Hai Phong Coastal zone

VNU Journal of Science, Earth Sciences 25 (2009) 20-31

Study on geotechnical characteristics o f H olocene soils with reference to geohazards in Kien An - Do Son,

Hai Phong Coastal zone

Dang Van Luyen*

Cullege o f Science, VNU

Received 5 January 2009; received in revised form 20 January 2009

A bstract. In recent years, after renovation policy launched by the Vietnamese government, the economy o f Kien An - Do Son, Hai Phong C o a sta l zone in the Northern Vietnam has quickly changed with high income and fast economical grow rate. With the economical development, the environment in many areas has been severely damaged. In addition, the so called “global change”

has also strengthened the natural hazards especially torpedo, storms surges, salt intrusion due to the sea lcvel change... increasing o f the losses caused by natural hazards in this vcry sensitive zone.

Based on the results obtained in site recent investigation some main geologicaỉ hazards vvere recorded and studied in detail such as ílooding, coastline and river bank erosion. The results of vulnerability assessment in this study will help in proposing suitablc remediai measures for hazards prevention, and also will help in realizing eíĩectively the implementation o f the socio- economic plans for the sustainable development o f this very sensitive Coastal zone.

1. I n tr o d u c tio n

K ien An - Do Son reg io n is located in the S outhw est o f H ai P hong city - o n e o f th e m ost im portant seap o rts o f V ietn am , a b o u t 100 kin east o f H ano i city. B oth K ien A n and Do Son nevvly had b eco m e m ain d istricts o f H ai Phong city instead o f sm all to w ns o f the su bu rban d istricts (F ig. ]).

■ Tel.: 84-4-38542905.

E-maiỉ: luyendv@vnu.edu.vn

Fig. 1. Location o f the study area.

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D .v . Luyeti / VN U Joumal ứf Science, Earth Sciences 25 (2009) 20-31 21

D o Son, H ai P hong is o n e o f im portant d o ors attractin g ío reig n in v estm en t in the N o rth ern V ietnam h aving m a n y in d u strial zones.

B esides, it is also a dy n am ic c e n te r for tourism , aq u ac u ltu re... so th at this rcgion h a s conílicts betvveen so cio -eco n o m ical d ev elo p m en t and en v iro n m en tal p ro tectio n . E ach y e a r it faces

\vith ab ou t 4-5 ty ph o o n s ca u sin g great losses, evcn deadths for the hab itants.

2. B a c k g ro u n d o n n a t u r a l & g eo lo g ical c h a r a c te r is tic s o f th e s tu d y a r e a

2.1. G e o g ìa p h ic a l cotưỉỉtion

L o cating in th e N E o f the R ed R iv e r D elta (R R D ), the study area is co m p rised o f 4 m ain types o f relief: k arts, re m a in e d lo w lìill, plain a n d Coastal m an g ro v e la n d vvith the area o f a b o u t 500 k m 2.

T errain h e re is very d iffcre n tiated \vith the g rad u al d ecrease in h eig h t in S E direction:

- K arts terrain : m ain ly d istrib u ted in N ui V oi w ith the elev atio n v a ry in g fro m 10 + 300m , the p la n t cov cr is n o t rcg u la rly distributed.

- R em ain ed lo w h ill te rra in is scattered distrib u ted in K ien A n in the N E -S W orien tation an d c o m p risin g terrigenous scdim ents. T h e elcv atio n is v ary in g fro m 15 -r 14 0 m In D o S o n this ty p e o f tcrrain also dev elo p ed in V an H oa, N u i T h ap an d C hoi M ong jo in te d in ran g c w ith th e N W -S E orientation.

- P lain te rra in is w id e ly d istrib u ted in the S outh an d th e S o u th e a st p a rts w ith elevation varying fro m 0.8 + 1 .2 m C o m po sitio n is m ainly c lay ey san d, silty clay o f allu v ial origin.

2.2. C lim a tic cond iíion

C lim ate in H ai P h o n g has trop ical - hu m id ch aracters \vith h igh in ílu e n c e o f th e sea. T h e

clim ate is sh arp ly d iv id ed into tw o seasons:

sum m er an d w inter. S u m m cr is usually hot and hum id, m any sh o w ers lastin g fro m M ay to O ctober (co n ta in in g 8 0 -9 0 % o f the annual rainfall). W in ter is colđ la stin g from N ov em ber to A pril o f the n e x t year. A nnual rain íall is

1 6 0 0 -1 8 0 0 m m H ig h h u m id ity (85-86% ).

H ai P hong is lo c ated in the area o f frequently o ccu rred sto rm s an d cyclones. There are 45 days o f stro n g w in d o r storm s during a year. B esiđes, m ild drizzles an d frogs are ử eq u en tly cxrcurred.

2.3. H yd ro lo g ica l co n d itio n s

A ll b ig riv e rs in H ai P hong are distribu taries o f T h a i B inh riv er sy stem w hich are flo w ing N W -S E w ith h ig h m eand ering and w ide san d b ars. M a in riv ers are B ach Dang, Cam , L ach T ray , V an U c, H oa, H an an d Thai Binh. T he d istrib u taries are T am B ac, D a Do, D a Bach. In av era g e, a t distance o f 2 0 k m along th e co ast th ere is a n riv er m outh flow ing into th e sea.

T h e riv e r d isch arg c is n o t cqually distrib u ted d u rin g th e y e a r r o u n d T h e ílo od ing seaso n co n tain s 7 5 -8 5 % o f discharge volum e (m ain ly in June, A u g u st and Septem ber) m ean w h ile the d ry seaso n contains o nly 15- 2 5 % (lo w est d isch arg e in M arch containing on ly 1%). T h e b ig m u d an d san d volum e carried d urin g flo o d seaso n m akes the quick siltation in th e h a rb o rs a n d high turb id ity o f seaw ater in all D o S o n beaches.

