DSpace at VNU: Sediment distribution and transport at the nearshore zone of the Red River delta, Northern Vietnam

Sediment distribution and transport at the nearshore zone of theRed River delta, Northern Vietnam a Faculty of Geology, University of Science, Vietnam National University, Hanoi, Viet Na

Sediment distribution and transport at the nearshore zone of the

Red River delta, Northern Vietnam

a Faculty of Geology, University of Science, Vietnam National University, Hanoi, Viet Nam

b Division for Marine Geology, Department of Geology and Mineral Resources, Viet Nam

c Royal Netherlands Institute for Sea Research (NIOZ), P.O Box 59, 1790, AB Den Burg, Texel, The Netherlands

Received 22 August 2003; received in revised form 27 February 2005; accepted 2 March 2006

Abstract

The coast between Ngason and Haiphong is largely formed by accretion of the Red River system In the region, five main surface sediment types (sand, sandy silt, silt, mud and sand at shoals) could be defined, which differ from one another in their sedimentary char-acteristics Sand dominates along the shoreline between 0 and 15 m water depth Down to a water depth of about 25–30 m, the sediment

is dominantly silt Further offshore the surface sediments are mainly sandy silt and sand of older units (Early-Middle Holocene, Late Pleistocene) Net sediment transport directions are defined by grainsize analysis according to the method of Gao and Collins [Gao, S., Collins, M., 1992 Net sediment transport patterns inferred from grain-size trends, based upon definition of transport vectors Sed-imen Geol 80, 47–60, 1992] At river mouths, directions of sediment transport are variable where the depths are shallower than 5 m From 5 to 10 m water depth, sediments are mainly transported southeastward at the Ba Lat, Lach and Day mouths, northeastward

at the Tra Ly mouth and eastward at the Thai Binh mouth Recently, the Hai Hau erosional shoreline is not supplied with sediment from the Ba Lat mouth and sediments are transported southwestward alongshore in the region shallower than 5 m The region of depths from 10 to 30 m is specified by southward sediment transport

 2007 Elsevier Ltd All rights reserved

Keywords: Grain size; Surface sediment; Sediment transport; Red River delta; Vietnam

1 Introduction

The Ngason-Haiphong area forms a part of the west

coast of the South China Sea (Fig 1) and has been largely

formed by accretion of the Red River delta system The

annual amount of sediment transported by the Red River

system into the South China Sea is about 82· 106

m3 In the wet season (from June to January), 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 These major river mouths represent very rapid accretion zones where sedi-ment accumulation rates exceed sea level rise (1–2 mm/ year) and tectonic subsidence (2 mm/year, Ngoi et al.,

2000)

The northern part of the Ngason-Haiphong coast (from

Ba Lat to Haiphong) has a diurnal tidal regime with an average amplitude of 2.5–3.5 m In the southern part, from

Ba Lat to Ngason, the tide is mixed with a diurnal domi-nance The average tidal amplitude is 2–3 m Waves usually have a dominant direction from the east-northeast during the dry season and from east-southeast during the wet sea-son The average and maximum wave heights are 0.7–1.3 m

1367-9120/$ - see front matter  2007 Elsevier Ltd All rights reserved.

doi:10.1016/j.jseaes.2006.03.007

*

Corresponding author.

E-mail address: ducdm@fpt.vn (D.M Duc).

www.elsevier.com/locate/jaes

and 3.5–4.5 m, respectively, but in severe storms wave

heights can reach over 5 m (Nhuan et al., 1996)

