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M O T S O N H A N X E T V E N H U N G T A C D O N G C U A C A N 3 A N G N U f i C H O D A U T I E N G D E N M O I T R U T j l N G K H U V U C
1. Mo dau
Ho Dau Tieng la mpt trong nhQng cong trinh thuy lpi ldn nhat nUdc ta, nam trong khu vpc Dong Nam Bp 3 khoangl 1°20'-12o00'vi bac va 106°10'-106o10' kinh dong, giua hai tinh Tay Ninh va Binh Duong, gan bien gidi vdi Campuchia. Ho co nhiem vu khai thac tai nguyen nddc mat thUdng Idu song Sai Gdn nham lam bien chuyen mpt each sau sac nen kinh te khu vuc Dong Nam Bp vdi viec cat lu trong miia mda, cap nude tudi (hdn 170.000ha), nddc cong nghiep, nUdc sinh hoat cho mpt vung rpng ldn bao gom cac tinh Tay Ninh, Binh Dddng, Long An. Tp. Ho Chi Minh va tham gia day man d ha du trong mua khd.
Cac quy trinh hoat dpng cua ho cd moi tUdng quan chat che vdi dieu kien thuy van. Vi vay nghien cdu, danh gia can bang nude ho la rat can thiet nhat la trong tinh hinh td ngay ho dupe khdi cong xay dung (12/01/83) de'n nay tai khu vdc ho chua cd cac hang muc khao sat va nghien cdu thuy van dang ke nao.
2. Can bang nude ho Dau Tieng
2.1 Can bing nddc lifu i/i/fc.'Theo sd lieu 26 nam td nam 1977 - 2001, trung binh hang nam luu vdc hdng nude nhan dUdc lupng mua 1870mm, tao ra dupe 2,03 ty m3 ddng chay, dp sau dong chay: 752mm, bdc hdi:1418 mm.
• TS. B u i Di?c T u a n
Phdn Vien KTTV&MTPlu'a Nam 2.2 Can bang nddc trong long ho: Theo sd
lieu 5 nam td 1997 - 2001 hang nam cd lupng nude de'n la 77.04 m7s. LUdng nddc ra cung 77.04 m7s bao gdm chay qua tran 11.77 mVs, cdng sd 1 la 20,74 m7s, cdng sd 2 la 33,82 m3/s, cdng sd 3 (3 nam td 1999 - 2001) la 4,11 mVs va ton that la 6.60 mVs..
So vdi thie't ke, Iddng ddng chay thddng ngudn dd vao hd Dau Tieng trong cac nam qua cd tri sd thap, va cd xu hudng ngay cang thap.
Dieu nay lien quan den xu the chung toan khu vuc cung nhu de'n tac dpng kinh te xa hpi nhu khai thac gan nhu triet de rdng tu nhien d dau ngudn.
3. Nhufng tac dpng cua can bang nuoc ho Dau Tieng den moi triidng khu vdc
3.1. Tac dong len che do khi hau vung ho Trong nhdng nam qua vung Dau Tieng vdi sU xuat hien cua hd Dau Tieng cd nhdng bien ddi nhat dinh ve mat khi hau. Sd bien ddi nay
• Tap san KHOA BBC & CONG NGHE 43
• diln ra kha philc tap lien quan den khong nhOng sii bien doi khi hau chung cua toan khu vpc ma con den su bien ddi cua cue bp dja phudng nhu sp xuat hien hd nude vdi mat thoang 270km2, su mat phan ldn rdng dau ngudn...
Den nay, do nhOng han che ve so lieu, nen nhufng thay ddi dinh lupng cac ye'u to khi hau trUdc va sau khi xay hd van chua dUdc xac djnh day dii nhung cd the dUa theo ket qua nghien cufu danh gia tac dpng moi trudng hd Trj An, la mpt hd chufa cd dieu kien tUdng tu cd the Udc doan nhusau:
- Phan bd am: Trong cac thang mua kho (td thang XII nam trudc de'n thang IV nam sau) dp am tUdng ddi cua khong khi sau khi cd hd tang len khoang 3 - 4% so vdi trudc khi xay hd trong khi trong cac thang mua mUa hau nhu khong cd sU thay ddi. Dieu do cd the hieu dUdc rang, trong mua mUa dp am khong khi khong chi phu thupc vao lupng bdc hoi nude td mat hd ma phan quan trpng hdn la do lUdng mUa chi phdi va dp am khong khi luon luon ldn tren dien rpng lam lu md vai tro cung cap hoi nude tii mat hd.