2.4. G eo lo g ica l co n d itio n s

T h e Q u a te m a ry sedim en ts overlying u n co m to rtab ly th c N eo g en e deposits are co m p o sed m ainly o f san d an d g ra v e k , so m etim e w ith lc n ses o f s ilt an d clay. The sedim en ts a re th ick cr to w ard s the sea, reaching up to 2 0 0 m in th ick n ess in the Coastal area. The

22 D.v. Luyen / V N U Ịounial oỊSãence, Earth Sciences 25 (2009) 20-31

upperm ost Q u atem ary sedim cn ts co n sist o f four form ations; L e C hi, V in h P huc, H ai H iuig and T hai Binh.

The Le C hi ío rm a tio n co n sists o f gravel, fine to m edium sand an d silty clay. V inh Phuc ío rm atio n is c o m p o sed o f a n u p w ard íin in g su ccessio n o f gravel an d clay ; the H ai H ung íb rm atio n co m p o sed m ain ly o f sand. Finally, the T hai B inh fo rm atio n is c o m p o sed o f an upw ard- fining u nit o f grav el, san d an d clay. So th at the Coastal zone o f th e R R D is considered as p ro pag atin g Coastal sy ste m ío rn ied m ain ly as a result o f riv er sedim ent supply.

The H olocene rn arin e te ư a c c s are b etw een 3 and 5 m ab o v e m ean sea level (M S L ) and the Coastal lovvland area lo cated se a w a rd from that te ư a c e is p red o m in an tly lo w er than 3 m (N ghi et al., 2000). T h e co astlin e is a d ro w n ed coastlin e follow ing the rise in m ean sea lcvcl o f som c 8 0 m over the last 10,000 years.

In this study, efforts are con cen trated in geotechnical ch aracteristics o f tw o upper geological divisions: H ai H ung an d T hai Binh form ations.

2.5. G eneral g e o te c h n ic a l ch a ra cte ristics G eotech nical characteristics o f typical soils in H olocene ío rm atio n s can b e d escrib ed as follows:

1. Fill material (an Qỉ)

T h e th ickn ess o f this ty p e o f sedim en ts is 0.5 -2 .0 m com p risin g m ain ly san d, clayey sand, clay m ixed \vith w a ste co n stru ctio n m aterials.

N ot suite for use as fíll m aterials.

2. Alluvial-swamp sedimenís o f Late

Holocene, U pper Thai Binh /o rm a tio n (tib Q 2 tb-J T hese sedim en ts are not w idely distrib u ted along sm all rivers and u su ally su b m erg ed vvith the thickness o f aro u n d l-3 n x C o m p o sitio n is niainly clayey m ud, clayey m u d containin g organics o f d ark grey or li-g rey in color.

3. A llu via l-m a rin e-sw a rn p se d im e n ts o f L ate Holocene, U pper T hai Binh /o n n a tio n (and) Qì tb i)

C o m p o sitio n is c lay ey m ud, silty -sa n d y m ud o f bro\vn ish g rey co n ta in in g o rg an ic s. lt is distrib u ted in L ac h T ray , V a n U c riv e r m ouths.

C o n sisten cy is v ery so ft to m e d iu m (B = 0.54 - 1.4), B earing c ap a city is v aried fro m 0 .5 -0 .7 K g/cm 2. (top p a rt) to 1.7-1.8 Kg/crrT. (b o tto m part).

4. M ỉirine se d im e n ts o f E a rly -M id d le Holocene, Upper Thai Binh /o n n a tio n (a Q: ỉ bi)

It is d istrib u ted in n a ư o w range fro m Do Son to V an Uc, aro u n d T hai Binh riv c r m ouths.

C o m position is silty san d o f g rcy color.

C onsistency is m e d iu m s tif f (B = 0 .6 5 ), B earing capacity is 2.2 K g /cm 2.

5. Aỉluvial sediments o f Late Holocene, Middle Thai Dinh/ormaíion (a Qi tb2)

This ty p c o f sed im en t is w idely d istrib u ted along V an Uc, T h ai B inh, H oa rivers.

C o n p o s itio n is in a in ly silty sand, silty clay o f h ro w n co lo r, very soft. V o id ratio is a b o u t 1.02-

1.43, high co m p re ssib ility vvith co efficicn t o f coiììp rcssion a = 0.0 7 8 -0 .0 2 6 cm 2/kg.

6. Alluvial-ìnarine sedim ents o f Late Hoỉocene.

L o w er T hai B in h /o r m a tio n (a m Q: tb ị)

S edim cn t co m p o sitio n is silty clay , silty san d o f brovvnish an d dark grey. It is w idely d istrib u ted in T h u y N g u y en , A n H ai, V inh Bao a n d inner H ai P hong. T h ick n ess is aro u n d I7m

7. M a rin e se d itn e n ts o f L ate H olocene, L o u v r Thai Binh /o rm a tio n (m Qi tbi)

S ed im en t co m p o sitio n is m ainly san d , silty sand o f yellovv, brovvnish y ello w an d brovvnish grey c o n tain in g sh e ll ữ a g n ie n ts. T h ick n ess is aroiind 2.6 n i

8. Alluvial sedivient o f Early-Middle Holocene, L o w er T hai B in h fiorm ation (mb Q ĩ/ : hhj)

T h e sed im en t is not seen on the s u ría c e but can be o b ser\red in b oreh oles w ith n iax im u m

D.v. Luyen / V N U lo u n m l o f Science, Earth Sciences 25 (2009) 20-31 23

th ick n ess o f 2 4 m C o m p o sitio n m u d d y sand, sandy m ud c o n tain in g organics, low consistency. B earing capacity is 0.5 k g /cm .

