DSpace at VNU: An analysis of coastal erosion in the tropical rapid accretion delta of the Red River, Vietnam

DSpace at VNU: An analysis of coastal erosion in the tropical rapid accretion delta of the Red River, Vietnam tài liệu,...

Journal of Asian Earth Sciences 43 (2012) 98–109 Contents lists available at SciVerse ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes An analysis of coastal erosion in the tropical rapid accretion delta of the Red River, Vietnam Do Minh Duc a,⇑, Mai Trong Nhuan b, Chu Van Ngoi a a b Faculty of Geology, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam Vietnam National University, Hanoi, 144 Xuan Thuy, Cau Giay, Hanoi, Viet Nam a r t i c l e i n f o Article history: Received November 2010 Received in revised form August 2011 Accepted 10 August 2011 Available online 13 September 2011 Keywords: Red River delta Shoreline Accretion Erosion Sediment transport a b s t r a c t 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 the evolvement 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 propagation is forming and connecting sand bars in front of the mouths Erosion coasts are distributed either between the river mouths (Hai Hau) or nearby them (Giao Long, Giao Phong, and Nghia Phuc) The evolvement of erosion is caused by wave-induced longshore southwestward sediment transport Meanwhile sediment from the river mouths is not directed to deposit nearshore The development of sand bars can intensively reduce the erosion rate nearby river mouths Erosion in Hai Hau is accelerated by sea level rise and upstream dams Sea dike stability is seriously threatened by erosion-induced lowering of beach profiles, sea level rise, typhoon, and storm surge Ó 2011 Elsevier Ltd All rights reserved Introduction The Red River begins from the mountains of Yunnan province (China) The Red River delta in Vietnam territory is formed by the Red and Thai Binh river systems, which is commonly called the Red River system (Fig 1) The delta is about 15,000 km2 It is a rich agricultural area and densely populated Along the coastline, the interaction between the sea and big rivers has created a typical tropical natural condition which is suitable for tourism, agricultural and aquaculture development The Red River delta develops in a very dynamic fluvial and marine environment The river basin is characterized by an alternation of wet and dry seasons producing a huge total annual suspended sediment load (Hoekstra and Van Weering, 2007) The delta is river dominated (Fig 2) The annual amount of sediment transported by the Red River system into the East Vietnam Sea is about 82 Â 106 m3 In the wet season (from April to September), about 90% of the annual sediment supply is transported through the various distributaries (Nhuan et al., 1996) Of the total amount of sediment supplied, 11.7% passes through the Van Uc and Thai Binh river mouths, 11.8% through the Tra Ly river mouth, 37.8% through the Red River (Ba Lat) mouth and 23.7% through the Day river mouth (Duc et al., 2007) ⇑ Corresponding author Tel.: +84 4912042804; fax: +84 38583061 E-mail address: ducdm@vnu.edu.vn (D.M Duc) 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd All rights reserved doi:10.1016/j.jseaes.2011.08.014 The northern part of the coast (from Ba Lat to Hai Phong) has a diurnal tidal regime with average amplitude of 2.5–3.5 m In the southern part, from Ba Lat to Day mouth, the tide is mixed with a diurnal dominance The average tidal amplitude is 2–3 m (Nhuan et al., 1996) Waves usually have a dominant direction from the east, northeast during the dry season (October–March) and from east, southeast during the wet season (April–September) The average and maximum wave heights are 0.7–1.3 m Wave heights in severe typhoons can reach over m (Nhuan et al., 1996) The large amount of sediments has made the delta a rapid continuous advancing to the sea Old shorelines of the delta are recognized through series of old sand bars, historical and anthropogenic proofs (Hoan and Phai, 1995) The delta was enlarged 20–30 km from the 10th to 15th century and 10 km from 15th to 19th century (Fig 1) Sediments supplied by the big mouths (Tra Ly, Ba Lat, Lach, and Day) are mainly deposited at shallow sea and form sand bars in front of the mouths They protect shorelines behind against wave and current attacks making a suitable condition for rapid accretion Most of sediments discharged from rivers deposit in front of the river mouths and causes rapid accretion As a consequence, severe shoreline retreat occurs at some other places due to sediment deficit Erosion coast in the Red River delta has length and area much less than those of accretion coast However, it has seriously damaged the coastal villages and made an obstacle for economic development in the region The distribution of erosion shoreline D.M Duc et al / Journal of Asian