KHOA HQC C 6 N G NGHfe
l\IGHIEI\l CUU S U VAIXI DOIXIG CUA HO XOI LOIM TREIXI SOIMG TIEIXI V A AIMH HUOIXIG CUA IXiCI TOI KHU VUC
c A u MY THUAlM
T6MTAT
Trong khuon kho hgp dong giira Cong ty Maunsell Pty LTD (cua Uc) va Trung tam Nghidn ciiu Dpng luc S6ng-Vidn Khoa hoc Thuy loi (cu),c6ngvi§c nghidn ciru hinh thiii song Tien khu vyc cau My Thuan da dugc ti^n hanh tu nSm 1996 tru6c khi xSy dung cdu, trong do co "Nghidn ciiu si; vSn d^ng cua hfl' x6i idn tr^n song Ti^n,..". Hd x6i Idn thupng luu c4u My Thu&n, di^m sSu nhfit sSu tdi 48 m. Vi nhung thay doi v^ vj tri cua hd xoi CO the se d ^ toi vide h? sSu day sdng t^i vi tri cdu, ndn dS nghidn ciiu cdc van dd vd: - lich sii sir van dong cua dong song A& tao ndn su co hep bd rpng sdng doan tir b^n pha tdi vj tri cdu; - si; co hpp Idng din nay co tac dpng lam sSu day song; - diem cong g^p trong Idng s6ng nSm tai b^n pha da anh huong vao hudng ddng chay lam sSu day sdng; - v] tri va hinh dgng cua hd xdi cd lidn quan ch^t chd vdi hinh d?ng mat bang ciia song va d§c bidt su co hep Idng din va vi tri cua do^in sdng cong. Do v^y su 6n djnh bd Bic phia thuong luu la ydu td then chdt lam giam thidu rui ro do nhung thay ddi vd vj tri hoSc hinh d ^ g cua hd xdi trong tuong lai gay ra.
Cho den nay thd sdng Tien khu vuc cau M? ThuSn dS 6n dinh, nhung do cac ydu td didn bidn sdng khong nhung theo quy luSt tu nhidn va ma cdn do tac dong con ngudi la rat phiic tap, vide theo doi di^n bien hinh thai sdng Tidn khu vuc clu M? Thudn cdn ndn dugc tiep tuc.
Tlrkboi-Su yan dong ciia hd xdi. hinh thai, guy luat tu nhidn.
L BAT VAN BE
Trong khudn khd hpp ddng giiia Cdng ty Maunsell P ^ LTD (ciia Uc) vd Trung tdm Nghien ciru Dpng luc Sdng - Vidn Khoa hpc Thuy lpi (cu) [5], cdng vide nghidn ciiu hinh thai sdng Tien khu vuc cau My Thudn da dupc tidn hanh trong nam 1996 phuc vu giai doan thidt ke chi tiet - lua chpn chidu sau mdng cau tai vi tri xay dung. Do dd van dd quan trpng trong cdng vide nghidn cihi Id ^'Nghi^n cuu su van dong cua hdxdi Idn tren sdng Tien va anh huong cua no toi khu vuc cau My Thuan". Trong bdi canh, long sdng phia thirpng lim cdu My Thuan khoang 700-900 m cd mdt hd xdi Idn, didm sdu nhdt sdu tdi 48 m (xem hinh 1- Sa hoa vi tri ciia hd xdi My Tliudn trdn sdng Tien). Hd xdi ndy ndm tai noi chidu rdng long sdng bi co hep nhat so vdi chidu rpng sdng thirpng va ha lim. Sy thay ddi vd hinh dang hodc vi tri ciia hd xdi nay cd thd ddn toi su ha thdp dp sdu chung cua ddy sdng t ^ khu vuc cau, do vdy can phai dupc nghidn ciiu ky ludng dd giam nii ro cho cau.
