Journal of Asian Earth Sciences 43 (2012) 98–109

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Journal of Asian Earth Sciences

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An analysis of coastal erosion in the tropical rapid accretion delta of the RedRiver, Vietnam

Do Minh Duc a,⇑, Mai Trong Nhuan b, Chu Van Ngoi a

a Faculty of Geology, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Namb Vietnam National University, Hanoi, 144 Xuan Thuy, Cau Giay, Hanoi, Viet Nam

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 November 2010Received in revised form 8 August 2011Accepted 10 August 2011Available online 13 September 2011

Keywords:Red River deltaShorelineAccretionErosionSediment transport

1367-9120/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jseaes.2011.08.014

⇑ Corresponding author. Tel.: +84 4912042804; faxE-mail address: ducdm@vnu.edu.vn (D.M. Duc).

The largest plain in the North Vietnam has formed by the redundant sediment of the Red River system.Sediment supply is not equally distributed, causing erosion in some places. The paper analyzes theevolvement and physical mechanism of the erosion. The overlay of five recent topographical maps(1930, 1965, 1985, 1995, and 2001) shows that sediment redundantly deposits at some big river mouths(Ba Lat, Lach, and Day), leading to rapid accretion (up to 100 m/y). Typical mechanism of delta propaga-tion is forming and connecting sand bars in front of the mouths. Erosion coasts are distributed eitherbetween the river mouths (Hai Hau) or nearby them (Giao Long, Giao Phong, and Nghia Phuc). Theevolvement of erosion is caused by wave-induced longshore southwestward sediment transport. Mean-while sediment from the river mouths is not directed to deposit nearshore. The development of sand barscan intensively reduce the erosion rate nearby river mouths. Erosion in Hai Hau is accelerated by sea levelrise and upstream dams. Sea dike stability is seriously threatened by erosion-induced lowering of beachprofiles, sea level rise, typhoon, and storm surge.

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1. Introduction

The Red River begins from the mountains of Yunnan province(China). The Red River delta in Vietnam territory is formed bythe Red and Thai Binh river systems, which is commonly calledthe Red River system (Fig. 1). The delta is about 15,000 km2. It isa rich agricultural area and densely populated. Along the coastline,the interaction between the sea and big rivers has created a typicaltropical natural condition which is suitable for tourism, agricul-tural and aquaculture development.

The Red River delta develops in a very dynamic fluvial and mar-ine environment. The river basin is characterized by an alternationof wet and dry seasons producing a huge total annual suspendedsediment load (Hoekstra and Van Weering, 2007). The delta is riverdominated (Fig. 2). The annual amount of sediment transported bythe Red River system into the East Vietnam Sea is about82 � 106 m3. In the wet season (from April to September), about90% of the annual sediment supply is transported through the var-ious distributaries (Nhuan et al., 1996). Of the total amount of sed-iment supplied, 11.7% passes through the Van Uc and Thai Binhriver mouths, 11.8% through the Tra Ly river mouth, 37.8% throughthe Red River (Ba Lat) mouth and 23.7% through the Day rivermouth (Duc et al., 2007).

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The northern part of the coast (from Ba Lat to Hai Phong) has adiurnal tidal regime with average amplitude of 2.5–3.5 m. In thesouthern part, from Ba Lat to Day mouth, the tide is mixed witha diurnal dominance. The average tidal amplitude is 2–3 m (Nhuanet al., 1996). Waves usually have a dominant direction from theeast, northeast during the dry season (October–March) and fromeast, southeast during the wet season (April–September). The aver-age and maximum wave heights are 0.7–1.3 m. Wave heights in se-vere typhoons can reach over 5 m (Nhuan et al., 1996).

The large amount of sediments has made the delta a rapid con-tinuous advancing to the sea. Old shorelines of the delta are recog-nized through series of old sand bars, historical and anthropogenicproofs (Hoan and Phai, 1995). The delta was enlarged 20–30 kmfrom the 10th to 15th century and 10 km from 15th to 19th cen-tury (Fig. 1). Sediments supplied by the big mouths (Tra Ly, BaLat, Lach, and Day) are mainly deposited at shallow sea and formsand bars in front of the mouths. They protect shorelines behindagainst wave and current attacks making a suitable condition forrapid accretion.

Most of sediments discharged from rivers deposit in front of theriver mouths and causes rapid accretion. As a consequence, severeshoreline retreat occurs at some other places due to sediment def-icit. Erosion coast in the Red River delta has length and area muchless than those of accretion coast. However, it has seriouslydamaged the coastal villages and made an obstacle for economicdevelopment in the region. The distribution of erosion shoreline

Fig. 1. Red River system and locations of old shorelines.

D.M. Duc et al. / Journal of Asian Earth Sciences 43 (2012) 98–109 99

can be a typical characteristic of the Red River delta. They areeither in the middle of the big river mouths (Hai Hau) or very closeto them (Giao Long, Giao Phong, and Nghia Phuc).

The paper has objective to outline shoreline change in the re-cent time and find out the physical mechanism of erosion in thetropical rapid accretion delta of the Red River. Recent evolvementof shoreline and the reasons are elucidated by the analysis oftopographical maps and nearshore sediment transport. Factorsaffecting shoreline retreat such as typhoon, sea level rise, and up-stream dams are studied to assess potential acceleration of erosionand its impacts to coastal structures.

