10th Asian Regional Conference of IAEG (2015)

Enhancement of coastal protection under the context of climate change: A case study of
Hai Hau coast, Vietnam

Do MINH DUC(1), Kazuya YASUHARA(2) and Nguyen MANH HIEU(1)(1) Department of Geotechnics, Faculty of Geology, VNU University of Science,
Vietnam National University, Hanoi
E-mail: ducdm@vnu.edu.vn(2) Institute for Global Change Adaptation Science, Ibaraki University, Japan
Abstract
Climate change and global warming have led to severe typhoons and sea level rise
(SLR) which may threat the stability of coastal structures. However, countermeasures toenhance coastal protection against SLR and severe typhoons have not appropriately
considered in Vietnam. This paper focused on the enhancement of coastal protection in
Hai Hau district – the most serious erosion coast in the North Vietnam. Erosion in Hai
Hau coast has occurred continuously since the beginning of 20th century with average
retreat rates of 10-15 m/y. The maximum rates reached to 40-50 m/y in some segments.
Sea level is considered to rise about 2 mm/y on average in Vietnam. The number andintensity of tropical cyclones have a complicated change with a tendency of much more
severe in recent years (2004-2013). Each year the accelerated rate of erosion due to SLR
is 0.1-0.3 m/y in the Hai Hau coast. SLR also causes larger wave pressure on theseadikes making them more unstable in typhoons and storm surges. In the projectedscenarios of SLR, erosion rates and scouring of dikes trough in Hai Hau coast werepredicted to increase sharply in the next few decades. Besides, typhoons induce wave
overtopping cause severe erosion of inner slopes of sea dikes and lead to dike broken.
Countermeasures to enhance coastal protection of Hai Hau district focus on using local
available materials, ecological engineering and geosynthetics measures. As a conclusionof the paper, to cope with future threats induced by climate change, solutions of multiple
protections in Hai Hau coast were proposed which include conventional structures (i.e.
dike, revetment, groins, mangrove) together with geotubes as submerged breakwaters
and vetiver grass.
Keywords: Erosion coast, sea level rise, typhoon, geosynthetics, ecological engineering,coastal protection
1.

Introduction
According to the fifth report of IPCC, averagetemperature of the global increased 0.890C in the
period of 1901-2012 and about 0.720C over the periodof 1951-2012. From 1993 to 2010, the rate of SLR
was very high at 3.2 mm/year. The SLR would also
raise the ground water level (GWL), thereby,engendering infrastructural instability along thecoastal zones (Yasuhara et al. 2007)
Many coasts around the world have sufferederosion as a significant hazard in the region such as in
Bangladesh, China, and the Southeast Asia. The
increase of erosion rate due to SLR can reach to
0.14–0.31 m/y in the coast of the Red River delta,
Vietnam (Duc et al. 2012). Coastal disasters in
Vietnam impact on human settlements andinfrastructure, which has become severe in terms ofmagnitude, frequency, and volatility (Takagi et al.,2013). There were some types of structures designed

in order to protect coastline. Chu et al. (2009)classified river and coastal structures according tomaterials used, including conventional methods andrelatively new ones. The most three traditional types
are earth-fill dike, masonry and concrete, and steel
sheetpiles or bored piles. In the past decade methods
using geotextile or geosynthetic materials andprefabricated concrete segment have been considered
and innovated.
Due to lack of investment, the current coastal
dikes still have to suffer overtopping seawater. In
such case, vetiver grass is a suitable application forprotection of inner coastal dike slope. The vetiver
hedgerows reduce soil loss on a slope by 62–86 % incomparison to the case without vetiver hedgerow(Donjadee and Tingsanchali 2013). Vetiver can also
be used in combination with other traditional
engineering solutions (Truong 1998; Truong et al.
2008).

1

10th Asian Regional Conference of IAEG (2015)
Mangrove forest is another measure against
coastal erosion, which has been applied in the coastof Hai Hau. A hundred meters of mature mangrove
can reduce 0.1 m of wave height (Mazda et al. 1997
and Quartel et al. 2007).
Application geotube is now popular worldwide
with its advantages such as easiness, costeffectiveness, rapidity of installation and durability(Koffler et al. 2008). Recently, owing to the high costof rubble mound coastal structures, the application of
geotube technology has become a serious
consideration (Shin and Oh 2007). They work as anefficient and environmentally friendly solution toprotect shoreline from erosion (Sheehana andHarrington 2012).

