Nghien cufu & Trao doi
DANH GIA KHA NANG Tl^ LAM SACH CAC SONG CHINH HUYEN CAN GlCf DlTCfl ANH
H I / O N GCUA NlTCfC THAI NUOI TOM
TS. Nguyin Ky Phiing - Dai hoc Khoa hoc Tff Nhien Tp. HCM TS. Nguyin Thi Bay - Dai hoc Bach Khoa Tp. HCM
Ung dung md hinh lan truyen chd't mot chieu tinh todn qud trinh lan truyen cdc chd't DO, B0D5 tit nUdc thdi nudi tdm ra cdc sdng chinh dng vdi cdc kich bdn khdc nhau. Tii dd ddnh gid khd ndng tif Idm sgch cua cdc sdng chinh.
1. Gidi thieu chung
Kha nang tiep nhan chat thai ciia mot ddng sdng dffdc hieu la tai Iffdng chat thai cd the dffa vao ddng sdng sao cho ddng sdng vIn cdn kha nang tff lam sach de dat yeu cau sff dung, nghTa la dat tieu chuan chat Iffdng qui dinh cho muc dich sff dung.
Kha nang tff lam sach (KNTLS) ciia sdng la kha nang loai bd, giam thieu cac chat d nhilm thdng qua cac qua trinh bie'n ddi vat ly, hda hoc, sinh hoc xay ra trong ddng chay.
Kha nang tff lam sach ciia sdng phu thuoc vao:
- Lffu Iffdng nffdc sdng - Van td'c ddng chay - Do sau
- Do dd'c
- Do ud'n khiic ciia sdng
- Thanh phan hda hoc cua nffdc - Nhiet do nffdc
- Thffc vat trong nffdc (Aquatic macro- phyte): rong reu, beo, cay co v.v.
-Sif thdng khi - Thdi gian
Kha nang tff lam sach ciia sdng gdm 2 giai doan:
(1) Qua trinh xao trdn (hay pha loang) thuan tiiy ly hoc giffa nffdc thai va nffdc sdng.
(2) Qua trinh Idioang hda cac chat hffu cd chffa trong nffdc sdng (qua trinh hda ly, sinh hda, sinh hoc).
Sif pha loang ndng do chat d nhilm thuan tuy ly hoc gdm 2 qua trinh cd ban sau:
(i) Chuyen tai: sff van chuyen cac chat do ddng chay cua nffdc,
(ii) Khuye'ch tan: sff phan tan hay van chuyen cac chat do sff xao tron trong nffdc.
Hai qua trinh nay lam cho chat d nhilm dffdc van chuyen xa ngudn thai va lam ndng do chat d nhilm giam di.
Song song vdi sif pha loang ndng do, cac chat d nhilm cd kha nang bi phan hiiy sau vung tie'p nhan nffdc thai do bi hap thu, thiiy phan, hoac phan ra sinh hoc. Vi du sd Iffdng vi sinh tff nffdc thai sinh hoat khi dffa vao ddng sdng se bi giam do dieu kien mdi trffdng khdng phii hdp (thay ddi nhiet do,...). Mot sdtac nhan hda hoc (cac chat hiJli cd khdng ben vffng, am- moni, ...) tff nffdc thai cung cd the bi chuyen hda do cac phan ffng hda hoc.
2. Noi dung va phfftfng phap nghien cffu a. Ngi dung nghien cdu
Ngffdi phan bien: TS. Nguyin Kien Dung
40 Tap chi Khi tiTc^ng Thuy van * Thang 5/2008
Nghien cihi & Trao doi
A'.- Xay dffng md hinh toan tren mang sdng tinh toan lan truyen chat.
- Ap dung tinh toan lan truyen DO, B0D5 tren mang sdng Can Gid vdi nhieu kich ban khac nhau, tff dd danh gia kha nang tff lam sach cua he thd'ng sd'ng nay.
b. PhUdngphdp nghien cdu
Viec nghien cffu kha nang tie'p nhan tai Iffdng d nhilm va kha nang tff lam sach dang dffdc quan tam nghien cffu chii ye'u dffa vao cac mo hinh toan. Cac bffdc chu ye'u tinh toan kha nang tie^p nhan ngudn thai bao gdm:
- Xac dinh trffdng van tdc ddng chay (lffu Iffdng ddng chay) bang phffdng phap do dac thffc dia hoac qua md hinh tinh toan thiiy life.
- Xac dinh cac ngudn thai (vi tri, thai Iffdng, ndng do thai,...)
