GII THIU BN NGUY C RI RO MI TRNG LU VC SNG B

INTRODUCING ENVIRONMENTAL RISK-PRONE MAP

OF SONGBE BASIN

Trn Tun T, H Quang Hi

Khoa Mi trng, trng i hc Khoa hc T nhin Tp.HCM, Vit Nam ---------------------------------------------------------------------------------------------------------------------------

TM TT

Bn nguy c ri ro mi trng c xy dng trong giai on cui cng ca cc phng php phn tch vn phng. i vi lu vc Sng B, bn ny th hin s tn thng v t ai do cc qu trnh suy thoi. Cc bin s quan trng nht nh hng n nguy c ri ro mi trng trong lu vc l thch hc, cc dng a hnh (cc qu trnh a mo, sn dc, mt dng chy) v thc vt. Cc phng php xy dng bn din tch c nguy c ri ro trong lu vc Sng B l lp bn cc qu trnh a mo, chng bn ny vi cc bn a cht, t v xi mn t, dc, mt dng chy v thc vt. Cc tc gi tin hnh khoanh vng cc qu trnh a mo v cc yu t ng lc hnh thi. Cc tc gi cng s dng cc ch s v xm thc ng, xm thc ngang v trt t. Ch gii bn biu din cc khu vc khng c ri ro v cc din tch c mc ri ro khc nhau: thp, trung bnh, cao v rt cao.

ABSTRACT

The environmental risk-prone map represents the final stage of a laborious analytical approach.

For Song Be basin, this map expresses the vulnerability of the lands to degradation processes. The most important variables that influence the risk-proneness of environment in the basin are lithology, landforms (geomorphic processes, slopes, drainage density) and vegetation. The methods used to draw up the map of risk-prone areas in Songbe basin are careful mapping of geomorphic processes, superposition of this map with the geological, soil and soil erosion maps and with slopes, drainage density and vegetation maps. The authors proceeded to regionalize the geomorphic processes and morphodynamic factors. The authors have also used indices for vertical erosion, areal erosion and landslides. The legend presents areas without risk and various areas with low risk, medium, high, very high.

1. GII THIU

Sng B l mt chi lu ln nht ca h thng sng ng Nai vi lu vc phn b ch yu trn din tch ca tnh Bnh Phc, c Lc, Bnh Dng v ng Nai. Trong cc chi lu ca sng ng Nai, Sng B c h thng thy in-thy li c thit k hon chnh nht vi bn bc

thang: Thc M, Cn n, Soc Phu Ming v Phc Ha. H thng thy in, thy li ny v ang cp in nng, nc ti, nc sinh hot, nc cho cng nghip, gp phn quan trng vo s pht trin kinh t x hi khu vc ng Nam B.

Trn lu vc Sng B nhng nm gn y xy ra cc hin tng tai bin a cht nh trt l t (Bnh Long, Phc Long, B ng), l qut (Lc Ninh, B ng), nt t (Lc Ninh, Bnh Long, Phc long, B ng), xi mn b mt, xm thc vch sng di p, bi t lng hCc tai bin ny ang c nguy c lm gim cht lng mi trng sng cng nh cht lng cc cng trnh (h, p). phc v cho cng tc qun l, gim thiu tc ng mi trng cn thit phi xy dng bn phn vng nguy c ri ro mi trng bng cc ti liu iu tra thc t v k thut tch hp d liu vin thm v GIS (IGIS).

2. C IM T NHIN S LC

Sng B c hai chi lu ln l Dak Lap v Dak Glum (phn b pha ng bc th trn Phc Long) vi hng chy v pha ty nam gn nh song song vi nhau. Sng chnh (tnh theo nhnh Dak Glum) c chiu di 331km, bt ngun t cao nguyn Xnar (cao trn 950m) v vo sng ng Nai Hiu Lim. Tng din tch lu vc l 7563km2, trong din tch thuc tnh Bnh Phc l 5034km2, k Lc 960.2km2, Bnh Dng 818.3km2, ng Nai 550.7km2. Lu vc sng B nm trong ta : 11o04'43-12o20'51 v Bc, l06o34'54-l07o3101 kinh ng.

Bng 1: Cc c trng hnh thi lu vc Sng B

c trng thy vn n v Gi tr Din tch lu vc km2 7563 Chiu di sng chnh km 331 rng trung bnh lu vc km 22,8 cao trung bnh lu vc m 280 dc lng % 5

3. NGUN TI LIU

Cc ti liu s dng thnh lp bn nguy c ri ro mi trng lu vc Sng B gm:

- Ti liu iu tra mi trng khu vc: cc kt qu lp bn a cht khong sn nhm t Lc Ninh -Phc Long, nhm t ng Xoi t l 1/50.000; ti nghin cu xi mn t khu vc ng Xoi- B ng; cc kt qua quan trc mi trng hng nm.

- nh hng khng ton sc khu vc Lc Ninh, Phc Long, ng Xoi, Chn Thnh t l 1/30.000. nh v tinh Landsat MSS 1983, TM 1992, TM 1998, ETM 1999. nh Radar khu vc nghin cu t l 1/250.000.