9. M arine sedim ents o f Early-M iddle Hoỉocene, U pper H a i H u n g fo r m a tio n ịm Q: ' hliỉ)

O utcrop o f th e se sed im en ts w ere ob serv ed in A n Hai, N o rth o f T h u y N gu yen, in the

p erip h e ry o f Phu D ien hill rang e and in bo reho les a t depths. T h ick n ess is v aried fro m 1- 16.6m (2 -2 .5 m in av erage). Sedim ent co m p o sitio n is silty sand , silty - clay ey s a n d

G eo tech n ical ch aracteristics o f som e su rface H olo cen e typical soils are show n in T able 1.

Table 1. Gcolcchnical cliaractcristics o f surface soils o f Holoccnc formations in Hai Phong arca

No. Properties Sym­

bol Unit

Holocene formalions and llicừ typical soils Silty clay

(mb Q i h h i )

Clay ( m Q Ỉ *

Claycy silt hhì) (am Q Ị s Itbi)

Silty clay (amb Qi’ lb;)

1 Componcut Sand % 26.79 16.62 13.2 19.3

2 Silt % 50,. 49.65 52.2 56.3

3 Clay % 22.8 33.73 34.6 24.4

4 Wa t ơ contcnt w % 43.6 33.7 50.7 39.0

5 Unit weight Y g/cniJ 1.74 1.85 1.65 1.8

6 Dry unit weiglit Yc g/cm3 1.22 1.38 1.0 1.3

7 Specific gravily A g/cm 3 2.7 2.7 2.73 2.75

8 Void ratio c 0.9 0.95 1.5 0.95

9 Porosity n % 52.3 48.8 59.0 54.6

10 D cg recof Saturatioi) s % 93 95 95 92

11 Liquid limit W L % 34.6 40.4 44.3 35.0

12 Plastic liniit W p % 22.1 22.0 25.0 20.0

13 Plasticity indcx Ip % 12.4 18.4 19.7 14.4

14 Liquidity indcx B 1.8 0.64 1.3 1.3

15 Cohesion c K g/cnr 0.054 0.13 0.059 0.06

16 Intcmal ữiclion anglc <p Dcgrcc 9 9 3 8

17 Coef. o f comprcssion 2|-J cm /Kg 0.09 0.073 0.09 0.057

18 Bearing capacity Ro Kg/cm2 0.45 0.55 0.40 0.60

2.6. G en era lized s o il p ro file

S oil p ro ílle w ith v ariatio n o f index p ro perties o f th e soils ag ain st d eplh s a t the site o f V an H uong h ig h -g rad e v illa are sh ow n in F ig ure 2. It is div id ed in to 4 lay ers in a 18 m depth borehole:

F ill is ch aracterized as san d y clay w ith gravel a n d bro k en brick . T he lay er is ranged from the g ro u n d su ría c e to th e đep th o f 2 m

S ilty cla y la yer (C L ), so ft to m edium extends from the dep th 2 m to 7 .5 m It is li-grey to d ark grey co lo u r w ith low plasticity. T he

w ater content is in ran g e b etw ee n 34% an d 40%.

T h e liq u id lim it (L L ) is b etw een 23% and 41% . In this layer, so m e organic m atters are found.

C la y la y e r (C H ) o f soft to m edium consisten cy lo c ates fro m the depth 7.5 m to 12m It is broNVTiish g ray colour to li-grey, grey colour w ith h ig h p lasticity . T h e w ater content is in the ran g e o f 4 0 % to 5 2% , the L L is o f 50% to 61% .

C la y la y e r (C H ) o f m e d iu m s tií ĩ co n sisten cy is fo u n d a t the d ep th ab ou t 12m and ex ten d to 18m until the en d o f borehole. The soil is g ray ish b ro w n in co lo u r an d high plastic.

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T h e vvater co n ten d is in ran g e o f 4 0 % to 50% , A cco rd in g to thc a u th o rs, the age o f the clay the LL is 55% to 65% . H ow ever th e re are so m e is estim ated 2 0 ,0 0 0 - 4 0 ,0 0 0 years from the lenscs o f fm e s a n d w ith in this layer. dating tech niqu e u s in g carb o n isotope,

14c.

Fig. 2. G cncrali/cd soil profilc and physical propcrties in Van Hương, Do Son, Hai Phong arca.

Note; laycr 1 ( Q'2m); fill,

laycr 2 (2-7.Sm): soíì to mcdium silty clay (cl), laycr 3 (7.5-12m): soft lo modium clay (ch),

laycr 4 (12-18m): mcdium sliíTclay (ch) soil classiíication in brackcts was madc according to Victnamcsc staiidards tcvn 5747:1993.

3. G e o lo g ic a l h a z a r d s 3.1. C o a sta l ero sio n

T h e N o rth east coast o f D o S o n h a d the a v c ra g e s p e e d o f ero sio n o f 5-14 nV year (D in h V u -B ach D ang an d alo n g ro ad N o. 14. A t B ach D an g m o u th area, d u rin g 6 0 y ears (1936-1996) an am o u n t o f 1055 h a o f a g ric u ltu re land w ith v eg etatio n covcr an d 2844 ha vvithout vegctation c o v er h ad b een lost. T h e av erag e ero sio n rate fo r th ree beach es in D o S on w as v ary in g from 0.3 6 - 0.45 nVyear [1], causing lost to the infrastructxưe and sea dyke sy stem (photos 1&2).

T h is h a z a rd co u ld b e p articu la rly exag gerated b y s to rm w ith h ig h S W L rise (p h o to s 4 & 5).

T h e av erag e rate an d vvidth o f ero sio n are sh o w n in T ab le 2.

