CAU HAI Tran Anh Tu, Vu Duy VTnh, Ng

MO PHONG THUY DONG hlJC VA MOT SO YEU TO DANH G U CHAT LLT'ONG N i r d c HE DAM PHA TAM GIANG - CAU HAI

Tran Anh Tu, Vu Duy VTnh, Nguyen HQn Cic

I. M O DAU

Trong cac darn pha d mien Trung Viet Nam, he dim pha Tam Giang - Ciu Hai (TG - CH) dugc biet den nhu la mdt tiem nang to ldn ddi vdi sy phat triin kinh tl - xa hdi viing nhimg hien nay dang phai ddi mat vdi su suy giam chit lugng mdi tmdng va cac gia tri tai nguyen thien nhien [1]. He thdng dim pha TG - CH cd toa do dia ly 16*^14'- 16°42' Vl do bic; 107*^22'-107°57' kinh do ddng, vdi chilu dai 70km va chilu rdng tir 1 den 10km; dien tich mat nude rdng 216 km^, va la dim pha rdng nhit Viet Nam ciing nhu khu vyc Ddng Nam A. Nham lam rd them biic tranh chit lugng nude cua he dim pha, cdng trinh nay da tien hanh nghien cim mdt sd ylu td (trim tich lo limg, tran diu) danh gia chat lugng nude. Dd ciing la mot trong nhiing ly do chinh ciia dy an hep tac song phuong 12EE6 giira Viet Nam va Italia trong giai doan 2007 - 2009 'Ddnh gid chdt luffng moi trudng, lich su vd xu the cda mgt so thuy vuc quan trgng ldm ca sd qudn ly: cdc ddm phd ven bd miin Trung Vigt Nam vd mgt so hd co liin quan". Cac tac gia chan thanh cam on Ban chu nhiem dy an 12EE6 da tao mpi dieu kien thuan lgi cho nghien cuu nay.

II. TAI LIEU VA PHU^ONG PHAP

• Cac tai lieu sd lieu quan trac nen khi tugng, thuy van tai tram Khi tugng - Thuy van Hue [3]; sd lieu dieu tra khao sat thuy van va chat lugng nude trong nam 2004 - 2007 cua Dy an 14EE5; 12EE6 dugc sir dyng lam sd lieu dau vao va hieu chinh md hinh.

Cac ban dd dia hinh, hai dd va cac tai lieu do dac dia hinh dugc sir dung de xac dinh dudng bd va tao ludi tinh cho md hinh.

• Sir dung phuang phap md hinh toan: de md phdng thiiy ddng lyc va mdt sd cac yeu td danh gia chat lugng nude, nghien cim nay da sir dung md hinh delft-3d (Vien Thuy lyc Ha Lan). Co sd toan hpc ciia md hinh ciing nhu cac phuang trinh co ban dugc md ta chi tiet trong tai lieu hudng dan sir dung md hinh [7]. Dieu kien bien dugc xac dinh tai bien md, vdi mdi tmdng hgp cu the cd the ap dung cac dieu kien bien khac nhau: myc nude; ddng chay; luu lugng. Cac bien ldng phia bien la: cua Thuan An va cira Tu Hien; ngoai ra cdn cd cac bien ldng sdng; sdng O Lau, sdng Huong va sdng Tmdi (hinh 1). Dac tnmg che dp thuy ddng lyc va chat lugng nude he dim pha TG-CH dugc md phdng trong hai miia, dai dien miia khd bat dau tinh toan tir OhOO, ngay 01/5/2004 va kit thuc liic 23h00, ngay 31/5/2004. Dai dien miia mua bit diu tinh toan tir OhOO, ngay 01/9/2005 va kit thiic liic 23h00, ngay 30/9/2005. Budc thdi gian de chay md hinh thuy ddng luc la 03 phiit. Nham giam thilu sy sai khac trong tinh toan cua md hinh den miic chap nhan dugc [5, 6], ket qua chay chuong trinh dugc so sanh vdi sd lieu quan trac de danh gia sy sai sd (hinh 2) hay sir dung sai so binh phuong trung binh (E) la mdt chi tieu quan trpng de danh gia dp chinh xac ciia md hinh:

trong dd: i = 1, n la sd lan quan trac dugc thyc hien Mm: la gia tri tinh toan tir md hinh tai thdi diem i

Tran Anh Tu, VQ Duy VToti, Nguyen Hu'u Cif

Ms: la gia tri quan frac tai thdi diem i

Gia tri E nhan dugc sau nhieu lin hieu chinh cac tham sd tinh toan bing 0,126 cho thiy md hinh dn dinh nhat. Day la mdt ket qua kha tdt doi vdi dim pha ndng va hinh dang keo dai phiic tap [2, 4].

