Regional Seminar on Environmental Geology, 1] - 13 November 1992, Hanoi
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CAT HAI ISLAND OF HAI PHONG
TRAN DUC THANH, NGUYEN DUC CU, » NGUYEN CHU HOI, DINH VAN HUY, Hai Phong Institute of Oceanology Vietnam ABSTRACT
The Cat Hai Island covers an area of 15 kon? andpopulated by 13,000 people For many years past, the south coast of the island has been subjected to violent erosion process with a
rate of 4.2 - 8 mly, maximum 25 m/y
The current status and causes of erosion in the island were investigated by Hai Phong Institute of Oceanology in the 1990-1991 period Various coastal geodynarnic investigation methods such as the hydrogeodynamic, morphodynamic, neotectonic ones together with modern techniques such as remote sensing, computer modelling, etc., were applied
The intrinsic cause of erosion is that the area has been sinking at the rate of 1.5 - 5 mmly without sediment compensation The direct and principal cause lies in the loss of about 467,000 rm ly sediments due to the great difference in velocity and duration of the flood tidal current and the ebb one The waves destroy the coast directly but play the secondary role
INTRODUCTION
The Cat Hai Island has a geographic coordinates of 20°47°22" N - 20°50'1 2" N and 106°50’30" E - 106°54’05" E Covering over 15 kmỶ, and with more than 13,000 population, the island is an importantpopulation and economic center Over the last few
decades, the erosion on the southern coast of the island has been very serious The coast
: line has been quickly retreated, dikes and embankments are always in dangers, threatening the life of the people Every year the municipality has expended a great
amount of man power and resources, to repair the dikes and embankments but without
high efficiency In 1990 - 1991, the Hai Phong Institute of the Oceanology carried out a survey on current status and investigated the causes of erosion in the island according to the coastline geodynamic methodology This paper present the initial results of studies, which can serve as orientations for further detailed investigations aiming at suggesting specific measures for preventing and controlling the erosion of the island
The results of survey and investigations:
1 Current status of erosion in.Cat Hai Island:
According to the stories told by the Cat Hai people, in early times the island was much larger than at present and in some places occupied by sea now there used to be sand dunes, markets and schools Interpretation of maps and aerial photographs shows that
Trang 2216 Regional Seminar on Environmental Geology, 11 - 13 November 1992, Hanoi
within 1938 - 1991 the shore line (the continental 0 m line) in the south of Cat Hai island has been pushed 250 - 600 m inland, while the marine chart 0 m line has been pushed 3.0
km away
At present, among 7 km of the south coast, 4.4 km are embanked with stones, 5.4 km are ©
in the state of instability due to erosion (from Hoa Quang to S.E of Hoang Chau) The
section of Ben Got (to the East) has been obviously built up since 1982, due to the construction of Ben Got Bridge that created the accumulation of sediment The coast section west of Hoang Chau has been continuously built up, creating recent sand bars
advancing northward The section south of Hoang Chau is now being built up and stable, but before 1965 was forcibly eroded
Over the past half century, the vulnerable section of Hoa Quang -’Gia Loc - Van Chan
has been eroded with an average rate of 4.2 - 8.0 m/y, maximally 11.3 m/y Since the
construction of the stone embankment (from 1990), the speed of shore line retreat in
Gia Loc is 1.4 m/y, in Hoa Quang - 2.7 m/y The waves have pushed the sand bar in the narrow bay between Van Chan and Gia Loc with a speed of 20 - 25 m/y during 1984 - 1991 Table 1 Erosion and banking rates in coast line sections of Cat Hai Island (m/y) Coast line Degree of 1938-1965 1965-1991 1938-1991 vox Erosion or ii banking Mean Max Mean Max Mean Max - Hoa Quang ab -5.6 -6.5 -2.7 -3.2 -4.2 -4.7 Gia Loc Continous 65 92 -14 -58 -42 RS Van Ch an an a erosion = 14 ˆ 2 13.9 = 96 = 10.4 = 8.0 * = 113 _