Hai Hau is a district in coastal zone of Nam Dinhprovince that has been formed by deposition processof the Red River delta system. The Hai Hau coast
includes 7 communes such as Hai Loc, Hai Dong,
Hai Ly, Hai Chinh, Hai Trieu, Hai Hoa, and Hai
Thinh. The shoreline is a straight line directing from
Northern East to Southern West in a distance of about
27 km (Fig. 1). The slope is 1:40 in near the shore,and it is from 1: 350 to 1: 200 at the depth of overthan 1 m. The slope decrease as the sea water depth
increases. The shoreline is covered by fine sand withthe thickness of 0.5 - 2m. That sandy layer is thinner
seaward. The tidal amplitude is 2.5–3 m. Waves havemain directions of East, Northeast in winter and East,
Southeast in summer. The average height of waves is
0.7 - 1.3m and reach to 3.2 m in storms.

Fig. 1 Location of the Hai Hau coast
then gradually became the main river mouth in Hau
2. Coastal erosion and the protection
Hau coast. After 1980s, the erosion was prone toin Hai Hau
decrease because the shoreline was protected by the
The erosion in Hau Hau coast occurred from 1905,
sea dike system. In the period between 1985 and 1995,
having close relation to the Ha Lan river mouth. The
the erosion intensity was more 1.5 times higher thanshoreline in Giao Long and Giao Phong was
the period of 1965-1985. Specifically, at hai Chinh –deposited between period of 1905 and 1930 withHai Hoa segment the erosion speed was 15-20 m/year.
speed of deposition reaching 200 m/year in some
Recently, the shoreline in Hai Thinh commune issegments. Nevertheless, during the period from 1965
being the most eroded segment in Hai Hau coast withto 1985 the shoreline was eroded. The Ba Lat mouth
the average speed of 400 m/year.

Fig. 3 Broken seadyke in Hai Hau

Fig. 2 Land loss due to erosion in Hai Ly

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10th Asian Regional Conference of IAEG (2015)
In the 1980s, sea dike in Hai Hau coast was
simply embanked by available soils which is easily
eroded by wave and storm surge in typhoons. To
reinforce the dike some conventional solutions wereused like T-groins, mangrove forest and tripods (Fig. 4).

The sea dike system in Hai Hau district was
intensively reinforced with the height of the dikeextending to + 4.5-5.5m, the foot of dike placing at
1.5m depth and concrete revetment covering outerslope.

Fig. 4 Conventional measures in Hai Hau coast
taken from two stations Hon Dau and another one at
3. Recognition of Climate Change in
Hai Hau coast, Thuy NN (1995) showed that the SLR
Vietnamwas 2.24 mm/y in Vietnam from 1950s to 1990s.
According to the MONRE (2009), the annual
It is clear that the number of typhoons landing
average temperature in Vietnam became higher about
Vietnam coast rapidly increased from 2005 up to now.
0.5 – 0.70C from 1985 to 2007. The annual average
Especially, the number of typhoons was 14, 13 andtemperature in the period of 1961-2000 was higher
19 in 2008, 2009 and 2013 respectively. In the period
than that of the period of 1931-1960.
of 1961-2004, the number indicated an unclear
Basing on data of 4 stations: Hon Dau (Quang
relation to climate change but complicated (Fig. 5).
Ninh province), Da Nang and Quy Nhon (Centre
Therefore, the number and intensity of typhoonspart) and Vung Tau (South of Vietnam), the relative
attacking Vietnam coast would be unpredictable insea level rise in Vietnam was 1.9 mm/year from 1960
the future.
to 2000 (Hanh & Furukawa, 2007). According to data

