Re: [Assam] [WaterWatch] Why does river Brahmaputra remain untamed?

Tue, 27 Dec 2005 18:29:18 -0800

<<<". Do geologists know how young the river is? Did the river start flowing at the time the Himalayas formed? Or was it a later phenomenon when a water body in Tibet burst its land container? It would be interesting to know.>>>

Nice to hear your Q/doubt..

I think your guess is correct -that Himalaya rose later,blocked the melt waters into vast lakes in East Tibet which eventually broke a barrier, took a westward bend and finally found a way down to our valley and joined the Burha Luit---which is and was the original  STRAIGHT course. That the Eastern Himalayas were/are rising is without a doubt.

They even found a fossil of Cow's jaw up a high peak near Bomdila.

The same looks like the casewith Ganga. Look where the Luit(Zomuna) meets her. Ganga carries on her original course --as if Zomuna came from nowhere.

mm


 

From:  Dilip/Dil Deka <[EMAIL PROTECTED]>
To:  Chan Mahanta <[EMAIL PROTECTED]>, [email protected]
Subject:  Re: [Assam] [WaterWatch] Why does river Brahmaputra remain untamed?
Date:  Tue, 27 Dec 2005 06:13:27 -0800 (PST)

Good article with facts and numbers.
  
Geologically Brahmaputra is known as a fairly new river though we call it "Burha Luit". Do geologists know how young the river is? Did the river start flowing at the time the Himalayas formed? Or was it a later phenomenon when a water body in Tibet burst its land container? It would be interesting to know.
  
 
  
The essence of the article is in the following two paragraphs. The statistics on embankments and canals that the article gave probably include the truibutaries and as I remember they were all temporary make-shift measures. Multidisciplinary investigation only as suggested by the author  will lead to a better solution. Wasn't the Brahmaputra Board dismantled? Is there talk of a new multidisciplinary organization? Anybody knows?
  
Dilip
  
 
  
"Why are we so helpless in containing the spates of Brahmaputra and coping
with its flood hazard? Why do our efforts go awry and all civil engineering measures end up in shambles?"
  
 
  
"It will therefore be advisable to embark
on a comprehensive multidisciplinary investigation of the
Brahmaputra valley in Assam to provide a rational knowledge-based perspective for the design of flood resilient mitigation measures."
  


Chan Mahanta <[EMAIL PROTECTED]> wrote:
  
       
At 11:40 AM +0000 12/27/05, [EMAIL PROTECTED] wrote:
  
Why does river Brahmaputra remain untamed?

Taking into consideration the reality of
the continuing tectonic movements on active faults, it should be evident that building of embankments and drainage channels in upper Assam would not bring lasting relief from flood hazards.





Come May and the wild river of the east is seized with spells of
unrelenting spate, attendant deluge, sediment deposition and
erosion. The flood hazard has been ravaging upper Assam unabatedly
since time immemorial. Explorers of ancient times – those who
established Praagjyotishpur in Kamrup province – must have discerned
in this river something very different from all other rivers of the
subcontinent, as evident from the singularly masculine name they
gave it – `Brahmaputra' or `the son of Brahma'. Despite gigantic
efforts and colossal expenditure (> Rs 15,000 million) in building
3647 km of embankments, 599 km of drainage channels and 431 km2 area
of soil conservation1, Brahmaputra continues to wreck havoc by
uncontrollable floods year
after year.

Records show that catastrophic floods occurred in 1954, 1962, 1966,
1972, 1973, 1977, 1978, 1983, 1984, 1987, 1988, 1991, 1993, 1995,
1996 and 1998. Upwards of 9600 km2 land, that is 12.21% of the
geographic area of Assam, is annually affected by floods. In 1998,
the flood which came in 4 frightening waves, deluged 38,200 km2 or
48.65% geographic area of the state, putting in peril the lives and
properties of 12.5 million people1.

Why are we so helpless in containing the spates of Brahmaputra and
coping with its flood hazard? Why do our efforts go awry and all
civil engineering measures end up in shambles? The answer, in my
opinion, lies in our failure to recognize the reality of active
faults and continuing crustal movements in this geodynamically
restless region. Understandably, the flood coping measures have
never been designed to accord with this recognition.