- Bn a hnh Gauss t l 1/25.000; bn UTM t l 1:50000.

Hnh 1: V tr lu vc Sng B

4. NI DUNG BN

Cc nghin cu khu vc trong nhng nm gn y cho thy cc tc ng mnh ca qu trnh a mo ng lc i vi mi trng lu vc nh bc tri b mt, trt l t, nt t, xm thc khe rnh, l qut... Cc qu trnh ny c xu th tng cng khi dn s trong vng gia tng cng vi cc hot ng cht ph rng v sn xut nng nghip lc hu. Kt qu l mi trng lu vc b suy thoi theo cc mc khc nhau. Ni chnh xc hn, l cc mc ri ro v s suy thoi v tn thng ca mi trng.

C th ni ri ro mi trng lu vc lin quan n cc nhn t gy tai bin bao gm: a cht (thnh phn, cu trc), a hnh (cc qu trnh a mo ng lc, mt mng dng chy), v lp ph thc vt. T vic phn tch cc thng tin nh tnh v nh lng chng ti tp trung xy dng bn nguy c ri ro mi trng trn c s cc qu trnh a mo ng lc. Bn nguy c ri ro mi trng thuc loi bn d bo tai bin mi trng, ni dung

chnh th hin trn bn ny l phn vng d bo cc khu vc c mc nguy c tai bin t nhin khc nhau da trn cc nhn t gy tai bin mi trng.

5. PHNG PHP THNH LP

Mi nhn t gy tai bin mi trng c t chc theo tng lp chuyn khc nhau, trong mi lp li c phn thnh tng loi vi trng s tng ng th hin mc ri ro mi trng. Cc lp phn loi c xem l cc tham s c bn tham gia vo ni dung ca bn nguy c ri ro mi trng. Bn nguy c ri ro mi trng thnh lp trn c s tch hp cc lp trng s chuyn trong mi trng GIS.

6. BN CHUYN

6.1. M hnh cao s (DEM)

M hnh DEM c phn gii 200m c xy dng t d liu ng ng mc a hnh t l 1/25.000 v 1/50.000, p ng ni dung thnh lp bn chuyn ( dc, hng sn, phn ct) t l trung bnh (1/100.000-1/200.000).

Hnh 2: M hnh s cao lu vc Sng B Hnh 3: Mng dng chy lu vc Sng B

6.2. Bn phn loi thch hc

Phn vng cc loi c da vo cc ti liu iu tra a cht hin c trong khu vc vi vic phn tch b sung t t liu vin thm. mc khi qut bn ny th hin cc loi chnh gm: cc trm tch gn kt, aluvi c, basalt c, basalt tr, v cc trm tch tr (aluvi, deluvi, m ly). Cc loi ny c s khc nhau c bn v cc ch tiu c l, b dy tng phong ha, b dy tng t. Nhn chung cc c lp v phong ha dy, ht mn (bt, bt st) nhy cm hn i vi qu trnh a mo ng lc.

6.3. Bn dc

dc a hnh lu vc c ngha quan trng i vi qu trnh thot nc. Lu vc c dc ln, khi ma nc theo cc sn dc thot nhanh xung sng chnh, ngc li dc nh nc thot chm hn. dc a hnh cng lin quan cht ch vi cng xm thc xi mn b mt sn. Cc sn c dc ln thng b xi mn mnh i khi km theo c cc qu trnh trt l.

Trong m hnh tnh ton, s thay i cao a hnh v hai hng x,y l thng s xc nh hng sn v ln dc a hnh ti mt im. y xem gi tr cao Z l mt hm s ca hai ta (x,y), c th biu din Z = f(x,y). Bn dc c tnh trn c s m hnh cao DEM lu vc.

Phn ln din tch c dc < 5o phn b ty nam lu vc. Nhng vng c dc > 5o tp trung trn cc vm basalt, trn cao nguyn ni la. Cc khu vc c dc ln hn 15o l ni c nguy c trt l t cao, phn b trn basalt c ng bc Phc Long. dc ln nht trong lu vc l 32,15o ti ni B R.

6.4. Bn phn ct ngang

Phn ct ngang trong lu vc D c tnh theo cng thc:

A

liD i

=

Trong li l di nhnh sng i. A l din tch lu vc. D ln nu trn lu vc c mt sng sui dy (H.3). D nh nu mt sng sui tha. Mt sng sui dy phn nh qu trnh xm thc ngang pht trin. Trong lu vc Sng B, mt sng sui dao ng t 1-5 km/km2, chim u th l mc 1 km/km2. Cc vm basalt c mt 2km/km2, trn sn cao nguyn vm pha ng bc lu vc, mt sng sui ti 4-5 km/km2.

6.5. Bn phn ct su

Bn phn ct su biu din chnh lch cao a hnh trong 1km2. Phn ct su phn nh mc chia ct a hnh theo chiu thng ng. Phn ct su ln thng dn n s pht trin cc h thng khe rnh xm thc v cc sn trt t, l.

