東北地域災害科学研究

第58巻（2022）

105

Shoreline change and impacts of coastal protection structures on Da Rang
River mouth and adjacent coast, south central of Vietnam
Nguyen Trong Hiep1, Hitoshi Tanaka2, Nguyen Xuan Tinh3, Nguyen Trung Viet4
Abstract
Da Rang River mouth, Phu Yen, Vietnam is a representative coastal area of Vietnam where recently
was observed a significant change on the morphology change. The morphology change was resulted byeither natural and human influences. Since 2016, several coastal constructions (jetties, groins,breakwaters) had been implemented to stabilize the mouth area and the morphology of the river mouth isnow very different from it in the last decade. This study, by satellite image analysis, provides the last
decadal shoreline change in the entire Phu Yen coast from Ganh Ba Seacliff to Da Nong River mouth. The
severe erosion can be found in region B where the old sand spits were both eroded in the mouth and pushed
into the river channel after the typhoon in 2017. The morphology of the Da Rang river mouth been recently
stabilized by new coastal structures constructed since 2018.
Keywords: Da Rang River mouth, Phu Yen Coast, coastal constructions, shoreline erosion.
1. INTRODUCTION
River mouths, estuaries or coastal regions in general are considered the centralized regions for socioeconomic development in almost all countries in the globe. The studies regarding coastal processes,
mechanism and potential coastal morphological change have been conducted broadly in the world in favourof coastal management, sustainable development, coastal protection and countermeasures against
catastrophes.
Vietnam is one of a few countries in the world having a long coastline along the country. This brings agreat deal of advantages for the development of many aspects such as economics, tourism, etc. However,
the responsibility of preserving the coastline stability by which affected overwhelmingly natural change
and nowadays mankind is also vital challenge. In recent years, many areas in Vietnam, particularly incentral region, have reported on the erosion occurring severely. Cua Dai and Nha Trang are two coastal
regions have had deep concern about coastal changes for several years lately. Viet et al. (2014) presented
the erosion of sandy beach on the right side of the Cai River mouth, Nha Trang City associated to thedifference of wave conditions in monsoon and non-monsoon periods. The study also discussed theinfluence of protrusive hotel similar to a groin that blocks the alongshore transport drift. Further analyses
on the evolution of Nha Trang Coast utilizing shoreline dataset extracted from time-average images ofvideo-camera monitoring system, were discussed in Thanh et al. (2015). Tanaka et al. (2017) investigatedsevere retreat of shoreline of the sandy beach on the left side of Cua Dai River mouth, Hoi An City, central
Vietnam. In addition, Nagasawa et al. (2016) discussed the reduction of sediment supply from Thu BonRiver basin to Cua Dai River mouth due to construction of dam reservoirs as well as excessive dredge ofsand and gravel along the river channels. Noshi et al. (2015) investigated changes of coastal morphology
associated with the elongation of a sandspit and longshore sediment accumulation at the jetty in Phan Rang
City, south central Vietnam.
(1) Graduate Student, Department of Civil and Environmental Engineering, Tohoku University, 6-606 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, 6-606 Aoba, Sendai, 980-8579, Japan(4) Professor, Department of Civil Engineering, Thuyloi University, Hanoi, Vietnam
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Similar to the coastal zones mentioned above, Darang River mouth and the adjacent coasts experienced
notable morphology and coastal changes induced by both natural and human effects. Hoang et al. (2015)
claimed the existence of the sand terrace in front of the river mouth created by floods contributing mainly
the sediment on both sides of the entrance. Hiep et al. (2020) explains the migration of the river mouth
before the sand mining in 2015 by mathematical solution of one-line model. In addition, Hiep et al. (2018)
mentioned the severe erosion of the sand spits of Darang River mouth after Typhoon in 2017. Since 2016,multiple coastal constructions (jetties, groins, breakwaters) had been implemented to stabilize the mouth
area and the morphology of the river mouth is now very different from it in the last decade. This study, byusing shoreline analysis, provides the recent morphology change in the Da Rang River mouth and its
adjacent coast over last decade.
2. STUDY AREA
The region including Da Rang River mouth and adjacent coasts in Phu Yen, Vietnam is one of thebiggest coastal area in the south-central coast of Vietnam. Da Rang River mouth is located in Tuy Hoa City,
Phu Yen Province, south central Vietnam. Figure 1 shows the location of the study area. Da Rang Rivermouth is the estuary of Ba River and known as the longest river in the centre of Vietnam with the length of
374 km, and total basin area of 13,900 km2. The left coast of Da Rang River mouth extends from the mouth
entrance to the Ganh Ba Seacliff, while the right coast is from the mouth to Da Nong River mouth (Figure1). This mouth area is usually affected by typhoons during the rainy season which extends from September
to December. The typhoon causes heavy rainfall resulting in floods which widens the river mouth. Thisriver mouth is deemed to be one of the most important area contributing to the development of Phu Yen
Province. The mouth area is now the shelter and harbour for more than 900 small and big fishing boats andvessels. Da Rang Opening is also an active and dynamic area that the severe deposition or erosion often
occurs in the river mouth. In 2015, The deposition was so severe that hundreds of ships were unable toenter or exit, raising concerns about navigation clearance. As a result, several sand mining events were heldto alleviate the problem. After 2015, consecutive coastal protection constructions were implemented inmouth area. These constructions had the objectives to prevent the erosions on the adjacent local coasts on
both sides of river mouth.