Fig. 5 Number of typhoons attacked Vietnam coast (1961-2014)3

10th Asian Regional Conference of IAEG (2015)
4. Impacts of Climate Change
4.1 Increase of erosion
Using the formula of Bruun (1962), accelerated
rate of erosion due to SLR in Hai Hau coast wasestimated.
L*R  0,001S
h*  B(1)exceeding rate during the periods 1985-1995 and1995-1999.
4.2 Scour
The physical model of Barnett and Wang (1988)
was used to estimate the rate of beach lowering in Hai
Hau.
h = 100Y x b / l(2)
Where: h – Rate of beach lowering (cm/y), Y Erosion rate (m/y), l - Width of beach from shorelineto the depth of mean sea level (m), and b - Height ofberm (m).
Recently, the beach lowering rate is very seriousin Thinh Long town with the value is 156 cm/year.
Meanwhile, the figure for Hai Ly, Hai Chinh, Hai
Trieu, and Hai Hoa is at high rate with 15-25cm/year.

in which: S - SLR (mm/y); R - the accelerated
rate of erosion due to SLR (m/y); L* and (h*+B) are
the width and vertical extent of the active cross-shore
profile.
Duc et al. (2012) showed that the accelerated rate
can reach to 0.17-0.25 m/y along the coast of Hai Hau,
and as raw estimation SLR can cause 10-50% of the
5. Impacts of extreme weather events
5.1 Typhoon-induced erosion
The retreat distance caused by extreme wave
heights can be estimated by the formula of Kriebel
and Dean (1993). Hai Hau coast experienced theerosion rate of about 100 m in a severe typhoon in1999 at Nghia Phuc coast (Duc et al. 2007). The
erosion rate can reach to 7.1 m when the wave heightis 4.25 m high and the duration is 2.4 hours. As aconsequence of climate change leading to stronger
variability of frequency and intensity of typhoons in
the Vietnamese coast, the extreme erosion rates canbe more often and severe in the future.
5.2. Wave overtopping and soil erosion
To estimate amount of overtopping water under
extreme condition being combination of storm surgeand highest tide level in Hai Hau coast, the formulaof van der Meer and Janssen (1995) was used, whichis as follows:
R
q
0.06
1

γ b ξ op exp( 4.7 c
) (3)
3
H
ξ
γ
γ
tanα
gH
s
op b f γ β γ v

are Hs = 3.2 m; T = 5.7 s; b = 1; f = 0.9; β = 1; υ =0.65 (Fig. 6). The results shown average overtopping
rate were illustrated in table 1.
Velocity of water flow on surface of inner slope iscalculated by Chezy’s equation as:v  C Ri

In which, Chezy coefficient (C) was determinedby Manning’s equation:

tanα
S op

and S op 

C

1 16
R
n

(5)

R

bh
b  2h(6)

Where:
v: mean flow velocity (m/s); C: Chezy coefficient; R:
hydraulic radius; i: slope of channel bed; n: roughness
coefficient (Pierre, 2012); b: width of flow (m); h:
depth of flow (m).
Materials used to build coastal dike in Hau Hau
coast are mostly clayey sand with low compaction.
Based on empirical relations between water velocity
and erosion rate for various types of soils (Fig. 7) of
Briaud (2008), erosion rates at the inner slope of theHai Hau dikes during a typhoon are shown in table 1.
Erosion is very severe at Hai Hoa, Hai Trieu, Hai
Chinh, and Hai Ly, especially in Hai Trieu whereinner slope was bare soils and no vertical concrete
wall to prevent wave running up. It shows a good
match with the fact of the Damrey typhoon, whencoastal dikes in Hai Hoa, Hai Trieu, and Thinh Long
were broken. After the typhoon, coastal dike in ThinhLong was rebuilt and the current one has much higher
resistance to overtopping-induced erosion.

s

In which: ξ op 

(4)

2 πH sgT 2

q: average overtopping rate (m3/s per m width); g:
9.81 ms-2 is acceleration due to gravity; Hs:significant wave height (m); α: average slope angle;
b: reduction factor for a berm; op: breakerparameter; Rc: crest freeboard (m); f: reduction
factor for slope roughness; β: reduction factor for
oblique wave attack and v: reduction factor due to avertical wall on a slope; Sop: Wave steepness; T:
period of wave (s);Data acquired from the Damrey typhoon in 2005
were used in the equation (3). The input parameters410th Asian Regional Conference of IAEG (2015)

Fig. 6 Input data of storm surge and wave in Damrey typhoon

Fig. 7 Estimation of erosion rate in soils at inner slope of coastal dike in Hai Hau coast(Original chart referred from Briaud 2008)
Table 1 Erosion rates caused by wave overtopping during typhoon at dike inner slopes

Section

Outer
slope
(deg.)