Tablc 2. Avcragc ratc and vvidth o f crosion at Bach Dang cstuary and Do Son bcachcs

No. Arca Averagc

crosion ralc (m/year)

The vvidúi o f crosion arca during 50 years (ra)

1 Dinh Vu 0,8 40

2 Bcach 1- Do Son

0,45 22.5

3 Bcacli 2- Do Son

0,36 18

4 Beach 3- Do Son

0,40 20

D.v. Luyen / V N U Ịountal o f Science, Earth Sáences 25 (2009) 20-3ì 25

Photo 1. Sea dykc in Ward 2, Ngoe Hai Communc, Do Son - Hai Phong uiidcr CTosion.

T h e Southvvest p a rt o f th e stud y area is h ig h l) p ro d u ctiv e in rice c u ltiv a tio n an d w ith a high po pulation dcnsity for a pred om inantly ru ra l com m unity. T he h u m a n activ ities su c h as:

shrim p an d físh p o n đ digging, san d exploitation, excavation vvith a largc íillin g m aterial volum e, m ang rove th ree cu ttin g ... not only inĩluence g eo d y n am ic p ro cesses b u t also all natural conditions. A t so m e p laces thc shrim p ponds w e re dig v ery n e a r to the d yke em bankm ent.

This could lead to sev ere d am ag e to the dyke em bankm ent.

T h is erosion situ atio n w ill b e particularly v u ln erab le in th e event o f accelera ted sea level rise.

Photo 2. Dong Hai fishcry port undcr thrcat o f crosion.

3.2. S a lin ity in tru sio n

S alin ity in tru sio n in th e Coastal zo n e is increasing due to fresh w ater ex tractio n for irrigatio n and d rin k in g w ater and due to the d a m co n stru ctio n s in the catchm ents.

A ccelerated rate o f the sea level rise also caiises a hig h er p en etratio n o f salin e w ater in to rivers as w ell as into the g ro u n d w ater sy stera

T h e in lan d ex te n d o f salin ity in tru sio n (1

°/oo an d 4 °/oo) in th e riv er sy stem v aries from B ach D ang to T hai B in h rivers (Tab. 3).

Table3. Distancc o f salinity inưusion from river mount (kni). (Ca V.T., 1989) [1].

River moutli Maximum Average Minimum (tan) (1°/qo) ( 4 ° /qo) ( 1 ° /qo) ( 4 ° /co) _______________

Bach Dang 35-40 30 25-30 20

VanUc 28 20 18 8 1

Thai Binh 26 25 15 5 1_______

A fter the H oa B in h reserv o ir operation, the 4 °/oo bo un daries h a s retre ated 5-10 k m fro m the sh ore line. It is n o te d th at for ag ricu ltu re cro p dam age occurs ab o v c 1 g/I ( o r l u/oo) a n d b e y o n d 4 g/1 rice crops arc n o t sustainable.

S alt in trusio n is th e m a in cau se fo r soil deg rad atio n in D o S o n area. This co m p o ses o f tw o p rocesses: the le ach in g o f salt p resen te d in the soils fro m th e ir form ation, n o t y et p ro p erly disso lv ed o u t b e c a u s e lying in low lan d a re a and salt intrusion c a u s e d b y in íiltratio n in to co arse g rain san d y layer w idely d istrib u ted a t the depths o f 8 0-1 20 c m an d so m etim es fo u n t even in th e su b -su rface (3 0 -5 0 cm )layer.

3.3. F loo d in g

A t p resen t ílo o d in g is one o f the hazard s cau sin g m o st n eg ativ e in ílu en ce in V ietn am develo pm en t, esp ecially in areas o f th e m o st econom ically activ e such as Red R iv er D elta (R R D ) an d M ek o n g D elta. T hcy h av e cau sed big h u m an as w ell a s econ om ical losses.

2 6 D.v. Luyen / VN U Ịoum al o f Science, Earth Sciences 25 (2009) 20-3Ì

Photo 3. Road in Do Son city was íloodcd during the Storm No. 7, Scpícmbcr 2005.

In th e R ed R iver (R R ) D elta p rov inces the ílo o đ in g a t p re se n t is m o st se v e re durin g sto rm su rg es th at hap pen ra th e r o ften (aro und 4-5 tim es/y c ar) in the areas o f Coastal zones. R ecent ex tre m e flo o d o ccurređ in the R R D in 1971 (ap p ro x . 1 in a h u nd ređ y ears) b ro k e RR dyke sy ste m in tree locations an d k illed th o u san d s o f peo p les a n d an area o f 2 5 0 0 0 h a w as ílooded, 2.7 m ill. p erso n s h ad suft'ered this sev ere flood hazard-

D u rin g the p a s t 100 y ears, a n um b er o f 26 h isto ric a l Aoods, p red o m in an tly riv ers A ooding w e re re c o rd e d in th e R R D p rovinces. M ost se v e re ílo o d s o ccu r d uring o ccasio ns o f h igh s to rm su rg es w h ich lift the sea w ater level and in h ib it th e d isch arg e o f high ru n -o ff fro m h eavy rain s d o w n p o u rs in the catch m en ts areas.

In ad d itio n , in m any lo cations in the R R D th e dykes are w eak er an d lo w er in th e relation w ith th e ừ ex ce ed in g w atcr levels. In the Coastal z o n c in o rđ e r to prev en t ílo o d in g fro m th e sea due to se a dy k e íailu re, a seco n d line o f sea dy k e is co n stricted b e h in d th e se a w a rd dyke.

M a n a g e d retre at strateg ies are ap p lied for the m o st c ritica l p a rts o f the R R D coast. T he estim atio n o f the total an n u al b u d g et for m a in te n a n c e o f sea an d estu ary dykes in the w h o le co u n try is U SD 1.54 m illions.

