(PAPER PRESENTED in IAG/AIG REGIONAL CONFERENCE 2011, ADDIS ABABA, ETHIOPIA – FEBRUARY)
Dr. Bhuban Gogoi
Principal, Tinsukia College, Assam, India.
principaltinsukiacollege@rediffmail.com
Abstract: The Brahmaputra valley, 750 km long and 80 km wide is located in the extreme north eastern corner of India. The Brahmaputra river with 640 km long in east-west alignment is flowing towards west of the valley sloping from altitude of 230 m in the east to 40 m to the west. The river has 103 tributaries, 65 in north and 38 in the south bank. It is a long narrow valley wider in western part of the north bank section and eastern part of the south bank section. The northern part of the valley, no matter of wide or narrow, has high slope to the main channel of the Brahmaputra. Bounded by mountains in the South and in the north with the Himalayas and its off-shoots having a profile it has a sudden break of slope like a hockey- stick specially in the north to release water with high speed flushing to the valley. The southern valley has low slope and it is characterized by meandering river channels with low hill headwaters supplying slow base flow. Spill floods occur here during excess rainfall periods. The rivers of northern valley are partially interconnected by anabranches and distributaries having more or less straight channels. These north bank rivers carry heavy sediment loads, form levees on the bank of the Brahmaputra and generally turn to the west. This causes high floods in the areas, upstream of these bends. Dry channels and sandy areas (char) are present in the northern valleys which are replenished by flash floods water during the rainy season. In marshy areas the valley becomes sluggish and frequent shifting of channels is common in the eastern part of northern valley. The south bank tributaries have high bank erosion due to meanderings and formations of palaeo-channels and ox-bow lakes. Flood is a common hazard occurring in the valley from historical time due to high rainfall in monsoon season and due to fluvio-geomorphology of the basin. Not only that flood occurs due to break monsoon and due to failure of dams and embankments, absence of embankments and avulsion of rivers channels. Flash floods occur specially in whole north bank valley, and south bank valley part in the west of Guwahati.. Spill floods occur in the broad south bank valley east of Guwahati. It is a recurrent event that takes life and property of the valley every year. This situation results from the combined effect of hydro-meteorology, tectonic and anthropogenic causes. It has also distant relationship with ENSO and periodic variations of monsoons. For management of flood, regional zonation is made as Northern Flash Flood Zone, Southern Flash Flood Zone west of Guwahati, Moderate Spill Flood Zone east of Guwahati, Active Flood Zone of the Brahmaputra and Flood Free Zone of hills and high lands. Mode of management may vary from zone to zone having some common methods to all like embankments, ring-bunds, inter- basin transfer, dams, drainage channeling, spur, dyke, etc. All these are to be judiciously used. A good number of flood protection measures are taken in the valley by construction of embankments about 5000 km long, drainage channeling for about 1000 km, protection and anti-erosion works for more than 500 km, etc. But these are not sufficient as compared to the menace. Flood abatement and adjustment through traditional and modern techniques must be used and developed. Watershed management and disaster management system may be highly effective to flood menace.
Key Words : Zonal monsoon, meridional monsoon, flash flood, spill flood, flood control, flood abatement, flood adjustment, watershed management, disaster management.
INTRODUCTION
Flood becomes an unfold tragedy of the Brahmaputra valley though it did appear with mixed blessings in the past. It is a problem widely talked about and is made responsible for continuous erosion of economy and social life of the state of Assam. Ironically there is yet to reach at a well-integrated detailed study of the problem but the situation demands urgent and immediate solution. The manifold dimensions and far-reaching effects of floods have neither been properly managed nor measured in the state though some effects be measured and perceived directly whereas some still are intangible. Flood of the Brahmaputra valley is mainly linked with the fluvial geomorphology of the Brahmaputra River and its tributaries fed by rains of the world’s wettest monsoon climate. The valley has the annual recurrent floods having three to four waves on average and nine at maximum during monsoon period. Floods have a far-reaching effect on the life of the valley from tangible loss of life and property to non-tangible great extent. Floods have great impact on geomorphic manifestations of the fluvial geomorphology and the valley at large. Human mismanagement adds another dimension to the occurrence of flood during recent days. Fluvial geomorphologists attempt to a great extent to study the menace of flood hazards for its proper management, but yet to achieve a lot in the world perspective. This paper seeks to investigate flood problem of the valley from view point of fluvial geomorphology and hydrometeorology on the background of recent environmental changes.
Location and Physical Set-up:
The Brahmaputra valley is located in between 900E and 960E and 270 N and 280N in east-west alignment in the south-central part of the Brahmaputra catchment area of the extreme north-eastern corner of India. It is the lowest part of the region with a long depressed surface like the bottom of a boat with one end flanked and bounded by the mountains of the Himalayas and its off-shoots in the east in a syntaxial bend keeping open the other end to merge with the plains of West Bengal and Bangladesh, part of broad Gangetic plain. The Brahmaputra valley covers 19 plain districts of Assam having an area of 63,600 km2 approximately 72% of the state’s area supporting 85% of state’s total population.
The Brahmaputra valley is a continuous plain extended on both bank of the main river Brahmaputra approximately in east-west alignment partially interrupted in midway by Karbi Anglong and elsewhere by some minor hills. The valley is 750 km long with a width of 80 km on average. The Brahmaputra River in the valley has as many as 103 tributaries (65 in north and 38 in south banks). The valley slopes down from 230m in the east towards 40m in the west. The valley is surrounded by high Himalayas in the north and east and Naga Hills,Karbi-Anglong, North Cachar Hills and Shillong plateau in the south. The northern valley slightly widens in the west and southern valley in the east. The 300m contour demarcates the valley boundary. The northern valley has high slopes with abrupt break to the valley due the presence of lofty Himalayas and the southern has relatively low gradient with the presence of low hills in the border of the valley. The valley in the west of Guwuhati is a ramp valley or a sag in character and east of Guwahati is a rift valley having WEW—ENE direction between the Main Boundary Fault of the Himalayas and the Naga Thrust in the Naga Hills in the south. The Valley topography consists of Piedmont zone (Bhabar zone), alluvial fans, valley plain and river channels where the Brahmaputra river channel occupies a considerable area of the valley. The plain has Old alluvial plain (Pleistocene) and new alluvial plain built up in recent times.
Climate, Vegetation, etc.:
The Assam valley enjoys sub-tropical monsoon climate having wet summer and dry winter. Rainfall comes due to south-west monsoon wind from the Bay of Bengal and Arabian Sea occurring relief rain being obstructed by the high Himalayas for seasonal change of pressure conditions over Bengal and north western part of India and the change of tropical oceanic air mass of the south-west monsoon. Due to orographic effect on local pressure system, low pressure in the plains and high pressure in the hills occur sub-tropical climate of modified type of cwg of Koppen. Rainfall occurs at maximum 335cm in the North Lakhimpur in the east ranging to minimum 129 cm in the middle Assam. Rainfall comes in the monsoon rainy season in the form both of normal and storm-rain when floods come due to the rise water levels above danger(flood) levels more or less coinciding with the occurrence of storms. Rainfall occurring in the hilly and mountainous tracts and plains give rise the water level of the Brahmaputra and its tributaries. Himalayan belt receives more or less high uniform rainfall than the southern hills. So flood events are also frequent and high in the northern part of the valley.
Upper catchment areas of all the tributaries of the Brahmaputra have mixed vegetations with both evergreen and deciduous trees covering more than 60% of land whereas Assam has 33% of land covered by forests at present all having a decreasing trend in coverage. Interception to run-off is good but of course in decreasing trend.
Fluvial Geomorphology of the Brahmaputra Valley:
Inventory of types of fluvial geomorphology of the Brahmaputra valley mainly are the fluvial regime, erosional and depositional activities and development of their landforms, channel avulsion, flood, sediment transport, etc. The Brahmaputra river originating from Tibet has a total catchment area of 5,80,000km2 out of which 1,94,413km2 falls within India and the Brahmaputra Valley covers 56,480km2 constituting 70% of the state’s total area of Assam in India. The valley covers the middle course part of the Brahmaputra for a distance of 640km whereas the valley is extended beyond for 750km in length. One of the fluvio-geomorphological features is the alluvial fans and alluvial cones located at the foothill belts of the hills and mountains of north and south originated due to deposition of rocks and boulders carried by rivers at their debouching points to the valley. Tributaries are engraving down deeply through these fans and cones or percolating underground to reappear at distance in the northern plain. Floods are rare here due to their high slopes ranging from 0.5 to 60 having a higher basin slope ranging from 5 to 210 (Dutta,2004) due to high slope of the mountains. This occurs considerably less in the south due to low mountains .The built up valley plains exist in between the channel of the Brahmaputra and alluvial fans and cones zones both in northern and southern banks washed regularly by floods. These plains are dissected by active floodplains of the tributaries. The Brahmaputra active flood zone is spreading over its braided channel, islands and sand bars and immediate areas of its low banks nearby. Rivers have high natural leeves that create drainage problems and develop perennial flood areas on banks and divert some tributary lower courses to run parallel to some distance to meet the main river at downstream. The Brahmaputra channel is highly braided and all the rivers are found more or less unstable and eroding their banks and shifting of channels. These two events occur highly during flood times. The Brahmaputra becomes highly flooding, bank eroding and widening its channel through increasingly braiding due to the impact of great earthquakes of 1897 and 1950 of which the later has more effects on landforms. The Brahmaputra becomes wider 2 to 3 times than before 1950. The valley has as many as 2930 wetlands consisted of palaeochannels, low marshy lands, tectonically subsided land, etc. which play very important role in the fluvio-geomorphology of the valley. Northern tributaries carry more sediments than the southern. 1950’s great earthquake has great impact on the metamorphosis of the Brahmaputra channel.
Some Fluvio-Geomorphologic Characteristic of the Brahmaputra and its Tributaries and the valley:
Watershed or catchment area of the Brahmaputra (total)-----5,80,000 km2
(1) China---- 2,93,000 km2
(2) India----- 1,95,000 km2
(3) Bhutan— 45,000 km2
(4) Bangladesh-47,000 km2
(5) At points in India (a) Pasighat Damsite ---2,44,700 km2
(b) Kobo----3, 05,000 km2
(c) Dhubri—5, 33,000 km2
Length of the Brahmaputra (total) ------2897 km
Tibet-----------1625 km
India-------------918 km
Bangladesh----354 km
Brahmaputra valley in Assam state—640 km
Gradient (slope) of the Brahmaputra river-
Tibet-------------------------0.00260 / km
India (mountain tract)—0.00190 /km (Tibetan border to Kobo)
India (plain course) ------0.00014 / km (Kobo to Dhubri)
Bangladesh-------------- 0.00009 to 0.00003 / km (from border to mouth)
Maximum Discharge/Highest flood level (HFL)/Danger level (DL), etc.
