Tap chi Khoa hoc va Cong nghe bien T9 (2009). So 4. Tr 39 - 50
TONG QUAN VE TINH HINH NGHIEN CtTU HIEN TUONG DONG RIP (RIP CURRENT) TAI CAC BAI BIEN VIET NAM VA TREN THE GlOl
NGUYEN BA XUAN
Tom tdt: Bdi bdo trinh bdy tong quan vi tinh hinh nghiin cini hien tuang dong Rip (rip current) xdy ra tgi cdc bdi biin cua Viet Nam vd trin Thi gi&i. Qua nghiin cim vd ddnh gid cdc tdi lieu thu thdp duac, chiing toi nhdn thdy rdng: ddng Rip khong phdi la mot hien tuang bi dn, md Id mot bo phdn dgc biit ciia ddng tdng hap, vdi huang chdy tdch ba ra khoi vd thudng gdy ra cdc tai nan chit ngudi khi dang tdm biin. Ddng Rip duac hinh thdnh vd luon luon biin dong trong viing dai song do sdt bd vd phu thuoc chu yiu vdo cdc diiu kiin ciia dia hinh ba, ddy vd sdng biin duac tgo ra do dnh hicdng ciia cdc cliiu kiin thai tiit thay ddi...Bdi bdo ciing trinh bdy tdm tdt ve nguyin nhdn vd ca che hinh thdnh ciia ddng Rip, cdch nhdn dgng vd huang ddn phuang phdp phdng chdng khi rai vdo viing hogt dong ciia ddng Rip.
I. M d DAU
Bai bao nay se trinh bay cac ket qua nghien citu budc dau cua chiing toi ve hien tugng dong Rip (Rip current), dugc danh gia la mgt hien tugng tu nhien rat nguy hiem va CO anh hudng rat ldn den cac hoat dgng kinh te va du lich d viing ven bien [1]. Hien tugng nay chua dugc nghien cim va danh gia d Viet Nam tit trudc den nay, nen sir hieu biet ve no con rat mo h6. Do vay, vdi mgt y nghia nhat dinh, bai bao nay co the dugc sit dung lam tai lieu tham khao va ph6 bien kien thiic hieu biet ve cac nguyen nhan va co che hinh thanh, ciing nhu each phong ch6ng dong Rip trong khi tam bien. Day la cac ket qua nghien cim budc dau nhung rat quan trgng ciia chiing t6i, nd se co tac dung tao co sd khoa hgc va dinh hudng cho nhiing nghien cim chuyen sau sau nay.
CUng qua cong trinh ndy, tdc gid muon bdy to su tuffng nh& den co Vien tru&ng Vien Hdi duffng hoc, PGS. TS. Vo Vdn Ldnh, Id mgt nhd nghien cuu hdi dwffng hpc ddu ngdnh cua Viet Nam vd cUng Id mot nguffi anh vd ngudi dong nghiep rdt thdn thiet vd mdu muc ciia chiing toi.
II. TONG QUAN VE TINH TINH NGHIEN ClTU HIEN TlTONG DONG RIP XAY RA TAI CAC BAI BIEN VIET NAM VA TREN THE G l 6 l
Dong Rip (Rip current) la mgt loai hinh ciu triic ddng chay tach khoi bd hudng ra
khoi, xay ra trong ddi sat bd viing sdng d l (surfzone) ciia cac bai biln, co hinh thai va kich thudc cua mgt luong nudc manh vdi be ngang hep chimg 15 - 30 m, bl dai tit bd ra chimg 100 - 150 m, ndi bat tren nen chuyen dgng chung ciia dai ven bd bdi toe do dong cao hon han va hudng chay khae biet mpt each tuong phan so vdi xung quanh. Thudng thudng cau true nay dugc tao thanh tit sir bite ngang ludng ddng chay dgc bd nen ngudi ta gpi no la dong Rip current (ddng xe toae ngang). Cd d6ng nghiep ciia chiing toi gpi la dong dirt ngang, dong rut, con ngudi dan dia phuong thi gpi la dong nudc xoay, viing xoay, ao xoay hoac dong nudc lira, thay chua th6ng nhat trong trong each gpi nen chung t6i d day tam giii ten gpi bing tieng Anh la ddng Rip hay ddi khi la Rip current.