Earth Sciences 43 (2012) 98–109 99 Fig Red River system and locations of old shorelines can be a typical characteristic of the Red River delta They are either in the middle of the big river mouths (Hai Hau) or very close to them (Giao Long, Giao Phong, and Nghia Phuc) The paper has objective to outline shoreline change in the recent time and find out the physical mechanism of erosion in the tropical rapid accretion delta of the Red River Recent evolvement of shoreline and the reasons are elucidated by the analysis of topographical maps and nearshore sediment transport Factors affecting shoreline retreat such as typhoon, sea level rise, and upstream dams are studied to assess potential acceleration of erosion and its impacts to coastal structures Materials and methods 2.1 Topographical maps A series of topographical maps are used to investigate recent changes of shoreline (Table 1) The maps were established using different co-ordinate systems and scales WGS-84 stands for World Geodetic System which is currently the reference system being used by the Global positioning system WGS-60 is one of the Fig The Red River in the chart of delta classification of Coleman and Wright (1975) 100 D.M Duc et al / Journal of Asian Earth Sciences 43 (2012) 98–109 Table List of recent topographical maps Year 1930 1965 1985 1995 2001 Co-ordinate Datum Ellipsoid – WGS-60 HN-72 VN-2000 VN-2000 – WGS-60 Krasovski WGS-84 WGS-84 former systems of WGS-84 HN-72 or Hanoi-72 is the system of Vietnam which was used before 2000 It used Krasovski ellipsoid and the datum (origin co-ordination) was transferred from Moscow to Hanoi It is not a spatial unified system and requires different formulas to convert to other systems VN-2000 is the current national geodetic system in Vietnam The origin co-ordination was installed in Hanoi VN-2000 is a unified system for the whole country Maps are scanned and then the shoreline in each map is digitized in MapInfo software The vector maps are then transformed to the same scale and datum which is used here as WGS-84 The converting procedures follow the instruction of the circular on ‘‘Guidelines for the Application of VN-2000 System’’, established on 20 June 2001 by the General Department for Land Survey (Ministry of Natural Resources and Environment) The 1930 map is separately analyzed Some national roads are assumed not to be changed and can be reference to overlay this map with 1965 map Because of uncertainty for 1930 map the change of shoreline from 1930 to 1965 has not high reality Shoreline in topographical maps is considered as the line between seawater and land when water level is at longterm mean tide The tide range at Hondau station (Quang Ninh province) is used for the North Vietnam where the longterm spring tide is 4.0 m Obviously the longterm neap tide is m Therefore shoreline Scale Note 1:250,000 1:50,000 1:50,000 1:50,000 1:50,000 Published Published Published Published 1978 1991 2001 2005 (Ba Lat mouth only) of the Red River delta in topographical maps is defined at the mean sea 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 the depths of 0–30 m was set up The distances between investigation points are 2.5 km and km in the depths of shallower and deeper than 10 m water deep, respectively (Fig 3) During the fieldwork small ships were used The position of sampling stations was determined using a GPS with an accuracy of 5–100 m A total of 52 sediment samples were taken by grab sampler This is a part of investigation in the Red River coast in 1996 and 2000 (referred to Duc et al (2007)) Grain size distributions of sediment samples were analyzed by means 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 samples containing particles smaller than 63 lm A thin-walled tube (ASTM, 2001) was manually inserted to surface sediment in tidal flat to take undisturbed geotechnical samples Six samples were retrieved along the coast in 2008 (Fig 3) Water content (W), bulk density (c), and grain density (D) of sediments are defined in the laboratory Porosity (n) of sediment is then defined as: Fig Sediment sampling locations D.M Duc et al / Journal of Asian Earth Sciences 43 (2012) 98–109 nẳ1 c qs ỵ 0:01Wị 1ị McLaren and Bowles (1985) proposed a hypothesis that relates two cases of grain-size trends to net transport paths According to this model, along the direction of net transport sediments can be either better sorted, finer and more negative skewed (measured in / units) or better sorted, coarser and more positively skewed The model has been re-examined by Gao and Collins, 1990 and 1992 They proposed a procedure to define two dimensional net sediment transport pathways, including some steps as follows: (1) comparisons of grain size parameters at a station with the ones at adjoining stations (the distance between them is not longer than a characteristic distance It