Stations discharge (cumecs) HFL(m) DL(m) Maximum lift (m)
Dibrugarh- -- 106.48 104.24 2.24
Neamatighat-- -- 87.37 85.04 2.33
Tezpur- -- 66.59 63.23 3.36
Guwahati- -- 51.37 49.68 1.69
Pandu- 72,794 49.87 48.77 1.10
Goalpara- -- 37.43 36.27 1.16
Dhubri- -- 30.36 28.00 2.36
Discharge characteristics of the Brahmaputra
Discharge at Pandu (Brahmaputra valley)
Maximum --- 72,794 cumecs (23.08.1962)
Minimum ---- 1,757 cumecs (20.02.1968)
Mean yearly maximum---- 51,156 Mean monsoon total
Shigatse---- 507 million m3
Pasighat----3979 million m3
Mean yearly minimum----4,420 cumecs
Discharge per unit area of watershed up to
T’sela D’s Zong (China)---------0.01 cumecs / km2
Pasighat (India)------------------ 0.023 cumecs / km2
Pandu (Brahmaputra valley)--0.03 cumecs / km2 (Assam)
Bahadurabad (Bangladesh)---0.032 cumecs / km2
Sediment Load Characteristics of the Brahmaputra
Mean Suspended load at Pandu during-
Yearly maximum discharge---400 million m. tons
Daily maximum discharge ---2.12 million m. tons
Mean yearly sediment yield in per unit area at
T’sela D’s Zong (China)---------100 m. tons / km2
Pasighat (India)------------------340 m. tons / km2
Pandu (Brahmaputra valley)--804 m. tons / km2 (Assam)
Bahadurabad (Bangladesh)-1,128 m. tons / km2
Mean annual rainfall of the Brahmaputra valley 230 cm (129-250 cm)
Total basin landuse in India --- Forests -------114992.08 km2
Agriculture ----50473.84 km2
Total basin population in India --------------30.4 million ( 143 persons / km2)
Brahmaputra valley population, Assam—22.66 million (403 persons / km2)
Some Fluvio-Geomorphological Characteristics of the Brahmaputra-
Bank- erosion ---1841.17 km2
Bank-fill------------ 105.55 km2
Total erosion------1735.62 km2 ------- (1912-1996)
Erosion on North bank—538.81km2,(1996-2008)
South bank—914.62 km2 (1996-2008)
Increase of Braiding Index---Average (Mode)PFI—5.896 (1997),
5.09 (2008)
(where <4=high, 4-19=moderate &>19=low)
Average BBI—4.47-------(1912-14)Brice Braiding Index
5.09-6.7 (1973-91)
Increase of average width—5.8 km (1912-28)
7.4 km (1963-75)
7.4 km (1996)increased by 1.6 km on average
& 8.0 km at maximum
Tributaries and their drainage characteristics of the Brahmaputra
Tributaries catchment area length average annual average annual
Km2 km Suspended load sediment yield
ha. m tons/km2
North bank
1.Subansiri------- 28,200 442 992 959
2.Ranganadi 2,941 150 186 1598
3. Buroi 791 64 16 529
4. Borgang 550 42 27 1,749
5. Jia Bharali 11,716 247 2,013 4721
6. Gabharu 577 61 11 520
7. Belsiri 751 110 9 477
8. Dhansiri 1,657 123 29 463
9. Noa Nadi 907 75 6 166
10. Nanoi 860 105 5 228
11. Barnadi 739 112 9 323
12. Puthimari 1,787 190 195 2,887
13. Pagladiya 1,674 197 27 1,883
14. Manas-Aie-Beki 41,350 215 2,166 1,581
15. Champamati 1,038 135 13 386
16. Gaurang 1,379 98 26 506
17. Tipkai 1,364 108 31 598
18. Godadhar 610 50 0.21 272
19. Jiadhal 937 118 _ _
South Bank (Right side)
1. Burhi-Dihing 8730 360 210 1129
2. Disang 3950 230 93 622
3. Dikhou 3610 200 34 252
4. Janzi 1130 108 16 366
5. Bhogdoi 920 160 15 639
6. Dhansiri 10,242 352 147 379
7. Kopili 13,556 297 118 230
8. Kulsi 400 93 0.6 135
9. Krishnai 1,615 81 10 131
10. Jinari 594 60 3 96
11. Dibang 12,270 164 - -
12. Lohit 28,280 350 - -
13. Kundil 1,178 55 - -
Form Ratio
Rivers Site Width (w) Depth (d) form ratio
in m. in m. w/d
Brahmaputra Pandu 1400 12.9 115
Jogoghopa 2180 9.1 240
Burhidihing river Margherita 145 2.6 55
Khowang 152 4.0 38
Meandering Geometry of some south bank tributaries
Rivers Width Bend-radius Meandering wave Meander
KM Km Length in KM Amplitude in KM
Burhi-dihing o.29 0.77 2.98 2.57
Disang 0.08 0.19 0.83 0.65
Mai-jan 0.21 0.56 2.21 1.97
Dibru 0.06 0.14 0.46 0.46
Noa-dihing 0.17 0.50 1.75 1.70
After Sharma (1993)
Flood Problem:
The Brahmaputra valley is one of the highest flood prone areas of the world. Annual recurrent floods occur in the valley due to the Brahmaputra and its tributaries. Annually 3 to 6 flood waves occur in the valley during rainy season. Rains come from normal monsoon and monsoon storms. Heavy rainfall brings heavy surface run-off and the rivers becomes spate and over bank flow of water makes flood in the nearby basin region. Over bank flow, flow through spill channels, breach of embankments, dam failure, drainage congestion, bank erosion and channel shifting are the mode occurrence of floods. Both flash floods of short duration and spill flood of relatively long duration occur. Monsoon rainfall, fluvio-geomorphology and breach of embankments and sudden clearance of reservoir are the main reasons responsible for the occurrence of flood.
History and Dimensions of Hazards:
Flood is a natural event that has been occurring in this region since historical times. Historical records exhibit dreadful floods that occurred for many times. Flood was devastating when occurred simultaneously associating with the shifting of channel course of rivers and also coinciding with earthquakes. Actually flood and earthquakes are twin sisters that come together to this region even today also, though earthquake frequency has been decreasing in this region since 1969 (Sharma, 1990).
The spatio-temporal dimension of pattern of the Brahmaputra valley flood is also to be taken into consideration. It is the narrow easternmost head of the Indo-Ganges-Brahmaputra (IGB) plain which is as narrow as only 80km (51-128 km) wide on average and surrounded by high off-shoots of the Arunachal Himalayas, Arakan Yoma and Shillong Plateau. The flood affected area and population respectively are 3.82 million hectares and 8.31 million at maximum in recent floods after 1950. The magnitude of flood so far recorded is found maximum in 1988. The loss of property and life and were high in 2004 due to sudden rush of flood water from nearby highlands for causes not normal ones.
Records of historical flood in the Brahmaputra valley are found in 1642, 1787, 1795, 1862, 1867, 1879, 1878, 1886, 1918, 1935, 1954, 1966, 1968, 1969, 1970, 1973, 1987, 1988, 1989, 1993, 2001, 2004 in Imperial Gazetteers of India (1908), historical books, official records, etc. The Inter-annual variation of flood during 1900-1940 was low due to influence of Zonal monsoon period (ZP) and periods before it (1871-1900) and after 1940 was high due to meridional monsoon period (MP). Zonal monsoon is stable monsoon period and ‘Meridional’ is less stable period. Therefore clustering of large floods occur during 1971-2005 and also occurred during 1871-1890 period (Parthasarathy et. al. 1987). This fact is aggravated by rising bed of the Brahmaputra due to the effect of 1950’s great earthquake.
For this reason both the low water level (LWL) and high water level (HWL) are rising after 1950 when the Inter-annual variation of water level is also high during this period. This is found consistent with the long term secular changes in monsoon rainfall and SST Index of ENSO (Monsoon) (Wright 1989; Fu and Fletcher, 1988; Parthasarathy et al. 1919). Occasional high flood is a symptom of this change. The isohyetal change is observed in N .E. India region and the location of peaks are found moving towards east. It may also have the variable effects of the wind system and physiography of the region.
Another character that the total flood prone area has not much variation except four peaks in 1954-56, ’72-’74, ’87-’89 and 2004 period within the last few years. Long-term flood data are not available for which the long-term characters cannot be observed. The palaeo flood study (Baker, 1994) on slack water flood deposits bring into light the fact that between 0-400 A.D. and 1000-1400 A.D. moderate flood occurred and extreme floods occurred within 400-1000 A.D. (Kale et. al., 1997). Studies on the deposits of Narmada, Godavari and Krishna rivers indicate that no floods having higher magnitudes than floods occurred after 1950 occurred during last several hundred years (Ely et. al., 1996). This indicates a general climatic change in case of monsoon throughout India and the world as a whole.
Flood Characteristics:
Some characteristics are noticed in the occurrence of floods in the valley as follows –
1. Flood comes in wet summer season associated with south-west monsoon rain from May to September. High flood, of course, are associated with break monsoon during retreating monsoon period of August and September. Occurrence of flood varies from 3 to 6 in number every year.
2. Distribution of annual flood in the Brahmaputra is negatively skewed whereas its north bank tributaries have positively skewed distribution of annual floods. Western south bank tributaries have negatively distributed annual floods and eastern south bank tributaries have mixed distribution, of course having a positive tendency.
3. Study of the highest floods of the Brahmaputra of 1988, 1989, 1998 and 2004; it appears that the floods of the Brahmaputra in mostly contributed by the flood waters of the north bank tributaries.
4. Ratio between magnitude of average monsoon flood and occasional extreme rare case varies from 1:3 to 1:4 (Kale, 1998).
5. Main generic type of flood (Ward,1978, p13) occurs due to both monsoon rainfall and rainstorm (Kale,1998). Vigorous monsoon rain incessantly occurring for a long number of days brings high spate of flood to the valley as a whole. Rainstorm flood occurs due to low pressure system developed in the Bay of Bengal which develops cyclonic storm and occurs heavy rainfall on its way moving towards north and north-east. The variability of distribution of rainfall by cyclonic storms occurs floods of varied magnitude in different tributary sub-basins. The break monsoon developed due to presence of low pressure system (LPS) in the Sub- Himalayan zone brings the highest floods in the Brahmaputra valley.
6. Number of flood peaks are large and flood duration is short in case of north bank tributaries.
7. Magnitude of flood of the Brahmaputra decreases towards downstream i.e. highest in upstream course.
8. There is no significant sign of increase of flood magnitude and events through times upto the present in the valley after 1950’s great earthquake from which point the height of the flood level is raised and flood events increase due to rise of the bed of the Brahmaputra (fig. ). Before then, there are found less number of annual flood events, but of course, proper records are not available.
9. Effect of flood increases through times due to increase of development activities in the floodplains and also due to increase of population through times. Increase of human settlements in the floodplains is also another aggravating factor.
10. Occasional floods due to reservoir failure, rushing down of water out of landslide dam bound reservoirs, flash flood, shifting of channel, dam failure or mismanagement, poor drainage both in hilly or plain region, breach of embankments, etc. occur. These accidental cases are found more frequent due to lack of proper management system of rivers lack of disaster management system in areas like Assam which is geomorphologically and tectonically more sensitive to flood and associated problems.
11. Flash floods are common in north bank plain, southern plain of west of Guwahati and Sadiya region. In the east of Guwahati, the whole of south bank plain of the Brahmaputra in under the occurrence of spill flood. The spilling occurs in both the channel of the Brahmaputra and the southern tributaries. The flash flood occurs only through tributaries of the north bank plain areas already mentioned.
12. Except climate, the closed and congested geomorphological set-up including compact river network and drainage of the region is another character for development of flood than other parts of India.
13. Flood is found from the study not related to the depletion of forests both in the valley and in the upstream areas, but only related to monsoon rain and fluvio-geomorphologic factors which is largely affected by the great earthquakes of 1897 and 1950 specially the later in the region.
14. Occasional local cyclonic storm and uneven distribution of rain in the valley occur flood in isolated areas or basins without affect on the Brahmaputra.
15. Long term secular changes of the monsoon rainfall (Zonal and Meridional monsoon) and SST Index of ENSO (Wright, 1989; Fu and Fletcher,1988; Parthasarathy et al., 1989) have affects on long term changes and stability of the monsoon rainfall in the Brahmaputra valley along with whole of India.
16. The inter annual variation of flood as already mentioned during 1900-1940 was low due to Zonal monsoon period (ZP) and perhaps period before it i.e.1871-1900 and after 1940 was high due to Meridional monsoon period (MP). Zonal monsoon is stable monsoon period and Meridional monsoon is less stable period. Therefore clustering of large floods occur during 1971-2005 and also occurred during 1871-1890 period (Parthasarathy et al,1987).
17. There is no variation in the size of flood-prone areas except five peaks in 1954-56, ’72-’74, ’87-’89, 2004 and 2007 as already mentioned.
Types of Flood:
Three types of flood occur in the Brahmaputra valley – flash flood, spill flood (long duration) and flood due to drainage congestion. Flash flood occur naturally due to rushing down of flood water through rivers from nearby high mountain of the great Himalayas in the north and through short plain courses of rivers flowing from high Shillong Plateau in the south bank plain west of Guwahati and artificially in all rivers due to sudden clearance of reservoirs and failure of embankments. Flash flood also occurs due to shifting of channel of rivers. This occurs for relatively short periods. Long period spill flood occurs in the Brahmaputra and its eastern south bank tributaries occur due to low slope and long plain courses of rivers flowing out of relatively low hills and mountains. Drainage congestion develops floods lasting for long periods. Drainage congestion occurs due sluggish flow or presence of no out let to drain out accumulated water.
Geomorphic Implications of flood:
Flood has some geomorphic implications. During flood, river gains enormous power to change the geomorphic processes within its domain (Baker and Costa,1987). The steam power (w=YQS/w=TV) to produce shear stress on banks and beds during flood are more than the extent of normal 42 at Pandu and 0.51 in the course between Pandu and Jogighopa. Unit stream power in Wm-2(w) of south bank tributary Burhidihing is 6. Stream power is low due to very low slope and high value of width-depth ratio. But in spite of low stream power, the flood can change the course of river, occur high bank erosion, change morphology, landforms and configuration of rivers, increase sinuosity, length, size and shape of river. Channel shifting is common in the Brahmaputra itself and tributaries during flood occurrence. Due presence of non-cohesive materials on the banks and plains and high mobility of bed and suspended materials in response to high discharge of water, the fluvial geomorphology of the rivers and the valley is frequently changing through channel shifting, formation of meanders and meander cut-offs, extension spill channels, development of new spill channels, bank slumping, stream capture (Spring,1935; Inglis, 1949; Coleman, 1969; Pandey,1980; Verma et al,1989; Goswami, 1985; GFCC,1986; Godbole, 1986; Sarma and Basumallick, 1986; Dubey, 1990; Gupta, 1995; Sing, 1996; Gilfellon, 1996; Kale, 1998 ). Valley is also highly silted and thereby landforms changed by the affect of flood.