Fig. 2. The Red River in the chart of delta classification of Coleman and Wright(1975).

2. Materials and methods

2.1. Topographical maps

A series of topographical maps are used to investigate recentchanges of shoreline (Table 1). The maps were established usingdifferent co-ordinate systems and scales. WGS-84 stands for WorldGeodetic System which is currently the reference system beingused by the Global positioning system. WGS-60 is one of the

Table 1List of recent topographical maps.

Year Co-ordinate Scale Note

Datum Ellipsoid

1930 – – 1:250,0001965 WGS-60 WGS-60 1:50,000 Published 19781985 HN-72 Krasovski 1:50,000 Published 19911995 VN-2000 WGS-84 1:50,000 Published 20012001 VN-2000 WGS-84 1:50,000 Published 2005 (Ba Lat mouth only)

100 D.M. Duc et al. / Journal of Asian Earth Sciences 43 (2012) 98–109

former systems of WGS-84. HN-72 or Hanoi-72 is the system ofVietnam which was used before 2000. It used Krasovski ellipsoidand the datum (origin co-ordination) was transferred from Mos-cow to Hanoi. It is not a spatial unified system and requires differ-ent formulas to convert to other systems. VN-2000 is the currentnational geodetic system in Vietnam. The origin co-ordinationwas installed in Hanoi. VN-2000 is a unified system for the wholecountry.

Maps are scanned and then the shoreline in each map is digi-tized in MapInfo software. The vector maps are then transformedto the same scale and datum which is used here as WGS-84. Theconverting procedures follow the instruction of the circular on‘‘Guidelines for the Application of VN-2000 System’’, established on20 June 2001 by the General Department for Land Survey (Ministryof Natural Resources and Environment). The 1930 map is sepa-rately analyzed. Some national roads are assumed not to be chan-ged and can be reference to overlay this map with 1965 map.Because of uncertainty for 1930 map the change of shoreline from1930 to 1965 has not high reality.

Shoreline in topographical maps is considered as the line be-tween seawater and land when water level is at longterm meantide. The tide range at Hondau station (Quang Ninh province) isused for the North Vietnam where the longterm spring tide is4.0 m. Obviously the longterm neap tide is 0 m. Therefore shoreline

Fig. 3. Sediment sam

of the Red River delta in topographical maps is defined at the meansea level of 2.0 m above the neap tide.

2.2. Sediment sampling and testing

A rectangle net of survey along the coast of Hai Hau at thedepths of 0–30 m was set up. The distances between investigationpoints are 2.5 km and 5 km in the depths of shallower and deeperthan 10 m water deep, respectively (Fig. 3). During the fieldworksmall ships were used. The position of sampling stations was deter-mined using a GPS with an accuracy of 5–100 m. A total of 52 sed-iment samples were taken by grab sampler. This is a part ofinvestigation in the Red River coast in 1996 and 2000 (referredto Duc et al. (2007)).

Grain size distributions of sediment samples were analyzed bymeans of sieve for the sandy fractions (sieve sizes: 2, 1, 0.5, 0.25,0.125 and 0.063 mm), and by means of pipette analysis for samplescontaining particles smaller than 63 lm.

A thin-walled tube (ASTM, 2001) was manually inserted to sur-face sediment in tidal flat to take undisturbed geotechnical sam-ples. Six samples were retrieved along the coast in 2008 (Fig. 3).Water content (W), bulk density (c), and grain density (D) of sed-iments are defined in the laboratory. Porosity (n) of sediment isthen defined as:

pling locations.

D.M. Duc et al. / Journal of Asian Earth Sciences 43 (2012) 98–109 101

n ¼ 1� cqsð1þ 0:01WÞ ð1Þ

2.3. Net sediment transport

McLaren and Bowles (1985) proposed a hypothesis that relatestwo cases of grain-size trends to net transport paths. According tothis model, along the direction of net transport sediments can beeither better sorted, finer and more negative skewed (measuredin / units) or better sorted, coarser and more positively skewed.The model has been re-examined by Gao and Collins, 1990 and1992. They proposed a procedure to define two dimensional netsediment transport pathways, including some steps as follows:(1) comparisons of grain size parameters at a station with the onesat adjoining stations (the distance between them is not longer thana characteristic distance. It represents the space-scale of sampling)to define unit vectors, i.e. if there is one or not a net sedimenttransport from the station to another; trend vector at a station isdefined by sum of unit vectors; (2) averaging trend vector at thestation and other ones of adjoining stations to remove noise anddefine transport vectors and (3) significance test on the transportvectors. The parameters of sorting, mean diameter of grain sizes,and skewness are considered to be equal importance in definingnet sediment transport pathways.