represents the space-scale of sampling) to define unit vectors, i.e if there is one or not a net sediment transport from the station to another; trend vector at a station is defined by sum of unit vectors; (2) averaging trend vector at the station and other ones of adjoining stations to remove noise and define transport vectors and (3) significance test on the transport vectors The parameters of sorting, mean diameter of grain sizes, and skewness are considered to be equal importance in defining net sediment transport pathways The grab sampler takes sediment samples from the bottom surface to the depth of 10–15 cm It may represent different time periods (e.g., a longer or shorter periods are taken into accounts at sites of higher and lower sedimentation rates, respectively) The characteristic distance in this area is assumed to be km, which is the longest distance between two adjacent sample stations The differences of sedimentation rates between stations of shorter than km apart are supposed to be small At about assumed characteristic distance, the difference in recent sedimentation rates achieved by 210Pb analysis in gravity cores (i.e cores and off the southwest 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 the shallow water it due to bottom fiction To a certain depth waves break and induce currents The currents then cause longshore sediment transport which is the main reason of coastal erosion Volume of sediment transport is estimated by CERC formula (US Army Corps of Engineers, 2002: Manual of Coastal Engineering) The potential longshore sediment transport rate, dependent on an available quantity of littoral material, is most commonly correlated with the so-called longshore component of wave energy flux or power: Pl ẳ Ecg ịb sin ab cos ab N=sị Eb ẳ cgb qgH2b N=sị ! c 2khb m=sị 1ỵ ẳ 2 sinh khb k ẳ 2p=Ls The amount of longshore sediment transport is expressed as the volume transport rate (Ql) which is estimated by the formula: Ql ẳ 2.3 Net sediment transport 2ị 3ị 4ị 5ị where Eb is the wave energy evaluated at the breaker line, ab the wave angle relative to the shoreline (°), Hb the wave height at breaking (m), cgb the wave group speed at the breaker line, Ls the wave length (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 the depth of wave break (m) 101 K Pl ðm3 =sÞ ðqs À qÞgð1 À nÞ ð6Þ where K is the experimental coefficient, equal to 0.39, qs the density of sediment grains (kg/m3), and n is the porosity of sediments Results 3.1 Shoreline change The overlaying of maps shows quantitative figures of shoreline change at the coast (Fig and Table 2) The Red River delta is intensively moved seaward at the big river mouths such as Ba Lat, Day, and Lach The distribution of erosion comes between accretion 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 (Table 2) The propagation of shoreline has close relation to the formation and enlargement of sand bars (Fig 5) A small bar (Vanh sand bar) was formed during the period from 1930 to 1965 The main direction of development is to the NE (left bank of the river) The mouth was then rapidly moved toward the sea (1965–1985) The Vanh bar was intensively extended, and two other bars (Ngan and Lu) were formed The accretion at the right bank was dominant The seaside 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 considered as the strongest development of the Ba Lat mouth Following that mechanism a new series of sand bars was formed during the period 1985–1995, which was then enlarging and connected to each other in 2001 The mouth developed symmetrically The Lach and Day river mouths have not typical mechanism of propagation as the Ba Lat mouth Beside the sediment budget transported from the Ninh Co and Day rivers, longshore sediments from erosion at Hai Hau is intercepted by river currents and deposited in between the mouths and sand bars However there are still some small creeks between bars Their channels change frequently and are easy to be filled up Therefore a large continuous area of tidal flat is formed at the Day and Lach mouths The average accretion rates were 95–110 m/y and 27–35 m/y, respectively Sediment is mainly deposited at the big river mouths, causing erosion in other places The erosion occurs either near the big river mouths or in the middle of them It has caused land loss of several villages (Fig 6) Nearby the Ba Lat mouth, the shoreline of 22 km in Giao Long and Giao Phong was eroded during the period 1930– 1965 The maximum retreat rate at Giao Phong was 24 m/y The erosion was even more severe in the period 1965–1985 with the average velocity of 1.5 times larger than it was in 1930–1965 However the erosion was interrupted in 1985–1995 along with the southward enlargement of the Lu bar A short segment of 2.5 km was recorded as weak erosion in 1995 The remaining 18 km of shoreline turned to be very strong accretion