The shorelines at the major river mouths are currently

expanding at a rate of about 15–100 m/year The newly

formed land constitutes a useful environment for aquatic

cultures and mangrove development The rapid accretion

in front of the river mouths causes widespread difficulties

for navigation On the other hand, sediment deficits in

the adjacent areas lead to shoreline erosion The coastal

erosion causes the loss of land, demolishment of

infrastruc-ture and expansion of saline intrusions The coastal zone

where rapid erosion prevails is from south of the Ba Lat

mouth (Giao Long) to the Hai Thinh commune (Hai

Hau district, Nam Dinh province) (Fig 1) The shoreline

regression in this region can reach 10–15 m/year Erosion

of the shoreline also takes place north of the Van Uc river

(Haiphong city) with lower intensity.234Th and210Pb

anal-ysis on a number of boxcores and gravity cores in front of

the Red River mouth indicates that the main deposition

takes place south of the river mouth Toward the north

the sea bed at a depth of 20–25 m is eroded during the

dry season (van den Bergh et al., this volume) There is a

regular southwest to southward bottom current in the

study area at a depth of 15–25 m (Nhuan et al., 1996;

Dan-kers, 2001), enabling transport of fine-grained sediments in

front of the Red River mouth towards the south

Detailed knowledge concerning the sediment

distribu-tion and transport pathways is very important in

understanding the accretionary and erosional patterns in

the coastal zone This paper presents some results of

sed-iment distribution and transport analysis along the

Ngason-Haiphong coastal zone between a depth of 0 and

30 m, in order to make a contribution in solving questions related to forecasting shoreline changes, landuse planning and mitigation of hazards caused by erosion and accretion

2 Methods and materials

Sediment samples were collected and obtained during the national research projects on sedimentology and geo-environment of the coastal zone of Northern Vietnam, car-ried out by Hanoi University of Science in 1996 and by the Vietnam – Netherlands ‘‘Red River Delta’’ joint research project in 2000

During the fieldwork of these projects, small ships were used The position of sampling stations in 1996 and 2000 was determined using a GPS with an accuracy of 5 and

100 m, respectively In 1996, 564 surface sediment samples were taken with a grab sampler The distance between sta-tions was 2.5 km in shallow areas (<10 m water depth) and

5 km in deeper water (10–30 m) In 2000 the fieldwork con-centrated on the front of the Red River mouth, covering the area between 1062002300E–1065703400E and 22250

4100N–195803900N Two successive fieldworks in the dry (March) and wet (August) season were carried out A grid

of 20 shallow penetrating echosounder profiles was retrieved Based on the interpretation of the acoustic pro-files, stations were selected for bottom sampling All the sampling stations were located at depths of over 8 m A total of 44 gravity cores (up to 2 m long) and 31 box cores (up to 20 cm long) were retrieved However, no gravity cores and box cores could be successfully retrieved at

THAI THUY

TIEN HAI

Giao Phong

HAI HAU Hai Loc

Hai Thinh

Den shoal Vanh shoal

Van

Ucm

outh

Thai Binhmouth

Diem Dien mouth

Tr a Ly mouth

Lan mouth

Balat

mout h

D a m o

Red River

NGASON

HAIPHONG

NAM DINH province

THAI BINH province

Ngo Dong River Ninh Co

River

Lach mouth

20 10’ o

20 20’ o

20 30’ o

20 40’ o

10

106 30’ o

106 00’ o

106 30’ o

106 00’ o

Accretionary shoreline Erosional shoreline LEGEND

Fig 1 Location of the study area.

places where sand or gravel constituted the surface

sedi-ments Twenty-one sub-samples were taken from the split

gravity cores for granulometry In addition, 49 sandy

sed-iment samples were retrieved at the Den shoal and Vanh

shoal (Fig 1)

In 1996, 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 (Table 1) In 2000, sieves were also used

for the sandy fractions of samples, but laser diffraction

analysis (Master Sizer, Malvern Instruments, Ltd.) was

used for the finer-grained fractions For comparison of

the two methods, 14 samples were analyzed by both

meth-ods (sieve/pipette and sieve/laser diffraction) The

correla-tion of grain size characteristics between both methods is

shown inFig 2

The coefficients of correlation (r) for the parameters median diameter (D50), sandy content, silty content and clayey content are 0.83, 0.94, 0.81 and 0.86, respectively These good correlations allow for a conversion of values obtained by laser diffraction to those of the pipette method The sediment was classified using the criteria for classifica-tion of sediments of the British Geological Survey