3.4. S to rm su rg e s

F ro m 1954 up to 1991 a b o u t 250 storrn surges an d c y c lo n e s la n d ed o r d ứ e c tly a íĩe c te d to V ietn am (F igs. 3). O ne fifth o f these has arrived to th e s tu d y area. A lm ost o f th e sto rm s o ccu rred d u rin g su m m er seaso n (July- N ov em b er) (Fig. 4).

Ơ > ờ > ữ > ỡ > ơ ) c r > ơ > ơ ) ơ ì ơ > ơ > ơ ) ơ >

Fig. 3. Numbcr o f stom is to victnam coast pcr ycar (1954-1991).

.Vumher oí siorms

Fig. 4. Nunibcr o f storms vs. months in tìie ycars.

D u rin g sto rm su rg es the se a w ater level (S W L ) raised m o rc than lm , am ong them 30%

o f sto rm s ra is e d th e S W L m ore than 1,5 m an d 11% o f th e m ra is e d th e S W L m ore th a n 2,5 m.

T h e area o f 4 0 k m in w id th fro m the seaco ast co u ld b e sev erely a ffe c te d T h ese kinds o f storm s also c a u se d A ooding a n d destruction to sea dyke system a n d inírastructure (Photos 3 to 6).

D .v . Luyen / V N U Ịoum al o f Science, Earth Sciences 25 (2009) 20-31 27

3.5. Sea level ríse (SLR)

The increased green h o u se effect is m aking the earth is w arm er, with m e ltin g ice in two poles in add itio n o f this the o v er-ex p lo itatio n o f underground w ater in u rb an areas m ak e rising sea levels.

A ccording to scen ario 1 (A S L R T 1 ) o f V ietnam C o asta l Z o n e V ulnerability A ssessm ent (C V C Z V A , 1996) w ith l m sea level rise and n o ad ditio nal p ro tec tio n m easures ab ou t 40.000 k m : o f Coastal zo n c o f V ietn am w ill be su bject to ann ual ílo o d in g [11]. A bout 10% o f lands in the K ien A n - D o Son, Hai P h o n g Coastal zo n e co u ld b e p artially o r alm ost com pletely affected.

Photo 4. Sea dykc Do Son ỉ during tíic storm No. 7, Scptcnibcr 2005.

Photo 5. Tourism facilitics and houses u n d ơ Uireat during Uic stomi No. 7, Scptcmbcr 2005.

Photo 6. The dcsưoycd inírastructurc by Storm No.

7, Scptcnibcr 2005.

A cco rd in g to th e research o f scientists an no unced recc n tly in th e \V orkshop on

“ C lim ate ch an ge issues and m an ag em ent o f to u rism d ev elo p m cn t o f u rb a n Coastal a re a s”

o rg an ized in D o S o n fro m 4-6 Jan u ary 2008, sp eed o f clim ate ch ang e in o u r co un try increasing. In 4 0 y cars, averag e tem p eratu re in V ic tn a m has in creased 0.6°

c

in 2 0 y ears and sea level has to rise in 6 c m m ore; storrns m ore p o w erfu l a n d stren g th , rain bcco m es stro n g er vvith m ore w ater, th e cy clc o f drou gh t an d heat longer th an in the la st dccades.

F o recast to th e y e a r 2 07 0 , tem p eratu res in th e N o rth w ill in c re ase 2-4°C , in the S o u th w ill increase 2-3°C . By th e y e a r 2 0 5 0 th e sea level

\vill ris e 3 3 cm m o re an d in 2 0 7 0 w ill in crease m o re th an 5 0 c m T h is is a grcat risk fo r the C o a sta l p rov inces o f V ie tn a m

Hovvever, S L R is not thc sudden c h an g e bu t a g rad u al p ro cess la stin g over tim e a n d space.

T h erefo re the ap p ro ac h sh o u ld b e grad u al and in asy m ptote o f ccolo gical env iro nm ent, ad apting to cope w ith the changcs o f natiưe.

E ngineering a n d n o n -en g in eerin g so lu tio n s to the changes m u st b e b u ilt flex ibility in acco rd an ce w ith th e grad u al increase o f the risin g in sea w ater level. It is n ece ssary for b asic research o f hydraulic, phy sical, env iro n m en tal co n d itio n s, cliniate, p ro m o tin g

28 ^{D.v. Luyen} / V N U )oum al ọ f Science, Earth Sciences 25 (2009) 20-31

the research a n d d ev elo p m en t o f slo p in g land...

to red uce the negative im p act i f any.

O ne o f the p ro jects in the strengthening, pro tectio n an d u p g rad in g o f th e sea dyke system s available fro m Q u an g N in h to Q uang N am is "S treng th ening , p ro tec tin g and upgrading the S ea D yke L in e N o. II in K ien T huy an d D o S o n d istricts, H ai P h o n g w ith total investm ent over 104 b illio n V N D , in v ested b y the D epartm ent o f A g ricu ltu re an d Rural D evelopm ent. T h e p rọ ịec t w as im plem ented w ith the g o al o f stren g th en in g and u p g rad e m ore than lO km in th e sea đ y k e lin e N o. II to prevent salt intrusion, h ig h tid es an d en sure rin g the dyke w ill b e safe at the d esig n ed level. This project is u n d er pro g ress d u rin g 20 08 -2010.

