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Contribution of the swash generated by low energy wind waves in the recovery process of a beach
impacted by extreme events: Nha Trang, Vietnam
Contribution of swash processes generated by low energy wind waves
in the recovery of a beach impacted by extreme events: Nha Trang,
Vietnam
Jean-Pierre Lefebvre†, Rafael Almar†,
†, Nguyen Trung Viet‡, Din Van Uu∞, Duong Hai Thuan‡,
‡, Lê Thanh
Binh+, Raimundo Ibaceta*, Nguyen Viet Duc
Duc@
†IRD-LEGOS
Université Paul
Sabatier/CNRS/CNES/IRD
Toulouse, France
jean-pierre.lefebvre@ird.fr
rafael.almar@ird.fr
‡ Water Resources University
Faculty of Marine and Coastal Eng.
Hanoi, Vietnam
nguyentrungviet@wru.edu.vn
duonghaithuan@gmail.com
∞ Hanoi University of Sciences
Departement of Oceanology
Vietnam National University
Hanoi, Vietnam
uudv50@gmail.com
+ Hydraulic Engineering Consultants
Hydrology and Environment Division
Hanoi, Vietnam
Lebinh.hec@gmail.com
*Universidad Técnica Federico Santa
@ Central Vietnam Construction and
Consultancy, JSC
Vietnam
nguyenvietducht1978@gmail.com
Maria, Valparaíso, Chile
raimundo.ibaceta@alumnos.usm.cl
www.cerf-jcr.org
ABSTRACT
Lefebvre, J.-P., Almar, R
R., Viet, N.T., Uu, D.V., Thuan, D.H., Binh, L.T., Ibaceta, R., Duc
Duc,N.V., 2014 Contribution of
swash processes generated by low energy wind waves in the recovery of a beach impacted by extreme events:
th
Nha Trang, Vietnam . In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13 International Coastal Symposium
(Durban, South Africa), Journal of Coastal Research, Special Issue No. 66, pp. xxx-xxx,
xxx, ISSN 0749
0749-0208.
www.JCRonline.org
Nha Trang beach experiences southerly sediment drift during winter monsoons and northerly during summer monsoons.
In addition, the area is likely to be impacted by tropical storms or typhoons. Due to the presence of islands at the south
east border of the bay, the strongest impact on the shoreline apart from extreme events is due to NE swell from October
to April. The
he mechanism responsible for the sediment drift generated by low energetic locally generated wind waves is
insufficiently understood. It involves the functioning of the swash zone for weak conditions. Two field experiments
were scheduled before and after tthe period of cyclonic activity. The aim of the first experiment was to describe the
site’s bathymetry and the geomorphology of the upper beach and hydrologic functioning of the bay. A new method of
measurements in the swash and surf zone based on processin
processingg of data extracted from HD video processing was tested
successfully for wind wave conditions in a reflective beach. Here, we present some data obtained during the field
experimentt at different time scales. The data provides a first quantification of the impact
mpact on sediment transport from
typical low energy condition
conditions which are encountered during spring and summer in Nha Trang.
ADDITIONAL
ITIONAL INDEX WORDS: swash zone, surf zone, low energetic waves regime,, video processing, typhoon..
INTRODUCTION
Vietnam experiences a tropical monsoon climate
climate, dominated
from April to September by southwest monsoon
monsoons and from
October to late March or early April, by northeast monsoon
monsoons.
From May to November, central Vietnam is likely to be impacted
by tropical storms or typhoons, with possible events occurring
outside this period (Takahashi, 2011; Nguyen-Thi
Thi et al., 2012). In
2013, the site was impacted in October by typhoon Nari and in
November by super typhoon Haiyan. Although regularly impacted
by these extreme events, the
he economy of Khanh Hoa province is
largely related to seaside tourism activities. At present, the hotel
and catering industry is intensifying in the vicinity of the beach of
Nha Trang. Although typically containing low energy
energy, these short
(largely wind-) waves tend to counterbalance the result of
southward drift by a transport of sediment. The swash zone likely
plays a significant role in this recovery, due particularly to its
infra-gravity oscillation (Masselink
Masselink and Hugues, 1998; Butt and
Russel, 2000).