- Tinh toan phan bd' ndng do cac chat d nhilm doc theo sdng va thay ddi theo thdi gian.
Tren the' gidi cd kha nhieu md hinh tinh toan thiiy life va lan truyen d nhilm tren sdng nhff Qual2E, WASP,... dffdc sff dung rat rong rai.
O Viet Nam cung da cd 1 so' md hinh thiiy life va lan truyen chat dffdc xay dffng nhff Hy- droGIS, VSAP,...
De danh gia kha nang giam thieu d nhilm, cai thien mdi trffdng do cac bien phap tren, chiing tdi nghien cffu ap dung cac md hinh thuy life va md hinh chat Iffdng nffdc tien tie'n. Ke't qua nghien cffu se gdp phan xay dffng cd sd khoa hoc cdng nghe de cai tao mdi trffdng nffdc trong lffu vffc.
1) Md hinh nghien cUu
Md hinh gdm 2 module chinh: module thiiy life va module lan truyen chat d nhilm.
- Module thiiy life: dffdc xay dffng tren cd sd he phffdng trinh Saint -venant, sai phan theo sd dd Upwind trung tam.
- Module lan truyen chat: sff dung phffdng trinh bao toan vat chat (cac ye'u to'sau: chuyen tai, khuye'ch tan, phan hiiy, lang,...) cho cac cbi tieu mdi trffdng nffdc nhff: DO, BOD, nhiet do, do man, cohform, chat ran Id Iffng,...
2) Ifng dung tinh todn tren mgng sdng thUc te Phan mem dffdc ffng dung tinh cho mang kenh, sdng huyen Can Gid. Trong dd cd tdng so'545 mat cat ngang tren 53 nhanh vdi 59 hdp lffu, va 16 diem thai. Khoang each Idn nha't giffa 2 mat cat la 2433 m va nhd nha't la 20 m, bffdc thdi gian tinh (t = 5s).
Dieu kien ban dau va dieu kien bien Dieu kien ban dau va dieu kien bien thuy life
O thdi diem ban dau (t=0), cho mffc nffdc tTnh z=0 va Q=0 tai tat ca cac mat cat tren mang sdng.
- Dieu kien bien:
Mang song dffdc tinh vdi 7 dau bien. Trong dd cd 3 bien tren (trong sdng) d phia Bac va 4 bien dffdi (cffa bien) d phia Nam.
Dieu Iden ban dau va dieu kien bien truyen chat
(J thdi diem ban dau (t=0), gia sff ndng do cac ye'u to'can tinh (do man, BOD, DO,...).
- Dieu kien bien:
Bang 1. Gid tri nong dg tgi cdc diem bien
So" hieu nhanh
1 36 38 39 53 49 51
BiSn 1 2 3 4 5 6 7
BOD (mg/l)
6 6 13,3
2 2 2 2
DO (mg/l)
5 5,1
5 5,1
8 8 8
- Dieu ki$n tgi diem thai:
Cac thdng sd nay bao gdm vi tri cac diem
Tap chi Khi ttf(?ng Thuy van * Thang 5/2008 41
Nghien ciJu & Trao doi
thai, thdi gian thai, lffu Iffdng thai va ndng do thai. Trong bai bao nay cac ngudn thai chinh la nffdc thai do nudi tdm tap trung tai cac Idiu vffc chinh la Binh Khanh, Tam Thdn Hiep, Ly Nhdn, An Thdi Ddng cho ca cac loai hinh nudi:
ruong Ilia, ban cdng nghiep, cdng nghiep.
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Chiing ta se xet sff bie'n ddi ndng do BOD va DO theo thdi gian tai ba khu vffc nudi tdm d Can Gid vdi cac kich ban Idiac nhau. Trong qua trinh nudi nffdc thai nudi tdm se dffdc bdm ra trffc tie^p vao sdng ngdi. So' lan bdm ra tiiy thudc vao loai hinh nudi. Trong khudn khd bai bao chiing tdi khdng dffa ra cu the lffu Iffdng thai va ndng do thai tff cac vudng tdm.
* Kich bdn Men trgng:
Theo danh gia, tinh toan thi the tich nffdc thai cac khu vffc nhff sau: (cho tat ca cac loai hinh).
Binh Khanh: 20.022.300m\
An Thdi Ddng: 35.785.00m^
Tam Thdn Hiep: 9.120.200ml Ly Nhdn: 25.632.200ml
* Kich bdn qui hogch 5000 ha nudi tdm:
Khi tang dien tich nudi tdm len SOOOha thi Iffdng nffdc thai tff cac vudng tdm ra sdng se tang len .
