MOT SO DAC TRU^NG PHAN BO
T R A MTICH BE MAT TREN
P H A NCHAU THO
N G A MVEN BO BIEN DONG
B A N GSONG ClTu LONG
Nguyen Trung Thanh^^'. Phiing Van Phach*^', Nguyen Hong Lan*'\ Daniel Unverricht^^' (1) Vien Dja chat va Dja vat ly Bien; (2) InstiUite of Geoscienees, Kiel University Abstract:
Distribution of some sedimentary characteristics on the subaqueous delta of Mekong Delta has been investigated sparsely. This research were carried out on
190 surface samples collected in the field trips in 2007 and 2008. Distributions of grain size parameters, sand content indicate that: (1) The general trend of surface sediment becomes finer from the river mouths towards the coast Bac Lieu, then becomes coarser around Ganh Hao river mouth and finally gets jiner towards Ca Mau Penisular; (2) five deposition regions were separated relativelv from Mekong river mouths to Ca Mau Peninsula consisting of the dominating sandv regions in front of Mekong river mouths and near the Ganh Hao river mouth, the dominating muddy regions occurs along the coast of Soc Trang-Bac Lieu and in
West of Ca Mau Peninsula, and the sand-mud deposition region from the middle place of Ganh Hao and Bo De to Hon Khoai Island.
1. Mddau
Chau thd Mekong la mot chau thd Idn thir ba cua Chau A bao gdm cae phan dien tich tren lanh thd Cam Pu Chia, Thai Lan va Viet Nam. Trong dd phan dien tich chau thd tai Viet Nam chiem den 74% va dugc ggi la ddng bang sdng Ciru Long (DBSCL). Khu vuc nghien ciru la phan chau thd n^ap nude thudc viing bien ven bd ddng bang sdng Cim Long (DBSCL) tir cira sdng Djnh An den ban dao Ca Mau (hinh 1) cd do sau khdng qua 20-25 m nude.
Viing bien ven bd DBSCL chju anh hudng Idn bdi che do thuy trieu vdi dac tmng ban nhat trieu cd bien do thiiy trieu khoang 3-4 m d khu vuc phia tmdc cac cira sdng [1]. Khu vuc phia tay ciia ban dao Ca Mau chju anh hudng thiiy trieu ciia khu virc Vjnh Thai Lan vdi che do nhat trieu cd bien do thuy trieu khoang 0.5 - 1 m [1]. Ddng thdi, che do ddng lire (sdng va ddng chay) tai khu vuc ciing chju sir chi phdi ciia che do gid miia, gid miia ddng bac trong miia ddng va gid miia tay nam trong miia he.
Nghien cim dac diem phan bd tram tich tai viing ven bien DBSCL cdn it. Trong De an thanh lap ban dd Dja chat va khoang san vimg bien ndng ven bd (1/500.000) tir 0 den 30 m nude do Tmng tam dja chat va Mdi trudng Bien thirc hien dudi su ehii tri ciia Nguyen Bieu [3], da thue hien lap tmdng tram tich trong khu virc ven bien DBSCL. Tuy nhien, cac van de chi tiet ve mot sd dac diem tram tich van chua dugc vach ro nhu phan bd kich thudc hat tmng binh, ham lugng eat, bim, do chgn Igc... Trong khi nhirng thdng tin dac diem tram tich nay tren viing chau thd ngam cd y nghia cho nhieu nghien cim lien quan nhu van chuyen, tich tu tram tich, d nhiem, sinh thai bien.. .Ranh gidi giiia chau thd ngam vdi them trong da dugc xac djnh thdng qua cae tai lieu dja chan ndng phan giai cao thu thap trong khu virc [4].
2. Tai lieu va phuang phap nghien cuii 2.1. Tdi lieu nghien ciru
Tai lieu diing cho bai bao nay la kit qua phan tich 190 mau trdm tich b l mat thu thap dugc trong hai chuyin khao sat biin ven bd DBSCL MEKONG 2007 va MEKONG 2008 trong khudn khd de tai hgp tac Viet Nam-CHLB Dire "Nghien ciru tiin hda dai ven biin ddng bdng sdng Ciru Long vd viing thim luc dia ki can trong Holocene-hiin dgi phuc vu phdt triin bin virng\ Cac mau tram tich nghien ciru chu ylu thudc phdn chau thd ngap nude tir cira Cung Hdu den ban dao Ca Mau (hinh 1).
