By Dr Phani Deka
W M. Davis, the renowned geomorphologist theorized that Landform is a function of Structure, Process and stage. The structure refers to the composition of the rock types and soil, its configuration, its shape and size, its slope or inclition and such other things. Once a structure is formed, it is subjected to various forces of erosion and deposition normally refers to as the process for a very long time and ultimately it reaches a stage at which we normally see the present formation.
Majuli Island has been declared as the largest river island in the world, toppling Marajo in Brazil, by Guinness World Records. The island is purely a fluvial landform and a result of the dymics of the vast Brahmaputra river system. It rises from the Brahmaputra basin and in course of time turned into a flat-level alluvial plain. An extreme flood, which occurred around 1750 was responsible for creation of the island. According to the first geographical report, Majuli was a cluster of 15 large and numerous small islands in 1792. At present, 18 have been included as stable or permanent. The geomorphology of this region is directly related to its physiographic characteristics. The island is bounded by the river Subanisri and her tributaries - Rangadi, Dikrong, Dubla,Chici and Tuni on the north- west, the kherkatia suti in the north-east and the main Brahmaputra river on the south and the south-west. Although there are some confusion about the dimension and area of the island, broadly it may be considered that the island extends for a length of about 80 km in the ne-sw direction and for about 10-15 km north to south direction in the middle. The total area is about 875 sq km. and it is 85 - 90 m above the mean sea level. Flood and erosion are the two major factors responsible for shaping the island into its present day topography. Quite a good number of scholarly and research studies covering various aspects of the island are available. The researchers and authors have done excellent jobs even though most of them lacks in suggesting a general model to mitigate the two basic problems of flood and erosion. Therefore, an attempt has been made to suggest a general geographical model for controlling erosion of this unique river island.
Process of erosion:
It is observed that during the last hundred years of 20th century, the island is shirking in size. The island lost around one-third of its area in the last 30-40 years due to frequent flooding and erosion. The average annual rate of erosion was 1.77 km 2 from 1917 to 1972, 1.84 km 2 from 1972 to 1996 and 6.42 km 2 from 1996 to 2001, indicating a gradual increase in the rate of erosion in later periods.
Erosion normally takes place as a result of the interplay between two forces – frictiol force and displacement of the centre of gravity. The tremendous frictiol force is created by the flow of large volume of water of the Brahmaputra especially in the summer monsoon season. Sub- aqueous flow of sediments and over steepening of soft alluvial bank with low threshold of erosion resistance are observed as the domint erosion characteristics of the island. The increase in sediment load in excess of competency of the river causes significant channel widening with resultant increase in differential rate of bank erosion on both the sides of the island. Available information indicates that active geomorphic transformations of the area, initiated by head erosion and channel migration triggered the diversion of origil flow of the Brahmaputra River to its present form.
A look at the map of the island reveals that the Brahmaputra flows from the north-easterly to the south-westerly direction and the resultant layout of the island is roughly east-northeast to west-southwest direction. Another characteristic of the island is its shape. The general shape of the island resembles to a river boat tapering to the east and west directions with widest extension in the middle. The principle of physics suggests that any object with a pointed front will have maximum frictiol efficiency (i.e. minimum friction) when it is placed against any moving force (be it wind, water, glacier etc) or when it is made to move forward through anything offering some kind of resistance (like- bullet train, boat etc). From the alignment and shape of the Majuli island, it can be said that it is more or less efficiently placed against the high speed flow of water of the mighty river Brahmaputra. From this point of view, the erosion activities on the island should have been much less. However, the efficiency of orientation and shape has been minimized by some other factors like- composition (soil), age of the soil structure, speed of flow of water, volume and ture of load, carrying capacity of the river, the gradient and bottom configuration of the river, number of tributaries surrounding the island and the slope of the river bank with the bottom of the river. Age-wise, the present day island is of very recent origin. It is formed entirely through fluvial process, the soil is very soft, mostly composed of very newer alluvium - silt at the top and sand below and as such the cohesive (binding) power is extremely low. All these have resulted in high erosiol activities on the banks. Depending on the speed, volume of water and gradient of the river, the load stratifies itself- heaviest one along the bottom (normally rolling), medium sized one in the middle and the finest one at the top. It is found that the medium sized load creates the maximum impact. The erosion is maximum in the bordering areas of silt and sand layers which normally occur at a few feet below the surface of the bank. The speed of erosion is enhanced by the steep slope created by the collapse of the soft top surface layer caused due to scouring of lower portion resulting in displacement of the centre of gravity of the top portion.
Recent actions initiated:
In November 1999, the Brahmaputra Board prepared a scheme for ‘Protection of Majuli
island from floods and erosion’ at a estimated cost of 86.56 crore. The scheme consisted of (1) immediate measures (2) The ‘Emergent Works’, basically involved construction and laying of RCC porcupine screens / dampeners. The ‘Administrative Approval and Expenditure Sanction’ was accorded to 41.28 crores and the estimate was subsequently revised to ' 56.07 crore. The works under Phase-I were taken up by the Board in March 2005 and completed in April 2011 at the cost of ' 53.40 crore. Important works executed under ‘Phase-I’ included Plugging / Closure of breaches ,(b) Raising and strengthening of embankment (c) Casting and laying RCC porcupine screens / spurs / dampeners on northern and southern sides of Majuli Island and (d) Construction and Repair of nose portion of land spurs at Sonowal Kachari and Kandulimari. In August 2011 the Project Appraisal Organization, Central Water Commission communicated acceptance of the Advisory Committee to the proposal for the estimated cost of ' 115.99 crore at the Price Level of the year 2010 for completion by March, 2014.
