東北地域災害科学研究79第55巻（2019）

Recent morphology change at Da Rang River mouth, Phu Yen, Vietnam
Nguyen Trong Hiep1, Hitoshi Tanaka2, Nguyen Xuan Tinh3, Nguyen Trung Viet4
Abstract
Da Rang River mouth, resembles many coastal areas in Vietnam, has been recently encounteringsignificant changes induced by either nature or human involvement. The instability of sandspit¶sformation at Da Rang River mouth can be detected by two major phenomena which are migration of theriver mouth after the 1993 flood and shrinkage of the south sandspit owing to human interference. Thisstudy, by utilizing one main source of satellite imagery: Landsat imagery, investigates the recent
morphology and its variation thoroughly.
Keywords: Da Rang River mouth, the 1993 flood, human interference, migration.
1. INTRODUCTION
Da Rang River mouth has a typically deltaic
shape that is common in the world. Since the past,
numerous studies about the formation of delta
coast had been made and yet still have been
researched extensively lately. This study focuseson the dramatic reversal of river mouthmechanism due to human involvement. Theobserved phenomena of Da Rang River mouth
are not obviously distinctive which wereconsidered in many prior studies: migration of theriver mouth and gradual disappearance ofSouthern sandspit. FitzGerald et al. (2000)
discussed fundamentally the mechanism of
ebb-tidal delta breaching at stable coasts where
dominant of longshore sediment transport andasymmetric accumulation of sediment on eithersides of river mouth leads to a severe deflectionof the main channel. In more specific study,
Patchanok Srivihok and Tanaka (2004), by using
aerial photos, stressed the seasonal migration of
Nanakita River mouth, Japan controlled by thedominance of longshore sediment transport. Satoet al. (2014) with abundant of bathymetric datacharacterized the formation and deformation ofTenryu River mouth, Japan trigged by floods, theformation of bar and trough topography boundwith frequency of storm waves and erosion as

Figure 1: Study area

result of imbalance longshore sediment transport.
The current situation of Da Rang River mouth issuspected that been causing by incidence of sandmining inside of river channel and the placementof beach protection construction on the South.
This has already demonstrated widely, namely,
Guangwei (2011) and Anthony et al. (2015)
emphasized the same point of view of coastal
erosion caused by the reduction of sedimentsupplied to coast owing to the impacts of major(1) Graduate Student, Department of Civil and Environmental Engineering, Tohoku University,
6-6-06 Aoba, Sendai 980-8579, Japan(2) Professor, Department of Civil and Environmental Engineering, Tohoku University, 6-6-06 Aoba,
Sendai 980-8579, Japan(3) Assistant Professor, Department of Civil and Environmental Engineering, Tohoku University,
6-6-06 Aoba, Sendai, 980-8579, Japan(4) Professor, Department of Civil Engineering, Thuyloi University, Hanoi, Vietnam

80dams and channel bed erosion triggered by sandmining.
Da Rang River mouth has received attentionof many studies for last few years due to its
complicated changes. Hoang et al. (2015)
claimed the existence of the sand terrace in frontof the river mouth contributing a great deal ofsediment on both sides of the entrance. Tanaka etal. (2016) indicated the elongation of the rightsandspit created severe erosion on adjacent area.
And more recent, Cuong et al. (2016) discussed
the nearshore hydrodynamics and sedimenttransport in Da Rang River mouth in a short termfrom 2015 to 2016 by abundant of field dataobtained within that time. However, theseresearches mainly did not reflect the most recentchange and such a debatable discussion between
mechanism variations is vague. Therefore, thisstudy will provide a comprehensive view on themechanism of Da Rang River mouth by using
Landsat images from 1988 to 2018.
2. STUDY AREA AND DATA COLLECTION
Da Rang River mouth is located in Tuyhoa City,
south central Vietnam, away from from Ho Chi
Minh City about 400 km in Northeast direction

Figure 2: Morphology variation at Da Rang River mouth (1988~2015)

81(Figure 1).The river mouth belongs to the largest
river in South Central of Vietnam, Ba River which
has the main stream of 307km in length and totalbasin area of about 13300 km2. At the entrance ofthe river mouth, the formation of the spits as theaccumulation of sediment beyond the shoulder ofbanks have existed for many decades. The
sandspits on both sides have been fed by sedimentsupplied from the river and incident waves.
However, the interference of human activities also
created such a big impact on this area which therate of sand mining has rised rapidly for last few
years. It is reported by the local government that in
the period from 2015 to 2017, at least 7 dregding
activities had been executed on different locations.
where the sand mining focuses on the upstream
river bed and the entrance of the river mouth.
Approaching waves are a vital factor on thevaried shapes of the sandpsits since seasonal
effects are highly noticeable. Da Rang Rivermouth is influenced by the monsoon climatic
regime; Northeast monsoon prolongs fromOctober to April and Southwest wind lasts fromMay to September.
The main source data of this study is Landsat
imagery obtained from U.S Geological Survey(USGS) database from 1988 to 2018 (190 photos).
Every images are rectified to one coordinate system
by linear transformation (Affine transformation)
having a baseline of 148 degree to the North.
Shoreline detection was also implemented to all
images by extracting the wet-dry line by themaximum gradient pixel and nearest neighbour
method. The drawback of using Landsat is its low
resolution which neglects the effect of tide onshoreline.
3. RESULTS AND DISCUSSION
3.1 Natural morphology variation of Da Rang
River mouth (1988 ± 2015)
Da Rang River mouth from 1988 to 2015
encountered an overwhelm change in its
mechanism. Figure 2 indicates clearly themigration of the river mouth from the South to theNorth after a big flood 1993. This implies theFigure 3: Morphology variation at Da Rang River mouth (2015~2018)