Because of its shallow, turbulent and unsteady nature,
measurements of the physical parameters in the swash zone using
conventional data collection systems can be highly problematic
(Longo et al., 2002; Jackson et al.,
al. 2007; Gómez-Pujol et al.,
2011). Recently, various field studies involving increasingly
complex experimental set-ups
ups have been conducted (Blenkinsopp
et al., 2011; Almeida et al.,, 2013). More than a decade ago,
different techniques taking advantage of video have been proposed
(Holman et al.,, 1993; Foote and Horn, 1999; Holland et al., 2001;
Vousdoukas et al., 2014).
The present paper provides a description of the first of two field
experimentss scheduled before and after the cyclonic season, one
during the summer monsoon, the other during the winter
monsoon. In order to investigate the role of the swash zone in the
Journal of Coastal Research, Special Issue No. XX, 2014
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Lefebvre et al.
recovery phases generated by low energy waves, different
techniques were used to collect data at different time scales (swash
event scale, tide scale and month scale). A new method based on
video monitoring of swash and surf zone was also tested for low
energetic, high-frequency conditions.
METHODS
Site presentation
A field experiment was conducted from 26th to 30th May, 2013
in Nha Trang, Vietnam facing the China Sea. Located in a semienclosed bay, the beach of Nha Trang (12°15'N–109°11'E) is
sheltered from SE winds and waves by Tre, Mieu and Tam
islands. Apart from extreme events, the beach is impacted by
waves generated locally by moderate south-eastern winds and by
north-eastern swells responsible for a strong southward longshore
sediment drift. The beach is 7 km long, oriented South-North and
composed by medium sand (φ = 2). The average slope in the
swash zone is of approximately 6°, and approximately 3° in the
surf zone. The tide is of micro-tidal varying from mixed to
diurnal.
Figure 1. Nha Trang bay. Site of experiment is circled in red.
Data
Instruments and Measurements
A suite of instruments were deployed along a cross-shore
transect from beyond the depth of closure to the swash zone
(Figure 2). The incident wave parameters were measured by two
AWAC (Nortek) moored together with two grain size analyzers
(LISST-25X, Sequoia Scientific, Inc) at depth 7.0 and 9.2m,
respectively. A pressure type wave gauge DNW-5M and a multiparameter sensor: water height, wave height and period, turbidity)
(OBS-3A, OSIL), both coupled with an electromagnetic current
meter (COMPACT) were moored at depth 4.8m and 2.6m,
respectively. Measurement of the bathymetry of the bay was
achieved with a dual frequency echo-sounder during the first day
of the field experiment. Daily topographic measurements of the
beach were conducted with a theodolite (Topcon, GTP101) and
differential GPS.
The experimental setting was completed by an alignment of 20
black painted metallic poles each with a 30mm wide red tape
adhered near their tops. They were deployed cross-shore from the
backshore toward the surf zone; the 13 landward-most poles were
spaced one meter apart and the last 7, two meters apart. The actual
position and elevation of the top of each pole was measured with a
theodolite. A high definition video camera (HDR-CX 250, Sony)
was used to monitor the waves along the poles over daylight hours
with an acquisition rate of 25Hz. A micro-profiler ADV (Vectrino
II, Nortek) was deployed in the vicinity of the poles, at either end
of the surf zone or at the highest point of the run up, following the
tide-induced displacement of the swash zone. Topographic
surveys were carried out three times a day with a theodolite from
the backshore up to approximately 1.5 meter depth. During two
days, hourly surveys of the elevation of the bed at each pole, was
conducted by manually measuring the elevation between the top
and the base of each pole. This pole-related measurement was
referenced with the 3 dimensional position of the top of the pole.