Girdled as it is, by the arms of the Eastern
Himalayan Syntaxis –
the knee-bend of the mountain ranges – the Assam terrane is
underthrusting northwards under the Arunachal Himalaya and, less
energetically, eastwards beneath the Indo-Myanmarese ranges2,3.
There is, therefore, very severe deformation and attendant faulting
and thrusting in the terrane caught between the Himalaya–PatkaiNaga
ranges and the Meghalaya massif. The drastic reduction of the width
of the alluvial plains from 350 to 300 km, respectively, in the
flood plains of Sindhu and Ganga to less than 100 km in the
Brahmaputra basin is not without significance. Coming of the
Meghalaya–Mikir blocks of the Peninsular Indian Shield closer to the
Himalaya explains this attenuation of the alluvial domain of the
Quaternary foreland basins.

The severe deformation of the Assam region is eloquently expressed
in its much faulted framework4,5. The E–W trending faults (Dauki
Fault, Brahmaputra-Mikir Fault), and the transverse
tear faults
(Kopili Lineament, Dhubri Fault, Dudhnoi Fault, Chidrang Fault, Um
Nagot Lineament) and thrusts (Dapsi Thrust, Barapani Thrust) are
among the many that dissect the terrane of the Meghalaya–Mikir
blocks. The E–W Dauki and Brahmaputra–Mikir Faults roughly demarcate
the southern and northern physiographic limits of the Meghalaya
Plateau which is a horst of sorts. The plateau stands as a ~ 2000 m
high physiographic eminence against the sunken 3–4 m high (above sea
level) Sylhat Plains in Bangladesh.

Most of these faults are seismically quite active4 as borne out by
the distribution of epicentres in the fault zones, such as the
locations of 7 earthquakes of M "  4.5 along the 26.5°

The catalogue of earthquakes of magnitude 4.5 and above in the
period 1897–1992 shows the higher (87%) seismicity in the Meghalaya-
Mikir blocks and adjoining Indo-Myanmarese range and northern
Bangladesh plains, compared to near absence in
upper Assam and lower
seismicity in Arunachal Himalaya6. In the Meghalaya block, the
  
western part (Tura region) riven with faults exhibits greater
seismicity as testified by major earthquakes of 1897, 1923, 1930 and
1943.

Gowd et al.6, Molnar7 and Gahlaut and Chander8 attribute the higher
seismicity of the Meghalaya–Mikir blocks to reactivation of what has
been postulated as a gently dipping shallow thrust or midcrustal
detachment. I believe that many E–W and transverse faults that
dissect the block (Figure 2) are active. Compared to the N3° E
orientation of SHmax in the Meghalaya–Mikir blocks, it is N26° E in
upper Assam (and N23° E in Peninsular Indian Shield)6.

The NNW–SSE  trending Kopili Lineament of high seismicity thus constitutes the  tectonic boundary between the two stress domains.

Significantly, it was in the belt of this Kopili Lineament that earthquakes have
occurred recurrently – one of M 7.2, three of 6.0 £  M < 7.0 and several of 4.5 £  M < 6. Doubtless, tectonic movements have been taking place time and again on the faults of the Kopili Lineament. This seismically active Kopili lineament cuts across the Brahmaputra about 35 km NW of Navagaon between Mikir Hills and the prolongation of Meghalaya massif.

Levelling observations9 made three times during 1910–197  indicated
that the blocks of the Guwahati–Dergaon section have been
consistently rising – the vertical velocity increasing progressively
from 0.3 mm/yr to 4.5 to 31 mm/yr at Dergaon (approximately 30 km
west of Jorhat) implying faster rise of the Mikir block. In the
Guwahati–Goalpara sector, the eastern side moved in the NNE
direction and the western part shifted SSW-ward during 1856–1938
(ref. 9). North of the Dauki Fault, the Meghalaya block has been
rising at the rate of 0.3 to 0.4 mm/yr (ref. 10).