Dong Rip CO y nghia khoa hpc va thuc tien rat ldn, quan trpng nhat la no thudng gay ra tai nan chit ngudi khi tam bien hoac lam viec d dai ven bien sat bd do ngudi tam bat ngd so y, kh6ng biet boi hoac khong biet each xu ly khi bi cu6n vao dong Rip. Theo MacMahan J.H., Thornton Ed. B. and Reniers J.M. [8], thi d bang Florida (My) s6 lugng cao nhat trong cac moi nguy hiem tu nhien la do ddng Rip gay ra, cdng dong xa hgi va cac CO quan quan ly phai tap trung den tren 80%o site luc de de phdng tai hga, xit ly va cap cim cho nhiing trudng hgp do ddng Rip gay ra tai cac bai bien. Theo dan lieu ciia Luschine J.B. (1991, [6]) va Lascody R.L. (1998, [3]) cac tac gia tren cdn cho biet sd ngudi thiet mang do ddng Rip cudn trdi tai bang nay con ldn han ca nhiing trudng hgp hda hoan, bao t6 hoac nhiing bien loan khac. Ngoai ra dong Rip con gay ra su bien doi ciia dia hinh bd va day bien d viing sat bd rat manh liet va ciing la mgt trong nhirng nguyen nhan gay ra nhiing bien doi moi trudng va sinh thai cue bg d vimg ven bien, de lai nhiing su kien kha bf an d6i vdi nhirng ai chua hinh dung dugc hoac chua phan biet ro rang ve su t6n tai cua dong Rip.
O cae Qu6e gia cang giau co, s6 ngudi di nghi dudng-du lich va tam bien cang tang, cac hoat dgng kinh te va xa hgi ven bd cang nhieu thi con sd tai hga do dong Rip gay ra cang eao. Cac nudc cd nen kinh te bien phat trien, dac biet la d My va d Uc, rit chii trgng nghien cim va canh bao dong Rip, hg da tim ra nhiing phuong phap du bao, canh bao ngin han va dai han, hudng dan each de phdng va xit ly cac tinh hudng khi bi dong Rip loi cuon. So bg chiing tdi cd the gidi thieu mgt s6 co quan thuc hanh nhiem vu nay nhu Sd Du bao Thdi tiet Quoc gia (National Weather Service-NWS) thuge Hiep hgi Nghien ciru Khi quyin va Dai duang My (NOAA). Trong nhiing Sd dia phuong ciia nd cd nhirng trung tam ehuyen nganh phuc vu du bao dong Rip ven bien, vf du nhu d cac Sd Du bao Thdi tiet Philadelphia, Florida, Wilmington, Wakefield,...kh6ng nhirng chi du bao ma ngudi.ta con to chirc nhirng hinh thitc eanh bao dong Rip rat bai ban va bat mit tai cac bai tam ven bd c6 Rip. Tai Sd Du bao Thdi tiet Melbourne ciing c6 mgt ca quan chuyen nganh ve canh bao ddng Rip eho cac bai tam du lich d Uc.. .Tai lieu edn cho biit, dac biet
d cac khu vuc nghi dudng ven biln va bai tim ludn lu6n cd cac dgi tuin tra, cim hg, cac vgng gac kiem soat, cac tim Pan6, ap phich canh bao Rip current.
0 Viet Nam, nhu chiing ta da thiy, kinh te cua dat nudc da cd nhirng budc phat trien nhanh chong va manh me. Nganh du lich Ik mgt trong nhiing nganh co tdc do phat trien vao loai cao va an tugng nhat. Cac khu du lich - nghi dudng d viing ven bien thuge Trung uong va dia phuong quan ly xuat hien ngay cang nhieu, so lugng ngudi co dieu kien di nghi dudng tang len dang ke, khach Quoc te ngay cang d6ng. Cac du an mdi ve du lich va nghi dudng con dang tren da tang trudng, gan nhu tinh ven bien nao ciing da va dang dau tu xay dung nhiing khu tam bien rat dep. Do la mgt dac diem va ciing la lgi the cua dat nudc ta. Tuy nhien qua mgt sd dgt khao sat tim hieu chiing tdi biet dugc t6n that do tam bien da xay ra kh6ng phai la ft. Ciing vdi dieu dd chiing t6i ciing nhan thay rang hau nhu chua noi nao hieu ro ve dong Rip va cd su quan tam den viec nghien cim, xae dinh va canh bao n6. Qua cac s6 lieu thu thap dugc trong eae dgt khao sat, ciing nhu th6ng qua cac thong tin thu thap duge tren mang Internet, ehiing t6i dugc biet rang, dong Rip hau nhu xay ra tren khap eae bai bien Viet Nam, tir M6ng Cai den dao Phii Qu6c. Do su hieu biet con qua mo hd ve hien tugng ddng Rip, nen viec t6 chirc canh bao va citu hg tai nan do dong Rip gay ra tai cac bai bien Viet Nam la rat bat cap va thieu co sd khoa hgc. Theo s6 lieu th6ng ke trong nam 2008, luc lugng tuan tra cap cim thuy nan tren dia ban TP. Viing Tau da vdt dugc gan 900 trudng hpp du khach bi dong Rip tan cdng. Tir dau nam 2009 den nay, luc lugng nay da vdt duge 266 trudng hgp.
