Risk of tu hien inlet closure in tam giang – cau hai lagoon journal of geology

Series B

No 13-14
1999

Special issuekjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA

'PROCEEDINGS

AND ABSTRACTS

OF THE INTERNATIONAL

WORKSHOP

GPA'99

TECTONICS, GEODYNAMICS AND
NATURAL HAZARDS IN WEST PACIFIC - ASIA
HA NO!, NOVEMBER 6-15,1999
Organised by Institute of Geological Sciences, NCNST of Viet Nam and IGCP Project 383.
In collaboration with Vier Nam National Committee for IGCP, Geological Association of Viet Nam,
Viet Nam Society of Rock Mechanics and Society of Tectonics (GA of Viet Nam)

DEPARTMENT OF GEOLOGY A & MINERALS

HA

NQJ

OF VI1!:TNAM

70. Qiao Sen, Chen Qiongying, Zeng Xiangfeng. Study on maximum entropy
principle for earthquake prediction and determining potential seismic source area

228

71. Panza G.F., Vaccari F., Romanelli F •. The lUGS-UNESCO rGCP Project 4f4:
Realistic modeling of seismic input for megacities and large urban areas

228

Part IV. NATURAL
C onvenors:

T rdn

Van

T ri

HAZARDS SRQPONMLKJIHGFEDCBA
& I. C h e sn o k o v a

72. Nguyen Trong Yern, Le Thi Nghinh, Doan Dmh Lam, Dao Thi Mien, Nguyen
Xuan Huyen, Phan Dong Pha. Flash flood in Northwest Viet Nam

233

73. Uromeihy A.. Evaluation of slope instability by the use of landslide

zonation maps with reference to landslide cases of Iran

241

hazard

74. Nghiem Hiru Hanh, Tran Minh Dan, Ngo Van Sj, Le Xuan Thu. Landslide in
some open pit coal mines in Vi~t Nam

244

75. Novikova T.I., Wen K.L., Huang B.S .. Numerical modeling of generation and
propagation processes of tsunamis and oceanic rayleigh waves excited by seismic
sources

25 AI

76. Nguyen Dich Dj, Do Van Tu, Dinh Van Thuan, Mai Thanh Tan. Relationship
between Quaternary sedimentation and landslide-rnudflow formation in Nothwest
vier Narn

256

77. Tran D(lC Thanh.

262a

Risk of Tu Hien Wet closure in Tam Giang - Cau Hai Lagoon

78. Chesnokova I., Koff G .. Assessment

Asian part of the Russian Federation

of risk of natural processes occuring in the
262

79. Nguyen Ttr Dan, Le Tram. Using remote-sensing data and GIS technology in
modeling geological hazard in mountainous region: the study case of M~ Dire,
Hoa Bmh

263

80. Ozer A., Phan Trung Trinh, Jaspar A., Risack S •. Study on the Hai Phong Bay
coastal evolution over a sixty year period using remote sensing data

266

81. Aurelio M.A. On-going studies in tectonics, geodynamics
the Philippines

267

and geologic hazards in

82. Doan Minh Tam. A review of landslide processes on the road network of Viet
Narn
:

268

83. D*u Hlen. I .andslide in Viet N am in the view of weathering research

268

84. Doan Van T u y e n , Tran Canh. The application
geological hazard studies at engineering sites

of electrical

tomography

in

85. Nguyen Van Ph6, Hoang Tuyet Nga, Doan Thu Tra, Study on the stability of
Thac Ba hydropower dam by using nuclear track detector method

269
270

Part V. PALEOEARTHQUAKE
A . H u ll & Y . K in u g a s a

C onveners:

86. Takashima I., Charusiri P., Kosuwan S., Won-in K .. TL-age dating results on
the Mae Chan segment of the Mae Chan fault, Northern Thailand: constraints for
Quaternary faulting

(*-)

275

Proceedings
and abstracts
of
the International
W orkshop GPA'99

Series

Journal
B, No.13-14/1999,

of GEOLOGY
p.262a-262b

RISK OF TUHlEN INLET CLOSURE
-IN TAM GIANG - CAU HAl LAGOON A
mAN
H d i P O O n g In s titu te o f O c e a n o lo g y ,
(F a x : 3 1 -7 6 1 5 2 1 ,

D O C 'l1 W f f i SRQPONMLKJIHGFEDCBA

246 Dd

N a n g S t r e e t , H d i P O O n g , VItI' N A M

E m a il: td th a n h @ h io .a c .v n )

I n t r o d u c t io n

With the area of 216 sq.km and the length
of 70 km, the Tam Giang - c a u Hai (TO CH) Lagoon occupies the most coastal length
of Thira Thien - Hue' Province:
Its great
values of natural resources have been used for
the local sectors of fishery, agriculture,
waterway- harbor and tourism - service. The
TO - CH Lagoon also takes a very important
function in coastal ecology and environment
that influences deeply to the survival of 300
thousand inhabitants in surrounding. It is
connected to the sea by two inlets of which
Tir Hien Inlet in the
south sometime is
suddenly closed. It is a heavy coastal risk that
needs to be controlled.