Lfng vdi kich ban nay thi the tich nffdc thai tffdng ffng se la
Binh Khanh: 74.100.000 m^
An Thdi Ddng: 76.950.000 m^
Tam Thdn Hiep: 14.775.000 m^
LyNhdn: 114.000.000 m l
Lffdng nffdc thai nay gan nhff gap ba lan so vdi phffdng an hien trang (280 trieu m^ so vdi 89 trieu m^).
* Kich bdn qui hogch 7000 ha nudi tdm:
Khi tang dien tich nudi tdm len 7000ha thi Iffdng nffdc thai tff cac vudng tdm ra sdng se tang len.
Lfng vdi kich ban nay thi Iffdng nffdc thai ra cac sdng se la:
Khu vffc Binh Chanh: 103.740.000ml Khu vffc An Thdi Ddng: 107.740.000ml Khu vffc Tan Thdi Hiep: 29.930.000ml Khu vffc Ly Nhdn: 159.600.000ml
Tdng cdng: 398.900.000m3(ga'p 4,5 lan so vdi phffdng hien trang)
Tdng Iffdng nffdc thai khi qui hoach 7000 ha trong 1 miia vu se Idn gap 4.5 lan so vdi phffdng an hien trang.
3. Danh gia ke't qua mo hinh
+ Nhan xet ve BOD:
- Khu vffc Ly nhdn:
Nhff vay ta tha'y Iffdng nffdc thai ra sdng Soai Rap tff Ly Nhdn la rat Idn va ndng do BOD thay ddi phu thuoc vao cac thdi ky xa nffdc ra rat rd net.
Ne^u trffdc mua vu nudi ndng do BOD la 2 mg/l thi sau mdi lan nffdc tff cac ao nudi tdm thai ra sdng thi ndng do BOD tang len kha rd net va sau do bi pha loang vdi nffdc sdng giam xudng kha nhanh, trung binh khoang 6mg/l - tai ndi gan vung thai (hinh 1) va trung binh khoang 3 mg/l - tai nhffng ndi xa vung thai hdn (hinh 2). Dieu nay chffng td kha nang pha loang d sdng Soai Rap la rat Idn. Ndng do BOD d sdng sau khi tie'p nhan ngudn thai tff cac ao tdm vIn dat Tieu chuan cha't Iffdng nffdc mat TCVN 5942-1995
42
Tap chi Khi ttf(;fng Thuy van * Thang 5/2008
Nghien cufu & Trao doi
Dd'i vdi phffdng an 5000ha thi ndng do BOD cua sdng sau khi tie'p nhan nffdc thai cua cac vudng tdm se tang cao, xap xi 10 mg/l - ngay tai khu vffc tie'p nhan nffdc thai (hinhl).
Sau dd, ndng do nffdc giam xud'ng. Ve phia ha lffu khu vffc tie'p nhan nffdc thai ndng do BOD cung tang cao sau mdi ddt nhan nffdc thai, tang de'n 5 mg/l (hinh 2).
Va phffdng an 7000ha thi ndng do BOD trong sdng tang rat cao sau mdi lan nffdc thai tff vudng tdm ra sdng. Tai cac khu vffc nffdc thai ra sdng ndng do len ddn 14mg/I (hinh 1), cdn d khu vffc ha lffu sau ndi nffdc thai ra song thi ndng do thay ddi it hdn, tang len 6mg/l (hinh 2). Va sau dd ndng do giam xud'ng nhieu do pha loang vdi nffdc trong sdng.
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"^* I—Hi&ifrang —aiOOha 14 T-
12 10
7000 ha
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Hinh 2. BOD tgi sdng Soai Rgp (xd Ly Nhdn )
BOD t^ mltcat2 rtarii 39
— H i f n trang — 5 i 7000 ha
— I 1 r 1 1 1 1 1
337 505 673 841 1009 1177 1345 1513 #
Hinh 3. BOD tgi sdng Soai Rgp (Cda Soai Rgp )
- Khu vffc Binh Khanh:
Tai khu vffc Binh Khanh ndng do BOD nen kha cao vi day la Idiu vffc nam phia Bac huyen Can Gid, chiu anh hffdng trffc tie^p nffdc tff thffdng lffu do cac boat dong kinh te'xa hoi.