The grab sampler takes sediment samples from the bottom sur-face to the depth of 10–15 cm. It may represent different time peri-ods (e.g., a longer or shorter periods are taken into accounts at sitesof higher and lower sedimentation rates, respectively). The charac-teristic distance in this area is assumed to be 5 km, which is thelongest distance between two adjacent sample stations. The differ-ences of sedimentation rates between stations of shorter than 5 kmapart are supposed to be small. At about assumed characteristicdistance, the difference in recent sedimentation rates achievedby 210Pb analysis in 2 gravity cores (i.e. cores 6 and 7 off the south-west coast of Hai Hau) is only 0.5 cm/y (van den Bergh et al., 2007).Therefore the assumption is acceptable.

2.4. Longshore sediment transport

Waves change the propagating direction when they reach to theshallow water it due to bottom fiction. To a certain depth wavesbreak and induce currents. The currents then cause longshore sed-iment transport which is the main reason of coastal erosion. Vol-ume of sediment transport is estimated by CERC formula (USArmy Corps of Engineers, 2002: Manual of Coastal Engineering).

The potential longshore sediment transport rate, dependent onan available quantity of littoral material, is most commonly corre-lated with the so-called longshore component of wave energy fluxor power:

Pl ¼ ðEcgÞb sin ab cos ab ðN=sÞ ð2Þ

Eb ¼qgH2

b

8ðN=sÞ ð3Þ

cgb ¼c2

1þ 2khb

sinh2khb

!ðm=sÞ ð4Þ

k ¼ 2p=Ls ð5Þ

where Eb is the wave energy evaluated at the breaker line, ab thewave angle relative to the shoreline (�), Hb the wave height at break-ing (m), cgb the wave group speed at the breaker line, Ls the wavelength (m), c the wave velocity (m/s), q the density of water (kg/m3), g the gravitational acceleration (g = 9.82 m/s2), and hb is thedepth of wave break (m).

The amount of longshore sediment transport is expressed as thevolume transport rate (Ql) which is estimated by the formula:

Ql ¼K

ðqs � qÞgð1� nÞ Pl ðm3=sÞ ð6Þ

where K is the experimental coefficient, equal to 0.39, qs thedensity of sediment grains (kg/m3), and n is the porosity ofsediments.

3. Results

3.1. Shoreline change

The overlaying of maps shows quantitative figures of shorelinechange at the coast (Fig. 4 and Table 2). The Red River delta isintensively moved seaward at the big river mouths such as BaLat, Day, and Lach. The distribution of erosion comes betweenaccretion segments.

The average velocity of accretion is 65 m/y (1930–1965), 84 m/y(1965–1985), and 60 m/y (1985–1995) at the Ba Lat mouth (Table2). The propagation of shoreline has close relation to the formationand enlargement of sand bars (Fig. 5). A small bar (Vanh sand bar)was formed during the period from 1930 to 1965. The main direc-tion of development is to the NE (left bank of the river). The mouthwas then rapidly moved toward the sea (1965–1985). The Vanhbar was intensively extended, and two other bars (Ngan and Lu)were formed. The accretion at the right bank was dominant. Theseaside of Lu bar in 1985 was 7.1 km away from 1965 shoreline,i.e. an advancing rate of 350 m/y on average. This period is consid-ered as the strongest development of the Ba Lat mouth. Followingthat mechanism a new series of sand bars was formed during theperiod 1985–1995, which was then enlarging and connected toeach other in 2001. The mouth developed symmetrically.

The Lach and Day river mouths have not typical mechanism ofpropagation as the Ba Lat mouth. Beside the sediment budgettransported from the Ninh Co and Day rivers, longshore sedimentsfrom erosion at Hai Hau is intercepted by river currents and depos-ited in between the mouths and sand bars. However there are stillsome small creeks between bars. Their channels change frequentlyand are easy to be filled up. Therefore a large continuous area of ti-dal flat is formed at the Day and Lach mouths. The average accre-tion rates were 95–110 m/y and 27–35 m/y, respectively.

Sediment is mainly deposited at the big river mouths, causingerosion in other places. The erosion occurs either near the big rivermouths or in the middle of them. It has caused land loss of severalvillages (Fig. 6). Nearby the Ba Lat mouth, the shoreline of 22 kmin Giao Long and Giao Phong was eroded during the period 1930–1965. The maximum retreat rate at Giao Phong was 24 m/y. The ero-sion was even more severe in the period 1965–1985 with the aver-age velocity of 1.5 times larger than it was in 1930–1965. Howeverthe erosion was interrupted in 1985–1995 along with the south-ward enlargement of the Lu bar. A short segment of 2.5 km was re-corded as weak erosion in 1995. The remaining 18 km of shorelineturned to be very strong accretion. The shoreline moved seaward100–430 m during the period 1985–1995. Another eroding coastis Nghia Phuc which situates nearby the Lach mouth. Erosion has ta-ken place since 1965 in the length of about 0.5 km. The retreat ratewas 8–10 m/y. The shoreline is now at the trough of sea dikes.

The most severe erosion is the coast of six coastal communes(Hai Dong, Hai Ly, Hai Chinh, Hai Trieu, Hai Hoa, and Hai Thinh)in Hai Hau district. The erosion is considered to start from thebeginning of the 20th century (1905) (Pruszak et al., 2002). It hasa close relation to the degradation of the Ha Lan river mouth (theformer main river mouth of the Red River system at that time).The clear evidence of Ha Lan mouth degradation can be found atGiao Long and Giao Phong shorelines where were continuously

Fig. 4. Shoreline change in Hai Hau coast and adjacent areas.