NghT rang, da den luc can giong len ldi keu ggi cac nha lanh dao, nha. quan ly vimg ven bien va du lich ve sir cap thiet phai nghien cim, tim hieu va xae dinh cac dieu kien tu nhien va m6i tradng ndi chung ciia mgt khu du lich-nghi dudng, coi do nhu la mdt tieu chi an toan va hien dai, trong do can dac biet quan tam den hien tugng ddng Rip tai cac bai tam.
III. NGUYEN NHAN VA CO CHE HINH THANH DONG RIP Of VUNG DOil SONG DO SAT BCJ
Ve CO ban dong Rip phu thuge vao song bien va dia hinh chi tiet eiia bd va day trong ddi song do. Sdng bien chita yeu td ngau nhien cao tao nen cho dong Rip mgt tinh chat ngau nhien trong qua trinh hinh thanh. Mat khac dong Rip ciing lai la mgt mdt yeu t6 dgng lire manh me ciia nudc bien viing ven bd, su t6n tai va hanh vi ciia nd phu thuge chat che vao cac yeu td thiiy thach dgng luc khac, dae biet la cac dieu kien gidi han ciia mgt bai toan bien. Cho den nay chua cd mgt co sd ly luan chfnh thing nao eho dong Rip, ca vl ly thuyit dgng luc hgc lin ly thuyit thing ke. Do su lien quan chat che vdi dgng luc hpc ddi ven bd nen nd thudng dugc coi nhu mgt khia canh trong thiiy thach dgng luc vimg ven bd
ndi chung va he ddng chay ven bd ndi rieng. Cac cdng trinh nghien cim vl dong Rip thuc ra ciing la su the hien nhiing net bien ddi ciia bai toan chung dd.
Theo cac tai lieu nghien ciiu ve ddng Rip thi vao nam 1941, lin diu tien loai hinh dong chay bien nay dugc luu y, da dugc ghi nhan va md ta bdi Shepard, Emery va Lafond
[10]. Sau khi phan tfch nhirng dii lieu do dae eac tac gia nhan dinh ring cudng do va kich thudc ciia dong Rip co lien quan mat thiet vdi do cao sdng, vdi dac diem lan tmyen va phan xa khi sdng truyen vao bd. Ve co ban vai trd va tinh chat cua ddng Rip dugc dinh doat bdi dang cau tnic dia hinh bd va day d viing sat bd. Ddng Rip thudng xay ra d noi co do cao cao nhat ciia mgt con song hay mpt tap hgp sdng khi trayen vao bd. Sau nhirng nghien cim va c6ng b6 khoa hgc mang tinh chat phat hien nay, nhirng canh bao ve dong Rip tai cac bai bien tren thuc te da dugc chu trgng han. Tuy nhien nhiing nghien cim tiep theo kh6ng thay tien trien may, mai cho den nam 1950, nghia la gan 10 nam sau, mdi lai CO cac cong trinh ciia Shepard va Inman [11, 12], tai thdi diem nay lan dau tien cac tac gia dua ra mgt khai niem mdi va tuong d6i hoan chinh ve hoan luu te bao (cell circulation) trong he dong Rip current (hinh 1).