The shoreline moved seaward 100–430 m during the period 1985–1995 Another eroding coast is Nghia Phuc which situates nearby the Lach mouth Erosion has taken place since 1965 in the length of about 0.5 km The retreat rate was 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 the beginning of the 20th century (1905) (Pruszak et al., 2002) It has a close relation to the degradation of the Ha Lan river mouth (the former main river mouth of the Red River system at that time) The clear evidence of Ha Lan mouth degradation can be found at Giao Long and Giao Phong shorelines where were continuously 102 D.M Duc et al / Journal of Asian Earth Sciences 43 (2012) 98–109 Fig Shoreline change in Hai Hau coast and adjacent areas Table Accretion and erosion at the Red River delta coast Segment Accretion Length (km) a Erosion Average rate (m/y) Area (ha/y) Length (km) Average rate (m/y) Area (ha/y) 1930–1965 Ba Lat mouth Ha Lan – Lach mouth South Lach mouth Day moutha 19.5 4.5 4.5 10.0 65 27 95 126.8 2.3 12.2 95 22 17.5 – – 12 15 – – 26.4 74.3 – – 1965–1985 Ba Lat mouth Ha Lan – Lach mouth South Lach mouth Day moutha 21 1.7 4.0 12.3 84 35 110 176.4 1.0 14 135.3 20 20.3 0.5 0.8 18 10 10 36 20.3 0.5 0.2 1985–1995 Ba Lat mouth Ha Lan – Lach mouth South Lach mouth Day moutha 21 0.5 4.0 12.5 60 28 100 126 0.2 11.2 125 2.5 21.5 0.5 1.5 2.5 11 0.6 23.7 0.4 0.6 The shoreline of the Day mouth in Ninh Binh province was not taken into account accreted with rapid rates (reaching to 100 m/year in some segments during the period of 1905–1930) (Fig 4) However the main river mouth was then shifted to the Ba Lat mouth and shorelines changed to erosion During the period 1930–1965 the maximum retreat rate was 22 m/y in Hai Ly and Hai Chinh communes The Hai Ly coast was then significantly eroded in 1965–1985 The average rate was 21 m/y At the same period the rates were m/y at Hai Dong coast and 11 m/y at Hai Chinh–Hai Thinh coast The south part of Hai Thinh commune was accreted Upto 1995, major parts of shoreline reached to the trough of sea dikes, i.e the water level at the mean tide touched the dikes Lateral movement is stopped Shoreline retreat is only realized at some segments in Hai Ly and Hai Chinh where the former dikes were broken and the locations of new dikes shifted landward Shoreline change at other parts of the Hai Hau coast cannot be estimated by topographical maps The evolvement of erosion is then recognized by the change of bottom topography and landscapes on the beach during low tide (see Section 3.2) Nowadays, the shoreline in Hai Dong has been changed to accretion (by eye-seeing and personal conversation with local authority for some recent years) However the erosion continues to increase in other segments The most severe erosion segment is now shifting to Hai Thinh commune It is very significant by a series of three photos taken at the same place at the coast of Hai Thinh commune from 2003 to 2005 Figs and show that a small tent for mineral exploitation was almost disappeared during 10 months (from 02 September 2003 to 25 July 2004) The shoreline retreated about 30 m Nine months later all the pine trees were destroyed The shoreline reached to the sea dike with a lateral movement of about 40–50 m (Fig 9) The result proves an actual situation of increasing erosion that is opposite to a remark of recent decrease of erosion (Pruszak et al., 2002) D.M Duc et al / Journal of Asian Earth Sciences 43 (2012) 98–109 103 Fig Land loss due to erosion in Hai Ly commune Fig Hai Thinh, 02 September 2003 Fig Hai Thinh, 25 July 2004 Fig Recent progress of the Ba Lat mouth 3.2 Nearshore topography change The erosion has caused a remarkable change of bottom topography along the coast The depth contour of m in 1985 is approximately matched with the one of m in 1965 (Fig 10) The m contour (if is considered as the middle between and m contours) was moved landward 1–2 km from 1965 to 1985 The contour then continued moving 1.5–3 km from 1985 to 1995 The maximum movement occurred at the south of Giao Phong commune An opposite sign of erosion is also recognized at the north part of Giao Phong commune where the m contour of 1995 intercepts the one of 1985 104 D.M Duc et al / Journal of Asian Earth Sciences 43 (2012) 98–109 Table Wave parameters at the Hai Hau station (01 January 1976–31 December 1994) (Pruszak et al., 2002) Fig Hai Thinh, 17 April 2005 Fig 10 Nearshore topography change 3.3 Sediment properties and net transport Laboratory testing of undisturbed soil samples (Table 3) shows that nearshore sand is medium sand with the porosity of 0.42– 0.49 Grain size parameters of surface sediments are shown in Table Based on grain sizes, two main types of sediments are defined such as sand and silt Sand is distributed along the shoreline in water depths of 3–5 m, except to the southeast of the Red River mouth, where