Contribution of swash processes in the recovery of a beach

Journal of Coastal

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

in the recovery of a beach impacted by extreme events: Nha Trang,

Vietnam

Jean-Pierre Lefebvre†, Rafael Almar†, Nguyen Binh+, Raimundo Ibaceta*, Nguyen Viet Duc

†IRD-LEGOS

Université Paul

Sabatier/CNRS/CNES/IRD

Toulouse, France

[email protected]

[email protected]

‡ Water Resources University

Faculty of

Hanoi, Vietnam

[email protected]

[email protected]

+ Hydraulic Engineering Consultants

Hydrology and Environment Division

Hanoi, Vietnam

[email protected]

*Universidad Técnica Federico Santa

Maria, Valparaíso, Chile

[email protected]

INTRODUCTION Vietnam experiences a tropical monsoon climate

from April to September by southwest monsoon

October to late March or early April, by northeast monsoon

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

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 economy of Khanh Hoa

largely related to seaside tourism activities. At present, the

and catering industry is intensifying in the vicinity of the

Nha Trang. Although typically containing low energy

(largely wind-) waves tend to counterbalance the result of

southward drift by a transport of sediment. The swash zone

plays a significant role in this recovery, due particularly to its

ABSTRACT

Lefebvre, J.-P., Almar, R

swash processes generated by low energy wind waves in the recovery of a beach impacted by extreme events: Nha Trang, Vietnam .

(Durban, South Africa),

Nha Trang beach experiences

In addition, the area is likely to be impacted by tropical storms or typhoons. Due to

east border of the bay, the strongest

to April. The mechanism

insufficiently understood. It involves

were scheduled before and after t

site’s bathymetry and the

measurements in the swash and surf zone based on processin

successfully for wind wave conditions in a

experiment at different time scales. The data provides

typical low energy condition

ADDITIONAL INDEX WORDS:

www.JCRonline.org

Journal of Coastal Research, Special Issue No. XX, 2014



processes generated by low energy wind waves


†, Nguyen Trung Viet‡, Din Van Uu∞, Duong Hai Thuan‡, Lê Thanh , Nguyen Viet Duc@

Water Resources University

Faculty of Marine and Coastal Eng.

Hanoi, Vietnam

[email protected]

[email protected]

∞ Hanoi University of Sciences

Departement of Oceanology

Vietnam National University

Hanoi, Vietnam

[email protected]

Universidad Técnica Federico Santa

Maria, Valparaíso, Chile

[email protected]

@ Central Vietnam Construction and

Consultancy, JSC

Vietnam

[email protected]

tropical monsoon climate, dominated

southwest monsoons and from

northeast monsoons. Vietnam is likely to be impacted

with possible events occurring

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

he economy of Khanh Hoa province is

activities. At present, the hotel

is intensifying in the vicinity of the beach of

energy, these short

waves tend to counterbalance the result of

southward drift by a transport of sediment. The swash zone likely

significant role in this recovery, due particularly to its

infra-gravity oscillation (Masselink and Hugues, 1998;

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.

2011). Recently, various field studies involving increasingly

complex experimental set-ups have been

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

Vousdoukas et al., 2014).

The present paper provides a description of the

experiments 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

Almar, R., Viet, N.T., Uu, D.V., Thuan, D.H., Binh, L.T., Ibaceta, R., Duc

swash processes generated by low energy wind waves in the recovery of a beach impacted by extreme events: In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13

th International Coastal Symposium

Journal of Coastal Research, Special Issue No. 66, pp. xxx-xxx, ISSN 0749

Nha Trang beach experiences southerly sediment drift during winter monsoons and northerly

In addition, the area is likely to be impacted by tropical storms or typhoons. Due to the

east border of the bay, the strongest impact on the shoreline apart from extreme events is due to NE swell

he mechanism responsible for the sediment drift generated by low energetic

insufficiently understood. It involves the functioning of the swash zone for weak conditions. Two field experiments

were scheduled before and after the period of cyclonic activity. The aim of the first

the geomorphology of the upper beach and hydrologic functioning

measurements in the swash and surf zone based on processing 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

t at different time scales. The data provides a first quantification of the impact on sediment

conditions which are encountered during spring and summer in Nha Trang.

ITIONAL INDEX WORDS: swash zone, surf zone, low energetic waves regime,

Contribution of the swash generated by low energy wind waves in the recovery process of a beach 1

generated by low energy wind waves


‡, Lê Thanh

Hanoi University of Sciences

Central Vietnam Construction and

[email protected]

Masselink and Hugues, 1998; Butt and

Because of its shallow, turbulent and unsteady nature,

physical parameters in the swash zone using

conventional data collection systems can be highly problematic

et al., 2007; Gómez-Pujol et al.,

field studies involving increasingly

ups have been conducted (Blenkinsopp

, 2013). More than a decade ago,

different techniques taking advantage of video have been proposed

, 1993; Foote and Horn, 1999; Holland et al., 2001;

The present paper provides a description of the first of two field

s 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

Duc,N.V., 2014 Contribution of

swash processes generated by low energy wind waves in the recovery of a beach impacted by extreme events: International Coastal Symposium

xxx, ISSN 0749-0208.

northerly during summer monsoons.

the presence of islands at the south

extreme events is due to NE swell from October

locally generated wind waves is

the functioning of the swash zone for weak conditions. Two field experiments

first experiment was to describe the

functioning of the bay. A new method of

g of data extracted from HD video processing was tested

we present some data obtained during the field

mpact on sediment transport from

encountered during spring and summer in Nha Trang.