- Phan bd nhiet: Su bien ddi nhiet dp khong khi vung hd lien quan de'n kha nang cua khdi lupng nude trong hd ve tang trO va dieu tiet mpt lUdng nhiet gap nhieu lan so vdi trudc khi cd hd, lam dieu hda nhiet dp trong chu ky ngay dem va nam, lam thay ddi cac gia trj cUc doan:
• Lam giam nhiet dp toi cao: Nhiet dp toi cao trung binh thdi ky td 1960 - 1968 (trUdc khi xay hd) la 32,70c, cdn thdi ky tii 1990 -1998 (sau khi xay hd) la 32,20c.
• Lam tang nhiet do tdi thap: Nhiet dp tdi thap trung binh thdi ky tii 1960 - 1968 (trUdc khi xay hd) la 22,00c, cdn thdi ky tii 1990 - 1998 (sau khi xay hd) la 23,30c.
- MUa: Tang sd lan mua nhd va cac hien tupng lien quan (sUdng, sUdng mii, may thap) so vdi trUdc khicd ho.
3.2. Tac ddng len che dp thuy van vung ho NUdc sdng Sai Gdn tai Dau Tieng vdi lUu
lupng trung binh nhieu nam khoang 62m7s dupe tich vao hd va dUdc dieu tiet theo cac kenh din, hoac xa qua dap tran. Khi nude chay vao hd, tdc dp ddng chay giam nhieu, tien dan tdi 0 m/s, muc nude tang cao. Su bdc hdi dien ra tren mat thoang hd khac vdi tren be mat luu vpc. LUu lUdng dieu tiet qua cac kenh va dap tran hoan toan khac vdi ddng chay tren song tu nhien. Nhiet dp khdng khi va dp am khong khi khu vuc cung thay ddi.
• MUc nircfc ho
- Muc nude miia lu: Trudc khi xay hd, mUc nude trung binh thang cac thang miia lu cua sdng Sai Gdn tai Dau Tieng bien ddi td 73- 131cm. Sau khi xay hd, mua lu la mua tich nUdc nen mpc nUdc trong hd tang dan td dau miia khoang 1.800cm de'n cudi miia khoang 2.450cm.
- Muc nUdc miia can: trudc khi xay hd, mUc nude trung binh cua song bien ddi trong khoang 59 - 75cm. Cdn sau khi xay hd, mua can la thdi gian hd cap nude cho ha du nen mUc nude giam dan tii dau de'n cudi miia, bien ddi td 2.450- 1.800cm.
• Dp sau ho: TrUdc khi xay hd, dp sau song khoang 6 - 7m. Sau khi xay hd, dp sau ho khoang 6 - 10m, cd ndi 20m.
• Mat thoang: Trudc khi cd hd, mat thoang nUdc sdng chiem dien tich khdng dang ke. Sau khi xay hd, dien tich mat thoang dat 27.000 ha.
• Tdc do dong chay: trUdc khi cd hd, tdc dp ddng chay trong sdng tu nhien cd the dat 1- 1,7m/s. Sau khi xay hd, tdc dp ddng chay trong hd rat nho, chitd 0 - 0,05m/s.
• HUdng chay: TrUdc khi xay hd, hudng chay ddng nUdc tru ng theo hudng cua ddng song. Sau khi xay hd, hudng chay trong hd khdng ddng nhat, phu thupc nhieu vao gid.
Tudi thp cua hd phu thupc nhieu vao lupng biin cat den hd. Lupng ngam cat trong nude sdng chay vao hd (g/m3) cd gia trj nhd trong miia can va ldn trong miia lu va thudng dat gia trjcUcdaivaothang IX hang nam, nhUsau:
I II III IV V V I VII VIII IX X X I XII
177 191.4 148,2 184,7 98.1 212,1 252,1 107,9 605 542 268,7 179,4
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• 3.3 Tac dpng len sUxam nhap man d ha du a. Dien bien man khi chua xay dung cong trinh Dau Tieng va Tri An.
Khi chUa co cac ho chute d thUdng nguon thi vi tri ranh gidi man tren he thdng song Dong Nai hoan toan phu thudc vao cudng do trieu va che dpthuyvanthUdng nguon.
• Tren song Dong Nai gidi han man S = 4%o dting lai d nga ba Long Dai va S = 1 %o dting lai d dudi Hda An.
• Tren sdng Sai Gdn: Sti xam nhap man tren sdng Sai Gdn (va Vam Co Dong) cd nhieu nguyen nhan, trong dd chu yeu la 2 yeu to: Trieu bien Ddng va ltidng ddng chay tti thUdng nguon.
Ndi chung, trong mua lu ddng chay tti thtidng nguon ldn nen man khdng anh hudng sau vao sdng va trong mtia can thi ngtidc lai. Gidi han man S = 4%o len de'n tren Lai Thieu va S = 1%o len den Thu Dau Mdt (cung vao nam 1977- 1978).