Flash flood occurs not only due to high slope of the mountains but also due to reckless dam clearance and dam failure and breach of embankment which occurs great devastation as in kapili river, Manas and Ranganadi. Manas was blocked by bed load at the mouth of the river debouching to the plains, high devastation occurred due to flushing out water towards vast surrounding areas. Again flash flood from Khasia-Jayantia hills of Shillong plateau through the Bolbola river (Jinari river) occurred great devastation during 2004 by floods and siltations. The Brahmaputra has shifting channel combinedly with the Dibang and Lohit and develop a new course through the southern boundary of the Dibru-Saikhowa National Park capturing the lower course of the Dholla-Dangari-Dibru river. It is an example of river avulsion or anabranching developing distributaries and building of river islands. Most of the courses of the tributaries of the Brahmaputra specially the northern ones are highly sensitive to change their courses.
Causes of Flood:
The causes of flood are discussed here. Flood occurrence is dependent on certain factors of hydrometeorological, fluviogeomorphologic, pedologic, tectonic, anthropogenic, etc.. Meteorologic, geomorphic and anthropogenic factors are the broad categories that needed to be examined in this region.
Hydro-meteorologic Factor:
Occurrence of rainfall, its frequency, intensity and distribution are essential factors for occurrence of flood. Rainfall occurs through monsoon rain, rain-storm and cyclonic effects, break monsoon, orographic lifting of air, etc.. Rainfall occurs due to (1) tropical storms and depressions, (2) active monsoon wind and (3) break monsoon. The tropical disturbances of storms and cyclones generally originate in Bay of Bengal and Punjab-Rajasthan landlows. Flood-generating rainstorms occur rainfall mainly on two major zones (Nandargi, 1996) - Ganga and Punjab plain and Central and northern half of the Peninsular India. But the maximum rainfall occurs in N.E. India - highest daily upto 1040 mm (Cherrapunji, June 1876) and high annual rainfall (50-1100 cm) due to convergence and integration of both branch of monsoon wind from Bay of Bengal and Arabian sea and also due to orographic effects on to the Arunachal Himalayas, Arakan Yoma, Shillong Plateau and others.
Again the affect of long term changes in the monsoon rain on flood is today increasingly recognized. There have been found four major rainfall period as 1880-1895, 1895-1930, 1930-1963 and 1963-1990 (Kripalani and Kulkarni, 1996) where 1880-1895 and 1930-1963 received above normal rainfall having very few drought and 1895-1930 and 1963-90 are periods having below normal rainfall with frequent droughts in India as a whole. This sort of periodicity also identified by Gregory (1989) by studying rainfall distribution at micro-regional level during 1871-1985 period. He identified the N.E. India as under above average rainfall condition during early and mid part of the studied period, whereas other region indicate wet period not similar to N .E. India’s period. Therefore the periodicity is also to be noticed while considering the rainfall factor.
EI Nino affect on these periodicity is established from many respects. It has a teleconnection with ENSO (EI Nino Southern Oscillation) (Bhalme and Jadhav, 1984; Parthasarathyet. al. 1991;Whetton and Rutherfurd, 1994) and anti ENSO (La Nina) (Ropelewski and Helpert, 1987). The relation of Southern Oscillation Index (SOl) with rainfall and flood was established (Bhalme and Jadhav, 1984). Though floods are correlated with Cold ENSO and drought with warm ENSO (Bum and Amell, 1993) and also the flood with La Nina and drought with ENSO (Kripalani and Kulkarni, 1996) yet remarkable relation between flood and EI Nino (ENSO) was already established (Kane, 1989; Kale et. al, 1996). Thus monsoon variability is found influensive in the mode of intensity and distribution of rainfall for annual, decadal or even for longer period.
Geomorphological factors:
Geomorphological factor is perhaps the greatest dominant one after the rainfall. The Brahmaputra basins is run row basins surrounded by high mountains as already mentioned facilitating the convergent rushing of water to the valley that definitely have high possibility of occurrence of flood. The geomorphic factor includes physiography, pedology, hydrology, fluvial processes, etc. which have considerable amount of affect on the flooding processes.
Physiography of the region is discussed in the foregoing point of physical set-up. In addition it is to be noted that if N.E. India is considered to be single region, its basin plain is consisted of only 24% of the total surface area through which more than 100% surface run-off i.e. the rivers have to carry away (100% of its (N.E. India) own surface run-off plus run-off of Tibetan catchment). The Brahmaputra basin is consisted of 5,80,000 Km2 catchment area out of which 1,94,413 Km2 falls in India. The valley north of the Brahmaputra is wider in the west and narrow towards east and the southern one is wider is the east and very narrow in the west having slightly wide corridor in Nagaon district in the middle. The watershed line for most of the tributaries to Brahmaputra except Trans-Himalayan rivers like Subansiri, Manas, etc. in the north is the crest line on the Middle Himalayan Range. This crest line is higher than the watershed or crest line in the south constituted by the crests of Arakan Yoma, Patkai Range and Shillong Plateau.
River and River channel character : Assam is a land of rivers. The valley length of the Brahmaputra is 720 Km with a channel length of 640 KIn. carrying waters from 103 tributaries (65 in north and 38 in south bank).
The important characters that have affect on flood are - the Brahmaputra along with the lower courses of some of its major tributaries are highly in the process of braiding, anabranching, migrating and enlarging through shifting of channel courses or bank erosion. Therefore large number of island, sand bars, point bars, chars, palaeochannels or ox-bow lakes or meander cut-offs, meander scrolls, sloughs, natural levees, back-swamps, sand splays, dry channels do exist in the midst, sides or over bank of the river channels of the Brahmaputra and its tributaries. The channels though are large in number, yet incapacitous to carry the rushing water from high hill slopes in time. Therefore within the same channel catchment, the upper course when found flooded with water, the lower course is in normal water stage or vice versa. This geomorphic character is conspicuous in the Brahmaputra Valley. Alluvial cones and fans are predominant in all the tributaries in their debouching points to the plain valley especially in case of north bank tributaries. Therefore large sand beds devoid of vegetations are developed in the foot hill plain which again facilitate the spreading of rushing flood water from the hills. Anastomosing of river channels are also developed in this belt. The flooding rivers have numerous spill channels attached to banks which facilitate flowing of flood water out the river channels to the plains.
The bed of the channels are not smooth, rather they are wavy having alternate pools and riffles. Again due to siltation, beds of some parts of the Brahmaputra and its tributaries are found quite above the general level of the plain. This character is sometimes developed due to the existence of embankments and facilitates the easy flow of water to nearby plain which are even lower than the channels or for breach of embankments, ringbunds or for abandonment or migration of channel towards the valley plain. The areas having full of swamps, palaeochannels facilitate the migration and shifting of channels especially in north bank (in the Dhemaji district by Gai Nadi and in parts of Kamrup valley). Sandy loamy soil of the most parts of the northern plain again offers favourable condition for channel migration and shifting. Back swamps and water logging areas develop drainage congestion problem and ultimately occur flood due to poor drainage. Channel migration, meander growth and avulsion can affect the channel character and water discharge. When meander grows, channel length increases, gradient decreases and the water storage is raised up for floods (Jorgensen et. aI, 1994). Meander avulsion in tributary courses though not frequent, is occurring during the flood.
Tectonic Affect: Though flood is a historical event, yet it is found deeply associated with the tectonicity of the region. Earthquakes occurred during flood times at maximum facilitate for breaching of embankments, natural levees, banks, etc. or landslide obstruction and sudden clearance, etc. The great tectonic events of 1897 and 1950 have great effect on the geomorphology of the region and on the intensity and frequency of flood events. Parts of the river courses were made uplifted or submerged downwards soaking with landslide sediments brought by’ rivers from the hills for which the rivers have to shift their channel or enlarge the size out of raising and rising up of their beds and water levels.
A survey shows that a length of 355 km in the south bank and 230 km in the north of the Brahmaputra were suffering from erosion during period between 1923-54 (RBA, 1980). An NEC study exhibits that the size of the Brahmaputra channel of 1967 was enlarged than 1930’s size by 2.49 km on average. The erosion on the north bank was to the extent of 1.57 km and the south, of l.44 km on average measured on maps through 64 cross sections throughout the length of 640 km. The shifting course of the Brahmaputra and its tributaries are spectacular which occurs annually or within even still short span of time. The tectonic activities of 1897 and 1950’s earthquakes raised up the bed of the Brahmaputra and its tributaries for several metres (Kale, 1998, P 241) which affect the flow and occur floods. The bed depositions have been doing the river braiding and the braiding also increases due to tectonic by for 1.5 times at minimum between 1913-76. The slope of the rivers especially the Brahmaputra is very less 13cm/km (17cm/km above Dibrugarh, 10 cm/km near Guwahati) on average. The historical factors exhibit a general southerly shifting of the course of the Brahmaputra, but a measurement by Survey of India done for NEC indicate increase of northward erosion than erosion on south bank and hence has a tendency slightly northerly shifting as mentioned above.
Hydrologic Factors: Hydrological factor though dependent on geomorphic and climatic characteristics, yet have to take into considerations as important factor for flood analysis and management. The excess flow of water develops flood which has input from rainfall, evaporation, interception, transpiration, infiltration, surface flow, etc. The surface flow combined with ground flow makes the river discharge and flood flow. The Brahmaputra basin spotted with natural elements like forests, hills and plains, different type of soil, rock beds, rocks have different amount of evaporation, evapo-transpiration, interception, infiltration, etc.. The daily water discharge of the Brahmaputra (Assam part) varies from place to place along its channel from its normal rate of change. It is dependent on fluvial geomorphology and regional climatic factor. Mean outflow at Jogighopa and inflow at Pashighat is of four times difference.
The share of contributions of the main tributaries to the Brahmaputra varies from tributary to tributary. The flow character of the north bank tributaries differs with that the south bank tributaries. It is noticeable from the daily storm flood hydro graphs. The leg time, peak time, flood-to-peak time are different. These all times are short in respect of north bank tributaries and slightly longer in case of south bank tributaries because of moderate speed and slow flow of water due to emerging out of relatively low hills in the south than the high in the north Great Himalayan Range.
The flood frequency study also reveals a high frequency and probability (Weibull Plotting Position) of flood in the north bank tributaries than the southern ones. The flood frequency of the Brahmaputra at different point vary - high in Nagaon, Majuli, Dhemaji and Golaghat districts and low in other parts. The former areas are proned to high frequency of flood due to presence of distributaries, anabranching off shoot channels and also for bottle necks, low banks, etc. Whereas the later ones are due to overbank and spill channel flow of water. Floods of Assam generally occur at maximum due to tributaries. The flood is generally occurred in association with bank erosion of the Brahmaputra and its tributaries at different point at different intensity and times, e.g. bank erosion, of course, sometimes occurs without flood.
Anthropogenic factors : Dam failure, wrong or no moderation of flood through dam or sluice gate, breach of embankments, clearance of forest in watershed areas, etc. are anthropogenic events which affects definitely the flood frequency and magnitudes. Siltation on bed of the rivers are generally made responsible for due to the presence of embankments, but it is found not 100 percent correct. For many times rivers channel courses are found abandoned due to siltation on beds without the presence of embankments. This is also true in respect of the Brahmaputra river. Its bed is raised and size is enlarged to a great extent, can not be made through the influence of two embankments that exists more than 10 to 12 km far away from each other. This embankment is also not continuous along the Brahmaputra and also made after 1954. Again the clearance of forest is made responsible for increase of flood frequency and magnitude wrongly. Study on flood of IGB plain is found that flood here is not increased due to change of land use and deforestation in the Himalayas (Chaphekar and Mhatre, 1985, P61) as conservationists claim these facts grossly exaggerated way (Messerli and Hofer, 1995). Only 7% of land in the hills is under jhum cultivation and 5 to 15% of land under human settlement. There is no convincing evidence of man-induced increase of flood frequency and magnitude in Ganga-Brahmaputra basin (Brammer, 1990, Messerli and Hofer, 1995). There are enough evidence of climatic change of regional and global scale for increase of flood frequency and magnitude during the last few decades (1940-2006).
