DSpace at VNU: Analysis of sediment distribution and transport for mitigation of sand deposition hazard in Tam Quan estuary, Vietnam

DSpace at VNU: Analysis of sediment distribution and transport for mitigation of sand deposition hazard in Tam Quan estu...

O R I G I N A L A R T I C L E

Analysis of sediment distribution and transport for mitigation

of sand deposition hazard in Tam Quan estuary, Vietnam

Do Minh Duc1•Dinh Xuan Thanh1• Dinh Thi Quynh1•Patrick McLaren2

Received: 3 November 2015 / Accepted: 11 March 2016

 Springer-Verlag Berlin Heidelberg 2016

Abstract Tam Quan estuary in the Binh Dinh Province of

Vietnam provides shelter for about 2000 boats Recently,

erosion and accretion associated with the mouth of the

estuary have badly affected marine transportation and

economic development An 850-m-long jetty was

con-structed to mitigate the hazard However, sand deposition

still continues to be an ongoing problem This paper aimed

to assess the reasons for sand deposition in the estuary

based on sediment distributions and transport pathways

Following an investigation of topography, geological and

hydrodynamic conditions, and sediment characteristics, an

analysis of sediment transport trends was undertaken

Results show that medium sand is distributed along the

coast from the shoreline to 2.5 m water depth Fine sands

are found in deeper areas of 10–13 m where there are also

medium sand deposits thought to be of ancient origin Sand

deposition has intensively occurred in the estuary due to a

dominant north-to-south longshore sediment transport

regime Sediment from this regime is presently trapped by

the jetty and deposited in the navigation channel As a

solution, it is suggested that a properly designed jetty

stemming from the headland on the north side of the

estuary could effectively control the patterns of sediment

transport enabling the sand to bypass the entrance, thereby

avoiding entrapment inside its mouth

Keywords Estuary Sediment  Sand deposition  Navigation channel  Sediment trend analysis

Introduction There has been extensive research worldwide on sediment distribution and transport in estuaries However, much of this work has focused on coastal plain estuaries Informa-tion on confined estuaries with steep slopes and floored by sand- and gravel-sized sediments is rather limited (Portela

2008) Moreover, morphologic changes in estuaries are occurring more seriously due to human and climate change impacts (Duc et al.2012; Zhang et al.2014; Chalov et al

2015; He et al.2015) which may lead to deposition and/or erosion hazards

An early recognition of grain-size trends and sediment transport (McCave 1978) showed how grain size along a beach can coarsen in the direction of net wave-driven transport The cause was ascribed to progressive winnow-ing out of the finer fraction and its dispersal offshore by tidal currents Other situations without strong tidal currents may allow the winnowed fines to be reintroduced to the beach further down drift and yield a fining trend The concepts to predict the relative changes that will occur in particle size distributions of sediments through erosion, transport, and deposition were first presented in McLaren (1981) followed by a more complete mathematical theory (McLaren and Bowles1985) On the basis of their theory, several methods to carry out sediment trend analysis (STA) have been developed The McLaren and Bowles approach

is one dimensional, whereby the changes in grain-size distributions along individual sample sequences are tested for validity with the Z score statistic to determine the preferred transport direction However, following

& Do Minh Duc

ducdm@vnu.edu.vn

1 Faculty of Geology, VNU University of Science, Vietnam

National University, Hanoi, 334 Nguyen Trai, Thanh Xuan,

Hanoi, Vietnam

2 SedTrend Analysis Limited, 7236 Peden Lane,

Brentwood Bay, BC V8M1C5, Canada

DOI 10.1007/s12665-016-5560-2

numerous testing of individual sample lines, a two

dimensional transport pattern is achieved A practical

assessment of this approach is discussed in Hughes (2005)

Gao and Collins (1991,1992) and Gao (1996) proposed a

two-dimensional vector approach to determine trends,

some elements of which were revised by Chang et al

(2001) A different vector approach altogether was

pro-duced by Le Roux (1994) and Le Roux et al (2002) A

summary of the various techniques is provided in Rios

et al (2003) and Poizot et al (2008)