McLaren and Bowles (1985) proposed a hypothesis that relates two cases of grain-size trends to net trans-port 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), afterwards (1992), they proposed a procedure to define two dimensional net sediment transport pathways, including some steps as follows: comparisons of grainsize parameters at a station with the ones at adjacent stations

to define unit and trend vectors, averaging to define of transport vectors and significance test of the derived trends

Assumptions taken into account: sorting, mean and skewness are considered to be of equal importance in defin-ing transport trends The sediment samples taken by grab sampler may represent different time periods (e.g., longer

Table 1

The number of analyzed samples

y = 2.3798x - 2.7154

R 2

= 0.6885

0 5 10 15 20 25 30

0 10 20 30

Laser diffraction

a

y = 1.4753x + 3.4639

R 2

= 0.8754

Laser diffraction

b

y = 0.9695x + 5.2816

R 2

= 0.6625 50

55 60 65 70 75

c

y = 0.7338x + 0.6377

R2 = 0.7369

10 20 30 40 50

Laser diffraction

Laser diffraction

d

5

Fig 2 Correlations of median diameter (in lm) (a) and sandy (b), silty (c), clayey (d) percentages based on analyses by laser diffraction and the pipette method.

or shorter periods are represented at sites of higher and

lower sedimentation rates, respectively) The characteristic

distance is assumed to be 5 km, which is the longest

dis-tance between two adjacent sample stations Therefore,

the differences of sedimentation rates between stations of

shorter than 5 km apart are supposed to be small

3 Sediment distribution at the nearshore zone of the RRD

Five main sediment types could be distinguished, and a

map of the surface sediment distribution was drawn

(Fig 3) Around the 30 m isobath in the central part of

the study area the sediment changes from silt to sandy silt

in offshore direction The sandy silt samples in the region

usually contain abundant shell fragments and have

green-ish grey colour To the contrary, sediment from the near

coastal zone has a dominant red colour The coefficient

of cation exchange (Kt= (Na++ K+)/(Ca2++ Mg2+)) of

greenish sediments is always above 1 and the pH varies

between 7.2 and 8.0 These values are typical for a shallow

marine environment and confirm the conclusions ofNghi

et al (1996)that silt, sandy silt and sand along the eastern

margin represent old sediment units of Early-Middle

Holo-ceneðQ1–2IVÞ and Late Pleistocene ðQ2IIIÞ

The characteristics of the most widespread old (Q2III,

Q1–2IV) and recentðQ3

IVÞ sediment units are shown inTables

2 and 3, respectively

3.1 Sand

The old units Q2III and Q1–2IV cover 3 large sandy areas

along the eastern margin of the study area (Fig 3) The

old sand unit is restricted to water depths of more than

25 m Most of the old sand unit is well sorted with an

aver-age Soof 1.297 Only three samples show moderate sorting

(So up to 2.516) This sand unit contains between 90 and

100% sandy particles The remaining part is silt D50 is

rather variable, with a maximum of 0.713 mm

Recent 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 The recent sand is very well sorted (So= 1.100–

1.318) and consists on average for 98.5% of sandy and

1.5% of silty particles D50 varies between 0.085 and

0.406 mm with an average of 0.135 mm, which is finer

grained than sand from the old sandy units

Sand at the Den and Vanh shoals in front of the Red

River mouth is characterized by very good sorting

(So= 1.116–1.287) and rather uniform D50 (0.125–

0.196 mm)

3.2 Sandy silt

Sandy silt exists only in the old sediment units It

distrib-utes widely at the eastern and southeastern margin of the

study area where the depth is over 25–30 m Most samples

of this type are moderately sorted (2.5 < S < 4.0), but

there are 24 well-sorted and 11 poorly sorted samples On average sandy silt is composed of 41% sand and 50% silt, whereas clay content is always less than 16.5% D50ranges from 0.020 to 0.062 mm with an average of 0.044 mm 3.3 Silt

Silt covers two narrow northwest-southeast trending strips pertaining to the old sediment units (the old and recent silts are not mapped as distinct units in Fig 3) The sediment is moderately sorted and the main compo-nents are silt (on average 69%) and clay (22%) Toward the northwest these strips merge into the reddish-coloured silt of the recent sediment unit

