Flood menace in North-East is a recurring event. Monsoon brings sorrow and grief for Arunachal Pradesh. Due to high annual rainfall and geological fragility of the region, every year, the floods hit the State and render many people homeless, devastate agriculture, destroy road communication, towns and other public assets.
The first flood disaster in Arunachal Pradesh was observed in township of Sadia, situated on North bank of Brahmaputra (virtually the headquarters of erstwhile NEFA) in 1950 due to change in course of Digaru river. Due to high seismicity, high annual rainfall and geological fragility of the region, the process of erosion and flood related disasters recur every year. Population concentration of Arunachal Pradesh is mostly on river valleys, the towns like Seppa, Naharlagun, Daporijo, Basar, Along, Yingkiong, Pasighat, Roing Tezu, Namsai, Miao etc. are on the banks of the rivers. The extent of damages caused by the flood is reported massive every year. Infrastructure development of the State has to go in tune with the existing characteristic behaviors of the river and therefore it is essential to develop a comprehensive master plan for structural and non-structural measures of flood management and its implementation is the foremost concern of the State.
(ii) Brief outline of basins and morphology of rivers:
Brahamaputra is a major international river covering drainage of 5,80,000 Sq.km, 50.50% of which is lying in China, 33.60% in India, 8.10% in Bangladesh and 7.80% in Bhutan. Its basin in India is shared mostly by Arunachal Pradesh ( 41.88%), Assam (36.33%), Nagaland ( 5.57%), Mehalaya ( 6.10%) Sikkim (3.75%) and West Bengal (6.47%). Within Arunachal Pradesh there are 10 major river basins consisting of 46 major and medium type rivers.
The 10 major basins are:
1) Tawang, 2) Kameng, 3) Dikrong, 4) Subansiri, 5) Siang, 6) Sisiri, 7) Dibang, 8) Lohit, 9) Tirap-Dehing, 10) Tissa river basins.
Numerous rivers originating from these basins ultimately drain to Brahmaputra River. This is a boon for the State for development of agriculture, power and industry sectors but at the same time these rivers have the destructive potentials unless certain preventive and protective measures are taken up in the State.
The Himalayan Rivers carry heavy sediment loads because of steep bed slope, soft and friable Himalayan rock. This is further aggravated by population growth with unscientific human activities on the valleys and high seismicity of the region. Consequent upon major seismic disturbance in 1950, large-scale landslide and heavy sediment transportation started. Rivers started braiding in the foothill area and this dynamic process is still actively continuing. Rivers of Arunachal Pradesh could be broadly classified into 3 (three) types namely:
(i) Hilly reach (incised rivers)
(ii) Foot hill submontance reach (boulder rivers) and
(iii) Flood plain (alluvial rivers).
Flood related problems mostly occur in foothill submontance reach and flood plains. Population concentration of the State is also on these reaches. In the theoretical perspective, there are no flood inundation problems in hilly and foothill submontance reaches. Nevertheless, problem persists in these reaches not because of flood inundation but because of erosion that is equally as destructive as floods. Massive bank erosion takes place in every monsoon destroying crops, livestock, roads and bridges, other public assets and flood problem of Assam could be attributed to soil erosion within Arunachal Pradesh. The heavy silt laden rivers coming down from steep slopes dissipate its energy at the flood plains (mostly foothill area of the State) and deposit silt on its beds due to which river water and excess silt spread overland causing braiding of rivers and submergence of agricultural land, towns and other public assets.
(iii) Flood Problems :
Due to high seismicity and geological fragility of Himalayan geology and high annual rainfall, the state is highly vulnerable to water related disasters. The water related disasters in Arunachal Pradesh could be broadly classified into following categories: -
Because of steep slope in mountainous areas combined with human interference in the catchments area, large-scale soil erosion and bank erosion occur in agricultural field and dwelling areas. The massive soil erosion and bank erosion in the river basins of Arunachal Pradesh is the primary reason for the flood problems in Assam. In fact, this has inter- state ramification and govt. of Assam with the govt. of India should also focus its attention on the catchment treatment mostly within Arunachal Pradesh.