sand extends down to the water depth of 15 m (Fig 11) The recent sand is very well sorted and consists on average for 98.5% of sandy and 1.5% of silt particles Silt is widely No Hs (m) Tp (s) h (°) ab (°) Duration (days) 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 0.57 1.22 1.67 2.04 0.51 1.13 1.79 1.99 2.48 3.20 0.80 1.16 1.91 2.50 4.25 0.59 1.32 1.60 2.03 2.52 3.25 0.50 1.19 1.74 0.54 0.47 1.28 1.76 0.39 2.93 5.14 6.49 7.00 3.00 5.13 5.94 6.82 7.18 8.20 2.65 5.00 6.00 7.00 8.25 3.27 5.12 6.00 7.47 7.79 9.05 3.11 5.44 6.79 3.20 5.80 9.36 11.00 5.00 87.5 81.7 81.6 77.0 65.0 63.6 73.2 68.8 67.6 63.5 43.0 43.0 43.0 43.0 40.0 20.0 18.4 26.0 16.7 18.4 24.0 À3.2 1.2 11.8 À22.6 À47.0 À63.4 À47.0 À70.0 44.5 50.3 50.4 55.0 67.0 68.4 58.8 63.2 64.4 68.5 89.0 89.0 89.0 89.0 92.0 112.0 113.6 106.0 115.3 113.6 108.0 À3.2 130.8 120.2 154.6 179.0 195.4 179.0 202.0 36.5 35.1 1.4 0.2 10.9 2.2 9.9 4.6 4.2 0.7 19.1 3.8 0.3 0.1 0.1 3.4 1.8 0.2 0.7 0.2 0.1 11.0 7.0 0.4 7.4 33.1 0.1 0.1 3.2 distributed along the coast stretching from northeast to southwest Most of the silt is poorly sorted The composition is dominated on average by 70% silt, 22% clay and 8% sand Besides, sandy silt distributes at the eastern and southeastern margin of the study area at the depth is over 25–30 m It is the old sediment units (Duc et al., 2007) A set of 52 sediment is used to define the net transport according to the method of Gao and Collins, 1992 The results in Fig 11 shows that the sediment from the Ba Lat mouth is not deposited nearshore, but moves seaward up to the water depth of 25 m It is very significant along the Giao Long – Ha Lan coast at the depth of 5–25 m In Hai Thinh shoreline, the sediment is transported along the coast southwestward In Giao Long–Giao Phong shoreline, the sediment is transported along coast northeastward The reason may be the northeast waves not have strong effect on the 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 the formula (6), with the wave monitoring data at Hai Ly from 1976 to 1994 (Table 4) The result shows that the sediment is dominantly transported southwestward by the northeast and east waves The Table Physical properties of nearshore sand No Sample H2 H4 H5 H6 H8 H11 Percentage of grain sizes (mm) 1.00.50 0.500.25 0.250.125 0.1250.063 > 0.063 0.2 0.0 0.9 6.1 0.1 0.8 2.7 2.8 92.8 3.7 97.9 1.6 92.0 94.1 0.4 94.3 0.1 93.4 3.0 1.0 1.0 1.2 0.5 1.9 2.2 2.1 4.9 0.6 1.5 2.3 Water content (%) Bulk density (g/cm3) Grain density (g/cm3) Porosity 26.8 29.5 35.0 34.5 34.7 29.5 1.96 1.92 1.87 1.86 1.85 1.94 2.66 2.67 2.68 2.68 2.69 2.67 0.42 0.44 0.48 0.48 0.49 0.44 D.M Duc et al / Journal of Asian Earth Sciences 43 (2012) 98–109 105 Fig 11 Net sediment transport pathways at the Hai Hau coast (1985 topography, map of surface sediment is extracted from Duc et al (2007)) volume is 654,078–801,078 m3/year in Hai Dong and Hai Ly section (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 during the period 1965–1985 Sediment transport changed to northeastward at this section in the context of 1995 topography The volume is 80,798 m3/y Discussion 4.1 Impacts of sea level rise IPCC (2007) indicated a clear trend of sea level rise (SLR) worldwide with the average rate of 1.8 mm/y over 1961–2003 A comparative study on impacts of SLR has confirmed that Vietnam is the most vulnerable country to sea level rise in Southeast Asia and one of top five most vulnerable countries in the world (Susmita et al., 2007) Relative SLR in Vietnam is mainly calculated from tide-gauge data collected at the four chief stations: Hon Dau (Quang Ninh province – North Vietnam), Da Nang, Qui Nhon (Center Part) and Vung Tau (South Vietnam) The longest tide data is achieved at Hon Dau station from 1960 to 2000 The SLR of 1.9 mm a year has been observed in this period (Hanh and Furukawa, 2007) Thuy (1995) analyzed two tidal gauges in the North coast, one is at Hon Dau and another is at Hai Hau The result shows that from 1950s to 1990s the average rate of SLR is 2.24 mm/y The recorded data of four chief stations shows that the increments in sea level varying from 1.75 to 2.56 mm/y along the coast of Vietnam in 50 recent years It is mm/y over 1993–2008 (MONRE, 2009) To estimate the increase of shoreline erosion the formula of the so-called Brunn’s rule (1962) is used The formula shows the relation between SLR and the increase of shoreline erosion as following: R1 ẳ 0:001S L mị h ỵB 7ị where S is the SLR (mm/y); R1 the exceeding rate of erosion due to SLR (m/y); L⁄ and (h⁄ + B) are the width and vertical extent of the active beach profile The results (Table 6) show that the increase of erosion rate can reach to 0.14–0.31 m/y along the coast of the Red River delta However the erosion rate depends on many factors such as human activity, change of direction of sediment flow, waves, and currents (Duc et al., 2007) It