Table 2Accretion and erosion at the Red River delta coast.

Segment Accretion Erosion

Length (km) Average rate (m/y) Area (ha/y) Length (km) Average rate (m/y) Area (ha/y)

1930–1965Ba Lat mouth 19.5 65 126.8 22 12 26.4Ha Lan – Lach mouth 4.5 5 2.3 17.5 15 74.3South Lach mouth 4.5 27 12.2 – – –Day moutha 10.0 95 95 – – –

1965–1985Ba Lat mouth 21 84 176.4 20 18 36Ha Lan – Lach mouth 1.7 6 1.0 20.3 10 20.3South Lach mouth 4.0 35 14 0.5 10 0.5Day moutha 12.3 110 135.3 0.8 3 0.2

1985–1995Ba Lat mouth 21 60 126 2.5 2.5 0.6Ha Lan – Lach mouth 0.5 4 0.2 21.5 11 23.7South Lach mouth 4.0 28 11.2 0.5 8 0.4Day moutha 12.5 100 125 1.5 4 0.6

a The shoreline of the Day mouth in Ninh Binh province was not taken into account.

102 D.M. Duc et al. / Journal of Asian Earth Sciences 43 (2012) 98–109

accreted with rapid rates (reaching to 100 m/year in some seg-ments during the period of 1905–1930) (Fig. 4). However the mainriver mouth was then shifted to the Ba Lat mouth and shorelineschanged to erosion. During the period 1930–1965 the maximumretreat rate was 22 m/y in Hai Ly and Hai Chinh communes. TheHai Ly coast was then significantly eroded in 1965–1985. The aver-age rate was 21 m/y. At the same period the rates were 5 m/y atHai Dong coast and 11 m/y at Hai Chinh–Hai Thinh coast. Thesouth part of Hai Thinh commune was accreted. Upto 1995, majorparts of shoreline reached to the trough of sea dikes, i.e. the waterlevel at the mean tide touched the dikes. Lateral movement isstopped. Shoreline retreat is only realized at some segments inHai Ly and Hai Chinh where the former dikes were broken andthe locations of new dikes shifted landward. Shoreline change atother parts of the Hai Hau coast cannot be estimated by topograph-ical maps. The evolvement of erosion is then recognized by the

change of bottom topography and landscapes on the beach duringlow tide (see Section 3.2).

Nowadays, the shoreline in Hai Dong has been changed toaccretion (by eye-seeing and personal conversation with localauthority for some recent years). However the erosion continuesto increase in other segments. The most severe erosion segmentis now shifting to Hai Thinh commune. It is very significant by aseries of three photos taken at the same place at the coast of HaiThinh commune from 2003 to 2005. Figs. 7 and 8 show that a smalltent for mineral exploitation was almost disappeared during10 months (from 02 September 2003 to 25 July 2004). The shore-line retreated about 30 m. Nine months later all the pine trees weredestroyed. The shoreline reached to the sea dike with a lateralmovement of about 40–50 m (Fig. 9). The result proves an actualsituation of increasing erosion that is opposite to a remark of re-cent decrease of erosion (Pruszak et al., 2002).

Fig. 5. Recent progress of the Ba Lat mouth.

Fig. 6. Land loss due to erosion in Hai Ly commune.

Fig. 7. Hai Thinh, 02 September 2003.

Fig. 8. Hai Thinh, 25 July 2004.

D.M. Duc et al. / Journal of Asian Earth Sciences 43 (2012) 98–109 103

3.2. Nearshore topography change

The erosion has caused a remarkable change of bottom topogra-phy along the coast. The depth contour of 3 m in 1985 is approxi-mately matched with the one of 5 m in 1965 (Fig. 10). The 2 m

contour (if is considered as the middle between 1 and 3 m con-tours) was moved landward 1–2 km from 1965 to 1985. The con-tour then continued moving 1.5–3 km from 1985 to 1995. Themaximum movement occurred at the south of Giao Phong com-mune. An opposite sign of erosion is also recognized at the northpart of Giao Phong commune where the 2 m contour of 1995 inter-cepts the one of 1985.

Fig. 9. Hai Thinh, 17 April 2005.

Fig. 10. Nearshore topography change.

Table 4Wave parameters at the Hai Hau station (01 January 1976–31 December 1994)(Pruszak et al., 2002).

No. Hs (m) Tp (s) h (�) ab (�) Duration (days)

1 0.57 2.93 87.5 44.5 36.52 1.22 5.14 81.7 50.3 35.13 1.67 6.49 81.6 50.4 1.44 2.04 7.00 77.0 55.0 0.25 0.51 3.00 65.0 67.0 10.96 1.13 5.13 63.6 68.4 2.27 1.79 5.94 73.2 58.8 9.98 1.99 6.82 68.8 63.2 4.69 2.48 7.18 67.6 64.4 4.2