3.6. E arthquake disa ster

T h e stud y area is situ a tc d in R ed riv e r deep fault zone sừ e tc h in g along R ed R iver fro m V ietnam -C hina b o rd er to th e E ast S ea with total length o f m ore th an 6 0 0 k m A cco rd in g to geophysical studies, th e Red riv er zo n e is still activated in th e p e rio d o f Q u atern ary - present w ith th e rig h t slid in g m ov cm en t o f 4 .5 -5 m in velocity an d th e N E p a rt o f R R D in still su bsiding o f av erag e rate 2 .m m / y e a r th a t’s vvhy R ed R iver tectonic z o n e h as th e av erag e stress- released eq u iv alen t w ith o f m ag n itu d e earthquake M s= 4.5.

A lth o u g h th e stu dy area is situ a te d far from the m ost seism ic activ ated re g io n (S on L a, Lai C hau) w ith M s = 6-8 b u t it is situ ated in a large b asin íĩlled w ith w eak zo n e d ep o sit (sand, silt, clay). U nder this special co n d itio n earthq uake can occur a t distance o f 5 0 0 -6 0 0 k m fro m the sou rce (ep icen ter region).

T hen m uch o f th e d estru ctio n can be occurred due to sig n iíica n t a m p liíĩc a tio n o f earthquake g ro u n d m otioiis b y this th ick so ft soil deposits. T h e sub soil o f T h ai B inh íò rm ation \vith v ary in g co m p o sitio n from silty

sand to sa n d y -c la y e y silt a lso e a sily to get into liqu id ĩíĩed co n d itio n u n d e r e a rth q u a k c s vibrations.

T h e m o st effectiv e m e a su re to m itigate seism ic risk in the u rb an area to b e taken into a cco u n t is in tro d u cin g the s u ita b le design req u irem en ts fo r new , esp ecially econom ically im p ortant con stru ctio n su ch as high rise buildings, p o rts and sca d y k e ... It is needed to ad opt in V ie tn a m th e s e is m ic design req uirem en ts (co des an d sta n d a rd s) as most quickly as possible.

4. V U ln e r a b ility a s s e s s m e n t o f th e K ien A n - D o S o n , H a i P h o n g C o astal z o n e

For ev alu atin g th e v u ln e ra b le lev el o f socio- natu ral sy ste m o f Coastal zo nc, a series o f in v estig atio n s an d calcu la tio n s in th e site have been estab lish ed in a g rid w ith a re a o f 1 km 2 an d 4 k m 2 eq u iv alen t to 1: 5 0 .0 0 0 and 1:

100.000 to p o g rap h ic m a p s c o v e rin g different regions w ith d iffere n t characteristics o f gcolo gical fo rm ation , to p o g rap h y , hydro- geology, a n d h u m an activ ities, ...

T h e in íb rm a tio n fro m fie ld stu d y com bining w ith c o lle c te d d a ta can b e g a th e re d in groups such as: T y p e o r categ o ry , history o f ap p earan ce, intensity, scale, signs a n d im pact o f the h a z a rd w ith v u ln e ra b le so c io - natural character. H a z a rd p re v e n tio n activ ities that have b een an d a re a p p lied stu d y th e c u rren t State and p red ic t th e p o te n tia l o f loss c a u se d b y the h u m a n a c tiv ity in th e a re a (irrigation, tran sp o rtatio n , sea, a n d m ining, tourism activ ities...) [10].

T h e d eterm in a tio n o f risk cau sed b y hazard to m arine a n d Coastal reg io n s c an b e carried out w ith fo llo w in g steps:

+ D e íìn e th e h a z a rd types and calculate grade o f ris k level cau sed b y eac h hazard basing

D .v . Luyen / V N U Ịoum al o f Science, Earth Sciences 25 (2009) 20-31 29

on different criteria such as type, intensity, íreq u en cy o f its o ccu rren c e b y the follow ing equation:

G = (F + A ) X M w hile: G = T o tal score,

F = Frequency,

A = Im pacted area , and

M = Potential dam ag c m agnitude.

T h e resu lt calcu lated are sh o w n in table 4.

T h e relativ e priority o f the princip al hazards is as follow sequ en t: Coastal erosion , salt intrusion, sto rm siư g e ílood, earth q u ak e an d sea level c h a n g e ...

Table4. Calculation o f the rclativc priority o f thc principal hazards[10]

T y p e o f Frcqucncy Impactcd Magnitude Total

ha/ards + arca X = score

Coastal 5 4 4 36

crosion

Salinity 3 5 4 32

intrusion

Storm 3 4 4 28

surge

Flood 4 4 3 24

Earthquakc 1 5 3 18

Sca level 2 4 2 12

change

+ R ecog nize the h a z a rd type a n d calcu late total p o in t o f risk lev el for eac h square:

D I , = SH , / SHib

w hile: D I ,: d an gero us level fo r eac h square, SH,: total p o in ts o f the i sq u are , and SH ,b: average dang eroiis d en sity for the w hole region.

T hen th e n ex t step is fillin g in the result in this square. Z o n in g o f risk lcvel for the Coastal zone can b e m ade o n the niap b asin g on the total p o in t o f eac h sq u are to g e th er \vith som e other criteria estab lish cd fro m site in v estig atio n for the Coastal zone.

B asin g o n the p o in ts o f sing le and in teg rated p o te n tia l h azards, th e zoning o f risk cau sed b y h aza rd s along the K ien A n - Do Son Coastal zo n e can b e div id ed into 4 zones a c c o rd in g D I, v alue:

DI, < 1 - L ow risk po tential

1 < DI, < 2 - ln term ed iate risk potential 1.7 < DI, < 3.4 - H igh risk potential DI, > 3.4 - V ery h ig h risk potential 4. Ị. Z o n in g o f p o te n tia l h a za rd s

Z o n e o f v ery h ig h risk poten tial hazards is contain ing at least 4-6 types o f disasters (erosion, sa lin ity in tru sio n, storm s, ílo o d and th e effect o f se a levcl rise (S L R ))... w ith DI value > 3.4 is n o t p resen te d in th e stud y area.