• Tren song Vam Co Ddng, gidi han man S = 4%o vupt qua tram thuy van Hiep Hda va S = 1 %o len tren tram thuy van Gd Dau Ha vao cac nam 1977 - 1978 dp man tai Ben Ltic dat 23,88g/l.
b. Diln bien man khi cd ho Dau Tieng.
• Tren sdng Dong Nai: Qua ddt xa thi nghiem cua ho Dau Tieng nam 1987 cho thay rang tac dgng lam giam dp man chi xay ra nhanh d nhting vung xa ctia song, tti Thu Dau Mdt de'n Lai Thieu. Tti Lai Thieu de'n Phu An, diln bien man v i n chu yeu phu thupc vao dp man tren sdng Dong Nai (vi du mtia khd nam 1986 mac du ho Dau Tieng cd sti cd' phai xa den Qmax = 860 m3/s nhung dp man chi giam nhanh tti Lai Thieu trd len thUdng nguon, tti Lai Thieu trd xudng dp man giam it, tren sdng Dong Nai dp man khdng giam. Dieu nay cd the giai thich bdi trieu tren sdng Dong Nai d vting ctia sdng cdn rat manh nen tac dpng cua hd Dau Tieng chi anh hudng nhting khu vpc xactia bien.
• Tren sdng Sai Gdn: Khi hd chtia van hanh vdi luu ltidng xa dn djnh Q = 20m7s gidi han man S = 4%o dting lai d Lai Thieu va thdi gian duy tri chi xuat hien vao nhting ngay trieu ctidng cua thang IV, va S = 2%o khdng cdn keo dai nhieu thang nhtitrudc, S = 1%o day liii xudng dtidiThu
Dau Mpt.
• Tren sdng Vam Co Ddng: Tti sau nam 1985 nhd ltidng ntidc hdi qui tti khu ttidi kenh
Ddng, khu ttidi kenh Tay va ltidng ni/cfc xa trtic tiep xudng sdng Vam Co Ddng qua kenh Phtidc Hdi, Ben Dinh... do dd tinh hinh nhiem man tren sdng Vam Co Dong dtidc cai thien rat nhieu. Tai Hiep Hda dp man S = 3,7%o chi xuat hien vao cudi thang IV va keo dai chi den 15-V hang nam.
c. Diin bien man khi cd hd Dau Tieng va Tri An.
• Tren sdng chinh Ddng Nai: Gidi han man 4%ova 1%o dtidc day Itiive ha du 8 -10 km, gidi han man 4%o bj day lui den Cat Lai. Cac hinh sau cho thay sti thay ddi dp man cua tram Cat Lai giufa nam 1983 (khi chtia cd hoat dpng cac hd va nam1989 khi cd cac hd Dau Tieng va Tri An.
• Tren song Sai Gdn: Dp man tren sdng dtidc cai thien. Gidi han man 4%o day lui tdi tan Thu Thiem, gidi han man 1%o d dudi xa ctia Rach Tra, tti Lai Thieu trd len dtidc xem la vung hoan toan ngot.
• Tren sdng Vam Cd: Tti Hda Hiep trd len da dtidc nggt hda.
3.4 Tac dpng den dan sinh kinh te
• Trong nhting nam qua hd da phat huy tac dung trong viec ttidi cho 93.000 ha cay trdng cung nhti cap ntidc sinh hoat va ntidc cdng nghiep cho cac khu dan cti cua cac tinh Tay Ninh, Long An, Binh Dtidng va Tp. HCM, cat ID sdng Sai Gdn trong mtia ID va tham gia day man d ha du cho ca song Sai Gdn lin sdng Vam Cd trong mtiacan.
• Man bj day lui xudng dudi Rach Chiec tren sdng Sai Gdn (va sau khi hoan thanh cdng An Ha tren kenh An Ha, man bj day xudng dtidi Cau Xang, lam ngot hoa toan bp kenh Thay Cai, Rach Tra) va xudng gan Ben Ltic, Long An tren sdng Vam Co Ddng. Do ranh man tren 2 sdng Vam Co Ddng va Sai Gdn bj day lui trong mua kiet, mot dien tich rdng 41.140 ha (Binh Dtidng 4.000 ha, Long An 16.599 ha, Tp. HCM 9.000 ha, Tay Ninh 11.649 ha) dupe nggt hoa tao ngudn, trong dd cd 34.140 ha dtidc ttidi bang trieu.
• Chat ltidng ntidc hd Dau Tieng ndi chung la tdt the hien ham ltidng cac chat trong ntidc trong hd chtia Dau Tieng nam trong pham vi ntidc sach (pH bien dgng trong khoang 6,7-7,0, SS trong khoang 6,2-30mg/l, g photpho:0,03-0,05 mg/l, N03: 0,02-0,05
COD:6,4-21,3 mg/l; BOD: 6,5-11,7mg/l;
• Tap sao KHOA HOC & CONG NGHE 45
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sat:0,2-0,7mg/l)...