Vd Imh vuc sdng ngdi, didn bien Idng dan vd hinh thai sdng Id ket qua cua su tuong tdc phiic tap giiia tdc ddng ddng chay va long ddn (bao gdm cac lop ddt set cdu thanh long sdng va bd sdng d doan
sdng nghidn ciiu). Trong dd, hinh dang cua hd xdi la ket qua tdng hpp chu y^u do cdc tidn trinh sau day: - Xdi tai nhiing doan long sdng bi thu hep [6] noi sdng cdng hep thi cang sdu; - xdi phat sinh tai khuc sdng cong, sdng cang cong (ban kinh cong cang nhd) cdng sdu [61. Ngoai ra cdn do cdc tien trinh khac nhu: - Cdc khu nudc cudn vd su xody gay ra tii su dao chieu cua ddng chay trong chu ky tiiuy trieu; - xdi hpp luu tir diem hpp ddng giiia raeh Sa Dec va Tien Giang [2]. Tuy kdt qua hinh tiianh va van ddng cua hd xdi cue bd la kdt qua tdng hpp, nhung trong nghidn ciiu can phan tich tiing anh hudng mdt cdch ridng bidt vd phan tich mdi lidn quan giua hinh thai trdn toan bd sdng Ti^n va hinh tiidi sdng Ti^n khu vtrc cdu My Thudn.
L sd Liu VA PHUONG PHAP NGHDi diu 1. Sd lidu nghidn ciiu
Ban dd dja hinh lich sir sdng Tien tii nam 1959 ddn ndm 1994 (36 ndm) da lam rd nguon gdc vd he cao dp vd dua vd hd cao dd qudc gia. Trac dpc ddy sdng Tidn tai My Thudn da dupc tiiii tiidp cho cac ndm 1962,1979,1986 va 1994. Sd lidu dieu ti-a caban nam 2008 vd ndm 2009 ciia Vidn KHTL Midn Nam [4]. Thdng tin vd s^t Id bd sdng cdc ndm gdn ddy, tir ndm 2007 d^n nam 2011.
' Vi?n Khoa hpc Thuy lpi Vi#t Nam
N 6 N G NGHIEP V A PHAT TRIEN N 6 N G T H 6 N - KY ] - THANG 5/2012
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2. Phuong ph4p nghiSn cmi
• Phuong phap phan tich, x i ly tong hop c4c so HSu thuc do tlia hinh, ban do lich sir, phSn tich Aiin bien, hinh thai song.
- Phuong phap chjp d|a hinh duijc s i dMng d<- phan tich diin bi^n trtn mat cat ngang, diJn bidn tuyen lach sau va duftng b(r. Cac m5t cat khi ch^P phai CO Cling ty le va ciing h j cao do quoc gia.
- Phuong phap phan tich cac quan hf hinh thai song.
10. KET guA NGHCN CUU Vii PHAN TKH THAV Udl HINH THAI SONG
1. Lich sii su dich chuydn h6 x6i
Trac dpc day song Ti^n tai My ThuSn da dupc thu thap cho cac nam 1962, 1979, 1986 va 1994 Ihi hien trong bang 1 [2). 13). Trac dpc song giiia cic nam 1962 va 1979 co su dich chuyen dang k^, nhung giiia cac nam 1979 va 1994 vi tri va hinh dang cCia ho xoi khong thay doi nhi^u. Trong thoi gian 1.5 nam ke tii nam 1%2 Vl tri sau nhat trong hd xoi da dich chuyen trdn 1000 m. Til do da khong dich chuydn niia.
2. Ljch sii su djch chuydn bir Bac so v6i su djch chuydn hd xdi
Su djch chuydn hit Bac so voi su dich chuydn ho xdi dupc thd hidn ft bSng 1. So sinh vj tri bo Bac nam 1977 v4 1979 cho thSy su dich chuyen gan 25 m trong 2 nam. Tn sd nay vd thuc ch^t khdng khac voi tri sd trung binh, m$c dii c6 xuat hidn tran Iii trong niim 1978. Didu niy chimg td su xoi bd l.a qui U-inh tii tii va lidn tuc tii nam nay sang nam k h a c .
Hmh 1. So ho9 vj tri ciia hd xdi My ITiu^n trdn sdng Tidn
Khoang cdch bd da dich chuyen tai MC 5 Tdc dd trung binh cua dich chuyen bd
Khoang each dich chuyen cua diem cudi dudng cao dd -40 m ve phia ha luu Tdc dp trung binh cua djch chuyen hd xdi
Bang 1 So sdnh su dich chuydn hd xdi sdu vd b a Bdc 1959-1977
309 m 17 m / ndm
1977-1992 291m 19 m/
nam
1992 -1996 20 m
5 m
1959 -1979
860m
43 m/ndm 1979 -1994
Om
Om/nam
2007-2on
2m
0m/n5m 3. So sdnh hd xdi tai My Thudn vdi cdc hd xdi
trong sdng Tidn
So sanh hd xdi tai My Thudn vdi cdc hd xdi Ion nhat trong sdng Tien dupc neu trong hinh 2; cdc hd xdi sdu cd xu hudng xuat hidn tai nhiing noi sdng hi CO hep ciing vdi dp cong gap.