The recent silt is widely distributed in a broad zone along the coast stretching from northeast to southwest Most of the silt is poorly sorted (So> 2.5), indicating quite unstable hydrodynamic conditions of sediment deposition The recent silt is dominated on average by 70% silt, similar

as in the old sedimentary units The average contents of clay and sand are 22% and 2%, respectively D50 varies from 0.018 to 0.051 mm

3.4 Mud Recent mud covers small patches in a water depth between 5 and 16 m The sediment is moderately to poorly sorted It consists mainly of silt (52%) and clay (44%) with minor amounts of sand (<8%) D50 is very small (0.005–0.011 mm)

In addition, there are a number of other sediment types, such as sandy gravel, gravelly sand, gravelly mud, slightly gravelly mud, muddy sand and sandy mud, all with low frequencies Gravelly samples were exclusively found in connection to the old sediment units

in deep water These sediments consist for 25–32% of gravel, 53–70% of sand and 3–14% of silt and clay They are poorly sorted (So= 2.1–6.2) D50 varies from 0.05 to 0.52 mm These types usually cover limited areas of sin-gle sampling stations, and have not been mapped in

Fig 3

4 Transport of recent sediments at the nearshore zone of RRD

The mean grain-size, sorting and skewness have been calculated using the /-scale The procedure for determining transport vectors has been programmed using Turbo Pas-cal (Duc et al., 2003) The characteristic distance is consid-ered as the longest distance between two survey stations and equals to 5 km The results (Fig 4) show the following: 4.1 River mouths

The transport vectors do not have dominant directions where the depth is less than 5 m This may be a result of strong variation of hydrodynamic factors at the river

mouths (flow, wave and tide induced currents, see van

Maren, 2004) From 5 to 10 m water depth, sediments

are mainly transported southeastward at the Ba Lat, Lach

and Day mouths, northeastward at the Tra Ly mouth and

eastward at the Thai Binh mouth

4.2 Hai Hau coast The coastal segment between Hai Loc and Hai Thinh has been considered as the northern coastal segment most severely affected by erosion Recently, the erosional retreat

Fig 3 Map of surface sediment distribution at the near shore zone of RRD.

velocity can reach over 20 m/year The coast is supplied

with sediment from the Ba Lat mouth However, the

dom-inant transport direction is perpendicular to the depth

con-tours to a depth of about 25 m Sediments are transported

along shore in southwestward direction in the coastal

waters shallower than 5 m

The depth interval from 10 to 30 m is dominated by

southward transport Along the offshore edge of the recent

sediment unit, the sediment is mainly transported

land-ward This is an artefact because here the recent sediment

consists of a very thin layer that has become mixed with

coarser, early-Middle Holocene sediments due to the

sam-pling method

5 Discussion

The old sandy sediment distributes at a water depth of over 30 m, with some lenses of gravel locally Gravel and sand are composed mainly of quartz and some lateritic lumps with various shapes The quartz gravels are of terres-trial origin and were transported by the Late Pleistocene-Early Holocene rivers Lateritic lumps were formed by the erosion of Late Pleistocene ðQ2IIIÞ weathering surfaces during the Holocene transgression and belong to the

Q2III-Q1IV formations As mentioned above, greenish sandy silt and silt belong to the units of Q1–2IV They were formed

in a period of rapid transgression The narrow strips of

Table 2

Characteristics of old surface sediment types of the nearshore zone

a S o is sorting coefficient.

b D 50 is median diameter (after Krumbein and Sloss, 1963 ).