The present status of the works envisaged are –( 1) Work is in progress of construction of 5 Boulder Spurs with geobags below Lowest Water Level (LWL) at a cost of 39.35 crores (2) Work is in progress of casting & Laying of RCC Porcupines (1,27,396 porcupines) at a cost of 72.14 crores (3) NIT was floated for procurement of geobags on 14th February 2012 for bank revetment with boulders and geo-bags at 4 (four) locations for a total length of 5190 m. (4) Proposed to be taken up in XII Plan the bank Stabilization at PGJH dyke by use of sand filled geo-mattress in a length of 3095m and (5) Some other works of Bank revetment at Jengrai, Raised Platform at 5 locations, Pilot Channel- 2 KM and construction of Check bunds at Sonowal- Kachari and Kandulimari are in various stages of implementation.
The Standing Committee of Experts on Majuli on its 6th visit of Majuli Island during the period from 6th to 8th November 2010, recommended for construction of guide bunds to channelize river flow with the view to protect Majuli Island from recurring floods and bank erosion. The standing committee of experts on Majuli various altertives for alignment of guide bunds are to be examined / studied through ‘Physical Model Study’ of Majuli Island set up at North Eastern Hydraulic & Allied Research Institute (NEHARI), North Guwahati. Fil selection is to be made on confirmation through ‘Mathematical Model Study’ assigned to Central Water and Power Research Station (CWPRS), Pune.
Physical model of Majuli Island set up at NEHARI Based upon the ‘Physical Model study’ carried out at NEHARI, Brahmaputra Board has come out with three altertives. Prelimiry design, assessment of quantum of work involved and estimation of fincial implication are in advanced stage of filization.
A geographic model
The list of recent works initiated and the future plan of action have been found to be alright so long the short term measures are concerned. However, a conceptual and comprehensive model for control of erosion considering the three basic dimensions- structure, process and stage of Majuli is yet to be taken up. It may be observed that the geographical aspects which are of great significance appear to have been grossly neglected in the attempts that have been taken up so far. Here, a geographic model is conceptualized following the principle of physics which states that the pressure is defined as the force applied divided the area of application. It can be seen that for a given force, if the surface area is smaller, the pressure will be greater. If a larger area is used, it amounts to spreading out the force, and the pressure (or force per unit area) becomes smaller. Pressure equals weight divided by Area and the weight of an object is a force caused by gravity. Therefore, it is seen that force is equal to weight of an object. In physics, mass is a property of a physical body. It is the measure of an object's resistance to acceleration when a force is applied. An object with mass creates a gravitatiol field around itself and every object with mass exerts a gravitatiol force. There is an intricate relationship among area, mass, weight, friction, gravitatiol force and centre of gravity (CG) or centre of mass or centroid. The geographers can easily measure these attributes for a given area. The significance of CG is well explained and demonstrated in the programme “Science of stupid” of Discovery Channel. There are many attempts all over the world to calculate CG at country and other selected levels considering different attributes. Recently, a group of researchers in the Ordince Survey of Great Britain have calculated the CG of Britain using 3D satellite maps.
The experts in the field of quantitative geography and fluvial geomorphology can provide valuable inputs in finding the CG of Majuli which is to be considered as the basis for any erosion control measures. After determining the CG, the line of gravity has to be drawn. The line of gravity is an imagiry vertical line passing through the center of gravity down to a point in the base of support. The position of the centre of gravity of an object affects its stability. The lower the centre of gravity (CG) is, the more stable will be the object. Two axes, one vertical representing the line of gravity exactly parallel to the flow of water considering the direction, speed, amount of load and the other at 900 to the former have to be drawn over the map of the island. Let the vertical axis be extended upstream. Two tangents touching the two extreme points of the land block Majuli to the right and the left have to be drawn to meet the vertical axis at a point. These two lines, the frontal tip of which will be against and parallel to the flow of the river will take the shape of a cone resembling the front portion of a boat or speed boat. Two dykes have to be constructed along the two tangents from the topmost point of the cone up to and a little beyond the points of contact of land areas. The two dykes may be constructed by dumping 3D toothed structures of considerable weight as those used in the Marine Drive in Mumbai, in Scotland and for reclaiming the land from the sea in the Netherlands. The force exerted by the flow of water on the two outward sides of the dykes will be at an angle less than 900 and it will have the tendency to be reflected outwards and downstream. The outcome of such action will generate lesser frictiol force on the dykes and ultimately result in lesser or no bank erosion. Therefore, construction of two cone shaped dykes, joining the apex facing upstream on the vertical axis passing through the CG and parallel to the flow of the river and the two extreme land points from the CG on the banks of the island may solve the problem of erosion in Majuli. It is to be remembered that this is a conceptual model based on the law of physics from geographical perspective and its practicability has to be assessed by the experts.
(Source: Adopted from the website- Maps of Majuli, A typical erosion pattern in Majuli (adopted))