T h e h igh risk h aza rd p oten tial zone d istrib u ted w id ely in the stu d y area including lovvland areas situ ated b o th side o f N goe X uyen an d Y en Son liill ran g e and stretching to K ien A n , w h ere th e rock s m ain ly are co m posed o f siltsto nes san d sto n e b elo n g to the interm ed iate ris k p o te n tia l zone.

Z o n e o f low risk h azard po tential is p resen te d in X u an S o n & N u i Voi hills, w here K ien A n F o rm atio n c o n tain in g the san dston e o f quartzite ty p es o r lim estones is m ainly distributed.

4.2. Z o n in g o f d e n sity o f vulnerable socio- n a tu ra l o b ịe cts

B ascd on C u tte r 's criteria (1996, 2000) and N O A A (199 9) w ith the an alysis resu lts o f the so cial o bject in the stud y area, 10 obịects socio- natural v u ln e ra b le c an b e determ ined concluding: 1- to u rist sites, 2- po rt, 3 - b o at p ark in g area, 4 - petrol station , 5- salt íĩeld, 6- ag ricu ltural p o n d , 7 -am o u n t o f boats and ships (in port, Coastal an d o íĩsh o re ), 9 - national

30 D.v. L uỵen / V N U Ịounial o f Science, Earth Sciences 25 (2009) 20-31

reserve p ark an d 10) h is to ric a l & c u ltu ra l place o f in terests...

F or the z o n in g o f v u ln e ra b le s o c io -n a tu ra l objects the fo llo w in g w o rk s n e e d to b e done:

+ T he so c io - n a tu ra l o b je c ts that are vu lnerab le sh o u ld b e re c o g n iz e d an d classified into diíTerent v u ln e ra b ility le v e l in a certain area. T hese v u ln e ra b le o b je c ts a re clo sely related w ith im p a c t an d p a rtic ip a tio n o f the h u m an such a s to u r p la ces, p o rts , aq u ac u ltu re ponds, facto ries...

+ C ounting p o in ts fo r v u ln e ra b le so cio- natural object in eac h sq u a re c o m b in in g w ith ex p ert’s k n o w led g c o n th c s tu d y arca.

A verage d e n sity o f v u ln e ra b le so cio -n atu ral objects (S O D lb) c an b e c a lc u la te d fo r th e vvhole area with form ula:

SO D Ib = X S O D i / N (i = 1,2.,3 n)

w hile: SOD, - p o in t o f v u ln e ra b le objects belo ng in g to the sq u a re ;

n-the n u m b e r o f sq u a re in th e to p o g rap h ical m ap o f the stu dy area.

B asing on th e d e n sity o f v u ln e ra b lc objects SO D i (m ax) a n d S O D i (a v e ra g c ) c o u ld b e identiíĩcd an d th e n th e v u ln e ra b ility o f social objects was c la ss iíie d in 4 c lasses acc o rd in g to vulnerable so cial fa c to rs (V S F ):

low d ensity o f V S F : < 2 .5

rath er high d e n s ity o f V S F : 2 .5 - 4 .0 hig h den sity o f V S F : 4 ..0 - 6 .0

very high d e n sity o f V S F .: > 6.0

Z o n in g a n d e v a lu a tio n o f v u ln e ra b le level for the so cio -n atu ral s y s te m c a n b e co n d u cted b asin g on th e o v e rly in g c o m b in a tio n an d analysis o f the a b o v e tw o c o m p o n e n t m aps.

The results o f V A a n a ly se s s h o w th a t in the K ien A n - D o S o n Coastal z o n e th e re a re 3 su b- zones o f d ifferent v u ln e ra b le d e g re e (V D ) such as: su b-zone o f h ig h V D , su b -z o n e o f interm ediate V D a n d s u b -z o n e o f lo w VD.

T h e m o st h ig h ly v u ln e ra b le deg ree o f VA su b -z o n e is b o rd e re d N g o e X u y en , Y en Son c o m m u n e s w h ich in clu des a re a s b eh in d th e sea d y k e D o S o n I in th e N o rth w e st a n d B ang La d y k e in th e Southvvest o f D o S o n pen in sular stre tc h in g fro m L ach T ray m o u th to V an Uc m o u th w ith a b o u t 10-15 k m w ide.

T h e n e x t su b -zo n e o f interm ediate v u ln e ra b le a re a s lo c k ed b y 1“ h ig h ly VA sub- zo n e to K ie n A n tow n. T h e last su b-zone w ith low d e g re e o f V A is X u an S o n & N u i V oi hills, m o s tly lo w h ill areas w ith ra th e r durable s a n d sto n e o f q u a rtz ite ty p e o r lim estone o f K ien A n fo rm atio n ..

5. D isc u ssio n s a n d r e c o m m e n d a tio n s

1. T h e H o lo c e n e so ils in K ien A n-D o Son area is c o m p risin g 4 ío rm a tio n s as H anoi, V inh P hu c, H ai H u n g a n d T h ai B in h w ith m axim um th ick n ess o f 2 5 - 3 0 m T w o u p p e r layers are d e p o sits o f sw a m p , s h a llo w m arin c o r alluvial o rig in s w ith m ain ly clay, silty clay or silty /c la y c y s a n d o f g rey o r g ra y ish brcnvn o f m e d iu m to h ig h c o n siste n c y w ith fine g rain c o m p o n e n ts v a ry in g fro m 73 -87 % .