Flood Zoning and Control : Depending upon geomorphic and drainage character, climatic influence, the Brahmaputra and Barak Valley may be divided into a number of homogenous Flood Zones - homogeneity being defined on mode of occurrence, flood characteristics, risk and vulnerability requirement of similar treatment for management, etc. The Flood Zones taken for discussion will be of meso type. The whole valley be under single macro type region and the third type - micro regions shall be identified on locality basis. These are included within meso regions. The homogenous Flood Zones (meso regions)are
1. Brahmaputra Active Flood plain
2. Flash flood zone of Northern plain and Southern narrow plain (West of Guwahati) of the Brahmaputra
3. Southern Flood Zone of the Brahmaputra east of Guwahati
4. Flood free zone
1. The Brahmaputra Active Flood plain is constituted by the very neighbour and nearest overbank part of the Brahmaputra river channel along with its islands, chars and bars within the channel length and breadth and the river channel or channels (in respect of braided parts) Here the area within the channel is the biggest and the land over bank part is not much wide. When the whole of Assam region receives high rainfall, this zone may have the occurrence of flood at the same time. But mostly the flood occurs in this zone at different parts at different times and intensities vary according to the variability of rainfall distribution over the whole region. Certain sections may have Flood occurrence according to excessive rainfall occurred over the upper course of the tributary basins of that section. Again it may also occur that other basin may have the occurrence of flood but the Brahmaputra may remain free from flood at that time. Thus fluvial geomorphology of this zone is an important factor to be studied very deeply. Geomorphological hydrology, tectonicity, hydrometeorology, etc. also are important factors to be taken in to consideration (Fig. 9). The Barak active flood zone is also behaving similar way to the Brahmaputra.
2. The Flash Flood Zone consists of the whole of North Bank Plain of the Brahmaputra active flood plain which also includes the Sadiya Region. Similar characteristic flood zone is also found is the narrow southern plain west of Guwahati in Assam. Though not contiguous, this area may also be included with in this zone due to similar flood behaviour through time and space. Thus this homogeneous Flood region may be divided into four sub regions- eastern and western sub-zones on the Northern flood zone divided by highlands of Tezpur, the South Bank narrow plain, west of Guwahati and the Sadiya Region. These regions are bordered by nearby highlands; have relatively short more or less straight tributaries where valley has sufficient slope and meandering rivers where valley is extra ordinarily flat; rivers having anastomozing and anabranching character at the foothill and flat mid-valley full of swamps; indefinite channels present where swamps exist; speed of water either in channels or during floods is very high; rushing of water through downstream or in sheets is frequent and flash flood is common; have also sluggish flow where Swamps do exits; sediment of the river is high; have large number of dry channels in winter and inundated channels in summer, flood intensity and magnitude are high and lag time and time of concentration are short in this region. Here requires the control of floods in the upper parts of the valley i.e. rivers through construction of dams, flood moderation project, and channel diversion in the foot hill zones. Embankment is less effective due to high bank erosion and bed siltation. Inter basin transfer within sub-basins may be done to decrease flood intensity and divert the flood concentration, but it may be a political issue, so should have political efforts too. Small dams in upstream courses of rivers may be effective and relieve the region from flood menace.
3. The Southern Flood Zone is consisted of the southern part of the valley of the Brahmaputra. This region is extended east of Guwahati upto Saikhowa in the eastern part of Assam. Though homogeneous in geomorphological and flood character; yet may be divided into some sub-zones depending on some slight variations. It may be divided into eastern and western zones having Karbi-Anglong as the divider. Common characteristics of these zones are long meandering and tortuous tributaries, having speed of water in channel or during floods slow, low amount of load, bordering with low hills at far distance, valleys are wide, some where rivers are sluggish in character at some seasons, full of palaeochannels, etc. The western sub-zone is highly spotted by large number of palaeochannel and anabranching channels. It is more or less like the shape of a bowl drained by the Kapili river surrounded by highland on its three sides and has more or less centripetal drainage system. The river in off season is found sometimes sluggishly flowing. The eastern sub-zone is normal except the easternmost part where the Brahmaputra exhibits ana branching character developing a major anabranch, the Ananta Nalla. Here river channel character varies according to varied soil character. Palaeo channels are common in the eastern part of this eastern sub-zone. The flood here occurs is of spill type and takes. enough time to move. Embankment are effective in the zone if scientifically constructed and managed. Bank protection and channel diversion project may also effectively solve some problems in this region.
4. The other parts of the Brahmaputra Valley i.e. hilly areas are flood free zones where occasional disasters may occur out of rushing of water downslope of hills, for which landslide may occur. Watershed management treatment is poor or completely not done in these areas which can lessen the volume of run-off in plain areas.
Flood Control Measures:
Common flood control measures are embankments, ringbunds, diversion of channel, drainage channelling, spurs, dyke, river dams, etc. which should be used judiciously. Measures may vary nom flood zone to flood zone suiting with the situation and nature of rivers and floods (Inglis, 1949, Hay, 1988,94; Singha, 1982; Kale, 1997; Rao, 1975; Chitale, 1978, etc.). The measures adopted in Assam are not sufficient and rather less scientifically planned. The risk factor is high in Northern Flood Zone and Barak valley zone. Strong and quick floods occur within these zones. So, moderation or diversion of flood force is necessary. Check dams, inter-basin transfer by inter-connecting canals at the foot of the hills (Borthakur, 1965) with sluice gates between basins, embankments, dredging and clearance of channel beds of the tributaries which is more necessary than the bed of the Brahmaputra, stabilization of banks, roads and rails having good number of culverts, drainage channelling and rescuer rapid action force for disaster operation are required in these high risk zones to minimize the risk factor.
The southern flood zone has moderate risk though the Kapili basin is under moderately high risk factor. Spilling of water through anabranches and palaeochannels from the Brahmaputra are frequent and dangerous. Embankments, culverts, ring-bunds, spurs, channel diversion, etc. will be important for this zone.
The active flood plain zone of the Brahmaputra has the overbank spilling and spilling through spill-ways. Embankments, ring-bunds, construction of spurs in human settlement areas, keeping completely or partially free the other areas will solve the flood problem in this zone. Stabilization of banks through construction of spurs and diversion of channels or current of water within the channel of the Brahmaputra will check further expansion of channel towards land.
As already mentioned, drainage channelling is required for the Barak and the Kapili valley zone. River dams at upper course of some major rivers are to be constructed for flood moderation. Dams proposed to be constructed on Dihang, Dibang, Lohit, Subansiri, Manas, Barak will definitely keep the water level of the Brahmaputra and the Barak low. The small tributary basin floods must be managed with the methods mentioned above for which physical as well as anthropogenic factors are to be understood to manage flood (Hey, 1994; Masserli and Hofer, 1995; Ward, 1978; CSE, 1991; CWPC, 1958, etc.)
References :
1. Baker, V. R. (1994) - ‘Geomorphological understanding of floods’ - Geomorphology Vol. 10 PP 139-156.
2. Bhalme, H Nand Jadhav, S. K. (1984) - ‘Southern Oscillation and its relation to the monsoon rainfall J. Climatology Vol. 4 PP 509-520.
3. Borthakur, M., (1965) -Irrigation, its necessity and advantages in Lakhimpur-Jour. of Assam Science Soc. vol. VIII, Gauhati
4. Brammer, H. (1990) - ‘Floods in Bangladesh-Flood mitigation and environmental aspects’ - The Geographical Journal vol. 156 PP 158-165
5. Bum, D. H. and Amell, N.W. (1993) - ‘Synchronicity in global flood responses’ -Jour. Hydrology Vol. 144 PP 381-404.
6. Chaphekar, S.B. and Mhatre, G.N. (1985) - Human Impact on Ganga River Ecosystem - Concept Publishing Company, New Delhi.
7. Chitale, S, V: (1978) - ‘Some thoughts on flood control problems in India’ - Irrigation and Power Jour. Vol. 36 PP 493-496.
9. CSE (1991)- ‘Floods, flood plains and environmental myths’ State of India s Environment -3. Centre for Science and Environment, New Delhi.
10. CWPC (1958) - ‘Floods; Are they on the increase?’ Central Water and Power Commission Publication, New Delhi.
11. Dutta, Lakshyahira (2003) – Locational pattern and Morphometric characteristics of Alluvial Fsans in the Himalayan Foothills of Assam-North Eastern Geographer-vol.33 No 1&2 Gauhati University pp46-54.
12. Ely, L.L., Engel, Y., Baker, V.R., Kale, V:S. and Mishra, S. (1996) - Changes in the Magnitude and Frequency of Late Holocene Monsoon Floods on the Normada river, Central lndia-Geol. Soc. Am. Bull. v. 108 pp 1134-1148
13. Fu, C. and Fletcher, J.1988 - Large signals of climatic variations over the ocean in the Asian monsoon region - Adv. Atmosphere Science, Vol. 5. PP389-404.
14. Gregory. S. 1989 - ‘Macro-regional definition and characteristics of Indian summer monsoon rainfall, 1871-1985’ - Int. Jour, Climatology, Vol. 9 PP 465-483.
15. Hey, R,D, 1988 - ‘Mathematical models of channel morphology’ - Modelling Geomorphological Systems -ed. M.G. Anderson, John Wiley and Sons Ltd. New York PP 99-125.
16. Hey, R,D, 1994 - ‘Environmentally Sensitive river engineering’ Modelling Geomorphological Systems - ed M.G. Anderson, John Wiley and Sons Ltd. New York.
17. Imperial Gazetteer of India 1908 - ‘The Indian Empire’ Oxford clarendon Press.
18. Inglis, C. C. 1944 - ‘The behaviour and control of rivers and canals’- Central water, Power, Irrigation and Navigation Research Station, Pune, Research Publication No. 13
19. Kale, V: S. 1998, - ‘Monsoon flood in India: A hydro geomorphic perspective’ - Memoir Geol. Society of India. No. 41 PP 229-256.
20. Kale, V:S.; Ely, L.L.; Engel, Y and Baker, V:R 1996 - Palaeo and historical flood hydrology, Indian Peninsulain Global Continental Changes: The Context of Palaeohydrology (ed) J. Branson, A.G. Brown, K.J. Gregory - Geol. Soc. Spl. Publication, London - Vol. 115 PP 155-163
21. Kale, V:S. 1997 - Fluvial Studies in India: A Brief Review J. of Geol. Soc. India - Vol. 49 PP 359-370.
22. Kane, R. P. 1989 - “Relationship between the southern Oscillation EINino and rainfall in some tropical and mid-latitude regions” - Proc. Indian Academy of Science - Vol. 98
23. Kripanali, R. H. and Kulkarini, A 1996 -Assessing the impacts of EI Nino and non EI Nino related droughts over India - Droughts Network News Vol. 8 PPII-13.
24. Messerli, B. and Hofer, T. 1995 - Assessing the impact of anthropogenic land use change in Himalayas - in Water and Quest for Sustainable Development in the Ganges Valley (ed) G. P. Chapman and M. Thompson, Mansell, New York PP64-89.
25. Nandargi, S. S. 1996 - Rainstorm studies for planning and development of water resources of the Indian region, unpublished Ph. Thesis submitted to univ. of Pune, quoted from Kale, 1998 ‘Monsoon flood..’
26. Pathasarathi, B., Sontakke, N. A., Munot, A.A. and Kothawale, D.R. 1997 -”DroughtsIFloods in the summer monsoon season over different - meteorological subdivisions of India for the period 1871-1984’’ – Jour. Climate -vol. 7 PP 57-70.
27 Parthasarathi, B., Rupa Kumar, K. and Munot, A. A. 1991 - “Evidence of Secular variations in Indian monsoon rainfall-circulation relationship” -Jour. Climate,. Vol. 4. PP. -927-938.
28 R.B.A.1980 - Report of the Rastitriya Bash Ayog-Govt. of India, New Delhi, Vol. 1&11.
29 Rao, K, L. 1975 - India s water wealth; Its assessment, uses and projects - Orient Longman, New Delhi PP. 150-165.
30 Rupelewski, C.F. and Halpert, M.S. 1997 - “Global and regional scale precipitation patterns associated with the E1 Nino/Southern Oscillation”, Monthly Weather Review - Vol. 115, PP 1605-1626.
31 Sharma, S. K. 1990 - “Seismicity and Seismic Risk in Northeast India” -Assam in 2001 A.D. (ed) P. Mahanta, D.U. Dibrugarh.
32. Sharma, Nayan (2010) – A Satellite Data Based Approach to Study Braiding Behaviour for Monitoring and Management of the Brahmaputra River-Souvenir-Landuse, Biodiversity and climate change-IGU Seminar,Cotton College, Guwahati, Assam, India-pp20-35.
33 Singha, C. P. 1982 - “Flood moderation in the Brahmaputra River by dredging” - Proc. CBIP Short-Ferm course on Flood control, Roorkee, Technical Report No. 31, PP 20-26.
34 Ward, R, 1978 - Floods - A Geographical Perspective -The Mac Millan Press Ltd. London.
35 Whelton, R. and Rutherfurd, I. 1994 - “Historical ENSO teleconnections in the eastern hemisphere”, Climate Change, Vol. 28, PP. 221-253.
36 Wright, P. B. 1989 - “Homogenized long period Southern Oscillation indices” - Int. Jour. Climatol. Vol. 9
PP. 33-54.