Trong lu vc mc chnh lnh a hnh ln nht l 100m/km2 tp trung ti ni B R v trn cao nguyn basalt c. Phn ct su lin quan cht ch vi h thng t gy tr. Trn bn phn ct su pht hin mt di ko di theo phng ty bc-ng nam c l lin quan ti mt t gy theo phng ny.

6.6. Bn ch s thc vt (NDVI)

Bn ch s thc vt th hin mc che ph thc vt trn mt t. Khu vc c ph thc vt dy s c ch s NDVI cao, ngc li khu vc c ph thp th c ch s NDVI thp. Cc khu vc c NDVI cao, qu trnh a mo ng lc (bc mn, ra tri, l qut) s gim. Cc khu vc c NDVI thp l nhng ni c ri ro cao.

Ch s thc vt lu vc Sng B c tnh ton t knh 3 v knh 4 nh v tinh Landsat TM nm 1998, 1999 theo cng thc:

3434

KKKKNDVI +

=

Bn ch s thc vt cho thy cao nguyn basalt c c NDVI trong khong 0,6-1. Khu vc gip ranh vi tnh ng Nai c NDVI l 0,5-0,8. Phn ln din tch cn li c ph thp.

7. BN RI RO MI TRNG

7.1. Xc nh trng s

Trong phn tch ny cc tc gi xem 5 bn thnh phn c tm quan trng nh nhau. Cc trng s c cho im theo thang bc 5 ng

vi cc lp phn loi trong tng bn chuyn .

7.2. Chng lp bn

Mi bn chuyn l mt lp d liu trong HTTTL theo h thng ta Gauss. Thc hin chng lp cng im trng s cc lp d liu s thu c bn nguy c ri ro mi trng dng raster, sau khi qut ha dng vector (hnh 4).

Bng 2: Phn lp bn thch hc TT Thuc lp Code

1 Cc trm tch gn kt 1 2 Trm tch Jura 2 3 Aluvi c 3 4 Basalt tr 4 5 Basalt c 5

Bng 3: Phn lp bn dc TT Thuc lp (o) Code

1 03 1 2 37 2 3 715 3 4 1525 4 5 >25 5

Bng 4: Phn lp bn phn ct ngang TT Thuc lp (km/km-2) Code

1 01 1 2 12 2 3 23 3 4 34 4 5 >4 5

Bng 5: Phn lp bn phn ct su TT Thuc lp (m/km-2) Code

1 025 1 2 2550 2 3 50100 3 4 100150 4 5 >150 5

Bng 6: Phn loi cc lp ch s thc vt TT Thuc lp Code

1 0.6 1 1 2 0.2 0.6 2 3 -0.2 0.2 3 4 -0.6 -0.2 4 5 -1 -0.6 5

Bng 7: Cc mc ri ro mi trng trn lu vc Sng B

TT K hiu Mc Din tch (km2)

T l (%)

1 1 Cc vng khng c nguy c ri ro 143.069 1.9

2 2a Vng ri ro thp 387.881 5.1

3 2b Vng ri ro trung bnh n cao 1714.9 22.7

4 3a Vng ri ro cao 2237.51 29.6 5 3b Vng ri ro rt cao 2104.89 27.8 6 4 Vng ri ro mnh m 975.749 12.9

Hnh 4: Bn ri ro mi trng lu vc Sng B

8. KT LUN

Khong 70% din tch lu vc c nguy c ri ro cao n mnh, 30% din tch cn li c nguy c ri ro trung bnh n thp.

a hnh cao nguyn basalt phn b pha bc lu vc b phn ct mnh, dc ln c nguy c ri ro mi trng rt cao n mnh m. y u th ca qu trnh xm thc khe rnh, trt l, nt t v nguy c xy ra l bn , l qut.

a hnh vm basalt c nguy c ri ro cao n rt cao vi u th ca hot ng nt t, xm thc git li nh cc thung lng cp 1, trt l t, xi mn v nguy c xy ra l qut.

a hnh c nguy c ri ro cao l cc ng bng aluvi c ni c cc tng trm tch gn kt yu, cc hot ng nhn sinh mnh m. Cc qu trnh a mo ng lc din ra y gm ra tri b mt, ra tri c rnh xi.

TI LIU THAM KHO

1. Trn Tun T, ng dng vin thm v GIS trong nghin cu c im mi trng a cht lu vc Sng B phc v cho cng tc qun l lnh th. Lun vn cao hc mi trng (2003).

2. Florina Grecu, Risk-Prone Lands in Hilly Regions: Mapping Stages in Applied Geomorphology. John Wiley & Sons, Ltd. (2002).

3. Marcel Suri, Jaroslav Hofierka, Soil Water Erosion Identification Using Satellite And DTM Data, Proceedings of the Fifth European Conference and Exhibition on Geographic Information Systems, EGIS 94. Utrecht: EGIS Foundation (1994), pp. 937-944.

4. Victor B. Serveiss. Applying Ecological Risk Principles to Watershed Assessment and Management. In: Environmental Management Vol.29, No.2 Springer-Verlag New York Inc (2002).