Tieu ban: Khoa hgc cong nghe Bien ISBN: 978-604-913-015-1
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Hinh 1: Khu virc nghiin ciru vd cdc diem lay mdu 2.2. Phuang phdp nghien ciru
Cac mau tram tich dugc phan tich va xac djnh kich thudc hat tai Vien Dja Ly thudc Dai hgc Tdng hgp Kiel, CHLB Diic vdi cdng nghe laze, ap dung nguyen ly tan xa anh sang dugc sir dung cho viec phan tich cac mau tram tich. Vdi thiet ke tdi uu ve quang hgc, thiet bj phan tich nay cho phep xac djnh kich thudc vat lieu tram tich nam trong dai 0.02-2000 |.im mot each hieu qua va thuan Igi cho viec tinh toan cac thdng sd tram tich nhu kich thudc hat tmng binh {x^), kinh thudc diem giiia (dso), kich thudc mdt I.
Viec lap ra cac so dd phan bd cac thdng sd tram tich, ham lugng cat, bdt set tren viing chau thd ngam dugc thue hien bang pham mem Sufer 8.0 vdi phep ndi suy Natural Neighbor cd cac thdng sd bat dang hudng (anisotropy) nhu ban kinh ndi suy 1.4 va hudng uu tien tuong irng vdi gdc 60 .Cac so dd phan bd nay se gdp phan cho ta thay xu hudng tich tu va van chuyen tram tich tren viing chau thd ngam ven bd DBSCL.
Kich thudc hat trung binh dugc tinh toan theo phuong phap moment, kich thudc hat dso dugc tinh toan tren dudng cong tich liiy, gia trj mdt I dugc tinh toan tren dudng cong phan bd.
X = exp
100
Trong dd .v^ la kich thudc hat tmng binh; / tdn sudt tinh theo phdn tram, mn kich thudc cac cap hat.
10 100 Kich throe hai \jim)
1000
Hinh 2: Biiii dd duang cong phdn bd mdu CM220308 05 vd kich thirac mdt 1
0 dso 50 100 150 200 250 300 350
Kich tliuoc hat (fim)
Hinh 3: Biiu dd dudng cong tich liiy vd kich thuac dso mdu CM220308 05
So dd hinh 2 bilu diln dudng cong phan bd va each xac djnh cac gia tri mdt. So dd hinh 3 the hien dudng cong tich liiy va each xac djnh gia trj kich thudc dso.
3. Ket qua va thao luan
3.1. Phdn bo cdc ddc diem trdm tich
Kich thudc hat tmng binh dugc tinh toan theo phuong phap momen da trinh bay d tren. Kich thudc hat taing binh Idn hon 60 |.im d khu virc mat trudc chau thd (delta front) phia tmdc cira sdng Djnh An, Tranh De va sau do mjn hon ve phia bien ven bd Sdc Trang-Bac Lieu. Kich thudc hat tmng binh Idn hon 45 )im, phan bd xung quanh cira sdng Ganh Hao va cd xu hudng giam dan tir khu vuc cira Ganh Hao den ban dao Ca Mau. Xu hudng thay ddi kich thudc vat lieu cho thay sir chiem uu the ciia tram tich mjn d phia Tay Nam khu vuc nghien ciiu va phia Tay ciia ban dao Ca Mau (hinh 4).
107°
85 - , 75 — 3
Hinh 4: Sa dd phdn bd kich thuac hgt trung binh
Kich thudc diem giira (dso) dugc irng dung rdng rai trong cac tinh toan van chuyen tram tich, ddng lire tram tich. Bdi dd, viec lap so dd phan bd kich thudc hat diem giira ciing can thiet va cung cap them mdt ldp thdng tin ve khu vuc nghien cim. Kich thudc hat diem giira thd hon 85 |um d phia tmdc cira sdng Djnh An, Tranh De va trd len mjn hon den gan cira sdng Ganh Hao. Viing xung quanh cira sdng Ganh Hao kich thudc hat dso la 55-95 )am, va mjn hon ve phia Tay Nam va khu virc phia Tay cua ban dao Ca Mau (hinh 5).