Dr. Bhuban Gogoi
Principal, Tinsukia College, Assam, India.
principaltinsukiacollege@rediffmail.com
Abstract: The Brahmaputra valley, 750 km long and 80 km wide is located in the extreme north eastern corner of India. The Brahmaputra river with 640 km long in east-west alignment is flowing towards west of the valley sloping from altitude of 230 m in the east to 40 m to the west. The river has 103 tributaries, 65 in north and 38 in the south bank. It is a long narrow valley wider in western part of the north bank section and eastern part of the south bank section. The northern part of the valley, no matter of wide or narrow, has high slope to the main channel of the Brahmaputra. Bounded by mountains in the South and in the north with the Himalayas and its off-shoots having a profile it has a sudden break of slope like a hockey- stick specially in the north to release water with high speed flushing to the valley. The southern valley has low slope and it is characterized by meandering river channels with low hill headwaters supplying slow base flow. Spill floods occur here during excess rainfall periods. The rivers of northern valley are partially interconnected by anabranches and distributaries having more or less straight channels. These north bank rivers carry heavy sediment loads, form levees on the bank of the Brahmaputra and generally turn to the west. This causes high floods in the areas, upstream of these bends. Dry channels and sandy areas (char) are present in the northern valleys which are replenished by flash floods water during the rainy season. In marshy areas the valley becomes sluggish and frequent shifting of channels is common in the eastern part of northern valley. The south bank tributaries have high bank erosion due to meanderings and formations of palaeo-channels and ox-bow lakes. Flood is a common hazard occurring in the valley from historical time due to high rainfall in monsoon season and due to fluvio-geomorphology of the basin. Not only that flood occurs due to break monsoon and due to failure of dams and embankments, absence of embankments and avulsion of rivers channels. Flash floods occur specially in whole north bank valley, and south bank valley part in the west of Guwahati.. Spill floods occur in the broad south bank valley east of Guwahati. It is a recurrent event that takes life and property of the valley every year. This situation results from the combined effect of hydro-meteorology, tectonic and anthropogenic causes. It has also distant relationship with ENSO and periodic variations of monsoons. For management of flood, regional zonation is made as Northern Flash Flood Zone, Southern Flash Flood Zone west of Guwahati, Moderate Spill Flood Zone east of Guwahati, Active Flood Zone of the Brahmaputra and Flood Free Zone of hills and high lands. Mode of management may vary from zone to zone having some common methods to all like embankments, ring-bunds, inter- basin transfer, dams, drainage channeling, spur, dyke, etc. All these are to be judiciously used. A good number of flood protection measures are taken in the valley by construction of embankments about 5000 km long, drainage channeling for about 1000 km, protection and anti-erosion works for more than 500 km, etc. But these are not sufficient as compared to the menace. Flood abatement and adjustment through traditional and modern techniques must be used and developed. Watershed management and disaster management system may be highly effective to flood menace.
Key Words : Zonal monsoon, meridional monsoon, flash flood, spill flood, flood control, flood abatement, flood adjustment, watershed management, disaster management.
INTRODUCTION
Flood becomes an unfold tragedy of the Brahmaputra valley though it did appear with mixed blessings in the past. It is a problem widely talked about and is made responsible for continuous erosion of economy and social life of the state of Assam. Ironically there is yet to reach at a well-integrated detailed study of the problem but the situation demands urgent and immediate solution. The manifold dimensions and far-reaching effects of floods have neither been properly managed nor measured in the state though some effects be measured and perceived directly whereas some still are intangible. Flood of the Brahmaputra valley is mainly linked with the fluvial geomorphology of the Brahmaputra River and its tributaries fed by rains of the world’s wettest monsoon climate. The valley has the annual recurrent floods having three to four waves on average and nine at maximum during monsoon period. Floods have a far-reaching effect on the life of the valley from tangible loss of life and property to non-tangible great extent. Floods have great impact on geomorphic manifestations of the fluvial geomorphology and the valley at large. Human mismanagement adds another dimension to the occurrence of flood during recent days. Fluvial geomorphologists attempt to a great extent to study the menace of flood hazards for its proper management, but yet to achieve a lot in the world perspective. This paper seeks to investigate flood problem of the valley from view point of fluvial geomorphology and hydrometeorology on the background of recent environmental changes.
Location and Physical Set-up:
The Brahmaputra valley is located in between 900E and 960E and 270 N and 280N in east-west alignment in the south-central part of the Brahmaputra catchment area of the extreme north-eastern corner of India. It is the lowest part of the region with a long depressed surface like the bottom of a boat with one end flanked and bounded by the mountains of the Himalayas and its off-shoots in the east in a syntaxial bend keeping open the other end to merge with the plains of West Bengal and Bangladesh, part of broad Gangetic plain. The Brahmaputra valley covers 19 plain districts of Assam having an area of 63,600 km2 approximately 72% of the state’s area supporting 85% of state’s total population.
The Brahmaputra valley is a continuous plain extended on both bank of the main river Brahmaputra approximately in east-west alignment partially interrupted in midway by Karbi Anglong and elsewhere by some minor hills. The valley is 750 km long with a width of 80 km on average. The Brahmaputra River in the valley has as many as 103 tributaries (65 in north and 38 in south banks). The valley slopes down from 230m in the east towards 40m in the west. The valley is surrounded by high Himalayas in the north and east and Naga Hills,Karbi-Anglong, North Cachar Hills and Shillong plateau in the south. The northern valley slightly widens in the west and southern valley in the east. The 300m contour demarcates the valley boundary. The northern valley has high slopes with abrupt break to the valley due the presence of lofty Himalayas and the southern has relatively low gradient with the presence of low hills in the border of the valley. The valley in the west of Guwuhati is a ramp valley or a sag in character and east of Guwahati is a rift valley having WEW—ENE direction between the Main Boundary Fault of the Himalayas and the Naga Thrust in the Naga Hills in the south. The Valley topography consists of Piedmont zone (Bhabar zone), alluvial fans, valley plain and river channels where the Brahmaputra river channel occupies a considerable area of the valley. The plain has Old alluvial plain (Pleistocene) and new alluvial plain built up in recent times.
Climate, Vegetation, etc.:
The Assam valley enjoys sub-tropical monsoon climate having wet summer and dry winter. Rainfall comes due to south-west monsoon wind from the Bay of Bengal and Arabian Sea occurring relief rain being obstructed by the high Himalayas for seasonal change of pressure conditions over Bengal and north western part of India and the change of tropical oceanic air mass of the south-west monsoon. Due to orographic effect on local pressure system, low pressure in the plains and high pressure in the hills occur sub-tropical climate of modified type of cwg of Koppen. Rainfall occurs at maximum 335cm in the North Lakhimpur in the east ranging to minimum 129 cm in the middle Assam. Rainfall comes in the monsoon rainy season in the form both of normal and storm-rain when floods come due to the rise water levels above danger(flood) levels more or less coinciding with the occurrence of storms. Rainfall occurring in the hilly and mountainous tracts and plains give rise the water level of the Brahmaputra and its tributaries. Himalayan belt receives more or less high uniform rainfall than the southern hills. So flood events are also frequent and high in the northern part of the valley.
Upper catchment areas of all the tributaries of the Brahmaputra have mixed vegetations with both evergreen and deciduous trees covering more than 60% of land whereas Assam has 33% of land covered by forests at present all having a decreasing trend in coverage. Interception to run-off is good but of course in decreasing trend.
Fluvial Geomorphology of the Brahmaputra Valley:
Inventory of types of fluvial geomorphology of the Brahmaputra valley mainly are the fluvial regime, erosional and depositional activities and development of their landforms, channel avulsion, flood, sediment transport, etc. The Brahmaputra river originating from Tibet has a total catchment area of 5,80,000km2 out of which 1,94,413km2 falls within India and the Brahmaputra Valley covers 56,480km2 constituting 70% of the state’s total area of Assam in India. The valley covers the middle course part of the Brahmaputra for a distance of 640km whereas the valley is extended beyond for 750km in length. One of the fluvio-geomorphological features is the alluvial fans and alluvial cones located at the foothill belts of the hills and mountains of north and south originated due to deposition of rocks and boulders carried by rivers at their debouching points to the valley. Tributaries are engraving down deeply through these fans and cones or percolating underground to reappear at distance in the northern plain. Floods are rare here due to their high slopes ranging from 0.5 to 60 having a higher basin slope ranging from 5 to 210 (Dutta,2004) due to high slope of the mountains. This occurs considerably less in the south due to low mountains .The built up valley plains exist in between the channel of the Brahmaputra and alluvial fans and cones zones both in northern and southern banks washed regularly by floods. These plains are dissected by active floodplains of the tributaries. The Brahmaputra active flood zone is spreading over its braided channel, islands and sand bars and immediate areas of its low banks nearby. Rivers have high natural leeves that create drainage problems and develop perennial flood areas on banks and divert some tributary lower courses to run parallel to some distance to meet the main river at downstream. The Brahmaputra channel is highly braided and all the rivers are found more or less unstable and eroding their banks and shifting of channels. These two events occur highly during flood times. The Brahmaputra becomes highly flooding, bank eroding and widening its channel through increasingly braiding due to the impact of great earthquakes of 1897 and 1950 of which the later has more effects on landforms. The Brahmaputra becomes wider 2 to 3 times than before 1950. The valley has as many as 2930 wetlands consisted of palaeochannels, low marshy lands, tectonically subsided land, etc. which play very important role in the fluvio-geomorphology of the valley. Northern tributaries carry more sediments than the southern. 1950’s great earthquake has great impact on the metamorphosis of the Brahmaputra channel.
Some Fluvio-Geomorphologic Characteristic of the Brahmaputra and its Tributaries and the valley:
Watershed or catchment area of the Brahmaputra (total)-----5,80,000 km2
(1) China---- 2,93,000 km2
(2) India----- 1,95,000 km2
(3) Bhutan— 45,000 km2
(4) Bangladesh-47,000 km2
(5) At points in India (a) Pasighat Damsite ---2,44,700 km2
(b) Kobo----3, 05,000 km2
(c) Dhubri—5, 33,000 km2
Length of the Brahmaputra (total) ------2897 km
Tibet-----------1625 km
India-------------918 km
Bangladesh----354 km
Brahmaputra valley in Assam state—640 km
Gradient (slope) of the Brahmaputra river-
Tibet-------------------------0.00260 / km
India (mountain tract)—0.00190 /km (Tibetan border to Kobo)
India (plain course) ------0.00014 / km (Kobo to Dhubri)
Bangladesh-------------- 0.00009 to 0.00003 / km (from border to mouth)
Maximum Discharge/Highest flood level (HFL)/Danger level (DL), etc.