10 3.20 8.20 63.5 68.5 0.711 0.80 2.65 43.0 89.0 19.112 1.16 5.00 43.0 89.0 3.813 1.91 6.00 43.0 89.0 0.314 2.50 7.00 43.0 89.0 0.115 4.25 8.25 40.0 92.0 0.116 0.59 3.27 20.0 112.0 3.417 1.32 5.12 18.4 113.6 1.818 1.60 6.00 26.0 106.0 0.219 2.03 7.47 16.7 115.3 0.720 2.52 7.79 18.4 113.6 0.221 3.25 9.05 24.0 108.0 0.122 0.50 3.11 �3.2 �3.2 11.023 1.19 5.44 1.2 130.8 7.024 1.74 6.79 11.8 120.2 0.425 0.54 3.20 �22.6 154.6 7.426 0.47 5.80 �47.0 179.0 33.127 1.28 9.36 �63.4 195.4 0.128 1.76 11.00 �47.0 179.0 0.129 0.39 5.00 �70.0 202.0 3.2

104 D.M. Duc et al. / Journal of Asian Earth Sciences 43 (2012) 98–109

3.3. Sediment properties and net transport

Laboratory testing of undisturbed soil samples (Table 3) showsthat nearshore sand is medium sand with the porosity of 0.42–0.49. Grain size parameters of surface sediments are shown inTable 8. Based on grain sizes, two main types of sediments are de-fined such as sand and silt. Sand is distributed along the shorelinein water depths of 3–5 m, except to the southeast of the Red Rivermouth, where sand extends down to the water depth of 15 m(Fig. 11). The recent sand is very well sorted and consists onaverage for 98.5% of sandy and 1.5% of silt particles. Silt is widely

Table 3Physical properties of nearshore sand.

No. Sample Percentage of grain sizes (mm)

1.0- 0.50- 0.25- 0.125- >0.50 0.25 0.125 0.063 0.063

1 H2 0.2 2.7 92.0 3.0 2.22 H4 0.0 2.8 94.1 1.0 2.13 H5 0.9 92.8 0.4 1.0 4.94 H6 6.1 3.7 94.3 1.2 0.65 H8 0.1 97.9 0.1 0.5 1.56 H11 0.8 1.6 93.4 1.9 2.3

distributed along the coast stretching from northeast to southwest.Most of the silt is poorly sorted. The composition is dominated onaverage by 70% silt, 22% clay and 8% sand. Besides, sandy silt dis-tributes at the eastern and southeastern margin of the study areaat the depth is over 25–30 m. It is the old sediment units (Ducet al., 2007).

A set of 52 sediment is used to define the net transport accord-ing to the method of Gao and Collins, 1992. The results in Fig. 11shows that the sediment from the Ba Lat mouth is not depositednearshore, but moves seaward up to the water depth of 25 m. Itis very significant along the Giao Long – Ha Lan coast at the depthof 5–25 m. In Hai Thinh shoreline, the sediment is transportedalong the coast southwestward. In Giao Long–Giao Phong shore-line, the sediment is transported along coast northeastward. Thereason may be the northeast waves do not have strong effect onthe coast because of the sand bars in front of the Red River mouth.

3.4. Longshore sediment transport

The volume of longshore sediment transport is calculated by theformula (6), with the wave monitoring data at Hai Ly from 1976 to1994 (Table 4). The result shows that the sediment is dominantlytransported southwestward by the northeast and east waves. The

Water content(%)

Bulk density(g/cm3)

Grain density(g/cm3)

Porosity

26.8 1.96 2.66 0.4229.5 1.92 2.67 0.4435.0 1.87 2.68 0.4834.5 1.86 2.68 0.4834.7 1.85 2.69 0.4929.5 1.94 2.67 0.44

Fig. 11. Net sediment transport pathways at the Hai Hau coast (1985 topography, map of surface sediment is extracted from Duc et al. (2007)).

D.M. Duc et al. / Journal of Asian Earth Sciences 43 (2012) 98–109 105

volume is 654,078–801,078 m3/year in Hai Dong and Hai Ly sec-tion (Table 5). The figure of Nghia Phuc section is 440,979 m3/y.Volume of southwestward transport at Giao Long section (1965)was 741,335. It gives an evidence of strong erosion in this area dur-ing the period 1965–1985. Sediment transport changed to north-eastward at this section in the context of 1995 topography. Thevolume is 80,798 m3/y.

4. Discussion

4.1. Impacts of sea level rise

IPCC (2007) indicated a clear trend of sea level rise (SLR) world-wide with the average rate of 1.8 mm/y over 1961–2003. A compar-ative study on impacts of SLR has confirmed that Vietnam is themost vulnerable country to sea level rise in Southeast Asia andone of top five most vulnerable countries in the world (Susmitaet al., 2007). Relative SLR in Vietnam is mainly calculated fromtide-gauge data collected at the four chief stations: Hon Dau (QuangNinh province – North Vietnam), Da Nang, Qui Nhon (Center Part)and Vung Tau (South Vietnam). The longest tide data is achievedat Hon Dau station from 1960 to 2000. The SLR of 1.9 mm a yearhas been observed in this period (Hanh and Furukawa, 2007). Thuy(1995) analyzed two tidal gauges in the North coast, one is at HonDau and another is at Hai Hau. The result shows that from 1950sto 1990s the average rate of SLR is 2.24 mm/y. The recorded dataof four chief stations shows that the increments in sea level varyingfrom 1.75 to 2.56 mm/y along the coast of Vietnam in 50 recentyears. It is 3 mm/y over 1993–2008 (MONRE, 2009).