82

mechanism is turned into a new one which is more
hydraulically efficient than the previous one due toa breakthrough by the flood. This new river mouthmechanism lasted for long period until late 2015.
The mechanism during this period was discussed
in detail by Hiep et al. (2016) which thepredominance of longshore sediment transport orriver mouth was cleared out thoroughly.
3.2 Morphology variation under impingement of
human involvement (2015 ± 2018)
In 2015, the closure of Da Rang River mouthnearly happened led to the urgent excavation tomaintain the navigation channel. But the sanddredging was overdone that sand and gravel of someplaces inside the river mouth were intentionally takenway. And the elongation of the right sandspit caused
a severe erosion on adjacent area that threatened thewhole area behind that. With the aim of both
reduction of wave heights at the shore andcollection of more longshore transport of sediment,
the former structures were 6 groins placed on the far
South of Da Rang river mouth and the latter wereseveral headland and detached breakwaters on thegroins. Breakwaters are detached, generally
shore parallel structures that reduce the amount ofwave energy reaching a protected area. They are
similar to natural bars, reefs or nearshore islands
that dissipate wave energy. The reduction in wave
energy slows the littoral drift, produces sedimentdeposition and a shoreline bulge or salient featurein the sheltered area behind the breakwater. Somelongshore sediment transport may continue along
the coast behind the nearshore breakwater.
However, the downside took place consequently
when the right sandspit was becoming smaller
until hit by a big typhoon in 2017. The entire
sandspits dissappeared and have not recovered
afterwards as indicated in Figure 3.
3.3 Longterm morphological change
A simple analysis was conducted to investigate
the longterm morphological change at Da RangRiver mouth by comparing the entire shoreline
data to the very first one. The expression of theanalysis can be seen at Eq.1 below:'y x, ty x, t  y0 x

(1)
The dark red area around 1993 in Figure 4emphasizes the severe erosion on the left sandpsit
which indeed the flood wiped out the entire left
sand spit. Then the left sandspit was not be able toreach its previous position and as a result deposited
to the Northern adjacent area. In the meanwhile,
the South sandspit accretion appeared after theflood and prolonged further to the opposite side.
This is because large sediment was attached on thisarea by ebb shoal in front of the river mouth andsand bypassing from the North side. The
elongation of the sandpsit also triggered theerosion beyond it and became more severe until
the 2017 typhoon emptied the entire Southsandpsit.
3.4 Analysis of river mouth charateristics

Figure 4: Long-term shoreline change at Da Rang River mouth

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To investigate the morphology change at Da
Rang River mouth by a quantitative appoarch but
not only by visual. Several vital quantities of theriver mouth are defined which are the center point,xC, yC; river mouth with, B; and area of eachsandspit, A. Plus, yearly maximum fresh water
discharge, Qmax from a hydrological station located
far upstream from the entrance about 37 km is alsobrought up to make a contrast with river mouthparameters. The definition of these quantities andtheir measured values are shown discreetly inFigure 5.
The value of the centerpoint regarding

longshore direction reveals the movement of rivermouth as the river mouth migrated to the left after
the 1993 flood and its reverse on which thesandspit was getting smaller. between 2015 and2018. With regard to crosshore direction, thespecific trend is not able to be detected; however
during the reduction period, the right sandspit wasintruding into the upstream due to incident waves
and lack of sediment from river mouth. The rivermouth width also fluctuated through the entire
period but 2 extreme values are those of two
catastrophical events.
The area of two sand spits showed how

Figure 5: Definition and analysis of river mouth characteristics

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dominant the growth of right sandspit was. Before
the 1993 flood, the river mouth was stable
although the area of two sandspit kept varying.
After getting struck by the flood, the North
sandspit was flushed almost entirely that the valueof it nearly reached the bottom, while the Southsandspit was faded away a smaller amount. The
flushed sediment then stayed offshore and in frontof the river mouth creating ebb shoal and bars. The
recovery started immediately afterwards but theshoal and bars and dominant longshore sedimentfrom South to North play their important roles todeposit plenty of sediment onto the right sandspit.
This is why the South sandspit developed by aremarkable amount, whilst the North one remained
constantly small. From 2015, the area of rightsandspit was reducing to the amount of the left
sandspit before they both went nothing after the2017 typhoon.
The maximum fresh discharge data majorly
referred to the flood events in Phu Yen and it isdistinct to detect the extreme value caused byintensity of the 1993 flood.
4. CONCLUSIONS
Da Rang River mouth has a typicalmorphogolical change of a deltaic coast which canclearly be seen by the shoreline position varition.
The shifting of the river mouth occurred when a
new mechanism had been formed after the flood in
1993. This mechanism stayed stable till 2015 andto break this regime, it is believed to take decades
or until next extreme event. However, since humanstarted getting involved in the natural process, theequilibrium of river mouth has broken making
severe erosion on two sandspits. With the lack ofsediment from both main sources, which aresediment discharge from river and longshore
sediment transport, it is presumed the recovery ofsandspit is a tall order.
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