The STA technique has still several uncertainties

asso-ciated with the methodology including transport model

assumptions, sample spacing, bias of the mean, variance

and skewness of grain sizes in the log-normal distribution,

random environmental and measurement uncertainties A

number of authors found their results to agree, either in

whole or in part, with a variety of other evidence including

direct measurements of processes, observations of

bed-form orientations and application of numerical modelling

(Livingstone1989; Lanckneus et al.1992; Van de Kreeke

and Robaczewska1993; Gao and Collins1994; Gao et al

1994; Aldridge1997; Bergemann et al.1998; Van Der Wal

2000; Mallet et al.2000; Duck et al.2001) However, some

authors found no agreement between the STA and outside

evidence (Flemming 1988; Masselink 1992; Guillen and

Jimenez1995) STA was accepted as a tool for

investiga-tion of coastal projects of US Army Corp of Engineers

(Hughes2005)

STA was used to indicate the sediment pathways inside

a lagoon which can be related to the sediments sources,

wind-related water circulation and wind directions

(Avra-midis et al.2008) STA in the integration of acoustic data

leads to a better understanding of the sedimentary,

mor-phological and biological processes in a shallow lagoon in

different spatial and temporal scales (Papatheodorou et al

2012) STA was also successfully applied for a study of

sediment pollution (McLaren and Singer 2008), coastal

change and sedimentation in estuaries (Van de Kreeke and

Robaczewska1993; Bergemann et al.1998; McLaren and

Beveridge2006; McLaren et al.2007; McLaren and Braid

2009; McLaren and Teear 2014)

The estuaries on the central coast of Vietnam, situated in

the south-west sector of the Gulf of Tonkin and the East

Vietnam Sea (Fig.1), are typical examples of such

sys-tems In recent years, navigation channels in the estuaries

of the central part of Vietnam have been facing severe sand

deposition Fishing boats are frequently unable to go in or

out of their harbours, resulting in a serious reduction in the

transport of aquacultural products Some of the most

typ-ical estuaries are My A (Quang Ngai Province), Da Rang,

Da Dien (Phu Yen Province), and Tam Quan and Lai Giang

(Binh Dinh Province) (Fig.2) As a contribution to

clarify the reasons for sand deposition in Tam Quan estuary based on the analysis of surface sediment distributions and transport and then propose suitable countermeasures

Study area

In the central part of Vietnam, high rainfall in the steep hinterland results in an abundance of rivers and their associated estuarine lagoons at the coast Rivers with large basin areas, usually larger than 500 km2, maintain open inlets during the whole year Because river discharge in the central coast is strongly seasonal, river mouths and tidal inlets tend to adjust to an equilibrium morphology associ-ated with low flow conditions during the relatively long dry period The morphology of estuaries formed under normal conditions may be altered dramatically by increased river discharges (Tung 2011) The sediment concentrations in the main rivers are usually around 50–150 mg/l, but during floods the concentrations are greatly increased (Eriksson and Persson 2014) This type of estuary with significant influence of river flood flows is dominant However, due to construction of dams for agriculture, water discharge of some small rivers is strongly interrupted and marine and coastal dynamics are more important than river dynamics

at the river mouths The coast is predominantly sandy as a result of alluvial accumulation, which nourishes the bea-ches and sandy barriers that form across estuary mouths and tidal inlets (Tung2011) The 100-m contour line is just about 10 km from the shoreline

Tam Quan estuary is located in Hoai Nhon District, Binh Dinh Province This is the centre of coastal economic development of the Binh Dinh Province where there is a harbour frequently containing more than 2000 fishing boats The navigation channel into Tam Quan estuary is about 150 m wide and 850 m long Faced with the adverse impacts of sand deposition, the local government, between

1998 and 2001, constructed a 400-m-long jetty extending seaward from the northern tip of a barrier beach on the south side of the harbour entrance However, the naviga-tion channel continued to infill with sand until 2004 after which, between 2006 and 2008, the jetty was elongated a further 450 m A technical explanation for its expansion does not, apparently, exist, and sand deposition has con-tinued to be a problem since 2010 (Fig.3) with the navi-gation channel requiring periodic dredging between September 2012 and March 2014 The dredged sand amounting to 61,600 m3 was used as filling materials for local construction Today, the sand deposition continues to

be a problem and dredging is frequently required to maintain the navigation channel of Tam Quan estuary The area surrounding the Tam Quan estuary is underlain