Cd quan hd chat chd giiia hd xdi va nhung dac trung doan sdng cong. Dirdng quan hd cho thay rdng dp sau ciia hd xdi hidn tai cd quan hd vdi ddc trung cua doan cong vd cd lidn quan ddn nhiing hd xdi khdc trdn sdng Tien [1], [4].
Hinh 2. Quan h$ giiia dO sdu hd xdi vd t? sd R/B CTrong ty sd R/B: - R la bdn kinh cong cua do?n
sdng; - B la bd rpng long sdng)
72 NONG NGHIEP VA PHAT TRIEN NONG THON - KY 1 - THANG 4
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4. Anh hudng ciia s^t co hep Idng sdng Bieu dd dudi day chi ra rang vi tri ciia hd xoi cd quan hd vdi sy co hep long sdng trong ndm 1994. Vj tri hd xdi gan phan hep nhat ctia long ddn. Didu nay la phii hpp vdi nhiing hinh vd tuong tu cho nhung nam khdc va tao ndn mdt quan hd giua vi tri co hep va vi tri cua ho xdi (Hinh 3).
Sii dung phuang trinh kinh nghidm co hep mot chieu de tinh todn va coi co hpp nhu la mpt yeu td ddi voi xdi cho thay rdng hd xdi hien cd cd the hoan toan do xoi CO hep. Phuong trinh la phuong trinh kinh nghidm va la du bao xdi trong trudng hpp nay, nhung xoi CO hep rd rang cd kha nang la mdt ydu td chinh trong su phat trien hd xdi.
Breusers and Raudkivi (1991) dua ra nhiing quan he (1) sau day de tinh xdi co hep:
y,=Y,(Bl/B2)' (1)
Trong dd:
- Yl vd Y2 Id dd sau trung binh phia thuong luu va tai vi tri co hep tuang ling.
- BI va B2 la bd rpng trung binh phia thuang luu va tai vi tri CO hep tuong img.
- oc Id he sd trong khoang tir 0,67 ddn 0,8.
^ ^500 ^ -ISOO -tOOO -SM 0 » 0 1000 Khung uch (m|
GhicM: -l|Usiu(iiij-piiil)ir^Ndin(in|
Hinh 3. Vj til ciia h6 xdi tuong quan vdi co h?p Idng sdng
Su hinh thanh vd van dpng cua hd xoi cue bd la k^t qua tdng hpp, thdng qua phan tich timg anh hudng mpt cdch ridng bidt, moi lidn quan giira hinh ti^di toan bd sdng Tidn vd hinh tiiai song Tien tai khu vuc cdu My Thuan; nhdn tiiay rdng d^ vi tri cua hd xdi sau khdng tiep tuc dich chuydn v^ ha luu cdn phai dn dinh thd sdng. Cdng vide nghien cim dn dmh thd sdng Tien khu vuc cau My Thudn duoc tien hdnh trudc khi xdy dung cau va du(>c xay dung trong giai doan thi cdng cau (tir Oiang 7/1997 d^n tiiang 5/2000) bao gdm: he thdng md han coc bao vd bd Bdc sdng Tidn (bdn pha cu), ke phia Nam sdng Tien (t?i vj tri Uiu hep nhat). Trong khudn kho hop ddng giiia Cdng ty Maunsell Pty LTD (cua Uc) va Vien Khoa hpc Thuy lpi (cu), cong vide nghien ciiu hinh thdi sdng Tidn khu vuc cau My Thuan da duoc tien hdnh, trong do phdn quan trpng nhat la giai phap cdng trinh dn dinh bd sdng - dn dinh the sdng.
5. Tinh hinh sat Id bd sdng nhOng nam gan day Tinh hinh sat Id bd sdng Tidn khu vuc cdu My Thuan van didn bien phirc tap, do do can phai theo ddi su thay doi vd the sdng, sat Id bd; dieu nay cd hen quan den su an toan cua cau My Thuan.