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Hinh 1. Sa do khu vuc nghien cuu

264

Tuyen tap TSi nguy§n v& M6I trudng bien. Tap XIV

Hinh 2. So sanh van toe d5ng chay (9/2005) giira tinh toan va quan trac III. KET QUA VA T H A O LUAN

3.1. Mo phong thuy d9ng lure

• Dong chdy: trong he dam pha TG - CH vao miia khd cd gia tri van tdc bien ddi trong khoang 0 den 40cm/s. Khu vyc dam Cau Hai tdc dp ddng chay dat trong khoang 0 - 6cm/s, la nhd nhat so vdi toan bd he thdng dam pha TG-CH. Trong miia khd ven bd dam Cau Hai gia tri van tdc ddng chay xap xi bang khdng. C) cira Tu Hien, do bi thu hep va ndng nen van tdc ddng chay chi dang ke trong pha trieu nit. O khu vyc dam Thuy Tii tdc dp ddng chay tnmg binh dat trong khoang 9 - 28cm/s, cyc dai dat 53cm/s, thdi gian nude ldn va nude rdng cham khoang 60 phut so vdi cira Thuan An va 20 phiit so vdi cira Tu Hien. Van tdc ddng chay khu vuc cira Thuan An la ldn nhat so vdi cac khu vyc khac trong he dam pha TG - CH, van tdc ldn nhat dat 67 cm/s trong pha trieu xudng va 45cm/s trong pha trieu len tai cira Thuan An. Van tdc ddng chay tai khu vyc Tam Giang dat trong khoang 3 - 1 0 cm/s, thdi gian nude ldn va nude rdng cham khoang 40 phiit so vdi cira Thuan An.

Vao miia mua van toe ddng chay trong he dam pha TG - CH cd gia tri trong khoang 0 - 102cm/s. Khu vuc dam Cau Hai la nhd nhat so vdi toan bd hp thdng dam pha TG - CH, tdc do ddng chay chi dat 0 - 15cm/s. 0 cira Tu Hien van tdc ddng chay ca khi trieu len va xuong diu ylu. 6 khu vyc dam Thuy Tii tdc dp ddng chay cyc dai dat 18cm/s, thdi gian nude dang va nude nit deu cham hon so vdi cac khu vyc cira Thuan An va cira Tu Hiln. Van tdc ddng chay khu vyc cira Thuan An la ldn nhat so vdi cac khu vyc khac trong he dim pha TG - CH. Van tdc ddng chay tai khu vyc Tam Giang bien ddi trong khoang 5 - 15cm/s, thdi gian nude dang va nude nit cham khoang 40 phut so vdi cira Thuan An. Trong pha trilu len, ddng chay tir biin chay vao he dam pha qua hai cira Thuan An va cira Tu Hiln, giiia dim Ciu Hai cd sy xao trdn manh. Tai dam Thuy Tii

Tran Anh Tu, Vu Duy Vinh, Nguyen Hihj CCf

trong pha trieu len ddng chay cd hudng ddng nam va pha trilu xuong cd hudng tay bae.

Khi trilu xudng, xuat hien cac ddng nit kha manh tai cira Thuan An, tdc dp ddng chay cyc dai dat tdi 102 cm/s va Thuy Tti 87 cm/s.

• Muc nude: vl miia khd myc nude trong dam pha Tam Giang - Ciu Hai bien ddng theo thdi gian va khdng gian do chiu sy chi phdi ciia dao ddng myc nude bien. Myc nude trong he dim pha frong miia khd dao ddng khoang 00-3 5 cm, tai cac cira sdng va cac khu vyc khac ciing khdng cd sy chenh lech dang kl, do trong miia khd luu lugng tai cac ciia sdng dd vao dam pha it. Ve miia khd, myc nude cac dam pha thudng thip hon dinh trieu ngoai bien, d Cau Hai la 25 - 30cm va d Tam Giang la 5 - 15cm. Dao ddng muc nude bien mang tinh ban nhat trieu d Tam Giang, ban nhat khdng diu d Thuy Tti va ban nhat khdng deu d Cau Hai - Tu Hien.

v l miia mua Iii, myc nude trong he dam pha dao ddng trong khoang 0 - 45cm, tai cac cira sdng va cac khu vuc khac cd sy chenh lech dang ke (tir 10 den 30 cm), do frong miia mua luu lugng tai cac cira sdng dd vao dam pha kha manh (luu lugng sdng Huong tmng binh 179 m^/s). Myc nude dam pha thudng ludn cao hon myc nude bien khoang 10 cm, dao ddng myc nude dam pha ciing bien ddi khdng tuong ddng vdi myc nude ngoai bien.