is hard to define the accurate contribution of SLR on the increase of erosion rate To have a raw estimation of SLR effect, the erosion rate at the south Hai Thinh commune is taken into account The rates were approximately and 11 m/y during the period 1965–1985 and 1985–1995, respectively It is about 40 m/y in 2005 Therefore SLR contributes 34% to the increase of erosion rate during the period 1965–1995 and 12% from 1995 to 2005 4.2 Impacts of tropical cyclones Tropical cyclone is a typical climatic event in the North Vietnam The so-called storm season often starts in June and ends in October About 13% of the total tropical cyclones attacked the country landed on the North coast Tropical cyclones, especially typhoons have caused many severe lost of properties and lives For instant, the typhoon PAT (23 October 1998) made 500,000 homeless and 90 death in the North coast The imprints of typhoons are recognized at the 22 m water deep by laminated sand layers between silty clay layers in a gravity core (van den Bergh et al., 2007) Storm surge due strong winds and heavy rainfall in a typhoon can reach to a height of 2.6 m (Table 7) This phenomenon always leads to serious losses The most recent Damrey typhoon landed in the high spring tide caused very disastrous damages on 106 D.M Duc et al / Journal of Asian Earth Sciences 43 (2012) 98–109 Table Volumes of longshore sediment transport a Location Beach slope Shoreline orientation (°) SW NE Total (m3/y)a Giao Long (1965) Giao Long (1995) Hai Dong Hai Ly Nghia Phuc 0.00800 0.00400 0.01500 0.01000 0.00455 44 76 42 42 42 1,000,002 275,402 878,094 716,962 483,375 À258,667 À356,200 À77,016 À62,884 À42,396 741,335 À80,798 801,078 654,078 440,979 ‘‘Plus’’ and ‘‘minus’’ are sediment transport to the SW and NE, respectively Table Increase of erosion rate due to SLR Section SLR (mm/y) h⁄ (m) B (m) L⁄ (m) Increase of erosion rate (m/y) Giao Phong Hai Dong Hai Hoa-Hai Thinh Nghia Phuc 2.24 2.24 2.24 2.24 5.4 7.0 10.4 5.3 2.0 2.0 2.0 2.0 556.7 821.6 1377.6 473.8 0.17 0.20 0.25 0.15 Table Heights of storm surge in severe typhoons No Typhoon Date of formation Landing place Storm surge height (m) PHILLIS ROSE RUTH JOE WARREN PAT DOT DAMREY 02 08 10 18 16 18 16 19 Nam Dinh, Ninh Binh Nam Dinh Thanh Hoa Hai Phong Thai Binh, Nam Dinh Hai Phong Hai Phong Nam Dinh, Hai Phong 1.10 2.56 2.50 1.94 1.15 0.78 1.92 2.50 July 1966 September 1968 December 1973 July 1980 August 1981 October 1988 May 1989 September 2005 sea dikes, mangrove, shrimp ponds, and infrastructure Hundreds thousand people had to emigrate According to the formula of Kriebel and Dean (1993) the retreat distance caused by extreme wave heights can be estimated as following: Rtị ẳ R1 et=T s ị mị R1 ẳ Hs W b hb =mo ị mị B ỵ hb Hs =2 Wb ¼  3=2 hb ðmÞ A T s ¼ 320  1 hb mo W b 1ỵ ỵ sị B hb g 1=2 A Hb3=2 ð8Þ ð9Þ ð10Þ ð11Þ where Hs is the significant wave height (m), Hb the wave height at breaking (m), hb the depth of wave break (m), Wb the width of breaking wave zone (m), B the height of berm (m), mo the beach slope, t the duration of extreme wave heights (h), A the sediment scale or equilibrium profile parameter (m1/3), R(t) is the retreat distance caused by extreme wave heights (m) The recorded wave heights during typhoons at Hai Hau tide station (1976–1994) were 3.2–4.25 m (Table 4) Table indicates that the erosion rate can reach to 7.1 m when the wave height is 4.25 m and the duration is 2.4 h The research of NCDC (1996) emphasized the increase in the number of tropical cyclones attacked Vietnamese coast during the period 1920–1994 The most recent statistical data of the annual number of tropical cyclones shows that the number of cyclones 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 to present (Duc et al., 2009) This matter cannot all be claimed on climate change However it is evidence showing that the variability of extreme events at the coast occurring more complicated Kleinen (2007) suggested an increase in occurrence and intensity of typhoons in the Western part of the Pacific, especially the ones that hit Vietnam The threat of typhoons on coastal zone in general and on sea dike stability in particularly is expected to be more serious in the near future The analysis of statistical data on typhoons of the National Centre for Meteorology and Hydrology (http://www.thoitietnguyhiem.net) shows that there were 86 typhoons directly hit the coast of the Red River delta over 1962–2010, i.e an average of two typhoons annually The return periods of typhoons with the intensity of equal or greater than 10, 11, 12, and 13 (Beaufort scale) are about 3, 5, 10, and 21.5 years, respectively (Fig 12) As recognized from the Damrey typhoon (September 2005), storm surge and wave-run up were the main reasons for sea dike destruction and coastal flooding The correlation between typhoon intensity and Fig 12 Return