Cac lan nffdc thai nudi tdm xa ra sdng ta tha'y ndng do BOD cd thay ddi nhffng khdng rd net, trff lan thai cud'i ciing lam ndng do BOD tang len va sau dd se hi pha loang vdi nffdc sdng (hinh3.)
Vdi kich ban 5000ha, ndng do BOD thay
ddi kha nhieu so vdi kich ban hien trang. Ne'u d phffdng an hien trang sau mdi lan nffdc thai ra sdng ndng do BOD trong sdng cd thay ddi nhffng khdng rd net, chi dao dong trong khoang 4 - 5 mg/l. Cdn d phffdng an nay BOD d sdng tang len den 6-7 mg/l va sau dd giam xudng d mffc 4 mg/l. Ve phia ha lffu sau ngudn thai, ndng do BOD cung thay ddi kha nhieu mdi lan thay nffdc, len de'n hdn. Chffng td rang ffng vdi phffdng an 5.000 ha qui hoach ro rang la nffdc thai ra sdng anh hffdng trffc tie'p de'n ndng do nffdc sdng trong tffng thdi doan (ffng vdi cac thdi gian xa thai ra sdng). Sau do ndng do trong
Tap chi Khi tufgTng Thuy van * Thang 5/2008
43Nghien cuTu & Trao doi
sdng giam xudng dang ke do pha loang va lan 8 mg/l (hinh 3) sau dd giam xud'ng xap xi truyen. 4mg/l. Vung dffdi khu vffc xa thai thi ndng do
Phffdng an qui hoach 7000ha ta tha'y ndng ^ ^ ^ ^^""^ ^°"g *ay ddi It hdn.
do sau mdi lan nhan nffdc thai tang len de'n gan
mgJ
BOD tai matcat23nhanh3
-Hientrang SOOOha 7000ha
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1 169 337 505 673 841 1009 1177 1345 1513 Si"
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Hinh 4. BOD tgi song Long Tau (Gdn ngd ba Dong Tranh) - Khu vffc An Thdi Ddng:
O khu vffc nay ndng do nen BOD ciia nffdc sdng xap xi 2 mg/l. Sau Idii nhan nffdc thai tff cac ao nudi, ndng do BOD cd thay ddi (tang len) ffng vdi cac thdi diem xa thai, dac biet la luc xa toan bo nffdc nudi tdm ra sdng (hinh 4).
Vdi phffdng an qui hoach 5000ha ndng do
BOD d sdng hau nhff khdng thay ddi gi so vdi phffdng an hien trang (hinh 4).
Phffdng an 7000ha, ndng do BOD trong sdng khdng cd bie'n dong gi nhieu so vdi phffdng an 5000 ha va hien trang (hinh 4).
Dieu nay ciing dl hieu vi nffdc thai ra d khu vffc nay it hdn ra't nhieu so vdi Idiu vffc khac.
BOD tai mat cat 9 nhanh 31
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1 169 337 505 673 841 1009 1177 1345 1513 3'"
Hinh 5. BOD tgi sdng Long Tau
DO tang len.
+ Nhdn xet ve DO:
- Khu vffc Ly Nhdn:
Ddi vdi Oxy hda tan (DO) cung cho ta ke't qua tffdng tff ve kha nang pha loang ciia nffdc sdng. Rd net nha't la d khu vffc Ly Nhdn (hinh 5,6). Nffdc thai tff cac viing nuoi tdm cd ndng do DO tha'p, sau khi thai ra song se dffdc pha loang va do be mat trao ddi khi Idn, ndng do
44 Tap chi Khi tifi^ng Thuy van * Thang 5/2008
Vdi phffdng an 5000ha, sff thay ddi DO trong sdng ffng vdi moi lan xa thai tffdng tif vdi phffdng an hien trang.
Va phffdng an 7000 ha bie'n ddi ndng do DO
d cac khu vffc nudi tdm tffdng tff vdi phffdng an
5000 ha, khdng cd gi bie'n ddi nhieu.
Nghien ciJu & Trao doi
D Otai mat cat 14 nhanh29
""3" I Hjgnfrang SOOOha 7000ha|
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1 169 337 505 673 841 1009 1177 1345 1513 gio
Hinh 6. DO tgi sdng Soai Rgp
DO tai mat cat 2 nhanh 39
mg/l — Hientang SOOOha 7Crj0ha
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— I 1 1
1 16S 337 505 673 841 1009 1177 1345 1513 gia
Hinh 7. DO tgi sdng Soai Rgp - Khu vffc Binh Khanh:
Vdi phffdng an hien trang thi DO cQng thay ddi tffdng tff nhff khu vffc Ly Nhdn nhffng khdng rd net bang.