A permanent video station made up of two video cameras
(IP7361 Vivotek) deployed on the same lamp-post, one pointing
to the North, the other installed before the field experiment. This
system allows a real time evaluation of changes in the intertidal
bed elevation. The description of the system and image
processing are detailed in Almar et al. (2014). The bed elevation
of the intertidal domain is obtained by plotting the averaged water
line.
Figure 2. Experimental setup. Left: AWAC and LISST 25X (B), pressure sensor and electromagnetic current meter (C), multiparameters sensor OBS-3A (D), poles and micro-profiler ADV (E). Right: photo of alignment of poles and the ADV micro-profiler.
Journal of Coastal Research, Special Issue No.XX, 2014
Contribution
ontribution of the swash generated by low energy wind waves in the recovery process of a beach
impacted by extreme events: Nha Trang, Vietnam
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Video processing
The wave height in the swash and surf zone simultaneously at
each pole was obtained at a sample rate of 25Hz using an
unsupervised method. In each frame of a video recording of 250s
duration, the red, green and blue component of the pixels located
on lines passing along each pole are extracted and stacked into a
enhancement of timestack. Initial
Figure 3. Normalization-enhancement
timestack (a) RGB intensity normalized stack (b) Mask obtained
by selection of ochre-red
red color band (c) enhanced stack obtained
by masking of the RGB intensity normalized stack (d).
matrix. The red component is used to detect the width of the tape
and to calculate the pixel to mm conversion factor for a given
pole.
In order to minimize the influence of variation of ambient light,
uncertainty between
een water with suspended sediment and bed,
variationn of bed color with saturation, shadow and reflection of the
pole on wet sand amongst other sources of perturbation, the stacks
are normalized and enhanced in hue and intensity. First, the
intensity of each red, green and blue raster is maximized
separately. The stack is then converted in Hue
Hue-Saturation-Value
coding and thee ochre to red band corresponding to the sediment
color is enhanced. A binary mask is obtained and the smallest gaps
are filled. This maskk is finally applied to the enhanced stack which
reduces the noise in the final stack and enhances the transitions
between water and pole (Figure 3). The water level is obtained by
detection of dark /light transition in the blue raster of the enhanced
stack. The wave spectral density is calculated from the temporal
water level. The bed is estimated by statistical analysis of the time
variations of detected water level. In order to limit the influence of
uncertainty of the method, only emerged episode of mor
more than one
second are considered, and location with at least 1/20 of time
emerged are considered as in the swash zone
PRELIMINARY RESULTS
The field experiment was carried out from neap to neap tide,
with a tide range varying from 1.2 to 1.8m. The wave
waves originated
mostly from the SE with a mean height of approximately 0.2m
(figure 4).
The wave spectral density obtained from the processing of
sequences of 250s of swash-surf video indicated a rapid
dissipation of 75% of the wave power at the transition betw
between the
surf zone to the swash zone (figure 5).and a peak period of 2.7s
which is consistent with the measurements by the AWACs
Figure 4. Mean
ean height in meters of waves measured at B
during the field experience (left) and percentage of occurrence
of the wave from a given direction (right)
(right).
(2.15s). A group period of 20s was observed in the upper swash
zone. At the upper part of the swash zone, the bed evolution
showed an accretion at the rate of 0.04mm s-1 (figure 6).
The hourly monitoring of bed elevation measured manually at the
eleven first poles of the alignment (swash and surf zone) during an
ebb tide (29/05 - tide range 1.47m) and a rising tide (30/09 - tide
range 1.36m) is presented in figure 4, the distance between poles
measured from the most landward one. During the two days, the
waves originated from the ESE with a mean height and period of
0.30m and 3s, respectively, the 29/05 and 0.19m and 6s, the 30/05.