To estimate the increase of shoreline erosion the formula of theso-called Brunn’s rule (1962) is used. The formula shows the rela-tion between SLR and the increase of shoreline erosion asfollowing:

R1 ¼ 0:001SL�

h� þ BðmÞ ð7Þ

where S is the SLR (mm/y); R1 the exceeding rate of erosion due toSLR (m/y); L⁄ and (h⁄ + B) are the width and vertical extent of theactive beach profile.

The results (Table 6) show that the increase of erosion rate canreach to 0.14–0.31 m/y along the coast of the Red River delta. How-ever the erosion rate depends on many factors such as humanactivity, change of direction of sediment flow, waves, and currents(Duc et al., 2007). It is hard to define the accurate contribution ofSLR on the increase of erosion rate. To have a raw estimation ofSLR effect, the erosion rate at the south Hai Thinh commune is ta-ken into account. The rates were approximately 0 and 11 m/y dur-ing the period 1965–1985 and 1985–1995, respectively. It is about40 m/y in 2005. Therefore SLR contributes 34% to the increase oferosion rate during the period 1965–1995 and 12% from 1995 to2005.

4.2. Impacts of tropical cyclones

Tropical cyclone is a typical climatic event in the North Viet-nam. The so-called storm season often starts in June and ends inOctober. About 13% of the total tropical cyclones attacked thecountry landed on the North coast. Tropical cyclones, especially ty-phoons have caused many severe lost of properties and lives. Forinstant, the typhoon PAT (23 October 1998) made 500,000 home-less and 90 death in the North coast. The imprints of typhoonsare recognized at the 22 m water deep by laminated sand layersbetween silty clay layers in a gravity core (van den Bergh et al.,2007). Storm surge due strong winds and heavy rainfall in a ty-phoon can reach to a height of 2.6 m (Table 7). This phenomenonalways leads to serious losses. The most recent Damrey typhoonlanded in the high spring tide caused very disastrous damages on

Table 5Volumes of longshore sediment transport.

Location Beach slope Shoreline orientation (�) SW NE Total (m3/y)a

Giao Long (1965) 0.00800 44 1,000,002 �258,667 741,335Giao Long (1995) 0.00400 76 275,402 �356,200 �80,798Hai Dong 0.01500 42 878,094 �77,016 801,078Hai Ly 0.01000 42 716,962 �62,884 654,078Nghia Phuc 0.00455 42 483,375 �42,396 440,979

a ‘‘Plus’’ and ‘‘minus’’ are sediment transport to the SW and NE, respectively.

Table 6Increase of erosion rate due to SLR.

Section SLR (mm/y) h⁄ (m) B (m) L⁄ (m) Increase of erosion rate (m/y)

Giao Phong 2.24 5.4 2.0 556.7 0.17Hai Dong 2.24 7.0 2.0 821.6 0.20Hai Hoa-Hai Thinh 2.24 10.4 2.0 1377.6 0.25Nghia Phuc 2.24 5.3 2.0 473.8 0.15

Table 7Heights of storm surge in severe typhoons.

No. Typhoon Date of formation Landing place Storm surge height (m)

1 PHILLIS 02 July 1966 Nam Dinh, Ninh Binh 1.102 ROSE 08 September 1968 Nam Dinh 2.563 RUTH 10 December 1973 Thanh Hoa 2.504 JOE 18 July 1980 Hai Phong 1.945 WARREN 16 August 1981 Thai Binh, Nam Dinh 1.156 PAT 18 October 1988 Hai Phong 0.787 DOT 16 May 1989 Hai Phong 1.928 DAMREY 19 September 2005 Nam Dinh, Hai Phong 2.50

Fig. 12. Return periods of typhoons.

106 D.M. Duc et al. / Journal of Asian Earth Sciences 43 (2012) 98–109

sea dikes, mangrove, shrimp ponds, and infrastructure. Hundredsthousand people had to emigrate.

According to the formula of Kriebel and Dean (1993) the retreatdistance caused by extreme wave heights can be estimated asfollowing:

RðtÞ ¼ R1ð1� e�t=Ts Þ ðmÞ ð8Þ

R1 ¼HsðWb � hb=moÞ

Bþ hb � Hs=2ðmÞ ð9Þ

Wb ¼hb

A

� �3=2

ðmÞ ð10Þ

Ts ¼ 320H3=2

b

g1=2A3 1þ hb

BþmoWb

hb

� ��1

ðsÞ ð11Þ

where Hs is the significant wave height (m), Hb the wave height atbreaking (m), hb the depth of wave break (m), Wb the width ofbreaking wave zone (m), B the height of berm (m), mo the beachslope, t the duration of extreme wave heights (h), A the sedimentscale or equilibrium profile parameter (m1/3), R(t) is the retreatdistance caused by extreme wave heights (m).

The recorded wave heights during typhoons at Hai Hau tide sta-tion (1976–1994) were 3.2–4.25 m (Table 4). Table 8 indicates thatthe erosion rate can reach to 7.1 m when the wave height is 4.25 mand the duration is 2.4 h.

