for the modal wave breaker height (Hb). For RTR
values <7 and Ω = 3, the intermediate stage low terrace bar/rip is
characterized whereas for RTR <15 and Ω = 5, the
morphodynamic ultra dissipative stage is characterized. For values
of RTR> 15, the beach is, in fact, a tidal flat.
Figure 1. Map of the area under study.
by a mesotidal regime and variable grain size distribution along its
8 km of coastline. During the low tide, sand bars and deep
channels can be identified in the area under study. At high tide, the
area favors the formation of rip currents, making the environment
susceptible to drowning accidents.
METHODS
The shoreline was mapped using aerial photography and
remote sensing and sub-divided in three areas (Figure 1) in order
to carry out high frequency surveys for 30 days in a row. The
beach morphology was obtained by applying the methods
proposed by Birkemeier (1985), with the aid of a total station,
model Topcom, a prism and a geologist's compass for the
alignment of the profiles. Readings were taken at points of
inflection in the beach sectors (berm, swash zone and surf zone).
Initial assessments of topographic profiles were determined
according to the benchmark established by the Diretoria de
Hidrografia e Navegação (DHN). Significant wave height and
period were obtained by visual observations, according to the
methodology proposed by Melo (1993). The determination of the
number of bars was carried out by using video images, according
to the methods proposed by Lippmann and Holman (1989). Some
studies, such as the ones published by Lippmann and Holman (op
cit.), Holman and Stanley (2007) and Guedes et al. (2009) have
shown that the preferential breaking of waves on the submerged
bars results in a pattern that can be detected by remote sensing,
since the foam on the surface, resulting from the break, is
characterized by bright colors which are very different from the
ones in adjacent areas. Composite images for temporal average of
pixel intensity values have been successfully used for estimating
the position of bars and flat morphology (e.g., by Lippmann and
Holman, 1989; Lippmann and Holman, 1990; Van Eckevort and
(2)
Omega (Ω) values were obtained using the average fall
velocity of sedimentary particles (Ws) extracted from Raudkivi’s
table (1990). Gibbs et al.’s equation (1971) was applied, including
the same corrections for natural grains calculated by Baba and
Komar’s equation (1976).
RESULTS AND DISCUSSION
Morphodynamic Behavior
Point 01
In Section 01, characterized by the presence of coarse
sediments in the low swash zone and in the initial portion of the
surfzone, the predominant morphodynamic state was a low tide
terrace, with RTR ranging from 3.6 to 8.6. The presence of coarse
sediments in that area can be attributed to the adjacent beach rocks
which are subject to erosion caused by physical and chemical
weathering and by wave action. The eroded material is available
in the surfzone and is subject to transport processes and local
remobilization by littoral drifts. Another possible source of coarse
sediments may be associated with material that is carried through
the mouth of the Coco River, especially in rainy periods, when the
highest outflow of the estuary is recorded. With respect to volume
variation, the location presented an average growth of the beach
envelope (profiles) of 11.72 m³/m, more significant variations
between the berm and the medium swash zone, especially on the
3rd and on the 4th monitoring day, and between the 26th and the
30th monitoring day. Subaqueous sandy bars were observed in
98% field surveys. The shallowest bar is located 117 m from the
reference level, approximately 89 m from the average position of
the waterline in the low swash zone. The deepest bar was formed
149 m from there and is positioned in the surfzone. The average
speed of migration of sand bars in the area under study was
Journal of Coastal Research, Special Issue 64, 2011
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Albuquerque et al.
This process is described by Woodroffe (2003), who pointed out
that the outer sand bars have less mobility at high tide. At
locations classified as low tide terrace, the swash processes can be
very important, forming large sand bars at low tide (King, 1972).
Point 02
Sand bar
Figure 2. Averaged images collected at the low tide.
Sand bars
Point 02, classified as morphodynamic low tide bar/rip
(RTR ranging between 3.06 and 5.25), shows high variability in
the beach envelope, especially at medium and low portions of the
profiles. The location is characterized by medium grain size in the
low swash and surfzone and by the presence of rip currents,
especially in the transition from high tide to low tide, with
transverse orientation of the coast. The mean volume of the beach
envelope was 13.11 m³/m. The main changes were recorded on the
2nd and 3rd monitoring day and between the 21st and the 30th
monitoring day, from 135 m to 170 m, forming scarps at 74 m.
The location is characterized by two sand bars: one at low depth,
located 85 m from the average position of the waterline, and the
other, at high depth, at 179 m from the waterline. At this location,
the average bar migration speed was 4.17 m/day, with the highest
peaks occurring on the 15th monitoring day and between the 19th
and the 20th monitoring day.
Point 03
Figure 3. Averaged images collected at the high tide
4.08m/day, with the highest peaks of migration between the 15th
and the 30th monitoring day. Values of significant wave height
(Hs) ranged between 1.1m and 1.5 m, and offshore bar migration
was observed. With values of Hs below 0.3 m, the bars migrated
to the coast. This observation agrees with the ideas proposed by
Short (1999): the formation of sand bars is associated with
significant changes in wave height. Short (op cit.) also highlights
that in a beach system, the inner bar has the largest mobility, given
that it is subject to stronger wave action. According to Calliari et
al. (2003), inner bars have higher mobility and are more unstable
than outer bars, since they require higher energy levels to be
mobilized. Video-imagery data collected at Point 01, obtained
from the transition from low tide (Figure 2) to high tide (Figure 3),
show the presence of two sand bars in the area, coinciding with
the beach profile data.