• Trao doi nude giica ddm phd vd bien:

Bang 1. Dac trung su trao dot nuo'c giira dam pha v^ bien

Miia

Dac trung Ciira Thdi gian chay (%)

Luu lugng chay ldn nhat (m^/s)

Luu lugng chay nhd nhat (m^/s) Can bing (%)

Chay vao Chay ra Chay vao Chay ra Chay vao Chay ra

Miia kho (5/2004) Thuan An

56,3 43,7 975,7 868,1 2,1 3,5 -^12,6

TuHien 62,5 37,5 15,2 10,4 0,0 0,0 +25,0

Mua mira Thuan An

17,2 82,8 1877,1 2215,7 17,3

•28,6 -65,6

(9/2005) TuHien

27,3 72,7

• 67,8 43,5

5,7 13,1 -45,4

Ghi chii: dau "-" chay ra; dau "+" chay vao.

Theo bang fren luu lugng ldn nhit tai cira Thuan An dat 975m Is va cira Tu Hien dat 15m^/s. v l miia khd, do luu lugng cac sdng rat nhd, lugng chay tir bien vao dam pha ldn. v l mua mua, do luu lugng cac sdng dd vao dam pha chiem uu the ve thdi gian chay ra phia biin, luu lugng ldn nhit tai cira Thuan An theo tinh toan ciia md hinh dat 1877m^/s va cira Tu Hiln dat 67mVs. Thdi gian chay ra t^ cira Thuan An gip 4,8 lin thdi gian chay vao va thdi gian chay ra tai cira Tu Hien gap 2,7 lan thdi gian chay vao frong miia mua. Theo tinh toan bing md hinh DUFFLOW narn 2000 [2] tai cira Thuan An gia tri cyc dai cua luu lugng dat 2400mVs va tai cira Tu Hien gia tri cyc dai ciia luu lugng dat 718mVs.

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Tuyen tap Tai nguyen va Moi trUcing bien. Tap XIV

3.2. Mo phong mot so yeu to danh gia chat luong nude

• Mo phong trdm tich la lung: ket qua tinh toan ddi vdi he dam pha TG - CH cho thay ham lugng tram tich lo limg (TTLL) trong he dam pha bien ddi phuc tap va thay ddi theo miia. Su bien ddng cua TTLL trong he dam pha phu thupc hoan toan vao lugng TTLL ciia cac sdng dua vao. Vao miia khd, TTLL dao ddng trong khoang 15 - 45mg/l, cac cira sdng Huong va O Lau cd gia tri ldn hon nhieu so vdi khu vyc dam Thuy Tii. O khu vyc dam Cau Hai, TTLL it bien ddi, do luu lugng ciia sdng dd vao dam nlid, gia tri TTLL trung binh dat 20mg/l. Ve miia mua, TTLL cac sdng dd vao dam pha kha ldn, cao hon nhieu so vdi miia khd, xung quanh khu vyc cira sdng Huong va Thuan An cd sy bien dgng manh, tai cira sdng Huong gia tri ciia TTLL ldn hon miia khd 3 lan (gia tri cyc dai dat 135mg/l). Khu vyc dam Cau Hai TTLL cd gia tri trung binh dat 57mg/l, gap 2,8 lan so vdi mua khd.

• Mo phong trdn ddu: vi tri gia dinh ngudn dd thai dau gan phia cira Thuan An va thdi diem tinh toan tran dau khi pha trieu len trong ca miia khd va miia mua, lugng dau do thai 50 tin.