The research of NCDC (1996) emphasized the increase in thenumber of tropical cyclones attacked Vietnamese coast duringthe period 1920–1994. The most recent statistical data of theannual number of tropical cyclones shows that the number of cy-clones does not have any clear trend during the period 1960–1990s. It had a significant reduction in number from 2000 to

2004 and then has been increasing very rapidly from 2005 up topresent (Duc et al., 2009). This matter cannot all be claimed on cli-mate change. However it is evidence showing that the variability ofextreme events at the coast occurring more complicated. Kleinen(2007) suggested an increase in occurrence and intensity of ty-phoons in the Western part of the Pacific, especially the ones thathit Vietnam. The threat of typhoons on coastal zone in general andon sea dike stability in particularly is expected to be more seriousin the near future.

The analysis of statistical data on typhoons of the NationalCentre for Meteorology and Hydrology (http://www.thoitietnguy-hiem.net) shows that there were 86 typhoons directly hit the coastof the Red River delta over 1962–2010, i.e. an average of two ty-phoons annually. The return periods of typhoons with the intensityof equal or greater than 10, 11, 12, and 13 (Beaufort scale) areabout 3, 5, 10, and 21.5 years, respectively (Fig. 12). As recognizedfrom the Damrey typhoon (September 2005), storm surge andwave-run up were the main reasons for sea dike destruction andcoastal flooding. The correlation between typhoon intensity and

Fig. 13. Correlation between storm surge height and intensity of typhoon.

Fig. 14. Estimated storm surge height and the return period (storm surge height isestimated by the regression equation in Fig. 13).

Fig. 15. Current concrete seadike system.

D.M. Duc et al. / Journal of Asian Earth Sciences 43 (2012) 98–109 107

storm surge (Fig. 13) indicates a unit increase of typhoon intensitycauses an increase of about 30 cm of storm surge height.

To assess combined impacts of typhoon and SLR on storm surgeheight, medium emission scenario (B2) is taken into account. TheB2 scenario expects that sea level in Vietnam will rise 30 and75 cm in 2050 and 2100, respectively (MONRE, 2009). As shownin Fig. 14, the return period of a 2.6 m storm surge height reducesfrom 20 years at the present to 9 years in 2050 and only 4.5 yearsin 2100.

4.3. Upstream dams

The Hoa Binh dam (fully operated in 1989) does not change theamount of water but reduced 56% of suspended sediment budgetin the downstream flow as recorded at Son Tay and Hanoi hydrolog-ical monitoring stations (Table 9). Sediment transported to the coastcan also derive from lateral and bottom erosion along the channels,but no monitoring data about suspended matters in river flows nearthe coast is recorded. Meanwhile the accretion rate decreased from84 m/year (1965–1985) to 60 m/year (1985–1995) at the Ba Lat

Table 8Erosion rate caused by an extreme wave height.

Section S (m) hb (m) Hb (m) B (m)

Giao Phong 4.25 6.96 3.15 2.00Hai Dong 4.25 9.23 4.10 2.00Hai Hoa-Hai Thinh 4.25 8.18 3.78 2.00Nghia Phuc 4.25 8.83 3.23 2.00

Table 9Average water, sediment discharge before and after Hoa Binh dam.

Parameter Station

Son Tay

1956–1988 1989–1994

Average water discharge (bill. m3/year) 112 106Average sediment budget (106 T/year) 117 65

mouth. It shows a tendency of reduction in coastal accretion dueto dam construction. In the near future, as Tuyen Quang, Son Laand other hydropower plants will be operating the sediment supplyfor the coast is going to decrease significantly. The reduction ofaccretion at the river mouths and more severe erosion in the HaiHau coast are irreversible.

4.4. Suggestion of coastal protection

The protection of the costal zone in the Red River delta is veryimportant because of high population density and economic bene-fits. Sea dike is the most important measure to protect the coast.Simple dikes of compacted soils were common in the 1980s. Thedikes are easy to be eroded and severely damaged in a typhoon.Such type of dikes is still at some parts of the coast in the Hai Chinhand Hai Dong communes. To reinforce the dikes groynes wereused. They were constructed by a chain of concrete tubes withdiameter of 1 m, thickness of 10 cm, and height of 1.5 m. The tubeswere filled up with sand bags and placed continuously at the depthof 0.5 m under beach surface. The distance between groynes is80 m. Mangrove forest is another measure against coastal erosion.A hundred meters of mature mangrove can reduce 0.1 m of waveheight (Mazda et al., 1997; Quartel et al., 2007). However it cannotbe used in areas of severe erosion. The sea dikes at erosion coasts,i.e. Hai Hau and Nghia Phuc have been intensively reinforced since1998, especially after the Damrey typhoon in September 2005.Dikes have mild slope of 1:2.2-3 with the height extended to+4.5 to 5.5 m and. The dike footing was placed at the depth of

mo D50 (mm) A (m1/3) T (h) R(t) (m)

0.0040 0.143 0.0798 2.4 6.60.0150 0.143 0.0798 2.4 7.10.0100 0.147 0.0840 2.4 3.10.0045 0.157 0.0872 2.4 3.6

Ha Noi

1995–1998 1956–1988 1989–1994 1995–1998

120 85 76 8951.5 71 45 –

Table 11Suggested preventive measures against coastal erosion.