7V-
(Dai song do) / LONGSHORE CURRENTS_^ J '(Dong doc ba)
(Bai bien)
Hinh 1: So d6 don gian ve he thong hoan luu t l bao ciia ddng Rip
Qua trinh hinh thanh mgt cau triic ciia hoan luu te bao cd thi hinh dung nhu sau:
Sdng bien tien vao bd nhu mdt front song vdi do cao ldn dan, nhat la d nhiing noi co ciu tao dia hinh la bai thoai thoai. Tuy thudc vao dieu kien thdi tiet va dia hinh day, khi din gan bd front s6ng tao thanh hai luong dong chay sat bd theo hai hudng doi nghich nhau, tach nhau ra. Nhu vay se co nhieu diem co co che tach ddng nhu vay tren mdt bai biln va ciing se cd nhiing diem tuong irng ma tai dd ddng chay dgc bd giao nhau. Tai dilm giao
nhau cua hai ludng dong dgc bd khac hudng, dong chay khung lai, hoa quyen vao nhau (ggi la feeder current) va cudi ciing chuyin hudng gap ra khoi tao thanh dong Rip. Dong Rip thudng thudng la ddng true giao vdi bd nhung ciing co nhieu trudng hgp xien bd thanh mgt gdc nhat dinh. T6c do dong chay cua Rip kha cao, trung binh dat den -1,00 m/s, CO tai lieu cho rang co thi len din 2,00 m/s, nhung chua dugc kiem chung [8]. 6 cudi dong Rip (phfa bien, noi ket thiic dong Rip nhung trong tai lieu cdng b6 lai ggi la " dau Rip "(rip head)), ddng chay yeu hon va toa ra bon phia nhu hinh bo hoa, mgt phan hoa vao vdi dong chay song dang tien vao bd va bat dau lai qua trinh cau tao ddng Rip. Toan bg qua trinh d6 tao thanh mgt vong khep kin ciia ddng chay song d vimg sat bd: dong chay dpc bd-dong Rip-dong s6ng tien vao bd, giira chiing la cae cau triic phu g6m feeder current va Rip head. Nd la mdt te bao ciia hoan luu trong he Rip-current. Khong phan ddi khai niem tren, nhung cac tac gia Putnam, Munk va raylor [9] lai cho rang nguyen nhan sinh ra ddng Rip la do sdng ldn ch6ng chat nhau, trudn cao len tren bai rdi riit xu6ng, qua trinh riit do md dau cho viec day nudc tit bd ra khoi thanh lu6ng cat ngang dong chay dpc bd, do chinh la dong Rip. Cach giai thich nhu vay ciing hgp ly, nd bd sung cho khai niem ve loai hinh ddng chay dac biet nay. Vdi nguyen nhan sdng ldn trudn bd thi dong Rip cd the xuat hien manh liet va dac biet tren mgt s6 bai bien, dudi ap luc ciia nhirng dieu kien thdi tiet xau, nhu bao t6 d ngoai khoi tao ra song ldn, song v6 bd, sdng limg trudn bd va nit xu6ng manh. Qua trinh dgng lire nay se day len bd va rut ra bien nhiing khoi lugng cat dang ke, trong nhieu trudng hgp lam cho bd va bai bien bien d6i tao thanh dudng mep bd CO cac miii cat nh6 ra (cuspate) va ldm vao (embayements) di lien nhau vdi kfeh thudc khac nhau. Ngudi ta thudng dua vao dau hieu do de canh bao kha nang t6n tai dong Rip.
Mac dii song, khiic xa ciia no vao dia hinh bd va day d sat bd qui dinh vi tri va cudng dp ciia dong Rip, cac khao sat cdn cho thay hoan luu te bao van xuat hien d nhiing bai bien co dia hinh bd va day tuong d6i phang va ddng nhat. Trong trudng hgp noi hoan luu te bao kh6ng cdn la he qua ciia phan xa song hay bien ddi dia hinh chi tiet nira [2].
Ngoai ra, eac khao sat cdn cho thay song ldn chi san sinh nhiing dong Rip manh, trong khi do song nho han thi sinh ra ddng Rip yeu hon nhung sd lugng cau tnic nay lai nhieu hon dang ke tren ciing mgt khdng gian bai bien. Dieu nay dat ra nhieu van de cho nhiing c6ng trinh nghien cim co che dgng lire cua ddng Rip ve sau.
Ddng Rip d cac bai bien khdng chi la hau qua ciia cac nguyen nhan do sdng bien be mat (s6ng do gi6) truyen vao bd. Cac yeu t6 Hai duong hgc nhu he ddng chay ven bd, thuy trieu va dong trieu, thay ddi muc nudc, v.v...ket hpp vdi eac dac diem khac biet vl dia hinh bd va day ven bd nhu cac bai ngim (bars), eac laeh trilu (inlets),... ciing la nguyen nhan gay nen nhung lulng ddng chay tach bd manh me ma nlu kh6ng luu y hoac dugc canh bao trudc chiing se la tai hga gay chit ngudi. Tai lieu nghien ciiu tren Thi gidi
da gidi thieu nhiing hinh anh khai quat ve thiiy trilu va dia hinh ven bd tuong itng de tao ra eac loai dong chay nguy hiem nhu vay [2, 8]. Tai lieu canh bao ciing da gidi thieu nhiing tai hga da xay ra trong cac trudng hgp tuong tu.