- Vao miia khd, khi xay ra sy cd tran dau, vet dau ndi (FO) tren mat nude nhanh chdng loang rdng ra ximg quanh vi tri gia dinh ngudn do thai. Sau 3 gid, vet dau dugc day len phia tay bae ciia he dam pha, nhung sau 7 gid vet dau cd xu hudng dich chuyen theo hudng ddng nam ciia he dam pha. Trong miia khd, che do thuy ddng lyc yeu nen vet diu ciing khdng thi dich chuyen len phia cira sdng 6 Lau va xudng khu dam Cau Hai. Sau 48 gid mat dp cua vet dau ndi giam manh va pham vi khdng gian bat dau thu hep din. Phai din 10 ngay sau khi say ra sy cd, lugng dau trdi ndi tren be mat nude trong he dim pha mdi phan buy gan het. Lugng dau bam (SO) ciing cd hudng phat triin tir dilm xay ra sy cd ve phia tay bae ciia he dam pha nhung tdc dp cham i hon so vdi vet diu ndi; pham vi khdng gian cua noi cd dau bam tang cham theo thdi i gian va md rdng vl phia ddng nam cua he dam pha. Sau 13 ngay, lugng dau bam i chim xudng day dim pha va bam vao ven bd dam each vi tri ngudn dd thai tir 5 -

;- 7km vl phia tay bic va ddng nam cua he thdng dam pha. Vet dau nay it bien ddi theo I khdng gian va thdi gian, ngay cudi cimg ciia md hinh tinh (30/5/2004) van cdn thay : - vet dau nay tdn tai.

i- • , , _

- Trong miia mua, che do thuy ddng lyc trong he dam pha dien ra manh hon so vai miia khd, cho nen vet diu ndi tren mat nude nhanh chdng loang rdng ra xung quanh

\ vi tri gia dinh ngudn do thai. Sau I gid, vet diu dugc diy len phia tay bae cua he dam 1 pha, nhung chi sau 5 gid vet diu cd xu hudng dich chuyen theo hudng ddng nam ciia

• he dim pha. Trong miia mua, tuy cd ehl dp ddng lyc manh hem miia khd nhung vet f diu cung khdng thi anh hudng len phia cira sdng O Lau va xudng dam Cau Hai. Sau I 32 gid vet diu ndi giam va pham vi khdng gian bat dau thu hep. Phai den 8 ngay sau khi xay ra sy cd, lugng diu trdi ndi tren bl mat nude trong he dam pha mdi bi phan buy gin hit. Lugng diu bam ciing cd hudng phat triin tir dilm xay ra sy cd ve phia ddng nam cua he dim pha; pham vi khdng gian ciia noi cd dau bam tang cham theo thdi gian va md rdng vl phia tay bic ciia he dim pha. Sau 10 ngay, lugng dau bam chim xuong day dim pha va bam vao ven bd dim each vi tri dd thai khoang 5km ve phia tay bic va ddng nam ciia he thdng dim pha. Vet diu nay it bien ddi theo khdng phia tay

Tran Anh Tii, Vu Duy Vinh, Nguyen HCfu CCf

gian va thdi gian, ngay cudi cimg cua md hinh tinh (30/9/2005) van cdn thay vet dau nay tdn tai. Do anh hudng ciia ddng chay manh trong miia mua, nen lugng dau bam it hon so vdi miia khd.

IV. KET LUAN

Ddng chay khu vyc he dam pha TG - CH frong miia khd chiu sy tac dgng chinh cua thuy trilu qua 2 cira Thuan An va Tu Hien, thdi gian chay vao chilm uu the tuyet ddi.

Trong mua mua, ddng chay ra chiem uu the tuyet ddi do bdi sy tac ddng ciia cac sdng miia Iii. Do dia hinh phirc tap vdi sy keo dai ciia thuy vuc nude nen toe do ddng chay rat khac nhau tren khdng gian mat nude cua thuy "vyc, tai mdt sd cira, lach hep van tdc ddng chay kha ldn khi trilu nit. Hudng ciia ddng chay hau nhu theo true chinh cua he dam pha. Trong pha trilu len vao miia mua thudng xuat hien cac xoay nhd d khu vyc dam c i u Hai. Myc nude trong miia khd chenh lech khdng dang ke so vdi myc nude bien nhung trong miia mua thi ngugc lai. Biic franh md phdng fram tich lo limg trong he dam pha cho thiy cd su biin ddi phtic tap va thay ddi theo mua. Vao miia khd, TTLL dao ddng trong khoang tir 15 - 45mg/l, khu vyc cira sdng cd gia tri ldn so vdi cac khu vyc khac. Miia mua, gia tri TTLL cao hon va bien ddng manh so vdi miia khd. Cac khu vuc cd ddng lyc ylu thi TTLL it bien ddi trong ca hai miia. Phan tich cac ket qua tinh toan cho thiy vdi cimg mdt lugng dau (gia dinh 50-tin) do thai tai cimg mdt vi tri: trong miia khd va miia mua dau bam va dau ndi khdng anh hudng len khu vyc cira sdng O Lau va xudng dim Ciu Hai ma chi anh hudng trong khoang each 5 - 7km so vdi ngudn dd thai theo hudng tay bae va ddng nam. Vet dau ndi di chuyen va phan huy frong miia mua nhanh hon trong miia khd. Vet dau bam tdn tai kha lau so vdi vet dau ndi trong ca miia khd va miia mua.