Stations discharge (cumecs) HFL(m) DL(m) Maximum lift (m)
Dibrugarh- -- 106.48 104.24 2.24
Neamatighat-- -- 87.37 85.04 2.33
Tezpur- -- 66.59 63.23 3.36
Guwahati- -- 51.37 49.68 1.69
Pandu- 72,794 49.87 48.77 1.10
Goalpara- -- 37.43 36.27 1.16
Dhubri- -- 30.36 28.00 2.36
Discharge characteristics of the Brahmaputra
Discharge at Pandu (Brahmaputra valley)
Maximum --- 72,794 cumecs (23.08.1962)
Minimum ---- 1,757 cumecs (20.02.1968)
Mean yearly maximum---- 51,156 Mean monsoon total
Shigatse---- 507 million m3
Pasighat----3979 million m3
Mean yearly minimum----4,420 cumecs
Discharge per unit area of watershed up to
T’sela D’s Zong (China)---------0.01 cumecs / km2
Pasighat (India)------------------ 0.023 cumecs / km2
Pandu (Brahmaputra valley)--0.03 cumecs / km2 (Assam)
Bahadurabad (Bangladesh)---0.032 cumecs / km2
Sediment Load Characteristics of the Brahmaputra
Mean Suspended load at Pandu during-
Yearly maximum discharge---400 million m. tons
Daily maximum discharge ---2.12 million m. tons
Mean yearly sediment yield in per unit area at
T’sela D’s Zong (China)---------100 m. tons / km2
Pasighat (India)------------------340 m. tons / km2
Pandu (Brahmaputra valley)--804 m. tons / km2 (Assam)
Bahadurabad (Bangladesh)-1,128 m. tons / km2
Mean annual rainfall of the Brahmaputra valley 230 cm (129-250 cm)
Total basin landuse in India --- Forests -------114992.08 km2
Agriculture ----50473.84 km2
Total basin population in India --------------30.4 million ( 143 persons / km2)
Brahmaputra valley population, Assam—22.66 million (403 persons / km2)
Some Fluvio-Geomorphological Characteristics of the Brahmaputra-
Bank- erosion ---1841.17 km2
Bank-fill------------ 105.55 km2
Total erosion------1735.62 km2 ------- (1912-1996)
Erosion on North bank—538.81km2,(1996-2008)
South bank—914.62 km2 (1996-2008)
Increase of Braiding Index---Average (Mode)PFI—5.896 (1997),
5.09 (2008)
(where <4=high, 4-19=moderate &>19=low)
Average BBI—4.47-------(1912-14)Brice Braiding Index
5.09-6.7 (1973-91)
Increase of average width—5.8 km (1912-28)
7.4 km (1963-75)
7.4 km (1996)increased by 1.6 km on average
& 8.0 km at maximum
Tributaries and their drainage characteristics of the Brahmaputra
Tributaries catchment area length average annual average annual
Km2 km Suspended load sediment yield
ha. m tons/km2
North bank
1.Subansiri------- 28,200 442 992 959
2.Ranganadi 2,941 150 186 1598
3. Buroi 791 64 16 529
4. Borgang 550 42 27 1,749
5. Jia Bharali 11,716 247 2,013 4721
6. Gabharu 577 61 11 520
7. Belsiri 751 110 9 477
8. Dhansiri 1,657 123 29 463
9. Noa Nadi 907 75 6 166
10. Nanoi 860 105 5 228
11. Barnadi 739 112 9 323
12. Puthimari 1,787 190 195 2,887
13. Pagladiya 1,674 197 27 1,883
14. Manas-Aie-Beki 41,350 215 2,166 1,581
15. Champamati 1,038 135 13 386
16. Gaurang 1,379 98 26 506
17. Tipkai 1,364 108 31 598
18. Godadhar 610 50 0.21 272
19. Jiadhal 937 118 _ _
South Bank (Right side)
1. Burhi-Dihing 8730 360 210 1129
2. Disang 3950 230 93 622
3. Dikhou 3610 200 34 252
4. Janzi 1130 108 16 366
5. Bhogdoi 920 160 15 639
6. Dhansiri 10,242 352 147 379
7. Kopili 13,556 297 118 230
8. Kulsi 400 93 0.6 135
9. Krishnai 1,615 81 10 131
10. Jinari 594 60 3 96
11. Dibang 12,270 164 - -
12. Lohit 28,280 350 - -
13. Kundil 1,178 55 - -
Form Ratio
Rivers Site Width (w) Depth (d) form ratio
in m. in m. w/d
Brahmaputra Pandu 1400 12.9 115
Jogoghopa 2180 9.1 240
Burhidihing river Margherita 145 2.6 55
Khowang 152 4.0 38
Meandering Geometry of some south bank tributaries
Rivers Width Bend-radius Meandering wave Meander
KM Km Length in KM Amplitude in KM
Burhi-dihing o.29 0.77 2.98 2.57
Disang 0.08 0.19 0.83 0.65
Mai-jan 0.21 0.56 2.21 1.97
Dibru 0.06 0.14 0.46 0.46
Noa-dihing 0.17 0.50 1.75 1.70
After Sharma (1993)
Flood Problem:
The Brahmaputra valley is one of the highest flood prone areas of the world. Annual recurrent floods occur in the valley due to the Brahmaputra and its tributaries. Annually 3 to 6 flood waves occur in the valley during rainy season. Rains come from normal monsoon and monsoon storms. Heavy rainfall brings heavy surface run-off and the rivers becomes spate and over bank flow of water makes flood in the nearby basin region. Over bank flow, flow through spill channels, breach of embankments, dam failure, drainage congestion, bank erosion and channel shifting are the mode occurrence of floods. Both flash floods of short duration and spill flood of relatively long duration occur. Monsoon rainfall, fluvio-geomorphology and breach of embankments and sudden clearance of reservoir are the main reasons responsible for the occurrence of flood.
History and Dimensions of Hazards:
Flood is a natural event that has been occurring in this region since historical times. Historical records exhibit dreadful floods that occurred for many times. Flood was devastating when occurred simultaneously associating with the shifting of channel course of rivers and also coinciding with earthquakes. Actually flood and earthquakes are twin sisters that come together to this region even today also, though earthquake frequency has been decreasing in this region since 1969 (Sharma, 1990).
The spatio-temporal dimension of pattern of the Brahmaputra valley flood is also to be taken into consideration. It is the narrow easternmost head of the Indo-Ganges-Brahmaputra (IGB) plain which is as narrow as only 80km (51-128 km) wide on average and surrounded by high off-shoots of the Arunachal Himalayas, Arakan Yoma and Shillong Plateau. The flood affected area and population respectively are 3.82 million hectares and 8.31 million at maximum in recent floods after 1950. The magnitude of flood so far recorded is found maximum in 1988. The loss of property and life and were high in 2004 due to sudden rush of flood water from nearby highlands for causes not normal ones.
Records of historical flood in the Brahmaputra valley are found in 1642, 1787, 1795, 1862, 1867, 1879, 1878, 1886, 1918, 1935, 1954, 1966, 1968, 1969, 1970, 1973, 1987, 1988, 1989, 1993, 2001, 2004 in Imperial Gazetteers of India (1908), historical books, official records, etc. The Inter-annual variation of flood during 1900-1940 was low due to influence of Zonal monsoon period (ZP) and periods before it (1871-1900) and after 1940 was high due to meridional monsoon period (MP). Zonal monsoon is stable monsoon period and ‘Meridional’ is less stable period. Therefore clustering of large floods occur during 1971-2005 and also occurred during 1871-1890 period (Parthasarathy et. al. 1987). This fact is aggravated by rising bed of the Brahmaputra due to the effect of 1950’s great earthquake.
For this reason both the low water level (LWL) and high water level (HWL) are rising after 1950 when the Inter-annual variation of water level is also high during this period. This is found consistent with the long term secular changes in monsoon rainfall and SST Index of ENSO (Monsoon) (Wright 1989; Fu and Fletcher, 1988; Parthasarathy et al. 1919). Occasional high flood is a symptom of this change. The isohyetal change is observed in N .E. India region and the location of peaks are found moving towards east. It may also have the variable effects of the wind system and physiography of the region.
Another character that the total flood prone area has not much variation except four peaks in 1954-56, ’72-’74, ’87-’89 and 2004 period within the last few years. Long-term flood data are not available for which the long-term characters cannot be observed. The palaeo flood study (Baker, 1994) on slack water flood deposits bring into light the fact that between 0-400 A.D. and 1000-1400 A.D. moderate flood occurred and extreme floods occurred within 400-1000 A.D. (Kale et. al., 1997). Studies on the deposits of Narmada, Godavari and Krishna rivers indicate that no floods having higher magnitudes than floods occurred after 1950 occurred during last several hundred years (Ely et. al., 1996). This indicates a general climatic change in case of monsoon throughout India and the world as a whole.
Flood Characteristics:
Some characteristics are noticed in the occurrence of floods in the valley as follows –
1. Flood comes in wet summer season associated with south-west monsoon rain from May to September. High flood, of course, are associated with break monsoon during retreating monsoon period of August and September. Occurrence of flood varies from 3 to 6 in number every year.
2. Distribution of annual flood in the Brahmaputra is negatively skewed whereas its north bank tributaries have positively skewed distribution of annual floods. Western south bank tributaries have negatively distributed annual floods and eastern south bank tributaries have mixed distribution, of course having a positive tendency.
3. Study of the highest floods of the Brahmaputra of 1988, 1989, 1998 and 2004; it appears that the floods of the Brahmaputra in mostly contributed by the flood waters of the north bank tributaries.
4. Ratio between magnitude of average monsoon flood and occasional extreme rare case varies from 1:3 to 1:4 (Kale, 1998).
5. Main generic type of flood (Ward,1978, p13) occurs due to both monsoon rainfall and rainstorm (Kale,1998). Vigorous monsoon rain incessantly occurring for a long number of days brings high spate of flood to the valley as a whole. Rainstorm flood occurs due to low pressure system developed in the Bay of Bengal which develops cyclonic storm and occurs heavy rainfall on its way moving towards north and north-east. The variability of distribution of rainfall by cyclonic storms occurs floods of varied magnitude in different tributary sub-basins. The break monsoon developed due to presence of low pressure system (LPS) in the Sub- Himalayan zone brings the highest floods in the Brahmaputra valley.
6. Number of flood peaks are large and flood duration is short in case of north bank tributaries.
7. Magnitude of flood of the Brahmaputra decreases towards downstream i.e. highest in upstream course.
8. There is no significant sign of increase of flood magnitude and events through times upto the present in the valley after 1950’s great earthquake from which point the height of the flood level is raised and flood events increase due to rise of the bed of the Brahmaputra (fig. ). Before then, there are found less number of annual flood events, but of course, proper records are not available.
9. Effect of flood increases through times due to increase of development activities in the floodplains and also due to increase of population through times. Increase of human settlements in the floodplains is also another aggravating factor.
10. Occasional floods due to reservoir failure, rushing down of water out of landslide dam bound reservoirs, flash flood, shifting of channel, dam failure or mismanagement, poor drainage both in hilly or plain region, breach of embankments, etc. occur. These accidental cases are found more frequent due to lack of proper management system of rivers lack of disaster management system in areas like Assam which is geomorphologically and tectonically more sensitive to flood and associated problems.
11. Flash floods are common in north bank plain, southern plain of west of Guwahati and Sadiya region. In the east of Guwahati, the whole of south bank plain of the Brahmaputra in under the occurrence of spill flood. The spilling occurs in both the channel of the Brahmaputra and the southern tributaries. The flash flood occurs only through tributaries of the north bank plain areas already mentioned.
12. Except climate, the closed and congested geomorphological set-up including compact river network and drainage of the region is another character for development of flood than other parts of India.
13. Flood is found from the study not related to the depletion of forests both in the valley and in the upstream areas, but only related to monsoon rain and fluvio-geomorphologic factors which is largely affected by the great earthquakes of 1897 and 1950 specially the later in the region.
14. Occasional local cyclonic storm and uneven distribution of rain in the valley occur flood in isolated areas or basins without affect on the Brahmaputra.
15. Long term secular changes of the monsoon rainfall (Zonal and Meridional monsoon) and SST Index of ENSO (Wright, 1989; Fu and Fletcher,1988; Parthasarathy et al., 1989) have affects on long term changes and stability of the monsoon rainfall in the Brahmaputra valley along with whole of India.
16. The inter annual variation of flood as already mentioned during 1900-1940 was low due to Zonal monsoon period (ZP) and perhaps period before it i.e.1871-1900 and after 1940 was high due to Meridional monsoon period (MP). Zonal monsoon is stable monsoon period and Meridional monsoon is less stable period. Therefore clustering of large floods occur during 1971-2005 and also occurred during 1871-1890 period (Parthasarathy et al,1987).
17. There is no variation in the size of flood-prone areas except five peaks in 1954-56, ’72-’74, ’87-’89, 2004 and 2007 as already mentioned.
Types of Flood:
Three types of flood occur in the Brahmaputra valley – flash flood, spill flood (long duration) and flood due to drainage congestion. Flash flood occur naturally due to rushing down of flood water through rivers from nearby high mountain of the great Himalayas in the north and through short plain courses of rivers flowing from high Shillong Plateau in the south bank plain west of Guwahati and artificially in all rivers due to sudden clearance of reservoirs and failure of embankments. Flash flood also occurs due to shifting of channel of rivers. This occurs for relatively short periods. Long period spill flood occurs in the Brahmaputra and its eastern south bank tributaries occur due to low slope and long plain courses of rivers flowing out of relatively low hills and mountains. Drainage congestion develops floods lasting for long periods. Drainage congestion occurs due sluggish flow or presence of no out let to drain out accumulated water.
Geomorphic Implications of flood:
Flood has some geomorphic implications. During flood, river gains enormous power to change the geomorphic processes within its domain (Baker and Costa,1987). The steam power (w=YQS/w=TV) to produce shear stress on banks and beds during flood are more than the extent of normal 42 at Pandu and 0.51 in the course between Pandu and Jogighopa. Unit stream power in Wm-2(w) of south bank tributary Burhidihing is 6. Stream power is low due to very low slope and high value of width-depth ratio. But in spite of low stream power, the flood can change the course of river, occur high bank erosion, change morphology, landforms and configuration of rivers, increase sinuosity, length, size and shape of river. Channel shifting is common in the Brahmaputra itself and tributaries during flood occurrence. Due presence of non-cohesive materials on the banks and plains and high mobility of bed and suspended materials in response to high discharge of water, the fluvial geomorphology of the rivers and the valley is frequently changing through channel shifting, formation of meanders and meander cut-offs, extension spill channels, development of new spill channels, bank slumping, stream capture (Spring,1935; Inglis, 1949; Coleman, 1969; Pandey,1980; Verma et al,1989; Goswami, 1985; GFCC,1986; Godbole, 1986; Sarma and Basumallick, 1986; Dubey, 1990; Gupta, 1995; Sing, 1996; Gilfellon, 1996; Kale, 1998 ). Valley is also highly silted and thereby landforms changed by the affect of flood.