, video processing, typhoon..

www.cerf-jcr.org

Journal of Coastal Research, Special Issue No.XX, 2014

2 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 semi-

enclosed 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.

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 multi-

parameter 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), multi-

parameters sensor OBS-3A (D), poles and micro-profiler ADV (E). Right: photo of alignment of poles and the ADV micro-profiler.

Figure 1. Nha Trang bay. Site of experiment is circled in red.

Journal of Coastal Research



Video processing The wave height in the swash and surf zone simultaneously at

each pole was obtained at a sample rate of

unsupervised method. In each frame of a video recording

duration, the red, green and blue component of the pixels located

on lines passing along each pole are extracted and stacked into a

matrix. The red component is used to detect the width of the

and to calculate the pixel to mm conversion factor

pole.

In order to minimize the influence of variation of ambient light,

uncertainty between water with suspended sediment and bed,

variation of bed color with saturation, shadow and reflection of the

pole on wet sand amongst other sources of perturbation,

are normalized and enhanced in hue and intensity.

intensity of each red, green and blue raster

separately. The stack is then converted in Hue

coding and the ochre to red band corresponding to the sediment

color is enhanced. A binary mask is obtained and the

are filled. This mask 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

second are considered, and location with at least 1/20 of time

emerged are considered as in the swash zone

PRELIMINARY RESULTSThe field experiment was carried out from neap to neap tide,

with a tide range varying from 1.2 to 1.8m. The wave

mostly from the SE with a mean height of approximately 0.2m

(figure 4).

The wave spectral density obtained from

sequences of 250s of swash-surf video indicated a rapid

dissipation of 75% of the wave power at the transition betw

surf zone to the swash zone (figure 5).and a peak

which is consistent with the measurements

Figure 3. Normalization-enhancement of timestack. Initial

timestack (a) RGB intensity normalized stack (b) Mask obtained

by selection of ochre-red color band (c) enhanced stack obtained

by masking of the RGB intensity normalized stack (d).


ontribution of the swash generated by low energy wind waves in the recovery process of a beach


The wave height in the swash and surf zone simultaneously at

sample rate of 25Hz using an

In each frame of a video recording of 250s

of the pixels located

lines passing along each pole are extracted and stacked into a

to detect the width of the tape

pixel to mm conversion factor for a given

the influence of variation of ambient light,

een water with suspended sediment and bed,

n of bed color with saturation, shadow and reflection of the

amongst other sources of perturbation, the stacks

are normalized and enhanced in hue and intensity. First, the

raster is maximized

separately. The stack is then converted in Hue-Saturation-Value

e ochre to red band corresponding to the sediment

and the smallest gaps

k is finally applied to the enhanced stack which

reduces the noise in the final stack and enhances the transitions

The water level is obtained by

detection of dark /light transition in the blue raster of the enhanced

The wave spectral density is calculated from the temporal

The bed is estimated by statistical analysis of the time

In order to limit the influence of

only emerged episode of more than one

second are considered, and location with at least 1/20 of time

RESULTS from neap to neap tide,

The waves originated

SE with a mean height of approximately 0.2m

The wave spectral density obtained from the processing of

indicated a rapid

dissipation of 75% of the wave power at the transition between the

peak period of 2.7s

which is consistent with the measurements by the AWACs

(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

The hourly monitoring of bed elevation measured manually at the

eleven first poles of the alignment (swash and surf zone) during

ebb tide (29/05 - tide range 1.47m) and

range 1.36m) is presented in figure 4,

measured from the most landward one. During the two days,

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.

At the beginning of ebb tide, the upper swash zone was

accreting (from reference to 3.9m seaward) and the surf zone

eroding. After mid-tide, (tide elevation of 0.3m) a significant

accretion was noted in the surf zone (poles between 7m to 8.9m

seaward from 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-2

enhancement of timestack. Initial

(a) RGB intensity normalized stack (b) Mask obtained

red color band (c) enhanced stack obtained

by masking of the RGB intensity normalized stack (d).

Figure 4. Mean 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)

Figure 5. Decay of wave spectral density at the

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

ontribution of the swash generated by low energy wind waves in the recovery process of a beach 3

period of 20s was observed in the upper swash

zone. At the upper part of the swash zone, the bed evolution

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

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

ESE with a mean height and period of

0.30m and 3s, respectively, the 29/05 and 0.19m and 6s, the 30/05.

At the beginning of ebb tide, the upper swash zone was

accreting (from reference to 3.9m seaward) and the surf zone was

tide, (tide elevation of 0.3m) a significant

accretion was noted in the surf zone (poles between 7m to 8.9m

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 2 h-1 (Figure 7).

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).

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.


4 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).




5

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

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 The work described in this publication was supported by the

Vietnamese Ministry for Science and Technology (BKHCN/NDT-

HD/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 7. Monitoring of variation of bed elevation in the swash-

surf zone.

Figure 8. Bed elevation of the inter-tidal domain estimated by processing of images from the video survey (left) trajectory of typhoon

Nari (right).


6 Lefebvre et al.

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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)

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Formatted: French (France)

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Contribution of swash processes in the recovery of a beach