Materials and methods
3.4 Collection of bank materials
Considering severity of river bank erosion problem in the last few years, six locations were selected for present study of geochemical evaluation of bank materials. The locations are:
A) Rohmoria, B) Dibrugarh, C) Nematighat, D) Majuli, E) Gamerighat, and F) Palasbari (Figure 3.6).
Figure 3.6 Major erosion sites of Brahmaputra River main stem in Assam (from records of Water Resources Department, Assam) and sampling locations
A brief description of the erosion problem in these areas is mentioned below.
Rohmoria (A): The village complex of Rohmoria is situated at a distance of 23 km to the east of Dibrugarh town. Earlier, there were several small villages, forests, bamboo groves, swamps, and cattle farms, which provided all commodities necessary for an affluent rural livelihood. Now, Brahmaputra has not only eroded away all those rich natural resources but also has been eroding the very villages of Rohmoria (Table 3.6).
Table 3.6 Extent of erosion in Rohmoria Area of land eroded 8,435 Ha
Bank line length 9 km
Average depth of erosion 800 m No of villages affected 16 No of house hold effected 1580
Population affected 23,000
Total bank area being eroded away by Brahmaputra from 1916 to 2009 along the 16 km segment around Rohmoria was about 108.48 sq. km. If this amount is divided by the time span of (2009 – 1916=) 93 years then the rate of erosion in this reach will be 1.17 sq. km per year. The rate of bank erosion in the south bank of Brahmaputra is the highest at Rohmoria. The main cause of erosion of Rohmoria is the development of a very large primary channel of Brahmaputra, which hits the bank at about 450near Balijan and flows along the bank for 12 km up to Nagaghuli area. This channel is now being reinforced by the flow of the Lohit that joins the same at Balijan.
Figure 3.7 Erosion sites of Rohmoria
Dibrugarh (B): Situated at upstream of Brahmaputra, Dibrugarh town is severely affected and vulnerable area of bank erosion (Table 3.7, Figure 3.8). Old records indicate that erosion was caused by the shifting of channel of river Dibru in 1880-1881, which was then meeting Brahmaputra downstream of Dibrugarh town. After the devastating earthquake in 1950, the erosive trend took a worst turn threatening the Dibrugarh town. Erosion was
alarmingly high during 1954--1955, when the entire south bank from Nagakhelia, west of Dibrugarh, up to Saikowa had been subjected to severe erosion and Brahmaputra was encroached upon by 2 km to the south till 1963. The names of the villages eroded away during this period were Mohmara, Mohmara Chise, Mohmara South, Sagali Pathar, Sagali Block, Mohmara Nepaligaon, Kalangaon, Kalani Mirigaon, Upper Barala, Barala Block, Baralagaon and Bahanagaon. Heavy erosion started in the year 1970-1971 in between Mathola and Maijan upstream of Dibrugarh town. The erosion was observed shifting upstream to Nagaghuli-Maijan area in the year 1991--1993 and then to the tea estates at Oakland during 1995 – 1997. Important erosive activity was located downstream of Dibrugarh during the year 1920 to year 1940, shifted upstream to Dibrugarh during 1950 to 1960 and then further upstream from the year 1970.
Table 3.7 Extent of erosion in Dibrugarh town
Annual flood effected area 835 Ha
Bank line eroded 25 km
Land area eroded (1967-2008) 491 Ha
Rate of erosion 4.72 m/year
Number of household lost (1988—2008) 768
Number of household affected 4,036
(WRD, 2008)
Figure 3.8 Erosion sites of Dibrugarh
Nematighat (C): Nematighat is facing severe problem from bank erosion (Figure 3.9) in recent years.
Figure 3.9 Erosion sites of Nematighat
Majuli (D): Majuli is one of the largest riverine islands in the world and the largest in Asia with a population of 0.16 million people and the site of Vaishnavite spiritual centers (Lahiri and Sinha, 2014). Constant and extensive land erosion particularly at southern and downstream edge by mighty Brahmaputra is the single most problem that threatens the very existence of the island and life and properties of its inhabitants (Figure 3.10).
Figure 3.10 Erosion sites and non-erosion sites of Majuli Erosion site
Erosion site Non-erosion site
Erosion site
Areal extent of Majuli has been decreased from 787.9 km2 to 508.2 km2 during 1915–2005 with 35.5% reduction and average erosion of 3.1 km2/y (Lahiri and Sinha, 2014).
Gamerighat (E): Gamerighat near Gohpur in north bank of Brahmaputra was a busy center during the British rule. Migration of Brahmaputra towards north has already covered large area. Severe bank erosion is taking place for the last few years (Figure 3.11).
Figure 3.11 Erosion sites of Gamerighat
Palasbari (F)
hbari
Figure 3.12 Erosion site of Palasbari
Among the 25 vulnerable and severe bank erosion sites as identified by the Water Resources Department, Govt. of Assam, Palashbari of Kamrup district is one of the most severely affected areas (Figure 3.12, Table 3.8) with erosion rate of 22 m/year.
About 16,037 ha area of land has been eroded in Palashbari site alone since 1954, affecting 12,530 households (WRD, 2008).
Table 3.8 Extent of erosion in Palasbari
Annual flood effected area 3,340 Ha
Land area eroded (1967-2008), (bank line length 25 km) 16,037 Ha
Rate of erosion 22 m/year
Number of household lost (last 10 years) 1,646
Number of household affected 12,530
Soil samples were collected at 10 cm depth and then at 30 cm intervals from vertical profile of exposed steep banks of erosion sites of the six locations. Multiple samples were collected from each location (Figure 3.13). Samples were collected from nearby non- erosion sites also.
Figure 3.13 Collection of samples from Palasbari erosion site
All samples were brought to laboratory and preserved till analysis. pH, organic matter, carbonate content, Sodium Absorption Ratio, Exchangeable sodium percentage, Cation exchange capacity, mineralogy and particle size were estimated.