Flash flood occurs not only due to high slope of the mountains but also due to reckless dam clearance and dam failure and breach of embankment which occurs great devastation as in kapili river, Manas and Ranganadi. Manas was blocked by bed load at the mouth of the river debouching to the plains, high devastation occurred due to flushing out water towards vast surrounding areas. Again flash flood from Khasia-Jayantia hills of Shillong plateau through the Bolbola river (Jinari river) occurred great devastation during 2004 by floods and siltations. The Brahmaputra has shifting channel combinedly with the Dibang and Lohit and develop a new course through the southern boundary of the Dibru-Saikhowa National Park capturing the lower course of the Dholla-Dangari-Dibru river. It is an example of river avulsion or anabranching developing distributaries and building of river islands. Most of the courses of the tributaries of the Brahmaputra specially the northern ones are highly sensitive to change their courses.
Causes of Flood:
The causes of flood are discussed here. Flood occurrence is dependent on certain factors of hydrometeorological, fluviogeomorphologic, pedologic, tectonic, anthropogenic, etc.. Meteorologic, geomorphic and anthropogenic factors are the broad categories that needed to be examined in this region.
Hydro-meteorologic Factor:
Occurrence of rainfall, its frequency, intensity and distribution are essential factors for occurrence of flood. Rainfall occurs through monsoon rain, rain-storm and cyclonic effects, break monsoon, orographic lifting of air, etc.. Rainfall occurs due to (1) tropical storms and depressions, (2) active monsoon wind and (3) break monsoon. The tropical disturbances of storms and cyclones generally originate in Bay of Bengal and Punjab-Rajasthan landlows. Flood-generating rainstorms occur rainfall mainly on two major zones (Nandargi, 1996) - Ganga and Punjab plain and Central and northern half of the Peninsular India. But the maximum rainfall occurs in N.E. India - highest daily upto 1040 mm (Cherrapunji, June 1876) and high annual rainfall (50-1100 cm) due to convergence and integration of both branch of monsoon wind from Bay of Bengal and Arabian sea and also due to orographic effects on to the Arunachal Himalayas, Arakan Yoma, Shillong Plateau and others.
Again the affect of long term changes in the monsoon rain on flood is today increasingly recognized. There have been found four major rainfall period as 1880-1895, 1895-1930, 1930-1963 and 1963-1990 (Kripalani and Kulkarni, 1996) where 1880-1895 and 1930-1963 received above normal rainfall having very few drought and 1895-1930 and 1963-90 are periods having below normal rainfall with frequent droughts in India as a whole. This sort of periodicity also identified by Gregory (1989) by studying rainfall distribution at micro-regional level during 1871-1985 period. He identified the N.E. India as under above average rainfall condition during early and mid part of the studied period, whereas other region indicate wet period not similar to N .E. India’s period. Therefore the periodicity is also to be noticed while considering the rainfall factor.
EI Nino affect on these periodicity is established from many respects. It has a teleconnection with ENSO (EI Nino Southern Oscillation) (Bhalme and Jadhav, 1984; Parthasarathyet. al. 1991;Whetton and Rutherfurd, 1994) and anti ENSO (La Nina) (Ropelewski and Helpert, 1987). The relation of Southern Oscillation Index (SOl) with rainfall and flood was established (Bhalme and Jadhav, 1984). Though floods are correlated with Cold ENSO and drought with warm ENSO (Bum and Amell, 1993) and also the flood with La Nina and drought with ENSO (Kripalani and Kulkarni, 1996) yet remarkable relation between flood and EI Nino (ENSO) was already established (Kane, 1989; Kale et. al, 1996). Thus monsoon variability is found influensive in the mode of intensity and distribution of rainfall for annual, decadal or even for longer period.
Geomorphological factors:
Geomorphological factor is perhaps the greatest dominant one after the rainfall. The Brahmaputra basins is run row basins surrounded by high mountains as already mentioned facilitating the convergent rushing of water to the valley that definitely have high possibility of occurrence of flood. The geomorphic factor includes physiography, pedology, hydrology, fluvial processes, etc. which have considerable amount of affect on the flooding processes.
Physiography of the region is discussed in the foregoing point of physical set-up. In addition it is to be noted that if N.E. India is considered to be single region, its basin plain is consisted of only 24% of the total surface area through which more than 100% surface run-off i.e. the rivers have to carry away (100% of its (N.E. India) own surface run-off plus run-off of Tibetan catchment). The Brahmaputra basin is consisted of 5,80,000 Km2 catchment area out of which 1,94,413 Km2 falls in India. The valley north of the Brahmaputra is wider in the west and narrow towards east and the southern one is wider is the east and very narrow in the west having slightly wide corridor in Nagaon district in the middle. The watershed line for most of the tributaries to Brahmaputra except Trans-Himalayan rivers like Subansiri, Manas, etc. in the north is the crest line on the Middle Himalayan Range. This crest line is higher than the watershed or crest line in the south constituted by the crests of Arakan Yoma, Patkai Range and Shillong Plateau.
River and River channel character : Assam is a land of rivers. The valley length of the Brahmaputra is 720 Km with a channel length of 640 KIn. carrying waters from 103 tributaries (65 in north and 38 in south bank).
The important characters that have affect on flood are - the Brahmaputra along with the lower courses of some of its major tributaries are highly in the process of braiding, anabranching, migrating and enlarging through shifting of channel courses or bank erosion. Therefore large number of island, sand bars, point bars, chars, palaeochannels or ox-bow lakes or meander cut-offs, meander scrolls, sloughs, natural levees, back-swamps, sand splays, dry channels do exist in the midst, sides or over bank of the river channels of the Brahmaputra and its tributaries. The channels though are large in number, yet incapacitous to carry the rushing water from high hill slopes in time. Therefore within the same channel catchment, the upper course when found flooded with water, the lower course is in normal water stage or vice versa. This geomorphic character is conspicuous in the Brahmaputra Valley. Alluvial cones and fans are predominant in all the tributaries in their debouching points to the plain valley especially in case of north bank tributaries. Therefore large sand beds devoid of vegetations are developed in the foot hill plain which again facilitate the spreading of rushing flood water from the hills. Anastomosing of river channels are also developed in this belt. The flooding rivers have numerous spill channels attached to banks which facilitate flowing of flood water out the river channels to the plains.
The bed of the channels are not smooth, rather they are wavy having alternate pools and riffles. Again due to siltation, beds of some parts of the Brahmaputra and its tributaries are found quite above the general level of the plain. This character is sometimes developed due to the existence of embankments and facilitates the easy flow of water to nearby plain which are even lower than the channels or for breach of embankments, ringbunds or for abandonment or migration of channel towards the valley plain. The areas having full of swamps, palaeochannels facilitate the migration and shifting of channels especially in north bank (in the Dhemaji district by Gai Nadi and in parts of Kamrup valley). Sandy loamy soil of the most parts of the northern plain again offers favourable condition for channel migration and shifting. Back swamps and water logging areas develop drainage congestion problem and ultimately occur flood due to poor drainage. Channel migration, meander growth and avulsion can affect the channel character and water discharge. When meander grows, channel length increases, gradient decreases and the water storage is raised up for floods (Jorgensen et. aI, 1994). Meander avulsion in tributary courses though not frequent, is occurring during the flood.
Tectonic Affect: Though flood is a historical event, yet it is found deeply associated with the tectonicity of the region. Earthquakes occurred during flood times at maximum facilitate for breaching of embankments, natural levees, banks, etc. or landslide obstruction and sudden clearance, etc. The great tectonic events of 1897 and 1950 have great effect on the geomorphology of the region and on the intensity and frequency of flood events. Parts of the river courses were made uplifted or submerged downwards soaking with landslide sediments brought by’ rivers from the hills for which the rivers have to shift their channel or enlarge the size out of raising and rising up of their beds and water levels.
A survey shows that a length of 355 km in the south bank and 230 km in the north of the Brahmaputra were suffering from erosion during period between 1923-54 (RBA, 1980). An NEC study exhibits that the size of the Brahmaputra channel of 1967 was enlarged than 1930’s size by 2.49 km on average. The erosion on the north bank was to the extent of 1.57 km and the south, of l.44 km on average measured on maps through 64 cross sections throughout the length of 640 km. The shifting course of the Brahmaputra and its tributaries are spectacular which occurs annually or within even still short span of time. The tectonic activities of 1897 and 1950’s earthquakes raised up the bed of the Brahmaputra and its tributaries for several metres (Kale, 1998, P 241) which affect the flow and occur floods. The bed depositions have been doing the river braiding and the braiding also increases due to tectonic by for 1.5 times at minimum between 1913-76. The slope of the rivers especially the Brahmaputra is very less 13cm/km (17cm/km above Dibrugarh, 10 cm/km near Guwahati) on average. The historical factors exhibit a general southerly shifting of the course of the Brahmaputra, but a measurement by Survey of India done for NEC indicate increase of northward erosion than erosion on south bank and hence has a tendency slightly northerly shifting as mentioned above.
Hydrologic Factors: Hydrological factor though dependent on geomorphic and climatic characteristics, yet have to take into considerations as important factor for flood analysis and management. The excess flow of water develops flood which has input from rainfall, evaporation, interception, transpiration, infiltration, surface flow, etc. The surface flow combined with ground flow makes the river discharge and flood flow. The Brahmaputra basin spotted with natural elements like forests, hills and plains, different type of soil, rock beds, rocks have different amount of evaporation, evapo-transpiration, interception, infiltration, etc.. The daily water discharge of the Brahmaputra (Assam part) varies from place to place along its channel from its normal rate of change. It is dependent on fluvial geomorphology and regional climatic factor. Mean outflow at Jogighopa and inflow at Pashighat is of four times difference.
The share of contributions of the main tributaries to the Brahmaputra varies from tributary to tributary. The flow character of the north bank tributaries differs with that the south bank tributaries. It is noticeable from the daily storm flood hydro graphs. The leg time, peak time, flood-to-peak time are different. These all times are short in respect of north bank tributaries and slightly longer in case of south bank tributaries because of moderate speed and slow flow of water due to emerging out of relatively low hills in the south than the high in the north Great Himalayan Range.
The flood frequency study also reveals a high frequency and probability (Weibull Plotting Position) of flood in the north bank tributaries than the southern ones. The flood frequency of the Brahmaputra at different point vary - high in Nagaon, Majuli, Dhemaji and Golaghat districts and low in other parts. The former areas are proned to high frequency of flood due to presence of distributaries, anabranching off shoot channels and also for bottle necks, low banks, etc. Whereas the later ones are due to overbank and spill channel flow of water. Floods of Assam generally occur at maximum due to tributaries. The flood is generally occurred in association with bank erosion of the Brahmaputra and its tributaries at different point at different intensity and times, e.g. bank erosion, of course, sometimes occurs without flood.
Anthropogenic factors : Dam failure, wrong or no moderation of flood through dam or sluice gate, breach of embankments, clearance of forest in watershed areas, etc. are anthropogenic events which affects definitely the flood frequency and magnitudes. Siltation on bed of the rivers are generally made responsible for due to the presence of embankments, but it is found not 100 percent correct. For many times rivers channel courses are found abandoned due to siltation on beds without the presence of embankments. This is also true in respect of the Brahmaputra river. Its bed is raised and size is enlarged to a great extent, can not be made through the influence of two embankments that exists more than 10 to 12 km far away from each other. This embankment is also not continuous along the Brahmaputra and also made after 1954. Again the clearance of forest is made responsible for increase of flood frequency and magnitude wrongly. Study on flood of IGB plain is found that flood here is not increased due to change of land use and deforestation in the Himalayas (Chaphekar and Mhatre, 1985, P61) as conservationists claim these facts grossly exaggerated way (Messerli and Hofer, 1995). Only 7% of land in the hills is under jhum cultivation and 5 to 15% of land under human settlement. There is no convincing evidence of man-induced increase of flood frequency and magnitude in Ganga-Brahmaputra basin (Brammer, 1990, Messerli and Hofer, 1995). There are enough evidence of climatic change of regional and global scale for increase of flood frequency and magnitude during the last few decades (1940-2006).