Vao tiiang 08/2007, theo dng Thanh. Chi cue trudng Chi cue Thuy Ipi tinh Vinh Long cho biet, tinh dd quyet dinh di ddi dinh Tdn Hoa. di tich lich su van hod vdo ddt lidn de trdnh sat Id bd. Ngoi dinh chi each chan cdu My Thuan gan 400 m vd phia thuong luu. Nam sau, tiiang 09/2008, ddt vdn Id nghiem trpng, nhat la ven sdng Tien, khu dinh Tan Hod. Ke dinh Tan Hoa - Vinh Long duoc tiii cdng xong nam 2001, vao dau tiiang 6 nam 2005, 48 m ke hi day ra song theo gdc nghieng 45 do. Dua ti-dn cac tdi lieu tiiuc do tiiang 12/ 2009 cho tiiay chdn ke bj xdi kha nhi^u, CO chd sau han den 4 m [4].
D^n nam 2008, ke nay da hi sup hoan toan. Do dd xdi Id b doan sdng cong cd hd xdi sau nay sdm hay mudn cung 1dm cdng trinh bi mat dn dinh.
Vdo ngdy 26/11/2011, da xay ra sat Id a ben phd My Thudn; mdt doan sdng Tien dai khoang 30 m tiiudc xa Hoa Hung bj sat Id 1dm 4 ho ddn ti-ong vimg phai di ddi khan cap. Trudc khi bi sat Id, tai day da xudt hidn nhung vet niit dn sdu vao dat li^n tir 1-2 m.
Tinh trang sat Id bd sdng Tidn h-ong nhimg nam gan day van didn bien phiic tap.
NONG NGHIEP VA PHAT TRIEN NONG THON • KY 1 - THANG 5/2012 73
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Ngodi nhimg nguydn nhdn didn bidn do quy lu^t hr nhidn trdn tiiupng luu cua hd thdng sdng Tidn vd sdng Hau, cdn cd nguydn nhdn do tdc ddng ciia con ngudi, do vi^c khai thdc cdt ti-dn lan. Vi vdy didu cdp tiiidt hidn nay la cdn khdo sdt bd sdng, thdm dd dd sdu, do ddng chdy dd giiip vide qudn ly Idng sdng, qudn ly vide khai thdc cdt, giir dn djnh tiid sdng, dn dinh cdu My Thudn dat hidu qud tdt.
IV. KET LUAN
Hinh thdi long sdng tai vj tri cdu vd thupng luu Id qud trinh l\rang tdc phiic tgp giira cdc ydu td ddng chdy vdi dja tdng sdt vd cdt cdu thdnh Idng sdng vd bd sdng. Mpt sd ydu td chinh nhu sau:
- Lich sir su van ddng cua ddng sdng dd tao ndn su co h?p bd rdng sdng, d o ^ tii bdn phd tdi v; tri cau. Su CO hep Idng ddn ndy cd tdc dpng 1dm sdu ddy sdng.
- Diem cong gdp trong long sdng ndm tai bdn phd da anh hudng vdo hudng dong chay, 1dm sdu ddy sdng.
- Ddng chay hi anh hudng tridu m^nh. Trong nhidu nam, su dao chidu ddng chay sdng do thay doi ddng trieu tao ndn nhiing vimg nhi^u l o ^ ddng, thdnh phan ngang va diing ciia van tdc ddng nudc vd nhirng vimg ti^p giap rdng. Nhimg di thudng nay lam sau ddy sdng.
- Ddng chay tir rach Sa Dec chay vao sdng Tidn tai b^n pha. Khi nghidn ciiu hidn trudng, ddng chay cua rach Sa Dec do dupc nhd han 10% ddng chay ciia sdng Tien. Tuy nhidn trong miia lu Uianh phan ndy tdng mdt each dang kd. Sir tuong tac ciia dong chay cua Rach Sa Dec va ddng chay trong sdng Tidn cd tiie la mpt ydu td gdp phan 1dm tdng dd sau ciia ddy sdng. Nhirng thay ddi hinh thai tuong lai ciia viing do vide xdi bd Bdc sdng Tidn khoang 15 km phia tiiupng luu cau, cung cd tiid lam tang thanh phdn ddng chay eiia Rjtch Sa Dec.
Nghidn ciiu hidn trudng dam bao rang dia hinh day sdng trong vimg lan can hd xdi da khdng thay ddi dang kd h-ong vdng 15 ndm, kd tii ndm 1979, mdc dii dd xay ra nhiing ti-dn lu tan suat lan hon 20 nam mdt lan trong nam 1991 va 1994. Tuy nhidn ti-ong tiidi ky 15 nam hi 1962, hd xdi da djch chuydn vd phta ha luu khoang 1000 m.