Sdng nhe
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Rip (urrent
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S6ng ldn
Rip current
^-f-l^ons .-^ong nho
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Ddng dgc bd" \ Ddng dgc bd
H i ^ set up (ap luc sdng cao)
Low set up (ap lire sdng thap) Low set up ( ap luc s6ng thap)
Bai bien
Hinh 2: So d6 co che hoan luu te bao kieu "Set-up"
Ly thuyet dgng lire hgc dong Rip ve co ban dugc xay dung tren khai niem ve ap luc hire xa song (radiation stress) ciia Longuet-Higins M.S.,Longuet-Higins M.S. & Stewart R.W. (1964, [3, 4]) - mgt loai dong dpng lupng vugt troi (excess flow of momentum) san sinh tir nhiing con song hoac mgt tap hpp song tien vao bd. Cac thanh phan ngang va dpc bd ciia ap lire song bite xa tao nen su chenh lech dp cao mat nudc bien trong kh6ng gian ven bd, trong do co noi mat nudc dang cao hon binh thudng (set-up) gan lien vdi noi co mat nudc giang thap (set-down). Su chenh lech muc nudc do tat yeu phai tao ra dong nudc chay tit khu vuc cd set-up cao den khu vuc set-up thap hoac khu vuc set-down (hinh 2). Do la eo che cau tao dong do song. Trong cau tao do, nhu da ndi d tren, tai nhirng diem giao nhau cua hai cau tao nghich dong se xuat hien feeder-current, xuat hien dilu kien va xu hudng tach bd eiia ddng dpe bd, xuit hien kha nang tao ddng Rip. Can luu y, mpt co chl thanh tao hoan luu tl bao nhu vita mo ta, c6 the xay ra trong trudng hpp cap hai con song ldn canh nhau, nhung thudng vin la trong khdng gian lan truyen cua hai tap hgp
canh nhau ma mdi tap hgp la mgt loat cac con song d6ng hanh. Moi tap hgp ay ciing tao ra nhiing khu vuc set-up (noi chiia do cao song ldn) va set-down (noi chiia do cao s6ng nho). Theo khai niem co ban vl ap lire biic xa sdng ven bd da phat trien nhieu cong trinh nghien cim ly thuyet va ban thuc nghiem doi vdi ddng chay ven bd, trong do hoan luu te bao va ddng Rip bai bien la nhiing yeu t6 dgng lire chi tiet. Cac bai toan thiiy dgng luc dugc giai nham nghien cim ddng Rip deu dugc quan triet day du khai niem ap luc bite xa song ven bd, chinh vi vay chiing deu mang nhirng khia canh, nhiing chi tiet khac vdi cac bai toan thuy thach - dgng luc th6ng thudng khac. De c6 mgt tong quan hoan thien han ve cac nghien cim van de nay chiing t6i nghT can nghien cim ky hon, se phan tfch va trinh bay sau hon trong cac bao cao tiep theo.
Dudi day chiing tdi se gidi thieu mdt sd loai ddng Rip tieu bieu da xay ra phd bien tai cac bai bien The gidi va Viet Nam ma co che hinh thanh ciia no co sir lien quan vdi cac dieu kien bien d6i ciia dia hinh bd va day bien. Co 4 loai dong Rip co ban sau day: dong Rip xae dinh (fixed Rip), c6 dinh (permanent Rip), titc thdi (fiash Rip) va di chuyen (traveling Rip), cu the giai thich nhu sau:
1. Loai xae dinh (fixed Rip): la loai dong Rip hinh thanh d nhiing nai c6 cac c6ng trinh nhan tao dupc xay dimg hoac cac cau tnic tu nhien nho ra bien, nhu: cau tau, dap chan s6ng hay cac mdi da.. .Dong rip xae dinh ciing thay xuat hien d nhiing viing song d6, noi t6n tai eac vdng nudc sau so vdi xung quanh. Ngoai ra, su hinh thanh ciia dong Rip nay con phu thupc vao cae dang dudng bd va cau tnic cua cac bar cat hinh thanh d viing sat bd. Vi tri tdt nhat de hinh thanh loai dong Rip nay la tai cac diem nam giiia cua cac miii nhgn nhd ra bien (cuspate). Loai dong Rip nay chi t6n tai d nhiing vi tri nhat dinh cho den khi cd sir thay ddi eiia dia hinh day sat bd. Sir thay ddi nay thudng la ket qua cua su thay d6i thdi tiet va thay ddi miia tai cac bai bien co vi tri dia ly khac nhau.