• Mat khac, ho cung tao mot so thay ddi bat loi nhu lam xuat hien hien tuong phu du6ng trong hd hien tupng lay thut, gia tang nhilm phen cf mpt so ndi gay bat ldi cho trong trot va xay cat.
4. Ket luan va kien nghj A. Ket luan
• Hd Dau Tieng la cdng trinh thuy ldi ldn nhat nUdc ta vdi dien tich mat thoang 270 km2, dien tich hdng nUdc 2700 km2, dung tich 1,5 ty m3. Trung binh hang nam lUu vpc hiing nUdc nhan dupe lUdng mua 1870mm, tao ra dupe 2,03 ty m3ddng chay, dp sau ddng chay: 752mm, bdc hdi:1418 mm; hang nam cd lupng nude den la 77.04 m3/s. Lupng nUdc ra cung 77.04 m3/s bao gom chay qua tran 11.77 m3/s, cdng so 1 la 20,74 m3/s, cdng sd 2 la 33,82 m3/s cdng sd 3 (3 nam tit 1999 -2001) la 4,11 m3/s va ton that la 6.60 m3/s.
• Trong nhufng nam qua hd da phat huy tac dung trong viec tudi cho 93.000 ha cay trdng cung nhu cap nude sinh hoat va nude cdng nghiep cho cac khu dan cu cua cac tinh Tay Ninh, Long An, Binh Duong va Tp. HCM, cat lu song Sai Gdn trong mua lu va tham gia day man 6 ha du cho ca sdng Sai Gdn lin sdng Vam Cd trong mua can. Sti xuat hien cua hd da cd tac ddng tich cue den mdi trudng khu vuc (lam tang muc nude ngam, lam khi hau ven hd bien ddi theo chieu hudng dn hoa hdn), ddng thdi cung cd mpt so it tac ddng tieu cue (lam xuat hien hien tUdng phu dudng, hien tupng lay thut, hien tupng gia tang nhilm phen).
• So vdi thiet ke, lupng ddng chay thUdng nguon dd vao hd Dau Tieng trong cac nam qua cd tri sd thap, va cd xu hudng ngay cang thap.
Dieu nay mpt mat cd the lien quan den xu the
chung toan khu vuc. Mat khac cd the lien quan den tac dpng kinh te xa hdi nhu khai thac gan nhu triet de rifng tu nhien, hoac chan ddng d dau ngudn. Hau qua cudi cung la hd khdng du nUdc de sd dung he't cdng suat thiet ke, anh hudng de'n tinh phucvu cua ho.
B. Kien nghi
• Lap cac tram thuy van chinh quy d cac cda sdng dd vao hd tai cac cau Tdng Le Chan, Sudi Ngd, Tha La de xac djnh chinh xac lUu lUdng vao hd vadubaolu.
• ThUc hien nghien cdu phu dudng va bdi lang Idng hd; thpc hien nghien cdu tac ddng mdi trUdng hd Dau Tieng.
TAI LIEU THAM KHAO
1. Nguyin Dinh Ninh, Nguyin Xuan Thanh, Bui Ddc Tuan, Nguyin Khac Sdn. Bp NN &
PTNT, 2.1998. Danh gia sa bp dong chay thuang nguon va chat lUOng nUdc long ho Dau Tieng nam 1997.
2. Bui Ddc Tuan. Tap san KHKT, Tdng cue Khi tuong Thuy van, thang 6/1997. Ho Dau Tieng vdi viec khai thac tai nguyen nude thudng luu song Sai Gdn.
3. Bui Ddc Tuan, 15th International postgraduate course, Roorkee University, India,
1987. Project report on Hydrologic design on dam Irrigation Project Burma,
4. Bui Ddc Tuan, de tai nhanh cua di tai KT02-15, Vien Ky Thuat Nhiet Ddi, 1994. Dac diem thuy van trUdc va sau khi co hd Tri An.
5. Bui Ddc Tuan, Nguyin Hdu Nhan, di tai nhanh cua di tai KT02-15, Vien Ky Thuat Nhiet Ddi, 1994. Budc dau nhan xet bdi ling ho Tri An.
6. Bui Ddc Tuan, Trung Tam Khi TUdng Thuy Van Phia Nam, Tp, HCM 1995. Bao cao ket qua khao sat so bp ddng chay nguon nude, dp sau day ho va chat luong nude ho chda Dau Tieng, Tay Ninh.
7. Bui Ddc Tuan, Le Mdc, Phung Chi Sy, TrUdng Van Hieu, Nguyin Van Trong, Trung Tam KTTVphia Nam, 2000. Dac diem khi hau va thuy van tinh Tay Ninh.
8. Bui Ddc Tuan, Trung Tam KTTVphia Nam, 2002. Can bang nUdc ho Dau Tieng Tay Ninh. •