ĐẶC ĐIỂM PHÂN BỐ CÁC DẠNG TRƯNG SÓNG TẠI BIỂN VEN BỜ TỈNH BÌNH THUẬN

S6 4; Tr 25 - 37 DAC DIEM PHAN BO CAC DAC TRimC SONG TAI VUNG BIEN VEN BO TiNH BINH THUAN LE DINH MAU Tom tat: Bdi bdo trinh bdy kit qud tinh todn truang do cao song hihi hieu Hs tgi

Tap chi Khoa hoc va Cong nghe biin T9 (2009) S6 4; Tr 25 - 37

DAC DIEM PHAN BO CAC DAC TRimC SONG TAI VUNG BIEN VEN BO

TiNH BINH THUAN

LE DINH MAU

Tom tat: Bdi bdo trinh bdy kit qud tinh todn truang do cao song hihi hieu (Hs) tgi ddi

ven bien tinh Binh Thudn img voi cdc truang gio miia dien hinh vd cue dgi Cdc dgc trung gio duac thong ke tic chuoi so lieu gio do dgc tgi trgm ddo Phu Quy (1987-2007) Cdc tham so song viing khai duac tinh bdng mo hinh - Dolphin Cdc tham so song viing nuac nong ven ba duac tinh bdng mo hinh SWAN Kit qud nghiin cicu cho thdy ddi ven bo tinh Binh Thudn hi tdc dong mgnh ciia song trong cd hai miia gio Dong Bdc vd Tdy Nam vai do cao song trung binh tgi khu vice ven bd Id Hs ~ 1 - 2 m Trong thai ky 1987 - 2007, khu vuc nghien cicu co gio mgnh cue dgi vai toe do V = 28 m/s, hudng NE, chiing gdy ra song co do cao cue dgi ngoai khai Id Hs ~ 5 - 7 m; vd gio V = 36 m/s, hudng SW, chiing gdy ra song co do cao cue dgi ngoai khai Id Hs~ 6 ~ 8 m Cdc miii ddt nho ra biin, cdc ddo ven ba, phdn bo do sau vimg ven

ba CO vai tro rdt quan trong trong viic phdn bo ndng luang song tgi khu vuc nghien cicu Kit qud tinh toan cdc trudng song trin phuc vu viic thiit ki, xdy dung cdc thiiy cong trinh cimg nhu cdc hoat dong kinh ti, du lich vd bdo vi moi truang viing ven bd khu vuc nghiin cicu

Bill bdo la tam ldng ghi ffn ciia tdc gid girl tdi PGS TS Vo Van Lanh (nguyen Vien Tnrdng Vien Hdi dwffng hoc) Ngirdi dd ddnh trgn cuoc ddi cong hien cho sir nghiep phdt trim nganh hdi dwffng hoc Viet Nam; cdng nhir trong viec ddo tao, boi dw&ng the he ciic nhd khoa hpc hdi dwffng tre trong do co tdc gid

- Nhirng thugt ngic chinh diing trong bdi: cdc ddc trung song, do cao song hicu hiiu (Hs), gio miia Dong Bdc (NE), gio miia Tdy Nam (SW), ddi ven ba biin, mui ddt

LMODAU

Tinh toan cac dac trung song tai viing bien ven bd da dugc tien hanh nghien cim tai nhieu vien thiiy luc tren The gidi nhu Trung tam Thiiy lire Ha Lan (Delft Hydraulics) vdi

bg chuang trinh SWAN (Booij and Holthuijsen, 1999); Vien Thiiy luc Dan Mach vdi bg chuang trinh MIKE 21, MIKE 3; Trung tam C6ng nghe ven bd cua quan dpi My vdi bg chuang trinh RCPWAVE (Ebersole, Cialone, and Prater, 1986) Tinh tdan cac dac trung sdng vimg khoi hien nay dugc ap dung rong rai nhit la m6 hinh WAM (WAMDI Group 1988); Dolphin (Mandal and Holthuijsen, 1985); STWAVE (Tolman, 1991) Ngoai ra cdn