Rd rdng rdng xdi bd Bdc sdng Tidn xay ra tiiudng xuydn tir ndm ndy sang ndm khdc ti"ong khi su thay ddi hd xdi chi xdy ra tiing tiidi ky. Hon niia vj tri vd hinh dgng ciia hd xdi cd lidn quan ch^lt chd vdi hinh d ^ g mat bdng cua sdng vd ddc bidt la su co h?p Idng dSn vd vj tri cua doan sdng cong. Do vdy sir dn djnh bd B^c sdng Tidn phia thupng luu la y^u td then chdt 1dm gidm thidu riii ro do nhiing thay d6i vd vj tri hodc hinh dgng ciia hd xdi trong tuong lai gay ra.
Cho ddn nay thd sdng Tidn khu vuc cdu My Thudn vdn dn djnh, nhung do cdc ydu td didn bi6 hinh thdi sdng khdng nhirng theo quy luat tir nhidn md cdn do tdc ddng con ngudi, ndn vide theo ddi didn bidn hinh thdi sdng Tidn khu vuc cdu My Thudn can ndn dupc tidp tijc. Ngodi ra chinh quydn dia phuong cung cdn quan ly ngdn cdm vide khai thac cdt tran lan trdn doan sdng nay.
TAI UEU THAM K H A O
1. Pham Dinh, 2006. Studies fa) Determine Dimensions for River Training Works in the Hanoi Segment of the Red River. Paper at Third International Conference on Scour and Erosion November 1-3/11/2006 Amsterdam, Pages. 198-199, (http://www.icse2006.org/).
2. Ld Ngpc Bich, 1991. Nghidn ciiu mdt sd van de didn bidn Idng sdng trong dieu kidn sdng ngoi Vidt Nam. Vidn Nghidn ciiu KHTL Nam bd.
3. Luong Phuong Hdu, Ld Ngpc Bich, 1993.
Nghidn ciiu ve tinh hinh bien dpng va quan hd hinh thdi sdng vimg ddng bdng sdng Cuu Long. NXB Ndng nghidp 9-1993.
4. Dinh Cdng San, 2009. Didu tia ddnh gid cac cdng trinh bao vd bd trdn hd thdng sdng Ciiu Long vd hd thdng sdng Sdi Gdn - Ddng Nai. Bdo cao dieu ti-a ca ban ndm 2008 - 2009, Vidn Khoa hpc Thu? loi midn Nam
5. Maunsell PLY LTD, 1996. Final Report on River Studies. My Thuan Bridge Project - Vietnam, Phase 3: Detailed Design and Documentation, October 1996.
6. Przedwojsky B. & Blazejewsky R., Pilarczyk K. W., 1995. River Traming Techniques, Fundamentals, Design and ^plications. A. A.
Balkema / Rott:erdam, Netiieriands / Brookfield, USA.
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STUDY OFTHE BIG SCOURHOLE MOVEMENT IN TIEN RIVERBED AND ITS EFFECTTO MY THUAN BRIDGE AREA
Pham Dinh Summary
In the framework contract between the Company Maunsell Pty Ltd (Australia) and the Center for River Engineenng Research • Institute of Water Resources (former), the morphology research in Tien river in Thuan bridge area has been conducted 1996 before the coiistiuction of bridge. These include "Study of large scour hole movement in Tien river ...". Large scour Iiole upstream of the My Thuan bridge, 48 meters deep as the deepest point. Because changes in the position of the scour hole could lead lo lower depth at the posiUon of die river bed at tiie bridge site, problems should have studied: - History of the movement of the river has created a surface conUaction wide river from the feny to the bndge position; the river width contraction has led to deep river impacts; - the bends in the river ferry was located at the dovmstream effects on the depth of tiie river bed; - the position and shape of the scour hole is closely related with die appearance by the river and in particular river width contraction led and location of the river bend. Thus the stability of the northern bank upstream of the key factors to reduce risks due to changes In posiUon or shape of the scour hole caused in the future. From research, building a stable world Tien river of My Thuan bridge area was conducted before and during bridge construction. So far Tien river position of My Tliuaii bndge area has been stable, but due to factors not only changes in the river as by a natural rule, but also human impacts is very complex, the Tien nver form monitoring and the My Thuan bndge area should also be continued.
Key words: Scour hole movement, nver morphology, naturalrule.
Ngudi phan bidn: PGS.TS. Trdn Qudc Tiiudng