Drivingfactors

Consequences Required measures Supporting measures regarding the erosion rate

<2 m/y 2–5 m/y 5–10 m/y >10 m/y

Typhoon Increaseerosion rate

Raise height of dikes Concreterevetment Land-use planning

Mangrove GroynesMangrove

Groynes Seadiketoe protection

Groynes Seadike toe protection BreakwaterInternal standby dike Evacuation

Sea levelrise

Instability ofseadikes

Sedimentdeficit

Table 10Rates of beach lowering at Hai Hau coast.

Location Beach slope Height of berm (m) Width of beach (m) Erosion rate (m/y) Rate of beach lowering (cm/y)

Hai Dong 0.0150 2.5 200 5.0 6.3Hai Ly 0.0150 2.5 180 12.0 16.7Hai Chinh 0.0150 2.5 250 15.0 15.0Hai Trieu 0.0150 2.5 225 20.0 22.2Hai Hoa 0.0100 2.5 210 21.0 25.0Hai Thinh 0.0100 2.5 260 7.0 6.7

108 D.M. Duc et al. / Journal of Asian Earth Sciences 43 (2012) 98–109

1.5 m. Revetment of polygonal pre-cast concrete with the mass of100–200 kg and chains of tripods were also used to reinforce dikes(Fig. 15).

Shoreline cannot keep lateral movement in front of the seadikes. A large amount of sediments on the beach is washed awaythat causes lowering of beach profile and scour at the toe of seadikes. The rate of beach lowering is approximately estimated bythe physical model of Barnett and Wang (1988). Assuming the vol-ume of sediment transported is similar to the value before the con-struction of the dike then the relation between erosion rate andrate of beach lowering can be referred as follows:

Dh ¼ 100DY � b=l ð12Þ

where Dh is the rate of beach lowering (cm/y), DY the erosion rate(m/y), l the width of beach from shoreline to the depth of mean sealevel (m), and b is the height of berm (m).

The calculation shows that rates of beach lowering are high atHai Ly, Hai Chinh, Hai Trieu, and Hai Hoa (Table 10) with the valuesof 15–25 cm/y. The current concrete sea dikes have foots placed atthe depth of 1.5 m. Therefore the dike’s foot can be destroyed in 6–10 years. Lowering of beach profile is the most serious threat tolong-term stability of sea dikes.

The current sea dike system is expected to suffer typhoons up tothe intensity of 10, landing during mean tide. Worse situationssuch as stronger typhoon with surge storms can destroy sea dikesand force local people to move to safe places. To adapt to coastaldisaster in such case the counter measures have to focus on all as-pects of protection, accommodation, and evacuation (Klein et al.,1999). However permanent immigration is impossible. Because itrequires the land capacity and jobs for almost people who haveonly known aquaculture, fishing, and salty production. Reinforce-ment to ensure the long-term stability of sea dikes is an essentialrequirement for socio-economic development. Depending on ero-sion rates the preventive measures are suggested in Table 11.

5. Conclusions

Tropical condition has brought the Red River a large amount ofsediments. Accretion is dominant and coastline of the delta isadvancing rapidly. However sediment supply is distributed

unequally. It is intensively deposited at the big river mouths (BaLat, Lach, and Day) and leads to the formation of sand bars in frontof the mouths. The rapid enlargement of sand bars reduces waveattacks and makes suitable environment for sediment depositionin channels between shoreline and bars. Shoreline is thereforeadvancing at both land and sea sides of the bars. When channelsare mostly filled up, river flows will bring sediments further offthe coast and form a new series of sand bars. This mechanism isperiodical with the return period of about 20 years.

Unequal distribution of sediments causes local erosion at thecoast of the Red River delta. Severe erosion is occurred either be-tween the river mouths or nearby them. The erosion is due tothe wave-induced longshore sediment transport and the deficitof sediment supply from river mouths. Shoreline erosion nearbythe river mouths (Giao Long, Giao Phong, and Nghia Phuc) dependsclosely on the evolvement of sand bars. It is strongly eroded whena new series of sand bars has just formed and then turns to be rapidaccretion when crest of the bars above the mean sea level. Erosionbetween big river mouths, i.e. Hai Hau coast, keeps being severe.The recent advancement of shoreline at the Ba Lat and Day mouthscannot help to reduce erosion in Hai Hau.

SLR and reduction of sediment supply due to upstream hydro-power plants are expected to reduce accretion and accelerate ero-sion along the coast of the Red River delta. Erosion lowers beachprofiles and threats seriously the stability of sea dikes. Moreovertropical typhoon and storm surge seem to be increasing in inten-sity. As the projected SLR of the medium emission scenario (B2)in Vietnam, the return period of the highest recorded storm surgeis 2 and 4 times shorter in 2050 and 2100, respectively.

Acknowledgements

The paper is funded by the National Foundation for Science andTechnology Development (NAFOSTED). The authors also would liketo thank the Asia Pacific Network for Global Change Research(APN) for the support through the Project coded CIA2009-06-Duc.

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