CO mgt sd md hinh tinh toan, du bao sdng trong eac dilu kien khac nhau dugc xay dung d

cac trudng dai hgc va vien nghien eiiu khac nhau vdi nhimg thi manh va nhiing han che rieng

Tai Viet Nam trong nhiing nam gan day da c6 nhilu chuang trinh cip Quoc gia va

du an hgp tac Qudc te tien hanh nghien ciiu cac qlia trinh thiiy-thach-dgng luc vimg ven

bd, trong do eo tinh toan cac dac trung song nhu dl tai KT.03.14, KHCN.06.08 (1996-2000), KC.09.05 (2001-2005) Du an Quoc tl "EU-Cim Long Projecf (1996-1998): nghien cim, du bao xdi Id tai bd bien Ganh Hao; du an Viet Nam-Thuy Dien (2004-2007): nghien cim xoi Id bd biln Hai Hau, Nam Dinh Tai dai ven bien Nam Trung bg tinh toan cac dac trung sdng da dugc tiln hanh bdi cac tac gia nhu: Biii Hdng Long, Le Dinh Mau (2003); Le Dinh Miu (2005a, 2005b, 2005c, 2006); Le Dinh Mau, et al (2007), nhung cong trinh tren da tiln hanh tinh song tai viing khoi va viing ven bd trong cac dieu kien tao song khac nhau Tuy nhien, tai viing bien ven bd tinh Binh Thuan hau nhu viee nghien ciiu, tinh toan song chua dugc quan tam diing mite Cac tac gia Biii Hong Long, Le Dinh Miu (2000) mdi tiln hanh tinh toan trudng sdng khiic xa trong cac trudng gio dien hinh tai vinh Phan Thiet

Kinh dQ

11.2

(ihiclii'i:

- : dUcVng dan); do

sfiudn)

11.0

lO.i

107.8 108.0 108.2 108.4 108.6 IC

(Theo ban do l^ 1^: 1/100.000 ciia Hai quan Nhan dan Vi^l Nam xuai bdn nam 1994)

Hinh 1: Dac diem khu vue nghien ciiu Trong bai bao nay, cac dac trung sdng tai khu vuc ven bd img vdi cac dilu kien tao song khac nhau da dugc tinh toan theo qui trinh nhu sau:

- Xae dinh cac trudng gi6 mua dien hinh va toe do gid cue dai tit chu6i so lieu gio

do dae tai tram dao Phii Quy (1987 - 2007)

- Xae dinh cac dac trung sdng ngoai khoi viing bien Binh Thuan img vdi cac trudng

gid khac nhau bang md hinh tfnh song bien sau - Dolphin

- Cac dac trung s6ng tai viing bien ven bd dugc tinh bang md hinh SWAN vdi dieu

kien bien la sdng ngoai khoi, gid dia phuong va phan bo do sau cua viing nghien cim

- Cac dac trung song tai vimg bien ven bd dugc tinh t6an trong dieu kien khong c6

su dang/riit muc nudc tao bdi cac trudng gio

Pham vi tinh toan ciia md hinh SWAN va dac diem khu vuc nghien cim dugc the

hien tren hinh 1

II NGUON SO LIEU VA PHlTONG PHAP TINH TOAN

1 Nguon so lieu

- So lieu gid dugc thu thap tit tram dao Phii Quy (1987 - 2007) tren 16 hudng

- Do sau tai khu vue nghien cim duge lay tir hai dd ty le: 1/100.000 xuat ban nam

1994 ciia Hai quan Nhan dan Viet Nam

- So lieu ve cac dac tnmg song do dac dugc lay tit dgt khao sat 12/2008 eua Vien

Hai duong hgc tai vinh Phan Thiet

Nhirng so lieu nay dung de phuc vu tinh toan va kiem chiing mo hinh

2 Phuong phap tinh toan

a Tinh todn cdc dac trung song bien sau

Mo hinh sd tri tinh song bien sau Dolphin (Mandal and Holthuijsen, 1985) dugc xay

dung tai Vien Thiiy lire Ha Lan cho phep tinh toan ket hgp song gid va song limg

Phuong trinh can bang nang lugng pho song hai chieu cho tradng hgp song gio tren hudng

eia:

^ ^ = S,i0) + ]s,if,e)df (al)

, * 0

6 day,

Ei(G) = mat do nang lugng phd ciia song gid tren hudng 9 (m^/rad.), Si(9) = tic do

bien doi cua Ei(9), neu Ei(9) nhd hon gidi han qui udc no dai dien cho trudng hgp s6ng

dang phat trien, ngugc lai dai dien cho trudng hgp s6ng limg (m^/rad/s) S2(f, 9) = tic do

chuyen giao cua nang lugng sdng limg 2 chiiu E2(f, 9) sang song gio cd cimg tan s6

(m^/rad/Hz/s); 9 = hudng s6ng tdi (rad); f = tan s l song (Hz)

Phuang trinh can bang nang lugng pho s6ng hai chiiu trong trudng hgp song limg tit

hudng 9 dugc trinh bay bang phuang trinh sau:

dF

^ = -Bs,,{f,e)-s,{f,e)-s,{f,e) (a2)

dt

6 day:

- E2(f, 9) = mat do nang lugng phd hai chieu cua thanh phan song limg vdi tan so (f),

hudng truyin (9) (mVrad/Hz/s); B = 1 nlu Si <0; B = 0 nlu Si >0; Su = tic do biln dii

ciia E2 tuong ling vdi Si khi Si co gia tri am (phan bd tan sd cua Si cd dau ngugc lai); 82

= tic do chuyin giao ciia nang lugng song limg E2 sang s6ng gid co cimg tan so, (gia thiet

gia tri E2 > 0) (m^/rad/Hz/s); S3 = toe do bien ddi cua nang lugng song limg gay bdi hieu

ling nudc nong (dimg cho trudng hgp nudc nong);

- So lieu dau vao cua md hinh la: vi tri diem tinh, trudng gio (toe do, hudng), thdi

gian tac dgng cua gid

Sd lieu dau ra ciia mo hinh la: do cao sdng hiiu hieu (Hs), chu ky ciia dinh pho song

(Tp), do phan tan ciia pho nang lugng song (coefficient of spectral directional spreading

-DSPR) va hudng sdng (9)

b Tinh todn cdc dac trung song viing nu&c ndng ven ba

Cac dac trung sdng tai viing bien ven bd dugc xae dinh bang mo hinh so tri tinh

song SWAN (Booij et al., 1999) SWAN dugc xay dung tren co sd phuang trinh can bang

hoat dgng pho (spectral action balance equation) nhu sau:

-N + ~C,N + —CN + -^C^N + —C,N^- (bl)

dt dx -' dy ' da ^ de ' a

d d a y ,

N = mat do ph6 hoat dgng (action density); t = thdi gian

Cx, Cy = toe do lan truyen ciia N theo tga do khdng gian x va y

Cc, Ce = t i c do biln ddi ciia N theo tan s6 tuong doi CT va hudng 9

S = S(a, 9) la ham nguIn (mat do nang lugng) dai dien cho qua trinh tao song

(generation), phan huy (dissipation) va tuong tac phi tuyen giira sdng - sdng (nonlinear

wave-wave interactions) va sdng-dong chay

Md hinh SWAN tinh hieu ling nudc nong, khiic xa, tao sdng do gi6 dia phuong, song bac diu, tuong tac song-song, ma sat day, s6ng do nhao, Pham vi ap dung mo hinh

la toan bg vimg biln ven bd tinh Binh Thuan tir 107.6°E din 109.0°E va tir 10.6°N den 11.3°N vdi kich thudc 6 ludi tinh la 1000 m x 1000 m (hinh 1)

S6 lieu dau vao ciia mo hinh la:

- Cac tham so song ngoai khoi: Hs, Tp, DSPR va 9 chiing dugc lay tit ket qua cua mo hinh tinh song Dolphin

- Trudng gio dia phuang (t6c do, hudng)

- Phan bo do sau cua vimg nghien cim

So lieu dau ra ciia mo hinh la: do cao s6ng hiiu hieu - Hs, hudng sdng trung binh - 9, chu ky cua dinh ph6 song - Tp, do dai sdng tmng binh - L

1 Dieu kien tu nhien

fl Ddc diem dieu kien tir nhien lien quan den sir phdn bo cdc dac trung song viing ven

b&

Dudng bd bien tinh Binh Thuan (Ca Na - miii O Cap) chay theo hudng Dong Bac (NE) - Tay Nam (SW), hinh thai va cau tao tuang doi don gian va ddng nhat Cac miii dat nho ra bien dien hinh la Ca Na, La Gan, MUi Ne va Ke Ga Dao ven bd co tac dgng manh len sir truyen song la Cu Lao Cau Cac vinh d day deu tuong doi can va hd nhu Phan Rang, Ca Na, Phan Ri, Phan Thiet Them luc dia d day tuong doi trai rong dudng dang sau

10 m gan nhu chay ngdai cac cira vinh, dudng 30 m chay song song va each dudng bd ~

20 km d phia Bac va ~ 50 - 60 km d phfa Nam (hinh 1) Do vay, nang lugng s6ng da bi suy giam nhieu khi tien vao khu vuc nay Do dudng bd chay theo hudng NE-SW nen nhirng khu vuc che chan sdng la cac miii dat nhd ra bien, tiiy theo miia gio ma cac miii dat

se che chan song cho cac vinh ven bd Ca sdng NE va SW deu tac dgng manh den khu vuc nay Rieng song trong bao se bi suy giam dang ke khi lan truyen vao khu vuc nay

Dai ven bien tinh Binli Thuan nam giiia 2 viing cd che do trieu khac nhau: Quy Nhon (nhat trilu khdng diu) va Viing Tau (ban nhat trilu khdng diu) Do tilp giap vdi viing biln Ba Ria - Viing Tau nen dai ven bd nghien ciiu nhit la khu vuc tir Phan Thilt trd vao chiu anh hudng nhieu ciia thiiy trilu Vung Tau Nhin chung, sd ngay nhat trilu dao dgng tir 10 den 15 ngay Thdi gian trilu dang thudng dai hon thdi gian trilu xudng Do

ldn trieu » 1.5 - 2.0 m, trong ky nudc cudng ndi chung bien do trilu it thay doi tren mdt dai bd bien dai

Viing bien nghien cim the hien ro tinh chat bien doi miia ciia chi do gio: gi6 miia

NE (vao miia dong) vdi toe do dat den 1 4 - 1 6 m/s va tan suat xuat hien den 20% thinh hanh tit thang 11 din thang 3; gid miia SW (vao miia he) vdi toe do dat din 1 2 - 1 4 m/s va tin suit xuit hien din 10% thinh hanh tit thang 5 din thang 9 Thang 10 va thang 4 la hai thang chuyin,tilp Gid mua mua dong manh hon gid miia miia he ca ve site gid lan ve thdi gian tac dgngi T6c do gid mua d6ng tang din tit thang 10, dat gia tri ldn nhat vao thang 12

- 1, sau d6 giani d4h.' Gid miia miia he bit dau vao thang 5 va dat gia tri ldn nhat vao thang