It is obvious that
the block delimited by seismically active faults
has been rising perceptibly though variably in space and time. The
uplift of the Meghalaya block is evident in the gorge that the
otherwise wide Brahmaputra has cut near Guwahati – in the place
where a N–S lineament traverses the river.

The uplift of the Meghalaya block – through the prolongation of
which the Brahmaputra has cut its channel – must have caused ponding
of the river. The continuing movements must be impeding its flow
with resultant accumulation of sediments in the upstream channel
(Figure 4). The deposition of great volumes of sediments – occurring
as islands and bars – in the channel has phenomenally reduced the
carrying capacity of the Brahmaputra. Flood waters are bound to
spill over and spread far and wide. The 1897 earthquake of M 8.7 had
caused not only the ponding of streams but also blockade of the
Brahmaputra.

According to Oldham11, there was a deluge
unrelated to
rains as a result of a barrier formation across the Brahmaputra,
downstream of Hathi- mura, accompanied possibly by subsidence of the
floor under the river. The 15 August 1950 earthquake (M 8.7) had
likewise drastically affected the gradient of this river, stopping
the flow temporarily and bringing about flooding and rapid
accumulation of enormous volume of sediments in the channel.

The lowest water level rose by 3 m as a result of the earthquake, and near Dibrugarh and downstream the channel was silted up 2.5 to 3 m. The scarp13 that one sees north-east of Dibrugarh is probably the
surface _expression_ of the fault that lifted up the downstream block
and caused ponding of the Brahmaputra.

Imagine a similar development overtaking the Brahmaputra valley in
the event of a major earthquake! Taking into consideration the
reality of the continuing tectonic movements on active faults, it
should be evident that building of
embankments and drainage channels
in upper Assam would not bring lasting relief from flood hazards.
The solution lies in channelizing discharge of the order of 48,160
m3/s (ref. 12) of the Brahmaputra through canals, aqueducts,
tunnels, etc. across the northward prolongation of the Meghalaya
block, particularly between Guwahati and Hathimura and other places
(west of Mikir Hills) where identified active faults cross its
channel.

The canals would have to be deepened periodically in order
  
to keep pace with the rate of uplift of the block. Absence of
channelization of the Brahmaputra in spate would always cause
ponding, leading to deposition of sediments, reduction in carrying
capacity and inundation of the flood plain. In my opinion, this is
what is happening to- day. It will therefore be advisable to embark
on a comprehensive multidisci-plinary investigation of the
Brahmaputra valley
in Assam to provide a rational knowledge-based
perspective for the design of flood resilient mitigation measures.

Goswami, S., Souvenir: National Workshop on Geodynamics of North-
eastern India, Dibrugarh University, Dibrugarh, 1998, pp. 36–41.
Mukhopadhyay, M. and DasGupta, S., Technophysics, 1988, 149, 299–
322.
Acharyya, S. K., Indian J. Geol., 1997, 69, 211–234.
Nandy, D. R. and DasGupta, S., Phys. Chem. Earth., 1991, 18, 1147–
1163.
Kayal, J. R. and De, R., Bull. Seismol. Soc. Am., 1991, 81, 131–138.
Gowd, T. N., Srirama Rao, S. V. and Chary, K. B., Curr. Sci., 1998,
74, 75–80.
Molnar, P., J. Geol. Soc. India, 1987, 30, 13–27.
Gahalaut, V. K. and Chander, R., Tectonophysics, 1992, 204, 163–174.
Rajal, B. S. and Madhwal, H. B., Himalayan Geol., 1996, 17, 17–32.
Chugh, R. S. and Valdiya, K. S., Indian J. Geol., 1989, 61, 1–13.
Oldham, R. D., Memoir Geol. Surv. India, 1899, 29, 1–379.
Goswami, Dulal, C., in Flood Studies in
India, Geol. Soc. India,
Bangalore 1998, pp. 53–75.
Proc. Intern. Symp. Neotectonics of South Asia, Survey of India,
Dehradun, 1986.

---------------------------------------------------------------------
-----------

K. S. VALDIYA

Geodynamics Unit,

Jawaharlal Nehru Centre for

Advanced Scientific Research,

Bangalore 560 064, India






  
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