Landslide is a common phenomenon in Arunachal Pradesh. Every year reports have been received from the districts regarding road blockages, mud slides in dwelling area, damages to irrigation structures and other public assets
(c)Flood inundation and siltation:
Foothill areas of Arunachal Pradesh are mostly the flood plains of major rivers originating from mountainous regions. The hilly rivers flowing down with high energy dissipates its energy in the foothill region. After dissipation of its energy the heavy silt carried by the rivers are deposited on its bed causing braiding and spreading overland. This is a common problem at Seijosa in East Kameng, Likhabali in West Siang, Pasighat in East Siang, Roing in Lower Dibang Valley, Tezu & Namsai in Lohit District, Diyun in Changlang District etc.
Considering the magnitude of flood devastation in the State, negligible achievement has been made so far through NLCPR, NEC and ACA funding. About 15 Km. wire created boulder guide wall, 28 Nos of spurs, 5 km plugging structures, 13 km embankment and 3 km pilot channel have been constructed. With these, an area of about 50 Sq.km has been protected covering certain flood prone area of Along, Jomlo and Kamki in West Siang District, Dirang in West Kameng District, landslide control to Tawang Monastery in Tawang District, Sagalee and NH52 (A) at Nirjuli in Papum Pare District, Pasighat in East Siang District etc. Such structural measures substantially moderate flood problems of specific area but do not give the total solution to the flood problem of the area.
(v) The Vision :
Holistic approach on flood management aspect is to be conceived. This would include structural and non-structural measures suiting to the local conditions of the area. Basin wise approach with due consideration of compartmentalized micro basin concept has to be taken into consideration.
1. Structural Measures :
(a) Immediate measures: Instances have occurred at Pasighat, Namsai, Tezu, Diyum and Seijosa that immediate plugging of breached embankments and application of brushwood for dampening the velocity of rivers to minimize further propagation of lateral bank scour were required. Such time-tested techniques are to be considered under the inventory of immediate measures. Arrangement of kits containing articles like engineering tools, gunny bags, life jackets, rubber boats etc. would be required for flood fighting during the flood.
(b) Short term measures: Having the flood vulnerable locality surveyed properly, comprehensive plan for engineering structure and agronomic measures with design discharge for 25-50 year return period depending on the area to be protected are to be developed. In the context of Arunachal Pradesh, several such engineering short term measures have been implemented and found successful in most of the cases like protection of Sagalee township, protection of NH52A at Nirjuli, protection of Pasighat township etc. In addition to the social benefits, these structures have given substantial protection to the area.
(c) Long term measures: Flooding is primarily due to continuous excess silt inflow over the years. The situation leads to rise of bed level, which in turn is the cause of overland spreading of floodwater. Control of excessive silt inflow is a time taking long-term measure. The multipurpose reservoir project with sufficient dead load provision is required wherever such project is conceivable. For other basins where multipurpose reservoir project is not feasible, catchments treatment as multi disciplinary approach watershed development is to be planned basin wise.
2. Non Structural Measures :
(a) Flood forecasting: Established Hydro meteorological stations in the North-East in general and particularly in Arunachal Pradesh is scanty. The established ones are also outdated and require modernization fit for real time forecasting. The participation of Indian Meteorological Department and the Central Water Commission is required for installation of as many stations as possible within Arunachal Pradesh. As Arunachal Pradesh covers about 42% of Brahmaputra’s basin within India, the hydro meteorological stations within Arunachal would help flood forecasting with fair degree of accuracy, which would be beneficial to Assam.
(b) Flood warning: Floods of Brahmaputra and Barak Basins have international and inter-State ramification. Proper networkings of co-basins are required for flood warning. The flash flood of Pasighat during 11th May’2000 is an example in which loss of lives and properties could have been minimized had there been proper networking and sharing of information to the downstream county.
(c) Flood Plain Zoning: Flood disaster happens to be a recurring event and contribute more that 80% of Nation’s agony. Emphasis shall be given to initiate survey on Flood Plain Zoning to delineate the prohibitive, restrictive and warning zones.