2. Loai CO dinh (permanent Rip): Dong Rip nay la loai dong cd hudng chay ra bien va mang tfnh chat tua dimg, nd dugc hinh thanh tren "dau" cua cac cau tnic cdng trinh va c6 the tdn tai quanh nam. Nhiing cau tnic nhu vay thudng la eac de chan song, mQi dat nhgn nhd ra bien hoac do nguyen nhan ciia mgt dong nudc manh ven bd tao nen.
3. Loai tu-c thdi (Flash Rip): Loai ddng Rip nay cd thdi gian t6n tai rat ngan (nho hon 10 phiit), nd dugc hinh thanh do sir anh hudng ciia song vd manh. Dieu nay c6 the giai thich, sdng limg vdi nang lugng ldn do bao gay nen dugc trayen vao bd va bi tieu tan dan. Dong rip Tiic thdi la rat khd du bao do sir anh hudng tam thdi ciia cac dieu kien san smh ra no, cung nhu su thay doi vi tri cua nd trong qua trinh phat triln.
4. Loai di dong (Traveling): Ddng Rip nay la mgt loai ddng Rip dugc hinh thanh rit phi biln, khi co sir tin tai cua mgt dong chay ven bd vdi cudng do manh. Dong chay
nay chuyen dpng theo hudng song song vdi dudng bd da cuin hiit dong Rip tach khoi vi tri phat trien ban dau ciia nd, giong nhu mpt ddng Rip di dpng tach khoi vimg hinh thanh va se ylu din di. Neu sdng va thiiy trieu cdn anh hudng tilp tuc thi ddng Rip nay se con phat triln, bdi vi, khi dd mpt dong Rip khac se dugc hinh thanh tir tren bai xudng bien, b6 sung cho ddng Rip da bi cuon di. Qua trinh nay co the tiep tuc phat triln su6t ngay, neu cac dilu kien hinh thanh cdn thuan lgi.
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Hinh 3: Ddng Rip loai Xae dinh (a) va loai Co dinh (b)
Do cd tam quan trgng doi vdi cac hoat dgng du lich bien, dac biet la hoat dgng du lich tam bien nghi dudng d viing ven bd, tren The gidi, dac biet la d cac nudc phat trien, ngudi ta da tien hanh nghien ciiu va du bao ddng Rip mdt each rat bai ban, nham tim ra cac giai phap tdi uu ve viee t6 chiie quan ly va canh bao ddng Rip tai cac bai tam. Ngoai viec tuyen tmyen, phd bien cac kien thiic ve su nguy hiem rinh rap eiia ddng Rip, viec hudng dan each nhan dang va phuang phap phdng tranh khi bi ddng Rip cuon tr6i ciing la mgt trong nhiing nhiem vu quan trgng hang dau:
- Su- nhan biet ve dong Rip: Ddng Rip xay ra tai cac bai bien khdng phai khi nao ehiing ta ciing cd the phan biet dugc, vi no la mgt hien tugng dpng luc bien ddi rat phirc tap, nen rat khd co the du bao trong mpi thdi diem va tai mpi noi cua eac bai tam. Tuy nhien, trong nhieu trudng hop cd the nhan biet su t6n tai eiia cae dong Rip tai cac bai bien bang mat thudng, chung ta co the chup anh, quay phim theo ddi dien bien ciia chiing de lam tu lieu nghien ciiu va canh bao tai cac bai tam. Tren nen chung tuong ddi ddng nhat cua nudc bien tai cac bai tam, cac ddng Rip thudng de lai nhirng dau vet khac biet ed the dl nhan biit, nhu: su xuit hien cua mpt vimg xao trpn lan tan hay mpt viing bpt nudc nudc tring x6a tren mat biln gin sat bd; mdt dau hieu ro net ve mau sac ciia nudc biln khi nhin tir tren cao xu6ng; mot dai hep tap trung nhirng rac radi va vat trdi n6i tren mat nudc, v.v... Du khach tam bien hoac nhirng ngudi lam nhiing cdng viec ven bien can quan sat va