TAI LIEU THAM KHAO

1. Nguyen Hiru Cir, 1999. Tong quan tinh hinh nghien cicu tdi nguyen vd moi trudng dam phd ven bd mien Trung Viet Nam. Tuyen tap Tai nguyen va mdi trudng bien, tap VI, trang 126-142. Nha xuat ban Khoa hgc va Ky thuat, Ha Ndi.

2. Nghiem Tien Lam, 2002. A Preliminary Study on Hydrodynamics of the Tam Giang - Cau Hai Lagoon and Tidal Inlet System in Thua Thien - Hue Province, Vietnam.

Master Thesis H.E. 105. 2002.

3. Sff Khoa h9c va Cong nghf Thira Thien Hue, 2004. Dgc diem Khi hau- Thuy vdn tinh Thica Thien Hue. Nha xuat ban Thuan Hoa.

4. Tran Anh Tu, Nguyen Hfru Cir, Georg Umgiesser, Mauro Frignani, 2006. Ap dung mo hinh SHYFEM de mo phong qud trinh thuy dong luc dam phd Tam Giang - Cdu Hai. Tap chi KH va CN Bien -Vien Khoa hpc va Cdng nghe Viet Nam, Phu tmong 1 (T.6), 2006; Ha Ndi, trang 69 - 78.

5. Georg Umgiesser, Tran Anh Tu, Francesca De Pascalis, Do Trong Binh, Nguyen Huu Cu, Mauro Frignani, 2007. Hydrodynamic modeling of the Tam Giang - Cau Hai lagoon. Vietnam. Tap chi KH va CN Bien -Vien Khoa hpc va Cdng nghe Viet Nam, Phu tmong 1 (T.7), 2007; Ha Ndi, trang 2 9 - 4 3 .

6. Georg Umgiesser, Donata Melaku Canu, Andrea Cucco, Cosimo Solodore, 2004.

268

Tuyen tap TSi nguyen va Moi tncdng bien. Tap XIV

A Finite Element Model for the Venice Lagoon Development. Set up. Calibration and Validation. Joumal of Manne Systems 51, 123-145.

7. WL I Delft Hydraulics (1999). Delft3D-FL0W User Manual Version 3.05, DelftSD- Waq User Manual Version 3.01. Delft3D-Part User Manual Version LO WL {Delft Hydraulics. Delft, Netherlands.

Summary

THE SIMULATION OF HYDRODYNAMIC AND WATER QUALITY ASSESSMENT BASE ON POLLUTED PARAMETERS IN THE TAM GIANG -

CAU HAI COASTAL LAGOON Tran Anh Tu, Vu Duy Vinh, Nguyen Huu Cu

Typical of the 12 coastal lagoons in the Centre of Vetnam, the Tam Giang-Cau Hai lagoon has been known so far to us as having great potentials for the development of regional socio-economy but being faced with problems of decreasing environment quality and natural resources values. To make clearly the picture of water quality in the Tam Giang-Cau Hai lagoon, this paper has assessed some parameters such as suspended sediment and oil spill. It is one of the main reasons for which the Vietnamese - Italian bilateral cooperative project I2EE6 for the years 2007-2009.

The simulation result has showed: hydrodynamic regime in Tam Giang - Cau Hai lagoon in dry season is influenced mainly by tidal regime through Thuan An and Tu Hien inlets, the inflow is dominated. In rainy season, it is influenced by current regime from rivers, the outflow is dominated. Suspended sediment in lagoon system change complex with season. In dry season, it is of 15 - 45mg/l, the it's value at Huong and O Lau river mouths were much higher than Thuy Tu area. In rainy season, the value of suspended sediment is much higher than that in dry season. There is big change of suspended sediment around Huong and Thuan An area. At Huong river mouth, suspended sediment in rainy season is higher 3 times comparing to dry season.

Calculated results analyzing of the discharge of the same amount of oil (supposed 50 tons) at the same location shows that in dry and rainy seasons, the cling oil and float oil do not affect to O Lau river and Cau Hai rammer area, they only affect in the distance of 5-7km of discharge source in north-west and north-east directions. The slick oil moved and decomposed in water in rainy season faster than in dry season. The cling oil exists in water longer than float oil in both dry and rainy season.