At low tide, the outer bar is susceptible to hydrodynamic
processes related to the surfzone. The shallowest bars are
subjected to the action of the swash processes and are more
susceptible to hydrodynamic factors in the high tide, assuming the
condition of a low tide terrace at low tide. From the images, we
can see that the white pixel in Figures 2 and 3 show that the bars
have a straight morphology and little three-dimensionality. The
intertidal bar is exposed at low tide; however, it becomes active at
high tide, providing an active wave breaking over it. In this
context, the surfzone plays a key role in the migration of bottom
features as the tide level changes. At high tides, high depths over
the bar crest make the second bar be less susceptible to the effects
of bottom currents and, thus, it begins to assume low mobility.
Point 03 showed an average pattern of bar migration around
5.67 m/day, between 154 m and 244 m, from the benchmark level.
The highest peaks were recorded between the 18th and the 20th
monitoring day. The location was classified as an ultradissipative
morphodynamic state, with RTR values between 6.7 and 9.0, and
volume changes in the package of profiles in the order of 12.77
m³/m. The highest percentages of change were recorded between
the 2nd and the 3rd monitoring day, and between the 21st and the
last monitoring day. In comparison with the other sites where
sampling was carried out, P03 was the one with the smallest
variations in the topographic profile. For dissipative beaches,
Dally and Dean (1984) and Sallenger and Howd (1989) have
suggested that the formation of bars and their migration processes
are caused by undertow. According to Wright and Short (1984),
dissipative beaches have a strong undertow circulation, where the
constant action of waves allows the mobility of sediments in the
surfzone and the swash zone, so the swash and wave breaking
processes are more energetic than the shoaling and are important
overall, even though they act on the profile for a short time span
(Bernabeu et al., 2002).
The three profiles we have studied assume distinct
morphodynamic states during periods of high and low tide. At
high tide, Points 01 and 02 take on a reflective character and, at
low tide, started to behave as intermediate morphodynamic states.
Profile 03 has both characteristics at high tide and dissipative ones
at low tide. Changes in beach topography are associated with the
time that the swash, the surfzone and shoaling processes act
significantly on the beach profile so that the morphological
changes decrease slowly and the ratio of sediment supply
decreases as well. According to Wright and Short (1984), beaches
can vary from one morphodynamic state to another, depending on
changes in the level of wave energy. Consequently, the
morphology of the bars can usually be changed considerably over
time on the same beach due to hydrodynamic changes (Guedes et
al., 2009). The average bar migration pattern of the three sites
(Figure 4), taking into account the results of the three sites, was
4.64 m/day. Thornton et al. (1996) and Gallagher et al. (1998)
Journal of Coastal Research, Special Issue 64, 2011
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Morphodynamic in mesotidal environments
REFERENCES
Figure 4. Bar migration analysis at three points during a month.
have considered that the seaward migration occurs under
conditions of more energetic incident waves, and driven by strong
currents offshore, which are at their maximum near the crest of the
bank. In the case of Futuro beach, this pattern is clearly observed
in Sections 02 and 03. In contrast, Hsu et al. (2006) have
considered that the onshore bar migration is observed when the
incident wave energy is moderate and the currents are relatively
weak. On Futuro beach, this behavior is described in Section 01,
which is characterized by weak currents in the surfzone.
CONCLUSION
Based on our observations, we can conclude that the
monitored points on Futuro Beach display different
morphodynamic behavior due to changes in the grain size.
Through video images, we observed the formation of two sand
bars in the profile located in Section 01. One of the bars was
located near the coast and played the role of a low tide terrace
during low tide periods. The other one was located in the outer
profile and took on a reflective state during the high tide. The
shallowest bar displayed higher mobility in relation to the external
one since it underwent variations due to the rise and fall of the
tide, the action of local currents. Regarding the migration pattern
of sand bars, higher values of significant wave height induced
seaward bar migration and low value onshore migration. The
highest migration peaks were recorded between 06:00 am and
08:00 am and between 11:00 am and 03:00 pm at all the sampled
points. These periods were characterized by having a more
turbulent surfzone, with significant wave heights ranging from 1.2
m to 1.5 m. The average speed of bar migration was 4.64 m/day;
the highest values were recorded at P03 (5.67 m/day) followed by
P02 (4.17 m/day). Peaks of low or no migration occurred mostly
in late afternoon surveys; at this time, the surfzone had a more
turbulent pattern due to strong winds blowing shoreward.
ACKNOWLEDGEMENTS
The authors thank the Laboratório de Geologia e
Geomorfologia Costeira e Oceânica (LGCO) at the Universidade
Estadual do Ceará for collaborating on the field work, the
Laboratório de Oceanografia Geológica (LOG) at the
Universidade Federal do Rio Grande for the logistic support,
CAPES for granting the scholarship for the field work and the
Conselho Nacional de Desenvolvimento Científico e Tecnológico
(CNPq) for granting a doctoral scholarship.