nhan biet nhiing dau hieu nhu vay de de phong ddng Rip tan c6ng, dac biet doi vdi nhiing ngudi khdng biet bai, boi yeu hoac tre em choi diia tren bai bien thieu su theo doi cua nhiing ngudi ldn. Tuy nhien, nlu gap tradng hgp bi dong Rip tan cdng do su so y, bat can, thi viec phong tranh co hieu qua c6 the dugc hudng dan nhu sau:
7 J,'i«fel?!?f(!''*ftSri ^ ^ ^ --^.fy' '••g'SiMgjMWjWBuBllMi]!^
Dong Permanant Rip c6 the keo dai hang thang, hang nam, dugc hinh thanh do cac cau true tir nhien co dinh nhu de chan song
Dong fixed Rip co the keo dai tir vai gia den vai thang, dugc hinh thanh boi cau true dja hinh cua cac bai cat boi sat ba
-^^'
Dong travelling Rip di chuyen dgc theo bai do CO su ton tai ciia dong chay ven bien du manh
Dong Flash Rip co the bat nga xuat hien va tieu tan nhanh, khi no dugc sinh ra bai sy don nuac bat nga va manh vao ba
Anh chup cac dong Rip dang chay qua duang song d6 va phan bo dan trai doc theo ba, chiing co the phu thuoc vao nhiSu ygu t6, trong do CO hinh dang dja hinh day sat ba
Anh nhm tu dat hen trong thoi ky trieu thap Kenh dugc nhin thay tren hinh da dugc hinh thanh do su phat \Q bar cat cua dong Rip hinh thanh doe ba
Hinh 4: Mdt s l hinh anh v l cac loai hinh ddng Rip tren cac bai biln Thi gidi
- Hudng dan each phong chdng:
- D6i vdi nhiing ngudi khi bi ddng Rip tin cdng cin phai thuc hien theo cac budc sau day: (1)- Khdng dugc hoang sg va phai het siic binh tTnh; (2)- Boi tha tr6i tu do de tiet kiem nang lugng, khdng boi eh6ng lai dong Rip theo hudng vao bd; (3)- D I thoat khoi dong Rip can boi xien qua dong Rip ra viing xung quanh, sau khi thoat khoi dong Rip, hay tim each boi tir tir vao bd; (4)- Neu khdng boi dugc thoat khdi dong Rip, thi phai sit dung phuong phap boi ndi hoac boi dung; (5)- Neu thay yeu site khdng boi dugc vao bd thi phai keu to va vay tay yeu cau su cap ciiu tit tren bd. Sau khi lay lai duge su binh tinh hoac khi ddng Rip da ylu din thi cd gang boi xien qua dong Rip va hudng thang vao bd.
Kh6ng nen hoang sg, vi biet rang dong Rip se cham diit va tieu tan d viing nude sau.
- D6i vdi nhiing ngudi d tren bd can thuc hien nhiing dieu sau: (1)- Phai than trgng de minh kh6ng phai la nan nhan khac tiep theo ciia ddng Rip; (2)- Can tim ngay su giiip dd ciia dgi tuan tra, cim hg bai bien; (3)- Neu kh6ng co dgi tuan tra ciiu hg bai bien, thi tim ngudi biet boi gidi hoac ngudi cd kinh nghiem di bien de cim hg; (4)- Noi vdi nan nhan each boi tha long, boi dung, khdng nen hoang sg; (5)- Cd the nem phao cim sinh xu6ng bien de hd trg cho nan nhan; (6)- Ggi khan cap den s6 dien thoai chuyen nghiep ciia dgi cim hg de giiip dd. Dudi day la mgt s6 hinJi anh chup dugc ve mdt s6 dong Rip tieu bieu xay ra tren The gidi (hinh 4).
IV. KET LUAN
Tit cac ket qua phan tfch chi tilt d tren cd thi nit ra mgt s6 kit luan sau day:
- Ddng Rip khdng phai la mgt hien tugng tu nhien bf in, ma thuc chit la mpt bo phan dac biet ciia ddng chay t6ng hgp, xay ra p h i biln d vimg ddi sdng d l va co hudng tach bd ra khoi vdi tdc dp kha manh, do do no rat nguy hilm va co thi gay ra cac tai nan chet ngudi khi tam biln.