Nhffng vdi phffdng an 5000ha thi sif thay ddi DO d khu vffc nay la dang ke sau mdi lan xa thai, nd lam ndng do DO giam xud'ng den gan
3 mg/l sau moi lan nffdc thai tff vudng tdm thai ra sdng, va sau dd tang len lai xap xi 5mg/l (hinh 7).
Va vdi phffdng an 7000 ha thi mdi lan xa thai DO cd the xudng rat tha'p gan 2mg/l va sau dd tang len lai xap xi 5mg/l.
mg/l'
DOt^ii m 8 t c a t 2 3 nhiainih 3 H i f n i t r ^ n g S i B O t e TiOilEi! h s
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1 1S9 337 505 813 841 n03 T177 1345 1513 giio
Hinh 7. DO tgi sdng Long Tau
S T 7
DO' fail «i # t e ^ t 3 nh^fi'hi 3'1 - H fSni frgng SOQSHs 7000 hs
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1 169 337 505 673 341 1009 1177 1345 1513
Hinh 8. DO tgi sdng Long Tau - Khu vffc An Thdi Ddng:
Vdi phffdng an hien trang thi DO ciing thay ddi tffdng tff nhff khu vffc Ly Nhdn nhffng khdng rd net bang. Va cac phffdng an khac thi
sff bie'n ddi DO Ididng dang ke.
Danh gia chung: Bang ket qua tinh toan vdi cac phffdng an khac nhau, bao cao da cho tha'y sff thay ddi ndng do nffdc sdng lffu vffc Can Gid trong sudt 1 vu mua nudi tdm. Day la bai toan
md phdng dau tien tff trffdc de'n nay vdi thdi gian dai nhff vay. Qua ket qua tinh toan cho tha'y ndng do BOD, DO trong sdng tai cac khu vffc tiep nhan nffdc thai tff cac vudng nudi tdm thay ddi Idia nhieu sau moi ddt tie^p nhan nffdc thai, sau dd ndng do BOD giam dan va DO tang dan do pha loang vdi nffdc song va anh hffdng ddng chay. Qua cac kich ban nhan tha'y rang kha nang tff lam sach d khu vffc nay cdn rat Idn ffng vdi cac qui hoach SOOOha, 7000ha.
Tap chi Khi tifgfng Thuy van * Thang 5/2008
45Nghien ciJu & Trao doi
Tai li$u tham khao
1. Bui Lai (Vien Sinh Hoc Nhiet Ddi), Nguyen Thi Kim Lan (Phdn Vien KM Tugng Thuy Vdn Vd Mdi Trudng Phia Nam), Le Thi Quynh Hd (Vien Ca Hoc lfng Dung), "Nghien cdu qud trinh d nhiem hUu cd trong ao nudi tdm su cdng nghiep".
2. Huynh Thi Minh Hang (2005), Bdo cdo De tdi: "Xdy difng chuang trinh vd tien hdnh quan trac mdi trudnng lUu vifc sdng Sdi Gdn - Dong Nai", Vien Tdi nguyen vd Mdi trUdng.
3. Le Thi Sieng vd CTV (2003), Bdo cdo tdm tat: "Nghien cdu dien bien mdi trUdng nUdc do hogt ddng nudi tdm tgi cdc tinh Bgc Lieu, Cd Mau dnh hudng tdi mdi trUdng vd de xuat cdc bien phdp khdc phuc ", Vien Khoa Hoc Thuy Lgi Mien Nam.
4. Nguyen Vdn Luc (1999), tap bdi gidng "Gidm sdt chd't lugng mdi trudng vd mdt so phuang phdp gidm thieu tdc ddng xa'u do hogt ddng nudi tdm", Vien Hdi DUdng Hoc Nha Trang.
5. UBND Huyen Can Gid (2005), "Mdt so ddc diem chinh ve dieu kien tU nhien, thuy sinh vdt, ngudn lgi thuy sdn vd phdn vung nudi thuy sdn cua huyen Can Gid"
6. UBND Huyen Cdn Gid (2005), bdo cdo: "Tdng ket 5 ndm ddu tU thdy lgi phuc vu nudi tdm ndm 2000 - 2004 vd ke hogch ndm 2005 -2010".
7. J.A.Cung, F.M.Holly, A.Verway (1980), Practical aspects of computational river hydraulics.
Pitman Advanced Publishing Program.