periods of typhoons 107 D.M Duc et al / Journal of Asian Earth Sciences 43 (2012) 98–109 Fig 13 Correlation between storm surge height and intensity of typhoon Fig 15 Current concrete seadike system Fig 14 Estimated storm surge height and the return period (storm surge height is estimated by the regression equation in Fig 13) storm surge (Fig 13) indicates a unit increase of typhoon intensity causes an increase of about 30 cm of storm surge height To assess combined impacts of typhoon and SLR on storm surge height, medium emission scenario (B2) is taken into account The B2 scenario expects that sea level in Vietnam will rise 30 and 75 cm in 2050 and 2100, respectively (MONRE, 2009) As shown in Fig 14, the return period of a 2.6 m storm surge height reduces from 20 years at the present to years in 2050 and only 4.5 years in 2100 4.3 Upstream dams The Hoa Binh dam (fully operated in 1989) does not change the amount of water but reduced 56% of suspended sediment budget in the downstream flow as recorded at Son Tay and Hanoi hydrological monitoring stations (Table 9) Sediment transported to the coast can also derive from lateral and bottom erosion along the channels, but no monitoring data about suspended matters in river flows near the coast is recorded Meanwhile the accretion rate decreased from 84 m/year (1965–1985) to 60 m/year (1985–1995) at the Ba Lat mouth It shows a tendency of reduction in coastal accretion due to dam construction In the near future, as Tuyen Quang, Son La and other hydropower plants will be operating the sediment supply for the coast is going to decrease significantly The reduction of accretion at the river mouths and more severe erosion in the Hai Hau coast are irreversible 4.4 Suggestion of coastal protection The protection of the costal zone in the Red River delta is very important because of high population density and economic benefits Sea dike is the most important measure to protect the coast Simple dikes of compacted soils were common in the 1980s The dikes 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 Chinh and Hai Dong communes To reinforce the dikes groynes were used They were constructed by a chain of concrete tubes with diameter of m, thickness of 10 cm, and height of 1.5 m The tubes were filled up with sand bags and placed continuously at the depth of 0.5 m under beach surface The distance between groynes is 80 m Mangrove forest is another measure against coastal erosion A hundred meters of mature mangrove can reduce 0.1 m of wave height (Mazda et al., 1997; Quartel et al., 2007) However it cannot be used in areas of severe erosion The sea dikes at erosion coasts, i.e Hai Hau and Nghia Phuc have been intensively reinforced since 1998, 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 Table Erosion rate caused by an extreme wave height Section S (m) hb (m) Hb (m) B (m) mo D50 (mm) A (m1/3) T (h) R(t) (m) Giao Phong Hai Dong Hai Hoa-Hai Thinh Nghia Phuc 4.25 4.25 4.25 4.25 6.96 9.23 8.18 8.83 3.15 4.10 3.78 3.23 2.00 2.00 2.00 2.00 0.0040 0.0150 0.0100 0.0045 0.143 0.143 0.147 0.157 0.0798 0.0798 0.0840 0.0872 2.4 2.4 2.4 2.4 6.6 7.1 3.1 3.6 Table Average water, sediment discharge before and after Hoa Binh dam Parameter Station Son Tay Average water discharge (bill m3/year) Average sediment budget (106 T/year) Ha Noi 1956–1988 1989–1994 1995–1998 1956–1988 1989–1994 1995–1998 112 117 106 65 120 51.5 85 71 76 45 89 – 108 D.M Duc et al / Journal of Asian Earth Sciences 43 (2012) 98–109 Table 10 Rates 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 Hai Hai Hai Hai Hai 0.0150 0.0150 0.0150 0.0150 0.0100 0.0100 2.5 2.5 2.5 2.5 2.5 2.5 200 180 250 225 210 260 5.0 12.0 15.0 20.0 21.0 7.0 6.3 16.7 15.0 22.2 25.0 6.7 Dong Ly Chinh Trieu Hoa Thinh Table 11 Suggested preventive measures against coastal erosion Driving factors Consequences Required measures Typhoon Increase erosion rate Instability of seadikes Raise height of dikes Concrete revetment Land-use planning Sea level rise Sediment deficit Supporting measures regarding the erosion rate 10 m/y Mangrove Groynes Mangrove Groynes Seadike toe protection Groynes Seadike toe protection Breakwater Internal standby dike Evacuation 1.5 m Revetment of polygonal pre-cast concrete with the mass of 100–200 kg and chains of tripods were also used to reinforce dikes (Fig 15) Shoreline cannot keep lateral movement in front of the sea dikes A large amount of sediments on the beach is washed away that causes lowering of beach profile and scour at the toe of sea dikes The rate of beach lowering is approximately estimated by the physical model of Barnett and Wang (1988) Assuming the volume of sediment transported is similar to the value before the construction of the dike then the relation between erosion rate and rate of beach lowering can be referred as follows: Dh ¼ 100DY Â 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 