7 - 8 Thudng thi eudng do gi6 miia miia he vao nita dau yeu hon vao nita sau cua miia Biln dgng ciia gid miia va gid tin phong vao miia d6ng the hien chii yeu trong dao dgng eua tic do gio, cdn hudng gid hiu nhu kh6ng thay d6i Dac diem cau true va dgng luc viing nghien cim cdn chiu anh hudng cua gid manh trong cac trudng hgp thdi tiet dac biet nhu bao va ap thip nhiet ddi Trung binh mgt nam cd 1.5 con bao di qua viing bien nay, thudng la vao thdi ky thang 8 den thang 12 Viing bien gan bd cdn chiu tac dgng cua gio dat-bien (breeze) Trong thdi gian ngay dem tdc do gid thudng bien doi (trir nhirng trudng hgp CO bao hoac ap thap nhiet ddi) nhu sau: ban dem va bu6i sang gid lang hay yeu, ban ngay tdc do gid tang va ve dem lai giam (Nguyen Kim Vinh, 1997)

b Dac diem che do gio qua phdn tich, thong ke chuoi so lieu gio do dac nhieu ndm

Toe do gio tai mgt vi tri ven bd cd vai tro quan trgng trong viec thanh tao song, dong chay, nudc dang, van chuyen bun-cat, xdi Id-bdi tu dudng bd, neo dau tau thuyen, De danh gia cac dac trung che do gid tai khu vuc nghien cim da sit dung ngu6n so lieu gio tai tram dao Phii Quy vdi chu6i s6 lieu gio do dac tit 1987 den 2007, tan suat do sd lieu la 6 tieng mgt lan tai cac gid trong ngay 1,7, 13, 19 gid Ket qua phan tfch, thdng ke cho thay:

- Thdi ky gio mua NE: co hai hudng gio chfnh la hudng NE chilm tin suit 34.83%, day la hudng xuat hien nhieu nhat khu vuc nghien cim, thdi gian hudng NE keo dai c6 the dat 137 ngay va hudng thii hai la NNE vdi tan suat xuat hien khoang 14%) vdi thdi gian duy tri cue dai la 49 ngay

- Thdi ky gid miia SW: co 3 hudng gio dai dien Hudng W vdi tan suit xuit hien 15.96%), thdi gian duy tri cue dai la 23 ngay; hudng WSW vdi tan suat xuit hien 10.09%,

thdi gian duy tri dai nhit la 14 ngay; hudng SW vdi tan suit xuit hien khoang 11.19%, co

thdi gian duy tri cao nhat co the dat 57 ngay

- Hai hudng gio gay song chii ylu la NE va SW tuong ling vdi mua gio NE va SW Cac dgt gid mua dien hinh ve thdi gian keo dai va cudng do la trudng gio vdi tie do V =

12 m/s (tuong duong gid miia cip 6) Tdc do gio cue dai V = 28 m/s xuat hien tai hudn^

NE va V = 36 m/s xuit hien tai hudng SW

Dac diem phan bd tdc do va hudng gid the hien tren hinh 2

WNW

W

WSW

Toe do gio (m/.s)

• <=2

• >2-4 [21>4-6

• >6-8

| ^ > 1 0 - 1 2

• >12-14

I l>14-16

a > 1 6 - 1 8

n > 1 8 - 2 0

• >20

Hinh 2: Phan bd t6c do va hudng gid tai tram dao Phii Quy (1987 - 2007)

2 Ket qua tinh toan cac dac trung song ngoai khoi viing bien Binh Thuan

Tren co sd thong ke ehudi sd lieu gid do dac tai tram dao Phii Quy (1987 - 2007), cac trudng gid sau day da dugc chgn de tinh tdan sdng:

- Gid mua NE dien hinh: V = 12 m/s, hudng = NE

- Gid miia S W diln hinh: V = 12 m/s, hudng = SW

- Gid cue dai hudng NE: V = 28 m/s

- Gid cue dai hudng SW: V = 36 m/s

Ap dung md hinh tinh sdng biln sau - Dolphin, cac dac trung sdng ngoai khoi vimg biln Binh Thuan dugc trinh bay tai bang 1

Bang 1: Cac dac trung sdng ngoai khoi viing biln tinh Binh Thuan

Toe do gio - V (m/s)

Do cao s6ng hiiu hieu - Hs (m)

Chu ky song - Tp (s)

He so phan tan eiia pho song - DSPR (o)