Figure 1. Phu Yen Coast and Da Rang River mouth
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Regarding hydrodynamic conditions, the diurnal tide of Tonkin Gulf is the main tidal cycle having aneffect on Da Rang River mouth. During the spring tide, the maximum that tidal range can reach is about
1.7 m and the minimum is nearly 0.5 m during the neap tide. The fresh water river discharge depends
mainly on the rainfall and flood regimes of the entire basin. In flood season, river discharge can increase
up to 2100 m3/s and the annual river discharge is about 275 m3/s. Similar to other coastal regions in central
Vietnam, incident waves are mainly dominant at the northeast direction during monsoon season. Waveheight is smaller dung non-monsoon season as the northeast wind becomes less dominant. The wave rosefrom the offshore wave data from WAVEWATCH III from 2006-2016 shows the dominance of northeast
monsoon drives the wave condition Phu Yen Coast (Figure 1)
3. MATERIALS AND METHODSThe Landsat Imagery retrieved from the U.S Geological Survey (USGS) database from 2013 to 2021 isutilized as the main dataset for this study. The dataset is a combination of images taken from different
satellite missions, Landsat 7 (EMT+), Landsat 8 (OLI), and Sentinel‐2 (MSI). Some images obtained from
Google Earth data base system were also collected for this study
Since Landsat Images are TOA images (Top-of-Atmosphere), all data must be pre-processed in order toextract the best quality of the image. Acquired images are then required to go through a pre‐processing
stage that involves cloud masking, pan sharpening and down-sampling. The pre-processing techniques
followed the same process introduced by Vos et al. (2019). Landsat data from USGS assesses the cloud
cover of each image by computing the percentage of cloud pixels on the image. Some images contain large
parts covered by clouds which are difficult to perform the shoreline detection. The cloud masking stage is
optional but necessary for discarding all images that are unable to analysis. In this study, a threshold is set
for evaluating cloud percentage of each image. The value of cloud cover of any image exceeding thisthreshold is excluded from the data. The resolution of Landsat 7,8 images can be enhanced to be optimalby merging high-resolution panchromatic and lower resolution multispectral imagery due to the availabilityof panchromatic band in these images (Tu et al, 2001). The resolution can be enhanced from 30 m to 15 m
per pixel. On the other hand, the resolution of Sentinal-2 is 10 m.
For shoreline detection, firstly, the separation between water and land zones in the image is implemented
using the modified normalized water index (MNDWI) expressed as follows
SWIR1  G(1)MNDWISWIR1  G
Where: SWIR1 is pixel intensity in short-wave infrared band and G is the green band. The range ofvalue is between -1 and 1. The shoreline is then detected by using K-mean clustering method. In this study,
the effect of tidal level at different capturing is not considered since the local beach slope is quite steep (≈0.1) and the possible error position error is likely smaller than the resolution of the image. The total error
in shoreline is considered to be equal to the resolution of image as 10 m.
The Shoreline change rate is obtained by applying the linear regression rate to all data sets of shoreline
position as followsyat  b(2)where a is slope of regression line (shoreline change rate); b is a constant.
4. RESULTS AND DISCUSSIONS
4.1 Overall shoreline change rate in Phu Yen Coast
Due to the different behaviours of shoreline change in different areas, the entire Phu Yen Coast is divided
into 4 sub-zones (A, B, C and D) from north to south. Zone B is the regions representing for the Da Rang
River mouth (Figure 1).