South of Erosion passing lì“ 56 +38 +63
Hoang Chau to banking Weak unstable Sung barking +0.7 +09 | +05 | +08 | +075 | +08 Continuous Heute Cons satonaive +23 +83 +2.9 +38 +2.4 +47 ° banking
By volumetric method, the amounts of sediment eroded and accumulated in different
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Table 2
Coast line section eriod Process Volume Average m°/y
Hoa Quang 1938-1965 Erosion 1,583,147 58,635 Gia Loc 1965-1991 ae 757,386 29,130 : 1938-1991 " 2,340,533 44,161 Gia Loc 1938-1965 "` | 2.508 841 92,920 Van Chan 1965-1991 : 1,815,399 - 69,823 : 1938-1991 : - 4324240 81,589 South of Hoang Chau 1938-1965 hiên 657,426 24,350 ; Hoang Chau 1938-1965 Banking 599407 22,200 1965-1991 5 1,109,714 42,681 1938-1991 1,709,121 32,248 Ben Got 1938-1965 š 36,818 1,364 ĩ 1965-1991 " 26,298 1,012 1938-1991 63,115 1,191 Total erosion: 7,322,225 m”
Annual erosion: 7,322,225 : 53 = 138,155 m7/y
(Hoa Quang - South of Hoang Chau) Total banking : 1,909,881 m°
Annual banking: 1,909,881 : 53 = 36,035 m”/y
Thus during 53 years, in average, along the coast line of Cat Hai
Island from Hoa Quang to southeast Hoang Chau about 138,000 m? of sediment a year
has been eroded and taken away
The results of calculation for the coastal slope (within the distance of 2.7 - 3 km from the shore line) show that from the submerged coastal slope on the side of Hang Day (eastern
side) 23,000,000 m” of sediment has been eroded, on the submerged coastal slope on the
side of Hoang Chau (the western side) 5,000,000 m e sediment has been accumulated At a result, over 53 years (1938-1991), 18,000,000 m° of sediment has been eroded, in
average 339,650 m° a year
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218 : Regional Seminar on Environmental Geology, 11 - 13 Novernber 1992, Haro
away every year As calculated by hydrogeedynamic method based on the measurements
of 1991, the average erosion is 341,000 m°/y (126, 000 m 3/y less)
This can be explained by the reduction of erosion due to the presence of the stone
embankment
2 Causes of erosion: ' 2.1 Intrinsic cause:
The intrinsic cause of the erosion in the island is related with the formation and development evolution of the island [2,3] The Cat Hai Island consists of a series of sand dunes 2 - 2.5 m high, separated from each other by channels, piled up by action of waves in river mouth condition rich in supply of sediments from the main land The island was formed 2,000 - 1,000 years ago in the condition of gradual sea level drop At that time, the Do Son peninsula was an island, not yet connected with the main land
The sediments of the Red river along its branches and then along the coast line has beer
transported to build up the island
Over the last 1,000 - 500 years and at present the sea level has been rising again with the rate of 1.5 - 2 mm/y [1,3], the supply of sediment diminished greatly, tides have intensified, the deltaic river mouth regime has been replaced by estuarine regime
Due to the rise of the base level and the intensification of the wave activities, the island is
in a dynamic environmental condition quite different from that when it was formed,
favorable for erosion The erosion include lateral destruction of the sea shore and
scouring of the coastal slope Not only the Cat Hai Island, but the Phu Dong, Dinh Vu
Islands have been also intensively eroded At Dinh Vu Island, the old sand dunes sonia
to those of Cat Hai Island have almost disappeared
Thus the intrinsic cause of the erosion in Cat Hai Island lies in the sividng at the rate 2 of
1.5 - 2 mm/y without sediment compensation during the present time in the Bach Dang river estuarine are a where there is a tendency of sea invasion and the sea dynamism is dominant over the river one Therefore erosion tends to be long lasting
2.2 Direct and principal cause:
The direct = principal cause of erosion is the loss of sediment in a great amount (about 467,000 m 3w) over a narrow littoral space of only over 5 km in length and 3 km in
breadth, which has led to the process of strong shore line destruction and coastal slope