- Nguyen nhan va co che hinh thanh ciia dong Rip la do su anh hudng chii ylu ciia dia hinh bd, day va che dp sdng phat sinh do su anh hudng ciia thdi tilt biln ddi. Dong Rip CO the dat eudng do cue dai trong tradng hgp khi cd sir kit hgp cong hudng vdi cac thanh phan ddng khac, nhu ddng trieu va dong gio thdi manh tit bd ra khoi. "
- Ly thuyet thiiy-thach dgng lire ciia co che hinh thanh dong Rip la do anh hudng cua ap luc bite xa va phan xa sdng trudn len bd va nit manh tao thanh ddng Rip.
- Mac dii la mgt hien tugng thuy thach ddng luc rat phiic tap va rit khd do dac nhung tren The gidi da nghien cim va dua ra cac phuong phap canh bao va dl phong tai
cac bai bien rat eo hieu qua. Q Viet Nam, dong Rip chua dugc nghien cim nen su hieu biet con rat mo h6, do dd din din tai cae bai bien Viet Nam chua dugc t6 chite canh bao va ciiu hg cd hieu qua, dieu nay co anh hudng rat ldn den cac hoat dgng du lich bien.
TAI LIEU THAM KHAO
1. Nguyen Ba Xuan, Le Phirdc Trinh, 2009. Tim hieu ve nguyen nhan va co ehe hinh thanh ddng Rip tai cac bai bien Viet Nam va tren The gidi ( bao eao ehuyen de ciia de tai ehua c6ng b6 )
2. Komar P.D, 1998. Beach processes and sedimentation 2nd ed., Prentice Hall, New Jercy, 544 ps.
3. Lascody R. L, 1998. East central Florida rip current program. Nat. Weather Service In-house Report, p. 10.
4. Longuet-Higins M.S, 1970. Longshore Currents Generated by Obliquely Incident Waves. 1,2 - Journal of Geophys. Res., 75, pp. 6778 - 6789 (1) and 6790 - 6901 (2).
5. Longuet-Higins M.S. and Stewart R.W, 1964. Radiation stress in Water Waves, a Physical Discusion with Applications, Deep-Sea Res., 11, pp. 529 - 563.
6. Luschine J.B, 1991. A study of rip current drownings and weather related factors, Nat. Weather Dig., pp. 13 - 19.
7. McKenzie R, 1958. Rip current systems. Joumal of Geology, 66, pp. 103 - 113.
8. McMahan J., Thornton Ed. B., Reniers Ad. J., 2006. Rip current review, Coastal Engineering, 53, pp. 191 - 208.
9. Putnam J.A., Munk W.H., Traylor M.A., 1949. The Predictions of Longshore Currents. Transactons of the Amer. Geophys. Union, 30, pp. 338 - 345.
10. Shepard F.P., Emery K.O., and Lafond E.C., 1941. Rip cun^ent: A Process of Geological Importance, Jour, of Geology, 49, pp. 338 - 369.
11. Shepard F.P., and Innman D.L., 1950. Nearshore Circulation Related to Bottom Topography and Wave Refraction. Transactions Amer. Geophis. Union, 31(4), pp.
555-565.
12. Shepard F.P., and Innman D.L., 1950: Nearshore Circulation - Procc. of the 1-st Coast. Eng. Conf, Amer. Soc, Civil Engrs., pp. 50 - 59.
OVERVIEW ON THE RESEARCH SITUATION OF THE RIP CURRENT AT THE SEA BEACHES OF VIETNAM AND IN THE WORLD
NGUYEN BA XUAN
Summary; This paper presents an overview of the research situation of the rip current occurred at the sea beaches of Vietnam and in the world. Through the study and evaluation of collected materials, it found that: the rip current is not a mysterious phenomenon, that is a special component of the general current having the direction apart from the coast and usually causes the death accidents when people swimming in the coastal sea. It was formed In the surf zone and often be in a fluctuation, which depends on the conditions of the terrain coast, sea bottom and sea wave created by the influence of the changed weather canddions...
This paper also presents a summary of the reasons and mechanisms formatted the rip current and the methods to identify and guide how to prevent the rip currents when falling in the zone of their action.
Ngdy nhdn bdi: 01 - 10 - 2009 Dia chi: Vien Hai duong hgc Nguoi nhan xet: PGS.TS. Nguyin Thg Sao