104° 105° 106° 107°
Hinh 5: Sa dd phdn bd kich thuac hgt diim giira d. 50
Tieu ban: Khoa hgc cong nghe Bien ISBN: 978-604-913-015-1
Phan bd cua ham lugng cat, bdt set chi ra rang sir chiem iru the cua mdi nhdm trong khu vuc nghien ciru. Ham lugng cat chiem uu the d phan mat tmdc cua sdng Djnh .'Xn la cao hon 60%. Ham lugng cat d nhieu noi tir cira sdng Ganh Hao den dao Hdn Khoai cao hon 30%. Ham lugng cat thdp hon 30% tir Hdn Khoai din phia tay ban dao Ca Mau tham chi thap hon 15% (hinh 6). Ham lugng bdt set bien ddi ngugc lai vdi ham lugng cat.
Hinh 6: Sa dd phdn bd lidm lugng cdt 3.2. Thdo ludn
Sir phan bd ham lugng cat chi ra tinh uu the cua ham lugng eat mjn d hai khu vuc: khu vuc phia tmdc cua sdng Djnh An va khu vuc xung quanh cira sdng Ganh Hao. Sir chiem uu the cua bdt set trong phan chau thd ngam dgc bd bien Sdc Trang-Bac Lieu va phan phia Tay cua ban dao Ca Mau.
Phan bd cua kich thudc hat tmng binh, kich thudc dso va kich thudc mdt 1 cho thay xu hudng tir cua sdng Djnh An den Ganh Hao tram tich tren be mat phan chau thd ngam bien ddi kha phire tap ed xu hudng chinh la mjn dan hi cira Djnh An den dgc bd bien Sdc Trang-Bac Lieu, sau dd thd len d khu vuc gan Ganh Hao, rdi mjn dan ve phia ban dao Ca Mau.
Nhirng ket qua nghien ciru mdt sd dac diem tram tich d day ket hgp vdi nhiing ket qua nghien ciiu dac diem cac thdng sd kich thudc hat mdt each chi tiet theo 12 tuyen eat ngang viing chau thd ngam ciia Nguyen T.T (2009) va Nguyen Tmng Thanh va nnk (2009) giup ta lap dugc mdt so dd khai quat ve uu the tich tu tram tich d viing chau thd ngam (hinh 7).
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Hinh 7: Xu the tich tu trdm tich
4. Ket luan
Tram tich cd xu hudng thay ddi mjn hon hi khu vuc cac cira sdng den khu vuc ven bd Bac Lieu sau dd trd nen thd hon d khu vuc xung quanh cira Ganh Hao va mjn dan ve phia miii Ca Mau va phan phia Tay ban dao Ca Mau.
Khu vuc tich tu trdm tich trong viing chau thd ngam cd the dugc chia thanh nam viing tich tu chinh: viing tram tich cat tich tu chiem uu the tmdc cac cira sdng cua sdng Mekong va khu vuc gan cira sdng Ganh Hao; viing bdt set tich ha chiem im the dgc bd bien Sdc Trang- Bac Lieu va khu vuc tir dao Hdn Khoai den phia Tay ban dao Ca Mau; viing tir giira Ganh Hao-Bd De den dao Hdn Khoai, cat va bdt set ciing tich tu, khdng cd uu the ro rang.
TAI LIEU THAM KHAO
1. Bang thuy trieu 2002. Tmng tam Khi tugng Thiiy van bien-Tdng cue Khi tugng Thuy van.
2. Nguyen Tmng Thanh, Biii Viet Diing, Phiing Van Phach, 2009. Mdt sd dac diem do hat va xu the tich tu tram tich tren phan chau thd ngam ciia chau thd Mekong. Cac cdng trinh dja chat va Dia vat ly bien. Tap X, trang 129-141.
3. Nguyen Bieu va nnk 2001. Bao cao dja chat ddi ven bd 0-30 m nude. Lun trii tai Cue Dja chat va Khoang san Viet Nam.
4. Phiing Van Phach va nnk, 2009. Nghien cim tien hda ddi ven bien ddng bang Sdng Ciru Long va viing them luc dja ke can trong Holocen-Hien dai phuc vu phat trien ben viing. Bao cao khoa hgc, luu tai Vien Dja chat vh Dja vat ly bien.
5. Nguyen.T.T., 2009. Surface sediment characteristics and sediment transport from Bassac River mouths to Ca Mau Peninsula (Mekong Delta). Master thesis.
Geoscienees library. Uni Kiel.