2. T h e re su lt o f th e in v e stig a tio n and the an aly sis s h o w th a t the s tu d y reg io n is m ainly in ílu e n c e d b y 6 types o f g eo lo g ical hazards n a m e ly : 1- Coastal ero sio n, 2 - salt intrusion 3- s to rm s u rg e & to rp ed o , 4 - ílo o d , 5 - earthquake an d 6 - se a le v e l ris e ... \vhich a re ranged in d e c re a sin g p rio rity order.

3. U p g ra d in g se a d y ke an d riv er dyke sy stem s b y ra is in g th e m in 5 0 cm m ore in h e ig h t to w ard s 2 0 2 0 to co p e w ith rising sea lev els is a n rem e d ial b u t n o t effective solution.

It is n e e d e d to n o te th a t u p g rad in g 1 km o f sea d y k e c o s ts a b o u t 10 b illio n s V N D , w hile c o n s tru c tio n o f o n e k m n e w ly b u ilt sea dyke co sts 10 tim e m o re - a b o u t 100 billio n VND.

D.v.

Luyen / V N U Ịo u m a l o f Science, Earth Sciences 25 (2009) 20-31 31

T h e co ast in the stu d y a re a h a s the ad v an tag e in good ad ap tatio n for th e ío rm a tio n and dev elo p m en t o f m a n g ro v e forests. It \vas reco g n ized that, d u ring the S to n n N o . 7 in 2005 lan d in g to R R D w ith level o f 12 all sea dykes vvith m a n g ro v e íorests p ro te c te d a re n o t b roken, m e an w h ile so lid ified co n cre te d y k e in H a i H au facing direct w ave is c o m p le tđ y d e s tr o y e d

A cco rd in g P h an N g u y en H o ng , a rese a rc h e r fro m H anoi T ea ch ers T ra in in g C o lleg e, the w av e en ergy cau sed by s to rm N o. 7 g o in g th ro u g h m ang rov e areas in B a n g L a, D o S o n (H ai P h on g) đ ecre ase d fro m 85 to 8 7 % a n d th a t though Cork íorest at V in h Q u a n g (H a i P hon g) has red u ced from 77 to 83%.

A strateg y for the d c v c lo p m c n t o f fo rest an d in an g ro v c ío rests in Coastal areas is a w ise, im po rtan t and su stain a b le o n e to m in im iz e the effects o f SLR.

4. Z o n in g o f risk cau scd b y h aza rđ s is ra th e r c o ư e sp o n d e n t w ith th e z o n in g o f so cial v u ln erab ility ía cto rs. T h e u se o f V A as a íĩrst step tovvards Integ rated C o a s ta l Z o n e M anagem ent (IC Z M ) is n ccc ssary , v a lu a b lc an d im p o rtan t p a rt in the b u ild in g o f S tra te g y an d A ction P lan for su stain a b le d e v e lo p m e n t in this se n sitív e Coastal zone.

A c k n o w le d g e m e n ts

T h e au th o r vvould lik e to a c k n o w le d g e th e íin an ce su p p o rt giv en to him b y V N U P rojects co ded Q T .0 8 .4 7 and Q G T Đ .0 7 .0 6 a n d to the D epartm ent o f G co lo g y , H an o i U n iv e rs ity o f S c ie n c e , V N U , w h e re th is re s e a r c h w a s c a rrie d o u t.

R e ĩe re n c e s

[1] V.T. Ca, Characteristics o f Flow and Saliniíy Intrusion in the Red River Delta.. M. Thesis.

AIT. Bangkok, Thailand, 1989.

[2] S.L. Cuttcr, et al., Revealing the Vulnerability o f People and Places: A Case Study o f Georgetown County, South Carolina, Atỉas o f the Association o f American Geographers 90, 4 (2000) 713 - 737.

[3] General Department o f Land Administratioìỉy Vietnam National Atlas, Hanoi, 1996, 163 pp.

[4] S.J. Malhers, J.A. 2^alasevicz, ỉiolocene Sedimentary Architecturc o f Ihc Red River Delta.

Vietnam Jour. O f Coastal Research 15 (1999) 314-325.

[5] N.M. Hung, N .v. Cu, L X . Hoan, Wave in Coastal Zone in Vietnam, National Marine Project, KHCN 10.06, Institute o f Mechanics, Hanoi, 2001,1-257. (in Vietnamese).

[6] T. Nghi, et al., Hoỉocene Sedimentary Evolution and Geodynamic Properties o f the Balat Area, Proccedings o f the ScientiGc Conf. on Red River Delta Research Program, ỉỉanoi, 2000,46-56.

[7] c . v . Ngoi, D .v. Luyen, Geostructural Characteristics and F;looding Hazards in Basin of Day River, Journal o f Science 18, 3 (2002).

[8] NOAA, Community Vulnerability Assessment Tool CD ROM. NOAA Coastal Services Center, 1999.

[9] M.T. Nhuan, et al., Report o f the Research on M aking Geochernicaỉ Environmental Map in the Shalỉow Sea Region o f Ngason- Haiphong (0-30 m deep), Scale 1: 50.000, (in VieUìamese), In Archive o f the Gcology and Marine Resources Center, Hanoi, 1996.

[10] D .v. Luyen, M.T. Nhuan, c .v . Ngoi, Vulnerabiỉity Assessment o f Socio-Naturaỉ systetns in Coastal Zone o f the Red River Delta, Procc. O f the Intcr. Workshops “Hanoi Geoengineering 2003 & 2004” 22 Dccember 2003 & 15 Octobcr 2004; Hanoi-Vielnam, 2004.

[11] T.D. Thanh, D .v . Huy, Coastal Development of the RRD in lỉolocenc, your. Resources and Environment, Hanoi, 2000.

[12] VCZVA, Vietnam Coastal Zone Vuỉnerability Assessment, Final Project Reporl, 1996.

[13] Website http://w w w . MONRE. pov.vn (3 April 2009).