sea level (m), and b is the height of berm (m) The calculation shows that rates of beach lowering are high at Hai Ly, Hai Chinh, Hai Trieu, and Hai Hoa (Table 10) with the values of 15–25 cm/y The current concrete sea dikes have foots placed at the 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 to long-term stability of sea dikes The current sea dike system is expected to suffer typhoons up to the intensity of 10, landing during mean tide Worse situations such as stronger typhoon with surge storms can destroy sea dikes and force local people to move to safe places To adapt to coastal disaster in such case the counter measures have to focus on all aspects of protection, accommodation, and evacuation (Klein et al., 1999) However permanent immigration is impossible Because it requires the land capacity and jobs for almost people who have only known aquaculture, fishing, and salty production Reinforcement to ensure the long-term stability of sea dikes is an essential requirement for socio-economic development Depending on erosion rates the preventive measures are suggested in Table 11 unequally It is intensively deposited at the big river mouths (Ba Lat, Lach, and Day) and leads to the formation of sand bars in front of the mouths The rapid enlargement of sand bars reduces wave attacks and makes suitable environment for sediment deposition in channels between shoreline and bars Shoreline is therefore advancing at both land and sea sides of the bars When channels are mostly filled up, river flows will bring sediments further off the coast and form a new series of sand bars This mechanism is periodical with the return period of about 20 years Unequal distribution of sediments causes local erosion at the coast of the Red River delta Severe erosion is occurred either between the river mouths or nearby them The erosion is due to the wave-induced longshore sediment transport and the deficit of sediment supply from river mouths Shoreline erosion nearby the river mouths (Giao Long, Giao Phong, and Nghia Phuc) depends closely on the evolvement of sand bars It is strongly eroded when a new series of sand bars has just formed and then turns to be rapid accretion when crest of the bars above the mean sea level Erosion between big river mouths, i.e Hai Hau coast, keeps being severe The recent advancement of shoreline at the Ba Lat and Day mouths cannot help to reduce erosion in Hai Hau SLR and reduction of sediment supply due to upstream hydropower plants are expected to reduce accretion and accelerate erosion along the coast of the Red River delta Erosion lowers beach profiles and threats seriously the stability of sea dikes Moreover tropical typhoon and storm surge seem to be increasing in intensity As the projected SLR of the medium emission scenario (B2) in Vietnam, the return period of the highest recorded storm surge is and times shorter in 2050 and 2100, respectively Acknowledgements The paper is funded by the National Foundation for Science and Technology Development (NAFOSTED) The authors also would like to thank the Asia Pacific Network for Global Change Research (APN) for the support through the Project coded CIA2009-06-Duc Conclusions Tropical condition has brought the Red River a large amount of sediments Accretion is dominant and coastline of the delta is advancing rapidly However sediment supply is distributed References American Society for Testing of Materials (ASTM), 2001 D1587 Standard practice for thin-walled tube sampling of soils for geotechnical purposes D.M Duc et al / Journal of Asian Earth Sciences 43 (2012) 98–109 Barnett, M., Wang, H., 1988 Effects of a vertical seawall on profile response In: Proceedings of the twenty-first international conference on coastal engineering American Society of Civil Engineers, pp 1493–1507 [chapter 111] Brunn, P., 1962 Sea-level rise as a cause of shore erosion Journal of Waterways and Harbor Division, American Society of Civil Engineers 88, 117–130 Coleman, J.M., Wright, L.D., 1975 Modern river deltas: variability of process and sand bodies In: Broussard, M.L (Ed.), Deltas, Models for 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Sciences 29 (4), 545–557 Website of National Centre for Meteorology and Hydrology  ... on the increase of erosion rate To have a raw estimation of SLR effect, the erosion rate at the south Hai Thinh commune is taken into account The rates were approximately and 11 m/y during the. .. emphasized the increase in the number of tropical cyclones attacked Vietnamese coast during the period 1920–1994 The most recent statistical data of the annual number of tropical cyclones shows that the. .. is intensively deposited at the big river mouths (Ba Lat, Lach, and Day) and leads to the formation of sand bars in front of the mouths The rapid enlargement of sand bars reduces wave attacks and