erosion The loss of sediment happens in the following mechanism: the waves stir up the
bottom and destroy the shore
The sea currents flowing with relatively high speed during the flood tide carry the suspended sediments and those stirred from the bottom along the two channels adjacent to the east and the west sides of the island During the ebb, only 1/3 the amount of sediment carried up along the Hoang Long Channel on the west side and 1/2 the amount of the sediment carried up along the Hang Day Channel (on the east side) turn back It means that only 2/5 the total amount of sediment carried up by the rising tide turns back with the ebb to the littoral area of Cat Hai Island Part of the sediments carried up is accumulated, forming sand bars near the east and northwest coasts A considerable part
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into the sea The main cause of this is that in the channels, the complex a consisting of the tide current), has the rising speed and time much greater tMan the
retreating speed and time The separating point of the alluvial current along the coast is off Gia Loc (deviated toward Huyen Channel) Thus the main and direct cause of coastal erosion of Cat hai Island is the big difference in speed and time between the flow current
and the ebb currents in the littoral area and along the two channels on the two sides of the island, causing a serious deficiency of the sediment for the area
The waves, although directly stir the bottom and destroy the shore, has only secondary role because their intensity are not high and their directions are nearly perpendicular to the shore(usually forming an angle of 5 - 20° with the normal of the shore line) Not for
the important role of the tide currents, in shallow water areas such as Cat Hai, the
coastal erosion would quickly create an equilibrated profile and the shore becomes
stable In fact, in Cat Hai area coastal erosion has been occurring for many years and always associated with bottom abrasion of the coastal slope
7 e ri
‘CONCLUSION
With an average rate of 4.2 - 8 m/y, maximal rate of 20 - 25 m/y, the southern coast of Cat Hai Island is strongly eroded Compared with the other localities subjected to erosion in
Vietnam, this rate is not the highest one It should be noted that the erosion process in
Cat Hai Island has been occurring for along time and the destruction of shore is
associated with bottom scouring with a rate of 1 - 3 cm/y By integrated studies with application of various methodologies for coastal geodynamic investigation, the causes of
erosion in the island have been preliminarily clarified The intrinsic cause is the
evolution of the river‘mouth environment from the sediment rich deltaic regime to the estuarine regime with serious sediment deficiency and uncompensated deepening with the rate of 1.5 - 2 mm/y associated with modern sea invasion The direct cause is the
deficiency of sediment in the littoral area with an amount of 467,000 mì The deficiency
is mainly caused by the rising tide currents having much greater speed and duration compared to the retreating currents Waves play the role of destroying the shore, stirring
up the sediments from the bottom, which are carried away by the rising tide currents and only 2/5 of which turn back with the retreating currents
Based on these preliminary study results, one can carry out further detailed investigations to find the effective solutions for preventing and eliminating of coastal
erosion here
REFERENC
1 Nguyen Tai Hoi, Bui Dinh Khuoc Ket qua ve: tinh toan cac dac trung dao dong muc nuoc ven bo bien Viet nam Hoi thao Viet Xo lan thu 2 ve khi tuong thuy van bien Tom tat bao cao Hai Phong, 1990 (Preliminary results of calculating the sea level
fluctuation characteristics in the littoral of Vietnam Second Vietnam-Soviet Seminar on
marine meteorology and hydrology Summary of Hai Phong paper, 1990)
2 Dinh Van Huy, Tran Duc Thanh, Nguyen Duc Cu, 1985 Ve su hinh thanh dao Cat
Hai Trong "Nhung phat trien moi khao co 1991" Uy ban khoa hoc xa hoi - Vien Khao co,