CONCLUSIONS AND RECOMMENDATIONS 9.1 A Brief Review of the Work Done
3.2 Study Area
The middle reach of the Brahmaputra river is considered for the present study. Within this river reach, different sites have been considered for different field observations and soil investigations. Fig. 3.1 shows the details of the study area considered along the Brahmaputra river. In the following paragraphs, the detail descriptions of different sites considered for the study is described.
3.2.1 River Gauging Stations
The historical stage hydrograph data analysis has been carried out for two river gauging stations of the Brahmaputra river. The river gauging sites at Guwahati city (26 10' ,91 44'0 N 0 E) and at Tezpur town (26 37' ,92 48'0 N 0 E) nearly 120 km upstream of Guwahati have been considered (Fig. 3.1). The Brahmaputra River is highly braided and complex in planform. However, due to presence of nodes (constrictions) at these two gauging sites, the river width is restricted and no major sand bars are present. At the northern bank up to Tezpur, Subansiri is the major tributary with drainage area of about 37,700 km2, while Dhansiri is at the southern bank with drainage area of about 12,250 km2. In between Tezpur and Guwahati, Kameng is the major tributary at the northern bank, having drainage area of nearly 14,450 km2. Kapili, the major tributary at the southern bank within this reach, has the drainage area of about 15,900 km2. The catchment area contributing flows up to Guwahati gauging station is about 424,100 km2. The average annual rainfall depth spatially varies from about 400 mm to 6,000 mm in some parts of the Brahmaputra river basin. However, the annual rainfall is 1,844 mm at Tezpur and 1,561 mm at Guwahati (Datta and Singh 2004). Based on the climatic conditions the annual rainfall system can be divided into three major parts: a) pre-
monsoonal rainfall (February to April), b) monsoonal rainfall (May to October), and c) winter rainfall (November to January). Up to 80% of the annual rainfall occurs as monsoonal rainfall. The average rainy days are 130 and 115 for Tezpur and Guwahati, respectively.
Fig. 3.1. Brahmaputra river at the North-East India showing the study sites (A-B is the river reach considered for satellite imagery analysis)
A
B
3.2.2 Submerged Jet Tests
Four field sites were selected along the banks of the Brahmaputra River (Fig. 3.2). One of the sites is located at the upper part of the Brahmaputra river, named Jamuguri (26 50'08"0 N,93 46'08"0 E). The other two sites are near Guwahati in the middle section of the river, and the last one at Nalbari.
Almost all parts of the river experience bank erosion at a very high rate (Sarma, 2004). River bed materials do not vary significantly within the study reach, with an average D50 of 0.20 mm (Dutta et al., 2008). Except for the red hilly formation, the river bank is mostly composite with sand (D50 = 0.2 mm) and fine silt (D50 = 0.06 mm) soils.
Sand layers are sandwiched between two fine silt layers. The hilly formations are formed with red coloured soil (D50 = 55µm) which have a sticky nature. Fig. 3.2 depicts the particle size distribution of some soil samples collected from the bed and banks along the Brahmaputra river. Local surveys demonstrate that this layer is more resistant than the other soils normally found in the alluvial flood plain. Only the Jamuguri site was monitored for river stage records, bank line survey and bathymetric survey during the rainy season (June to September, 2007 and 2008).
3.2.3 Seepage Erosion Measurements
After investigating the river bank along the Brahmaputra river, four sites has been found appropriate to measure the seepage erosion. As most of the cases, the bank erosion is sum of the fluvial and seepage erosion. Therefore it is very important to identify the sites where, the seepage erosion is predominant. The four locations are: Dolgobinda (26 11'25"N, 0 91 43'26"E), Amingaon (0 26 10'55"N, 0 91 41'24"E), Sualkuchi 0
(26 09'50"N, 0 91 34'26"E) and Singimari (0 26 10'42"N, 0 91 35'15"E). Fig. 3.3 shows 0 the field photographs collected during the seepage erosion measurements at these sites.
0 20 40 60 80 100
0.01 0.1 1 10
Particle size (mm)
% Finer
Sample 1 from river bed Sample from sand layer in river bank Sample 2 from river bed
0 20 40 60 80 100
0.001 0.01 0.1 1
Particle size (mm)
% Finer
Sample from silt layer in river bank
Fig. 3.2. Particle size distribution of the soil samples collected from: (a) the river bed and sand layer in river bank, (b) the silt layer in river bank.
3.2.4 Site for Model Validation
Detailed hydrographic and river bank line survey was conducted at the upper reach of the Brahmaputra river at Jamuguri, India. One braided loop of the river is in this site. The site
(a)
(b)
is about 70 km upstream of Tezpur town. Figure 3.4 shows the geographic location of the site. The river flows from east to west at this location. Morphological studies using multi- date satellite imagery show that after 2004 extreme flood event, a large river bend has been formed at the location. Till the year 2009 the river bank was under severe threat of erosion. It also has been found from the satellite imagery study that a bank area of 2.4 km2 was eroded out during the flood season of 2004. The radius of the centerline of the bend is about 2460 m and the channel widths at the upstream and downstream are 630 m and 1126 m, respectively (Karmaker and Dutta, 2009).
The developed mathematical bank erosion model has been validated for this channel bend with detail morphological observations over a period for the years 2005 to 2008. Moreover, the 45 channel bends from the middle reach of the Brahmaputra river have been selected for further evaluation of stochastic prediction of the model. However, the details of these locations and observations are discussed in Chapter 8.
Fig. 3.3. Field photographs showing the seepage undercut and collapse along the Brahmaputra river bank: (a) collpase after seepage erosion (b) seepage undercut.
(a)
(b)
Fig. 3.4. Study river reach at Jamuguri (Arrows show the flow direction, numerals indicate the survey transects)