the Kim Son Formation and the Phu My, Ben Giang and

Hai Van complexes (Fig.4) Quaternary marine sediments

are found mainly in the north as well as in small areas to

the south of the estuary which appear as low relief within

the confines of the valley Also on the south side, there are

riverine–marine–swampy sediments River sediments are

characterized by sand and gravel as well as cobbles

con-taining a large variety of lithologies Coastal dunes,

10–20 m high, 0.2–1.5 km wide and 2–5 km long, are

found along large portions of the coastline

Waves are subject to seasonal changes with north-east waves dominant from October to April During June to September, south and east waves dominate whereas May is a transitional season during which time waves are low and their directions irregular Average wave heights are from 1.2 to 1.7 m with maximum heights reaching 12 m during typhoon conditions Annually, south-east, south and east waves occur 47, 24 and

13 %, of the time, respectively (Table1) For about 60 % of the time, dominant wave heights range from 0.51 to 1.50 m; larger waves (1.51–2.50 m) occur 20 % of the time

Fig 1 River system and tidal

inlets in the central coast of

Vietnam

Tides are irregular and diurnal Tides more than 155 cm

occur only 1 % of the time Long-term maximum spring

and neap tide ranges are 107 and -93 cm, respectively

Storm surges have been recorded at a maximum of 1.7 m

Tam Quan estuary receives water from only three small

streams, and turbidity measurements show that sediment

input from these rivers is negligible Data of bottom

cur-rents (about 1 m above the sea floor) was retrieved during a

week in two periods of October 2012 and June 2013,

respectively (Fig.5) The currents were monitored at two

stations (Fig.6) which were set up at 6 m water depth The

results show a dominant direction of currents from north to

south in October Maximum velocity was recorded at about

27 cm/s with an average of 13 cm/s In June, current

directions are mixed between north–south, south–north and

east–west Velocities are 8 cm/s on average However,

east–west current velocities reached 30–37 cm/s for some

short times due to strong south-west wind-induced waves

Methods

A total of 130 sediment grab samples were collected along

the Tam Quan coastline to a depth of 20 m (Fig.6) Based

on rectangular grid, samples were from 150 to 250 m apart

and were positioned by GPS to an accuracy of ±5 m

Grain-size distributions were obtained by sieving the sandy

fraction (sieve sizes: 2, 1, 0.5, 0.35, 0.25, 0.18, 0.15, 0.125,

0.1, 0.074 and 0.063 mm, i.e -1.0, 0, 1.0, 1.51, 2, 2.47,

2.74, 3.0, 3.32, 3.76 and 4.0 u) and the grain-size

parameters of mean, sorting and skewness were calculated

in u units (Folk1966,1980)

The mineral composition was analysed by thin section using an optical microscope Representative portions of about 15 g from each sample were thoroughly washed several times with water followed by a bath of dilute hydrochloric and sulphuric acids to clean the grains of any limonite coating or stains After drying, the material was sieved through a half-millimetre sieve The portion of grains smaller than 1 mm in size was separated by means

of bromoform having a specific gravity of 2.83 After each process, the sands were weighed Permanent slides of the light and heavy portions of these sands were made for later microscopic determination using Canada balsam as the imbedding medium The mineral composition was esti-mated by using point counting

McLaren and Bowles (1985) demonstrated that when two sediment samples (d1and d2) are taken sequentially in

a known transport direction (e.g from a river bed, where d1

is the up current sample and d2is the down current sample), the sediment distribution of d2may become finer (case B)

or coarser (case C) than that of d1; if it becomes finer, the skewness of the distribution must become more negative Conversely, if d2 is coarser than d1, the skewness must become more positive The sorting becomes better (i.e the value for variance decreases) for both cases If either of these two trends is observed, sediment transport from d1to

d2can be inferred If the trend is different from the two acceptable trends, the trend is unacceptable, and it cannot

be supposed that transport between the two samples has

Fig 2 Tidal inlets characterized by severe sand deposition in the central coast of Vietnam

taken place In the preceding example, where the transport

direction is unequivocally known, d2(s) can be related to

d1(s) by a function X(s), where s is the grain size The

distribution of X(s) may be determined by:

d2ð Þ ¼ ds 1ð Þ X ss ð Þ

X(s) provides the statistical relationship between the two deposits, and its distribution defines the relative probability

of each particular grain size being eroded, transported and deposited from d1to d2 The shape of the X(s) distribution relative to the shapes of the d1(s) and d2(s) distributions

Fig 3 Sand deposition in navigation channel of Tam Quan estuary.