In fact, Tu Hien Inlet consists of two
subinlets, named as Vinh Hien larger in the
north, and LQc Thuy very smaller in the
south. Two subinlets are separated by a sand
beach ridge 3 km long and from 2-2.5m high.
The lagoon is connected to the sea by the
larger subinlet directly or by the smaller
subinlet and small channel behind the sand
beach ridge. Mentioning the closure of Tir
Hien Inlet it means that of the larger subinlet.
When the larger subinlet is opened, the

smaller one is closed by sedimentation in
general. When the larger inlet is closed, the
people have to dredge the smaller one to get
the water way for smaller ships and boats to
the sea. The historical document shows some
evidences for the status of Ttr Hi~n Inlet
closure and opening on the table 1.

S ta tu s

Table

1. S t a t u s

o f T I J H ie n I n le t c lo s u r e

Year
Before
1404
Before
1811
1823
1844
Before
1953
1959
1979
1984
1990
. 1994 -

1404
1811

1953

Larger one
Unique inlet of the lagoon
Existed
Gosed
Opened
Closed
Opened
O..,£ened
Closed
Opened
Closed

oresent days

Opened
Go sed

The Thuan An Inlet was opened in 1404
and the feudal government decided to block it

2.62.

a

a n d o p e n in g

Subinlets
Smaller one
Not existed

Closed

Closed
Opened
Closed
Opened (dredged)
.,
and to maintain Tir Hien Inlet in 1469. It


roves the fact that Tir Hien Inlet began to be
closed after the opening of Thuan An Inlet.

Consequences
Falling in the years of Tu Hien Inlet
closure, the flood - inundations are higher in
number and intensity and their consequences
are more heavy. During last 50 years, there
were six very heavy floods - inundations of
which five have felt in the closed duration of
the Tv Hien Inlet. The floods .. inundations
have made serious consequences such as the
loss of human life, livestocks, crops, damage
of infrastructure

(houses,
roads,
gates,
bridges, boats ..etc ..), coastal
erosion .and
pollution. For example, there were nearly A
th o u s a n d
dead people
in the flood inundation generated from a typhoon in
October, 1985.
Due to the closure of Tu Hien Inlet. the
waterway for marine
transportation
and
fishery is blocked. Hundreds small ships and
boats have had to pass an additional distance
of 40 km for sailing to the sea through Thuan
An Inlet in the north of lagoon. This risk has
damaged seriously the sea fishery economy
and impacted negatively
to the life of
fishmen.
The closure of Tu Hien Inlet has freshened
the lagoon water, degraded
the lagoon
ecosystem, decreased biodiversity, damaged
brackish
aquaculture
and
changed

the
catching patters in the lagoon. This event has
also decreased the water quality, promoted
the pollution, especially oil and organic
matters by restricting the water circulation.
Every year, there are about 1.1 million tons of
suspended matters discharged into the lagoon.
From them, 30 percent are transported to the
sea through the inlets. The closure of Tu Hien
Inlet
strengthens
the sedimentation
and
shallows the water body that leads to quick
filling of the lagoon. Finally, the deep
changes in coastal environment, ecology and
natural resources concerning the closure of Ttr
Hi en
Inlet
make
unsustainable
the
development of local socio-economy.

Cause
The intrinsic cause of Ttr Hien Inlet
concerns the natural evaluation of TG -CH
Lagoon. Previously, it was the unique inlet,
and Phii Cam River was the main one running

into the south part of lagoon. Later, Phii Cam
River was ruined, and Huong River running
into the north part of lagoon become the
largest one. Based on
the new dynamic
balance, a new inlet named Thuan An was
opened in 1404 in
front of Hirong River
mouth. Since this time, T tr Hien has been
being the secondary inlet and sometime is
closed by sedimentation. The direct cause of
Tu Hien Inlet closure is the weakening of the
flow passing
the inlet and the violent
concentration of longshore sandy drift in the
inlet
area.
The
mathematical
model
calculation shows that a total number of
500,000 ton sand/year are transported to Tir
Hien - Chao May coastal part by the
eastsouthward
wave
longshore
drift.
Meanwhile a westnorthward local longshore
drift transports about 900,000 ton sand/year
from Chao May site to Tu Hien Inlet. This

local concentration makes the closure of T tr
Hien Inlet in the late stage of northeast wind
season from December to April, generally, It
falls in the time of ending the flood flow and
changing the prevailing wave direction. After
a closing duration from 6-11 years, the inlet
can be opened in the coincidence with a
heavy flood and violent coastal erosion in the
early stage of northeast wind
season. The
opening duration is from 4 to 20 years.

Conclusion
As a heavy geological hazard, the closure
of Ttr men Inlet makes a negative impact to
coastal environment and ecology, and the
great losses of economy and people's life.
This hazard happens also in some other
coastal lagoons in Central Viet Nam. Its
intrinsic cause is related to the geological
evolution of the lagoon created by the
combination of endogenous and exogenous
factors.
The direct cause
concerns
the
interaction between the longshore sand drift
transported by wave and the eroded tidal and
flood current through the inlet. The control
of Tir Hien Inlet closure must be regarded as

a prior
issue
of TO
CH Lagoon
management.

REFERENCE

1. Trdn H(re Th~nh, 1997. Environment
impact of the closure and displacement of
inlets of the Tarn Giang - Cau Hai Lagoon.SRQPONMLKJI
M a r in e

resources

and

e n v ir o n m e n t.

196. Sci and T ech. P ub. H ouse,

I V .- 1 8 5 -

H d 8 ¢ i.

262b