Crest
freeboardRc(m)

Lengthof inner
slope
(m)

Inner
slope
(deg.)Hai Dong
Hai LyHai Chinh
Hai Trieu
Hai Hoa
Thinh Long

14
14
13
14
15
14

1.80
1.60
1.35
1.40
1.60
1.60

5.2
7.5
7.0
5.0
4.2
7.2

25
25
25
33
27
30

6.

Geotechnical monitoring for
Climate Change adaptation
6.1. Ground water level (GWL)
monitoring system
In order to monitor GWL under the dike, amonitoring system was installed in Hai Dong
commune. The data taken from the system will beconnected to tide level in the area. The system includes
two sensors to be assembled in two boreholes whichare 10 and 12m in depth. Being combined with tidelevel data in the study area, the data shows relationshipbetween fluctuation of the tide level and GWL waspresented in Fig. 8. Generally, every fluctuation in tidelevel triggers corresponding fluctuation in GWL in a
linear relation. Corresponding with the amplitude ofOvertoppingflow(l/s per
m)
81
115
137
164
150
115

Water flow
velocity (m/s)

Erosion rate
(cm/hr)

Bare
soil

Grass
covered

Bare
soil

Grass
covered

2.48
2.46
2.88
4.53
4.40
2.75

0.66
0.66
0.77
1.21
1.17
0.73

70
70
120
1000
800
100

Very low
Very low
Very low
Very low
Very low
Very lowtide oscillation from 0.047 to 1.70m height, the GWL
fluctuates between -1.54 and -0.313m. Therefore,
elevation difference of GWL and tide level in themonitored period changes from 1.59 to 2.01m.
Considerably, the data gotten by the sensor 1 is always
higher than sensor 2 which vaies from 8.7 to 39.7cm.
The difference may be come from two reasons: Firstly,
the sensor 1 was installed closer to the shoreline than
the sensor 2; Secondly, layer 3 (clay soil) is located at
higher elevation at position to install the sensor 2 sothat it leads to a hysteresis in changing of GWL.
Due to this relationship, GWL can be interpolated.
It is forecasted that the ground under sea dike will besaturated when seawater level reaches to 2.5m high.

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10th Asian Regional Conference of IAEG (2015)

Fig. 8 Groundwater level monitoring system in Hai Dong commune
6.2.

Pore water pressure (PWP)
monitoring system
Calculations to determine PWP under the groundthrough data of GWL are somewhat incorrect becauseof concerned factors like tide, wave and stratum.
Therefore, in order to have a precise insight into
variety in PWP in the dike body and ground, a PWP
monitoring system was installed in the area (Fig. 9).
Equipment used for the system is provided by
Slope Indicator. Sensors are Vibrating Wire (VW)
type possessing a high accuracy in range of pressure
from 0.7 Bar to 35 Bar. Totally, 7 piezometers were
installed in and under the sea dike.

The piezometers are located at different depth andisolated from each other to establish a net of multilevel PWP inside and under the dike.
PWP at the same depth but different positions
inside and under the dike are different. PWP in thesame borehole but different levels are also different.
The deeper piezometers are located the higher PWP
value they show. North-east monsoon strongly
impacts on changes in PWP, inducing higher PWP
even in lower tide level conditions.

Fig. 9 Relationship between tide level and change in PWP in Hai Hoa

610th Asian Regional Conference of IAEG (2015)
7. Geotechnical measures for climatechange adaptation
The use of only a single countermeasure such as
the dyke reinforcement described above isinsufficient for long-term protection, particularly
against severe weather conditions following storm
surges or typhoons. As one solution to extremely

disastrous events, multiple protection can beproposed as shown in Fig. 10, which depicts threecombined countermeasures: an off-shore wave-eating
facility, near-shore measures (mangrove plantation is
popular in the developing countries), and a dike
reinforced with vetiver grass and locally available
techniques and materials.

Fig. 10 Multiple protection and adaptation to climate change of coasts with different severity of erosion
Education, Culture, Sports, Science, and Technology,
8. ConclusionsJapan
Hai Hau coast has been undergoing severe erosion.
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