The area of severe sand deposition was about 150 m long and 100 m

wide, which started from a point of 120 m from seaside head of the

jetty in May 2010 The navigation channel was 50 m wide at the

south side near the jetty; boats could go in and out during spring tide Sand deposition was then significantly enlarged and the navigation channel shifted to the north side near the rock mountain in February 2014; boats can also go in and out during spring tide

determines the behaviour (stability) of the sediments There

are five defined categories for behaviour: (1) net erosion,

(2) net accretion, (3) equilibrium, (4) total deposition type

1 and (5) total deposition type 2 (McLaren and Singer

2008; McLaren and Braid 2009)

There is now a large body of literature that uses or

discusses sediment trend analysis (STA) (e.g Gao and

Collins1991,1992; Gao1996; Chang et al.2001; Le Roux

1994; Le Roux et al 2002; Hughes 2005; He´quette et al

2008; Poizot et al 2008; Duc et al.2012; McLaren 2014;

McLaren and Teear 2014) As a result, a number of

methods have been developed to apply the theory to derive

transport pathways For this paper, the STA was carried out

following the descriptions provided in McLaren and

Bev-eridge (2006) and McLaren et al (2007)

Results Sediment characteristics

Based on their characteristics (Table2) and spatial distri-bution, sediments were classified into four types which include nearshore coarse-medium sand, nearshore fine sand, offshore medium sand and offshore fine sand (Figs 7,8,9) These are described as follows

Nearshore coarse-medium sand

This sediment type extends from the coast line to water depths of 2.5–3.0 m which is at a distance of about 200 m from the shoreline Sediments have mean grain diameters

Fig 4 Geological settings at Tam Quan estuary

Table 1 Frequency (%) of

wave heights at the nearshore of

Tam Quan estuary

Wave height (m) N NE E SE S SW W NW Sub-total

\0.25 0.00 0.13 0.38 0.16 0.04 0.00 0.00 0.01 0.71 0.26–0.5 0.07 1.28 2.65 1.95 1.60 0.10 0.39 0.03 8.08 0.51–1.5 1.26 18.86 9.24 5.6 21.23 3.46 0.00 0.04 59.69 1.51–2.5 0.93 16.41 0.33 0.11 1.29 0.52 0.00 0.06 19.65 2.51–3.5 0.43 7.56 0.03 0.01 0.03 0.02 0.00 0.03 8.11 [3.5 0.34 3.37 0.02 0.00 0.00 0.00 0.00 0.01 3.76 Total 3.03 47.61 12.65 7.83 24.19 4.10 0.39 0.18 100

Fig 5 Characteristics of currents at north shore of Tam Quan estuary (a from 30 September to 5 October 2012; b from 4 June to 9 June 2013)

Fig 6 Sediment sampling locations

of 0.5–2.6 u, average value of 1.2 u Sorting coefficients

are from 0.4 u (well sorted) to 1.1 u (poorly sorted),

average value of 0.8 u (moderately sorted) Similarly,

skewness varies from coarse skewed (-0.1 u) to strongly

fine skewed (0.5 u) The average value is near

symmet-rical (0.1 u) These characteristics suggest an

environ-ment of highly variable hydrodynamic conditions, which

is mainly controlled by the formation and migration of rip

currents, especially during time period of north-east

waves (from October to April next year) (Trinh et al

2011; Tung 2011)

Nearshore fine sand

Offshore from the nearshore coarse-medium sand, this sedi-ment type ranges from 2.5–3.0 to 10–12 m water depths to a distance of 200–800 m from the shoreline Mean grain diameters vary from 2.4 to 3.3 u, with an average value of 2.8 u Sorting coefficients are in a narrow range from 0.5 u (moderately well sorted) to 0.9 u (moderately sorted), aver-age value of 0.6 u (moderately well sorted) Skewness varies from strongly coarse skewed (-0.4 u) to near symmetrical (0.1 u), average value of -0.3 u (strongly coarse skewed)

Table 2 Characteristics of surface sediments in at the nearshore of Tam Quan estuary

Sediment Grain-size parameters (u) Mineral composition (%)