Flood Zoning and Control : Depending upon geomorphic and drainage character, climatic influence, the Brahmaputra and Barak Valley may be divided into a number of homogenous Flood Zones - homogeneity being defined on mode of occurrence, flood characteristics, risk and vulnerability requirement of similar treatment for management, etc. The Flood Zones taken for discussion will be of meso type. The whole valley be under single macro type region and the third type - micro regions shall be identified on locality basis. These are included within meso regions. The homogenous Flood Zones (meso regions)are
1. Brahmaputra Active Flood plain
2. Flash flood zone of Northern plain and Southern narrow plain (West of Guwahati) of the Brahmaputra
3. Southern Flood Zone of the Brahmaputra east of Guwahati
4. Flood free zone
1. The Brahmaputra Active Flood plain is constituted by the very neighbour and nearest overbank part of the Brahmaputra river channel along with its islands, chars and bars within the channel length and breadth and the river channel or channels (in respect of braided parts) Here the area within the channel is the biggest and the land over bank part is not much wide. When the whole of Assam region receives high rainfall, this zone may have the occurrence of flood at the same time. But mostly the flood occurs in this zone at different parts at different times and intensities vary according to the variability of rainfall distribution over the whole region. Certain sections may have Flood occurrence according to excessive rainfall occurred over the upper course of the tributary basins of that section. Again it may also occur that other basin may have the occurrence of flood but the Brahmaputra may remain free from flood at that time. Thus fluvial geomorphology of this zone is an important factor to be studied very deeply. Geomorphological hydrology, tectonicity, hydrometeorology, etc. also are important factors to be taken in to consideration (Fig. 9). The Barak active flood zone is also behaving similar way to the Brahmaputra.
2. The Flash Flood Zone consists of the whole of North Bank Plain of the Brahmaputra active flood plain which also includes the Sadiya Region. Similar characteristic flood zone is also found is the narrow southern plain west of Guwahati in Assam. Though not contiguous, this area may also be included with in this zone due to similar flood behaviour through time and space. Thus this homogeneous Flood region may be divided into four sub regions- eastern and western sub-zones on the Northern flood zone divided by highlands of Tezpur, the South Bank narrow plain, west of Guwahati and the Sadiya Region. These regions are bordered by nearby highlands; have relatively short more or less straight tributaries where valley has sufficient slope and meandering rivers where valley is extra ordinarily flat; rivers having anastomozing and anabranching character at the foothill and flat mid-valley full of swamps; indefinite channels present where swamps exist; speed of water either in channels or during floods is very high; rushing of water through downstream or in sheets is frequent and flash flood is common; have also sluggish flow where Swamps do exits; sediment of the river is high; have large number of dry channels in winter and inundated channels in summer, flood intensity and magnitude are high and lag time and time of concentration are short in this region. Here requires the control of floods in the upper parts of the valley i.e. rivers through construction of dams, flood moderation project, and channel diversion in the foot hill zones. Embankment is less effective due to high bank erosion and bed siltation. Inter basin transfer within sub-basins may be done to decrease flood intensity and divert the flood concentration, but it may be a political issue, so should have political efforts too. Small dams in upstream courses of rivers may be effective and relieve the region from flood menace.
3. The Southern Flood Zone is consisted of the southern part of the valley of the Brahmaputra. This region is extended east of Guwahati upto Saikhowa in the eastern part of Assam. Though homogeneous in geomorphological and flood character; yet may be divided into some sub-zones depending on some slight variations. It may be divided into eastern and western zones having Karbi-Anglong as the divider. Common characteristics of these zones are long meandering and tortuous tributaries, having speed of water in channel or during floods slow, low amount of load, bordering with low hills at far distance, valleys are wide, some where rivers are sluggish in character at some seasons, full of palaeochannels, etc. The western sub-zone is highly spotted by large number of palaeochannel and anabranching channels. It is more or less like the shape of a bowl drained by the Kapili river surrounded by highland on its three sides and has more or less centripetal drainage system. The river in off season is found sometimes sluggishly flowing. The eastern sub-zone is normal except the easternmost part where the Brahmaputra exhibits ana branching character developing a major anabranch, the Ananta Nalla. Here river channel character varies according to varied soil character. Palaeo channels are common in the eastern part of this eastern sub-zone. The flood here occurs is of spill type and takes. enough time to move. Embankment are effective in the zone if scientifically constructed and managed. Bank protection and channel diversion project may also effectively solve some problems in this region.
4. The other parts of the Brahmaputra Valley i.e. hilly areas are flood free zones where occasional disasters may occur out of rushing of water downslope of hills, for which landslide may occur. Watershed management treatment is poor or completely not done in these areas which can lessen the volume of run-off in plain areas.
Flood Control Measures:
Common flood control measures are embankments, ringbunds, diversion of channel, drainage channelling, spurs, dyke, river dams, etc. which should be used judiciously. Measures may vary nom flood zone to flood zone suiting with the situation and nature of rivers and floods (Inglis, 1949, Hay, 1988,94; Singha, 1982; Kale, 1997; Rao, 1975; Chitale, 1978, etc.). The measures adopted in Assam are not sufficient and rather less scientifically planned. The risk factor is high in Northern Flood Zone and Barak valley zone. Strong and quick floods occur within these zones. So, moderation or diversion of flood force is necessary. Check dams, inter-basin transfer by inter-connecting canals at the foot of the hills (Borthakur, 1965) with sluice gates between basins, embankments, dredging and clearance of channel beds of the tributaries which is more necessary than the bed of the Brahmaputra, stabilization of banks, roads and rails having good number of culverts, drainage channelling and rescuer rapid action force for disaster operation are required in these high risk zones to minimize the risk factor.
The southern flood zone has moderate risk though the Kapili basin is under moderately high risk factor. Spilling of water through anabranches and palaeochannels from the Brahmaputra are frequent and dangerous. Embankments, culverts, ring-bunds, spurs, channel diversion, etc. will be important for this zone.
The active flood plain zone of the Brahmaputra has the overbank spilling and spilling through spill-ways. Embankments, ring-bunds, construction of spurs in human settlement areas, keeping completely or partially free the other areas will solve the flood problem in this zone. Stabilization of banks through construction of spurs and diversion of channels or current of water within the channel of the Brahmaputra will check further expansion of channel towards land.
As already mentioned, drainage channelling is required for the Barak and the Kapili valley zone. River dams at upper course of some major rivers are to be constructed for flood moderation. Dams proposed to be constructed on Dihang, Dibang, Lohit, Subansiri, Manas, Barak will definitely keep the water level of the Brahmaputra and the Barak low. The small tributary basin floods must be managed with the methods mentioned above for which physical as well as anthropogenic factors are to be understood to manage flood (Hey, 1994; Masserli and Hofer, 1995; Ward, 1978; CSE, 1991; CWPC, 1958, etc.)
References :
1. Baker, V. R. (1994) - ‘Geomorphological understanding of floods’ - Geomorphology Vol. 10 PP 139-156.
2. Bhalme, H Nand Jadhav, S. K. (1984) - ‘Southern Oscillation and its relation to the monsoon rainfall J. Climatology Vol. 4 PP 509-520.
3. Borthakur, M., (1965) -Irrigation, its necessity and advantages in Lakhimpur-Jour. of Assam Science Soc. vol. VIII, Gauhati
4. Brammer, H. (1990) - ‘Floods in Bangladesh-Flood mitigation and environmental aspects’ - The Geographical Journal vol. 156 PP 158-165
5. Bum, D. H. and Amell, N.W. (1993) - ‘Synchronicity in global flood responses’ -Jour. Hydrology Vol. 144 PP 381-404.
6. Chaphekar, S.B. and Mhatre, G.N. (1985) - Human Impact on Ganga River Ecosystem - Concept Publishing Company, New Delhi.
7. Chitale, S, V: (1978) - ‘Some thoughts on flood control problems in India’ - Irrigation and Power Jour. Vol. 36 PP 493-496.
9. CSE (1991)- ‘Floods, flood plains and environmental myths’ State of India s Environment -3. Centre for Science and Environment, New Delhi.
10. CWPC (1958) - ‘Floods; Are they on the increase?’ Central Water and Power Commission Publication, New Delhi.
11. Dutta, Lakshyahira (2003) – Locational pattern and Morphometric characteristics of Alluvial Fsans in the Himalayan Foothills of Assam-North Eastern Geographer-vol.33 No 1&2 Gauhati University pp46-54.
12. Ely, L.L., Engel, Y., Baker, V.R., Kale, V:S. and Mishra, S. (1996) - Changes in the Magnitude and Frequency of Late Holocene Monsoon Floods on the Normada river, Central lndia-Geol. Soc. Am. Bull. v. 108 pp 1134-1148
13. Fu, C. and Fletcher, J.1988 - Large signals of climatic variations over the ocean in the Asian monsoon region - Adv. Atmosphere Science, Vol. 5. PP389-404.
14. Gregory. S. 1989 - ‘Macro-regional definition and characteristics of Indian summer monsoon rainfall, 1871-1985’ - Int. Jour, Climatology, Vol. 9 PP 465-483.
15. Hey, R,D, 1988 - ‘Mathematical models of channel morphology’ - Modelling Geomorphological Systems -ed. M.G. Anderson, John Wiley and Sons Ltd. New York PP 99-125.
16. Hey, R,D, 1994 - ‘Environmentally Sensitive river engineering’ Modelling Geomorphological Systems - ed M.G. Anderson, John Wiley and Sons Ltd. New York.
17. Imperial Gazetteer of India 1908 - ‘The Indian Empire’ Oxford clarendon Press.
18. Inglis, C. C. 1944 - ‘The behaviour and control of rivers and canals’- Central water, Power, Irrigation and Navigation Research Station, Pune, Research Publication No. 13
19. Kale, V: S. 1998, - ‘Monsoon flood in India: A hydro geomorphic perspective’ - Memoir Geol. Society of India. No. 41 PP 229-256.
20. Kale, V:S.; Ely, L.L.; Engel, Y and Baker, V:R 1996 - Palaeo and historical flood hydrology, Indian Peninsulain Global Continental Changes: The Context of Palaeohydrology (ed) J. Branson, A.G. Brown, K.J. Gregory - Geol. Soc. Spl. Publication, London - Vol. 115 PP 155-163
21. Kale, V:S. 1997 - Fluvial Studies in India: A Brief Review J. of Geol. Soc. India - Vol. 49 PP 359-370.
22. Kane, R. P. 1989 - “Relationship between the southern Oscillation EINino and rainfall in some tropical and mid-latitude regions” - Proc. Indian Academy of Science - Vol. 98
23. Kripanali, R. H. and Kulkarini, A 1996 -Assessing the impacts of EI Nino and non EI Nino related droughts over India - Droughts Network News Vol. 8 PPII-13.
24. Messerli, B. and Hofer, T. 1995 - Assessing the impact of anthropogenic land use change in Himalayas - in Water and Quest for Sustainable Development in the Ganges Valley (ed) G. P. Chapman and M. Thompson, Mansell, New York PP64-89.
25. Nandargi, S. S. 1996 - Rainstorm studies for planning and development of water resources of the Indian region, unpublished Ph. Thesis submitted to univ. of Pune, quoted from Kale, 1998 ‘Monsoon flood..’
26. Pathasarathi, B., Sontakke, N. A., Munot, A.A. and Kothawale, D.R. 1997 -”DroughtsIFloods in the summer monsoon season over different - meteorological subdivisions of India for the period 1871-1984’’ – Jour. Climate -vol. 7 PP 57-70.
27 Parthasarathi, B., Rupa Kumar, K. and Munot, A. A. 1991 - “Evidence of Secular variations in Indian monsoon rainfall-circulation relationship” -Jour. Climate,. Vol. 4. PP. -927-938.
28 R.B.A.1980 - Report of the Rastitriya Bash Ayog-Govt. of India, New Delhi, Vol. 1&11.
29 Rao, K, L. 1975 - India s water wealth; Its assessment, uses and projects - Orient Longman, New Delhi PP. 150-165.
30 Rupelewski, C.F. and Halpert, M.S. 1997 - “Global and regional scale precipitation patterns associated with the E1 Nino/Southern Oscillation”, Monthly Weather Review - Vol. 115, PP 1605-1626.
31 Sharma, S. K. 1990 - “Seismicity and Seismic Risk in Northeast India” -Assam in 2001 A.D. (ed) P. Mahanta, D.U. Dibrugarh.
32. Sharma, Nayan (2010) – A Satellite Data Based Approach to Study Braiding Behaviour for Monitoring and Management of the Brahmaputra River-Souvenir-Landuse, Biodiversity and climate change-IGU Seminar,Cotton College, Guwahati, Assam, India-pp20-35.
33 Singha, C. P. 1982 - “Flood moderation in the Brahmaputra River by dredging” - Proc. CBIP Short-Ferm course on Flood control, Roorkee, Technical Report No. 31, PP 20-26.
34 Ward, R, 1978 - Floods - A Geographical Perspective -The Mac Millan Press Ltd. London.
35 Whelton, R. and Rutherfurd, I. 1994 - “Historical ENSO teleconnections in the eastern hemisphere”, Climate Change, Vol. 28, PP. 221-253.
36 Wright, P. B. 1989 - “Homogenized long period Southern Oscillation indices” - Int. Jour. Climatol. Vol. 9
PP. 33-54.
No comments:
Post a Comment