Figure 5. Decay of wave spectral density at the boundary
between surf zone and swash zone (red). The bed elevation is
represented in blue with the circle indicating the position of
the poles 5 to 9 (from left to right). The dashed line marks the
tidal height at the moment of the measurement
measurement.
At the beginning of ebb tide, the upper swash zone was
accreting (from reference to 3.9m seaward) and the surf zone was
eroding. After mid-tide,
tide, (tide elevation of 0.3m) a significant
accretion was noted in the surf zone (poles between 7m to 8.9m
seaward from
rom reference). At the end of ebb tide, an accretion was
measured at every emerged pole. During 8 hours of the ebb tide,
the 10m portion comprising the swash zone and a part of the surf
zone accreted at the rate of 0.8cm m-22 h-1 (Figure 7).
Journal of Coastal Research,
Research Special Issue No. XX, 2014
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Lefebvre et al.
Figure 6. Monitoring of variation of bed elevation in the swash- surf zone from pole #5 to #9 (top to bottom). The water height
detection is plotted on the timestack in yellow and the detected bed in red (centre). The corresponding water spectral density is shown
with bed elevation (for pole located in the swash zone) (left).
Journal of Coastal Research, Special Issue No.XX, 2014
Contribution of the swash generated by low energy wind waves in the recovery process of a beach
impacted by extreme events: Nha Trang, Vietnam
From the video survey system, we extracted at a different date,
the bed elevation in the intertidal zone along a cross-shore line
close to the field experiment (figure 8). The area was accreting
from May to October. After 12th October, significant erosion
occurred, probably due to the North-East wave regime associated
with winter monsoons. In the night of 13th and 14th October, Nha
Trang experienced strong waves associated with the passage of
5
typhoon Nari. The effect of this event caused an accretion that
counter-balanced the erosion generated by the NE swell of the
winter monsoon. During the night of 10th and 11th November, Nha
trang was impacted by waves generated by the passage of super
typhoon Haiyan (figure 9).
DISCUSSION AND CONCLUSIONS
During the first field experiment of our two year project, the
hydrodynamics of the site was described both by conventional
techniques (mooring of wave gauges and current meters,
bathymetry and topography surveys) and by less usual and low
cost methods (video survey of the beach, high frequency
monitoring of swash and surf zone). The processing of
information extracted from surveys allows a quantification of
variation of bed elevation of the intertidal domain, in a radius of
approximately 1 km around the video survey station. The setting
successfully resisted to two major tropical storms induced by the
passage of typhoons Nari and Haiyan.
The measurements of bore propagation in the swash and surf
zone by video acquisition of poles proved to be efficient. It
provided consistent wave spectral density for location spaced
every meter. The water height was detected precisely enough to
allow a wave to wave analysis of the run up and run down. The
simultaneity of measurements at different locations is inherent to
the method. The bed can be detected even between short laps
between a run up and run down event. The field experiment
confirmed the sediment transport landward due to the swash
generated by weak wave. Finally, the asymmetry of sediment
transport in the surf and swash zone for the rising and ebb tide
during similar wave conditions was shown.
ACKNOWLEDGEMENT
Figure 7. Monitoring of variation of bed elevation in the swashsurf zone.
The work described in this publication was supported by the
Vietnamese Ministry for Science and Technology (BKHCN/NDTHD/2013/110). We thank the National Institute of Oceanography
of Nha Trang for its help in the preparation and carrying out of
this field experiment.
Figure 8. Bed elevation of the inter-tidal domain estimated by processing of images from the video survey (left) trajectory of typhoon
Nari (right).
Journal of Coastal Research, Special Issue No. XX, 2014
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Lefebvre et al.
Figure 9. Monitoring of the location of field experiment: left: before and after typhon Nari (13 October): 12/10(a), 14/10 (b), 16/10(c)
and 18/10 (d) and right: before and after typhoon Haiyan (10 November): 9/11 (e), 10/11 (f), 11/11(g) and 12/11 (h).
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