Table 3

Characteristics of recent surface sediment types of the near-shore zone

old silt constitute evidence for the former existence of old

river channels prior to the Holocene transgression

Sandy bars in front of the Ba Lat mouth were formed

during the regression that started at the end of the Middle

to the Late Holocene after the maximum transgression in

the Middle Holocene The landward side of the bars is

now accreted and the offshore side is locally eroded The

recent sediments always have a typical red colour that

orig-inates from lateritic suspended sediments of the Red River

system

Reliable bedload sediment transport pathways obtained

by using the method ofGao and Collins (1992)can only be

used where sediments are deposited in the same period of

time Grab samples contain a mixture of sediments from

depths ranging between 0 and 10 cm At places where

recent sediments constitute only a thin layer, the samples

would contain both recent fine-grained sediments and

coarser-grained older sediments, causing chaotic patterns

in the transport pathways This situation is met at the

off-shore edge of the Red River delta, where resulting sediment

transport pathways are mainly in landward direction

According to Mathers and Zalasiewicz (1999), the northern part of the Ngason-Haiphong coastal zone (from Haiphong to the Thai Binh mouth) has a tide-dominated morphology and the coast from the Thai Binh mouth to the Day mouth is wave dominated The coast from Hai-phong to the Thaibinh mouth has a ratio of tidal range (TR) to wave height (H) of more than 3, corresponding with a tide-dominated coast (Davis and Hayes, 1984) The remaining parts of the Ngason-Haiphong coastal zone correspond to a mixed tide-wave-dominated coast with a TR/H of between 1 and 3 However most of the sediments are transported from the larger river mouths to near shore areas Therefore the present coasts at the Red River, Day and Tra Ly mouths represent mixed tide-wave-dominated coasts with strong riverine influence, in particular during the wet season The Hai Hau coast zone with a straight shoreline, a dominance of longshore sediment transport and rapid erosion behaves like a high-wave energy coast The sediment from the Red River is largely transported

in southern direction and does not reach the coastline of the Hai Hau district The completion of a large dam

30 30

25 5

25 20

25 20

15

30

10

Day mouth Lach mouth

Ba

Lat m

outh

Tra Ly mouth

Diem Dien mouth Thai Binh mouth

Lan mouth

H

hau

shor el e

Net sedimet tranport pathways

Fig 4 Sketch of net sediment transport pathways.

(Hoa Binh dam) in 1987 upstream of the Red River system

has led to a significant decrease in the amount of sediment

supplied in front of the Red River mouth Therefore it is

likely that present coastal erosion in the Hai Hau area is

partly due to a decrease in sediment supplied by the Red

River mouth Moreover, the Ngo Dong river, which was

dammed in 1955, losts its importance as a major branch

of the Red River towards the end of 19th century, when

the erosion in the Hai Hau district started It is not clear

whether the decreased activity of this branch has been

caused by natural developments or by man-made

channel-ing works (Vinh et al., 1996)

6 Conclusions

The main conclusions can be summarized as follows:

1 Five main surface sediment types in the

Ngason-Hai-phong coastal zone could be defined, namely sand,

sandy silt, silt, mud and sand at shoals They differ from

one another in their sedimentary characteristics (So, D50,

sand, silt and clay contents) Sand and silt exist in both

old and recent sediments in the study area, but the latter

are distinct in their red colour and low contents of

car-bonate fragments

2 The recent ðQ3

IVÞ sandy surface sediment is deposited along the shoreline at a depth of between 0 and 5 m,

except near the Ba Lat mouth, where sand reaches a water

depth of 15 m Further offshore down to a depth of about

25–30 m, the sediment becomes silty with lenses of mud

Further offshore the old surface sediments of sandy silt

ðQ1–2

IVÞ and sand ðQ2

III-Q1IVÞ are widely distributed

3 At the river mouths, the directions of net sediment

trans-port are variable in water depths shallower than 5 m

From 5 to 10 m water depth, sediments are mainly

trans-ported southeastward at the Ba Lat, Lach and Day

mouths, northeastward at the Tra Ly mouth and

east-ward at the Thai Binh mouth At the Hai Hau erosional

shoreline, sediments are transported southwestward

alongshore in the region shallower than 5 m The region

of depths from 10 to 30 m is specified by southward

sed-iment transport

Acknowledgements

The writers thank WOTRO (NWO, the Netherlands) and the Vietnam Fundamental Scientific Project coded 7.8.12 who funded this research

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