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Figure 2. Shoreline position change in each zone

Figure 33. Sh
FiShorelineli change
hrate (2013~2021)(2013 2021)
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The shoreline change of Phu Yen Coast are shown in Figures 2 and 3. The shoreline changes in Zone
A, B, D shows no significant change since these areas are almost straight sandy beaches. The small
deposition can be found the Ganh Ba Seacliff and Da Nong River mouth. In the Da Nong River mouth,
due to the construction of the jetty in the past, the sand spit of this river mouth is similar to the restricted
spit which it can not elongate. The remarkable changes are found in Zone B where Da Rang River mouthis located. The severe erosion spot with the change rate approximately 40 m/y was the position of the sandspit. As seen in Figure, the elongated sand spit was gradually eroded until it was disappeared after theTyphoon in 2017. The deposition on the southern coast of Da Rang River mouth was due to the constructionof breakwaters and groins in this area from 2016. The highest deposition rate is at the position of jetty after
constructed in 2019. The sand accumulation is also detected at the locations of breakwaters and groins. Thedeposition was resulted from the formation of tombolo.
4.2 Morphology
gy change
g in Da Rangg River mouth (Zone B)
According to Hiep et al. (2018, 2020), after the constructions of detached breakwaters and groins on
southern coast, and riprap on the northern coast at the river mouth, the sand spits were eroded severely until
they were disappeared caused by the Molave Typhoon in 2017. Figure 4 shows that the sediment from sandspits was actually over-washed and pushed into the river mouth by extremely high waves to form two
sediment strips attached on the embankment inside the river mouth. The natural subsequent recovery of thesand spits at the old locations were not observed after typhoon due to the coastal infrastructures on bothsides of the river mouth preventing the alongshore drift to transport sediment to the mouth (Hiep et al,2018). Due to the intensity of storm waves, the detach breakwaters and groins were also destroyed partly
by the typhoon in 2017. To stabilized the morphology in the river mouth, the local government continued
to constructed 5 more breakwaters beyond the position of the groins and 2 jetties on both sides of the rivermouth. After the completion of the new coastal structures, the formation of tombolo at the new breakwaters
and the deposition at the position of two jetties were observed.

Figure 4. Sand spit in Da Rang River mouh before and after the Molave Typhoon in 2017 (Zone B)
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The current morphology of mouth entrance can improve the flushing behaviour of the river during flood
events. However, the wide opening can result to wave intrusion propagating inside the river mouth. Thiscan trigger erosion inside the river and possible complicated cross-shore sediment transport induced byhigh and storm waves, especially inside the Dong Tac Fishing port where diffracted wave can propagate
inside the port. The formation of subaqueous channel-spit was also discussed in several river mouth after
the construction of jetties caused by the tidal transport current induced by ebb flows (Hein et al., 2019).
Therefore, the detailed sediment transport inside the river must require further monitoring in the rivermouth and thorough analysis in the near future.
5. CONCLUSIONSThe shoreline evolution of Phu Yen Coast and Da Rang River mouth in recent years is investigated bythe satetllite image analysis. Significant change of shoreline from 2013 to 2021 was detected at the locationof Da Rang River mouth where the sand spits was dissapeared at the mouth entrance and pushed inside thechannel after the historic typhoon in 2017. The morphology of the river mouth been recently stabilzed by
new coastal structures constructed since 2018. The opening of the river mouth and new construction ofjetties requires further analyses in the future since wave-induced sediment transport inside the river mouth
can be more complicated when waves can propagate further upstream.
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