12 2.1 6.3 27.2

28 6.6 10.1 27.0

36 9.5 12.2 26.8

3 Dac dilm phan bo cac dac trung song tai vung bien ven bd tinh Binh Thuan

So lieu diu vao ciia md hinh SWAN la cac dac trung sdng ngoai khai (lay tir ket qua tinh toan cua md hinh Dolphin), gid dia phuong va phan bo do sau Ket qua tinh toan dugc the hien tren cac hinh 3 ^ 6 :

Kin ft dp

11.2

Ohi (hli:

: hifdng sdng

- : 6]iiing dang d()

cao so'ng (m)

1 1 0

•a

10.8^

10.6

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r-108.1

Hinh 3: Trudng do cao song hiru hieu tai viing bien ven bd tinh Binh Thuan trong dgt gio

miia NE dien hinh (Gid: V = 12 m/s; Hudng = NE)

11.2-;

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." hufifng sdng

- : diid'ng dung do

cao so'ng (m)

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Hinh 4: Trudng do cao song him hieu tai vimg bien ven bd tinh Binh Thuan trong dgt gio

mua SW dien hinh (Gi6: V = 12 m/s; Hudng = SW)

Kinti do

11.2-;

(ihichu:

: hu(?ng sdng

- : di/iVng dang dO

cao so'ng (m)

11.0-5

10.8 ^

HAM TAN

10.6

107.6 107.8 108.0 108.2 108.8 109.0

Hinh 5: Trudng do cao song hiiii hieu tai viing bien ven bd tinh Binh Thuan vdi tmdng

gid cue dai (Gio: V = 28 m/s; Hudng = NE)

1 1 2

-11.0

10.1

10.6

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: hif((ngsdng

- : difdng dang dO

cao sóng (m)

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Hinh 6: Trudng do cao sdng hiiu hieu tai vimg bien ven bd tinh Binh Thuan vdi trudng

gid cue daị (Gio: V = 36 m/s; Hudng = SW)

Nhan xet:

- Dudi tac đng ciia dgt gid miia NE, cap 6 (V=12 m/s, hudng NE) (hinh 3), ta thay:

toan dai ven bien Binh Thuan cd Hs ~ 1.0 - 1.5 m, hudng NE-Ẹ Dudng dang do eao song

Hs = 0.5 m dm sat dai ven bd; Hs = 1.0 m chay bao cac miii dat, dao ven bd va cira cac

vinh; Hs = 2 m chay each xa bd Khu vuc phia Bac Binh Thuan bi sdng NE tac dgng manh

hon khu vuc phia Nam (do phan bd do sau ciia dai ven bd va qua trinh sdng khue xa) Khu

vuc phfa Nam cac miii Ca Na, La Gan, Miii Ne, Ke Ga tuong đi kin sdng Nhin chung,

trong miia gid NE dai ven bien Binh Thuan hi song tac dgng tuong d6i manh

- Dudi tac dgng ciia dgt gid mua SW, cap 6 (V = 12 m/s, hudng = SW) (hinh 4), ta

thay: tren toan dai ven bien, cac viing, vinh ciia Binh Thuan bi tac dgng manh ciia song

vdi Hs ~ 1.0 - 1.5 m, hudng SW-S Tuong tu mua gid NE, dudng dang do cao song Hs =

0.5 m 6m sat dai ven bd; Hs = 1.5 m chay bao cac miii dat, dao ven bd va ciia eae vinh; Hs

= 2 m chay each xa bd So vdi mua gio NE (ciing cudng do gio), song hudng SW tac dgng

manh hon din khu vuc Binh Thuan Khu vuc phia Bac Binh Thuan bi song SW tac dgng

nhe hon khu vuc phia Nam (do phan bo do sau ciia dai ven bd va qua trinh sdng khiic xa)

Khu vuc phfa Nam cac miii Ca Na, La Gan, Miii Ne, Ke Ga bi sdng tac dgng manh; ngugc

lai, khu vuc phfa bic tuang dii kin song Dai ven bd phia Nam cac vinh song ylu hon dai