Baba, J. and Komar, P., 1981. Measurements and analysis of
settling velocities of natural quartz grains. Journal of
Sedimentary Petrology, 51 (2), 631-640.
Bernabeu, A.M.; Medina, R. and Vidal, C., 2002. An
Equilibrium profile model for tidal environments. Scientia
Marina, 66(4), 325-335.
Birkemeier, W.A., 1985. A User’s Guide to ISRP: The
Interactive Survey Reduction Program. Vicksburg,
Mississippi: U.S. Army Corps of Engineers, Instructions
Report CERC 84-11, 101p.
Calliari, L.J.; Muehe, D.; Hoefel, F.G., and Toldo Jr., E.E.,
2003. Morfodinâmica praial: uma breve revisão. Revista
Brasileira de Oceanografia, 51, 63-78.
Dally, W. and Dean, R.G., 1984. Suspended sediment
transport and beach profile evolution. Journal Waterway,
Port, Coastal and Ocean Engineering, 110, 15-33.
Davis, J.C., 1973. Statistics and Data Analysis in Geology.
New York: Wiley-Interscience, 550p.
Dean, R.G., 1973. Heuristic models of sand transport in the
surf zone. Proceedings of the International Conference on
Coastal Engineering Dynamics in the Surf Zone (Sidney,
N.S.W.), pp. 208-214.
Gallagher, E. L.; Elgar, S., and Guza, R.T., 1998. Observations
of sand bar evolution on natural beach. Journal of
Geophysical Research, 103, 3203-3215.
Gibbs, R.T.; Mathew, M.D. and Link, D.A., 1971. The
relationship between size and sorting velocity. J. Sedim.
Petrology, 41, 07-18.
Guedes, R.M.C.; Calliari, L.J. and Pereira, P.S., 2009. Beach
and surfzone morphodynamics of Cassino beach measured
using vídeo imagery and beach profiling. Pesquisas, 36(2),
165-180.
Hoefel, F. and Elgar, S., 2003. Wave-induced sediment
transport and sandbar migration. Science, 299, 1885-1887.
Holman, R.A. and Sallenger Jr., H., 1993. Sand bar generation:
a discussion of the Duck experiment series. Journal of
Coastal Research, 15, 76-92.
Holman, R.A. and Stanley, J., 2007. The history and technical
capabilities of Argus. Coastal Engineering, 54, 477-491.
Hsu, T.; Elgar, S, and Guza, R.T., 2006. Wave-induced
sediment transport and onshore sandbar migration. Coastal
Engineering, 53, 817-824.
King, C.A.M., 1972. Beaches and Coasts. London: Edward
Arnold, 570 p.
Komar, P.D., 1976. Beach Process and Sedimentation. Upper
Saddle River, N.J.: Prentice-Hall, 429p.
Lau, J. and Travis, B., 1973. Slowly varying stokes waves and
submarine longshore bars. Journal of Geophysical Research,
78 (21), 4489-4497.
Lippmann, T.C. and Holman, R.A., 1989. Quantification of
sand bars morphology: a video technique based on wave
dissipation. Journal of Geophysical Research, 94, 995-1011.
Lippmann, T.C. and Holman, R.A., 1990. The Spatial and
temporal variability of sand bar morphology. Journal of
Geophysical Research, 95 (C7), 11.575-11.590.
Masselink, G. and Short, A.D., 1993. The effect of tide range
on beach morphodynamics. A conceptual beach model.
Journal of Coastal Research, 9,785-800.
Journal of Coastal Research, Special Issue 64, 2011
2015
Albuquerque et al.
Melo, E., 1993. The sea sentinels project: watching waves in
Brazil. Proceedings of the 8th Symposium on Coastal and
Ocean Management (New Orleans, EUA), pp. 505-517.
Raudkivi, A.J., (ed), 1990. Loose Boundary Hydraulics. New
York: Pergamon Press, Inc., 538p.
Sallenger, A.H. and Howd, P.A., 1989. Nearshore bars and the
break-point hypothesis. Coastal Engineering, 12, 301-313.
Short, A.D., 1999. Beach hazards and safety. In: SHORT, A.D.
(ed), Handbook of beach and shoreface morphodynamics:
Editions by Jonh Wiley & Sons, pp. 293-303.
Thornton, E.B.; Humiston, R.T., and Birkemeier, W., 1996.
Bar/trough generation on a natural beach. Journal of
Geophysical Research, 101(C5), 12,097-12,110.
Van Enckevort, I.M.J. and Ruessink, B.G., 2001. Effect of
hydrodynamics and bathymetry on video estimates of
nearshore sandbar position. Journal of Geophysical Research,
106 (C8), 16969-16979.
Woodroffe, C.D., (ed), 2003. Coasts: Form, Process and
Evolution. New York: Cambridge University Press, 623 p.
Wright, L.D. and Short, A.D., 1984. Morphodynamic variability
of surf zones and beaches: a synthesis. Marine Geology, 56,
93-118.
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