Mean So Sk Quartz Felspar

Kali

Biotite Muscovite Plagioclaz Heavy

mineral Nearshore

coarse-medium sand

0.5–1.8 (1.2)a 0.4–1.1 (0.8) -0.1 to 0.5 (0.1) 70 15 6 4 3 2

Nearshore fine sand 2.4–3.3 (2.8) 0.5–0.9 (0.6) -0.4 to 0.1 (-0.3) 75 10 4 4 5 2 Offshore medium sand 1.1–1.9 (1.7) 0.7–0.9 (0.8) -0.1 to 0.3 (0.1) 80 12 1 2 3 2 Offshore fine sand 2.1–3.1 (2.5) 0.6–1.3 (0.7) -0.5 to 0.1 (-0.1) 80 12 1 2 3 2

a 0.5–1.8 (1.2): Min–Max (average)

Fig 7 Sediment distribution at Tam Quan estuary

Offshore medium sands

Found in depths ranging from 10–12 to 20 m, these

sedi-ments have mean grain diameters of 1.1–1.9 u, with an

average value of 1.7 u Sorting coefficients are slightly

varied from 0.7 to 0.9 u (moderately sorted) Skewness

changes from coarse skewed (-0.1 u) to fine skewed

(0.3 u) The average value is near symmetrical (0.1 u)

Offshore fine sands

These sediments have mean grain diameters varying from 2.1 to 3.1 u, average value of 2.5 u Sorting coefficients are in a large range from 0.6 u (moderately well sorted) to 1.3 u (poorly sorted), average value of 0.7 u (moderately well sorted) Skewness varies from strongly coarse skewed (-0.5 u) to near symmetrical (0.1 u), average value of

Fig 8 Distribution of modern sediment grain-size mean value at the nearshore zone

Fig 9 Distribution of modern sediment sorting value at the nearshore zone

-0.1 u (coarse skewed) Like the offshore medium sands,

the offshore fine sands are also found from 10 to 12 m to m

of water and both sediment types have a similar colour to

the nearshore sands, but their grain-size characteristics fall

into much narrower ranges

In terms of mineral composition, sediments are quite

homogenous (Table2) Quartz dominates with contents of

75–80 % followed by kali feldspar which ranges from 10 to

15 % Other minerals include small amounts (2–6 %) of

bio-tite, muscovite, plagioclase and heavy minerals These findings

suggest that the hydrodynamic conditions do not lead to any

significant differentiation in mineral compositions and that all

the sediment types are likely derived from a similar source

Sediment transport at the nearshore zone of Tam

Quan estuary

The results of the STA, as shown Fig.10, are listed as

follows:

• Sediments are not transported from the coastline to

shallow water (up to about 2.5 m water depth) on either

side of the Tam Quan estuary This supports the

concept that both northern and southern parts of the

beach are relatively stable and that the predominant

wave direction is almost perpendicular to the shoreline

An exception occurs in a short segment of the Tam

Quan north shore (Fig.8), where the coast is known to

have eroded in recent years

• The dominant transport behaviour of sediment in the north part of Tam Quan estuary is net accretion at water depths of 2.5–10 m It shows relative significant strength of southward sediment transport which matches well with the dominance of north–south bottom currents in the area Moreover, the net accretion indicates a likely increase of available sediments for transporting southwards Around the Truong Xuan headland, this mode was also found at the water depths

of 5–10 m but changes to net erosion at shallower water depths of 2–5 m Change in shoreline orientation at the headlands leads to the formation of a high wave energy surf zone in the shallow water Sediment samples were unable to be collected in the shallow water surrounding the rocky headland precluding the determination of the transport regime at 0–2 m water depths in this area

• Around the Tam Quan estuary, sediments are trans-ported past the Truong Xuan headland generally as net accretion but with occasional sample sequences show-ing net erosion or equilibrium

• Sediment trends were undefined in much of the navigation channel, most likely the result of recent dredging in the area Sediments are transported along the south side of the jetty in equilibrium and net accretion Near jetty’s head, sediments do not show any transport

• In the south part of the estuary, sediment trends were not defined at the water depths of 0–5 m near the jetty (300–1000 m southwards) In the other parts (from 3 to

Fig 10 Transport of modern sediments at the nearshore zone