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Braiding process and bank erosion in the Brahmaputra River
International Journal of Sediment Research 26 (2011) 431-444
Braiding process and bank erosion in the Brahmaputra River M. P. AKHTAR1, NAYAN SHARMA2, and C. S. P. OJHA3
Abstract The present work explores relations between stream power, braiding intensities and bank erosion in certain stretches of the Brahmaputra River. In this paper, an objective approach is presented to enable quantitative assessment of spatio-temporal behaviour of channel braiding process of the Brahmaputra River by using the Plan Form Index and corresponding estimation of stream power to establish a behavioural pattern of variability of potential energy expenditure. The braiding index is compared for discrete years to understand the morphological behaviour. Subsequently, a real time estimation of stream power for certain stretches of Brahmaputra River is done in order to analyse its variability in braiding intensity and bank erosion. The paper presents the dynamic behaviour of the channel pattern of the Brahmaputra River System in Assam valley of India over a time span of 18 years. The procedure addresses the selection of input parameters from digital satellite images, comprising scenes for the years 1990, 1997 and 2007 with specific dates, from Dhubri near Indo-Bangladesh Border to Upper Assam. Deployment of GIS technique has been made to extract the required parameters to derive Plan Form Indices for the entire study reach. Stream power estimation is done for corresponding latest floods and for corresponding dates of image scenes. The study indicated that due to consistent aggradation of riverbed inducing temporal declination of stream power, there is an occurrence of wide spread braiding. This in turn incurs substantial yearly land loss due to bank erosion, caused by flow concentrations due to temporal evolution of multiple channels in the Brahmaputra River. Key Words: River plan form, Braiding indices, Bank erosion, Stream power
1 Introduction The Brahmaputra River is the largest river in the Indian subcontinent and ranks fifth in the world in terms of discharge. The specific yield from its catchment area is one of the highest in the world due to incidence of very high rainfall on a narrow drainage basin. Significant areas of prime inhabited land are lost every year to river erosion in the Brahmaputra River basin. Furthermore, unrelenting bank erosion process has caused channel widening which created navigation bottle-neck zones in the Brahmaputra River due to inadequate draught during non-monsoon. For efficient management of bank erosion problem spanning over hundreds of kilometre length along the Brahmaputra River, the need has arisen for a convenient scientific methodology which can aid systematic monitoring of bank-line changes, help prioritization of erosion zones, and facilitate maintenance of navigation for all-weather fairway. The Brahmaputra River flows through the Indian state of Arunachal Pradesh for 278 km, mostly across the Himalayas, where it is called Dihang or Siang. The Dihang debouches onto the plains at Pasighat (elevation 155 m). Near Kobo in Assam, 52 km downstream from Pasighat, the Dihang is joined by two large rivers - the Lohit and the Dibang River, and from hereafter the river is known as the Brahmaputra 1
Ph.D Student, Dept. of WRD and M, I.I.T. Roorkee, India, E-mail: [email protected] Prof., Dept. of WRD and M, I.I.T. Roorkee, India, E-mail: [email protected] 3 Prof., Dept. of Civil Engg., I.I.T. Roorkee, India, E-mail: [email protected] Note: The original manuscript of this paper was received in Feb. 2011. The revised version was received in Nov. 2011. Discussion open until Dec. 2012. International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444 - 431 2
River. The Brahmaputra River flows for about 670 km through the province of Assam, where it receives 103 tributaries - 65 on the right (north) bank and 38 on the left (south) bank (Sarma et al., 2002). The mighty Brahmaputra River along with the well-knit network of its tributaries controls the geomorphic regime of the entire region, especially in the Brahmaputra River valley. The erosion phenomenon is evident on the Brahmaputra River channel for a stretch of 270 km from Panidihing Reserve Forest to Haloukonda Bil, as studied with the help of Survey of India toposheets (1914 and 1975) and Indian Remote Sensing (IRS) satellite imagery (1998). The stretch covers the world’s largest river island, “Majuli” under extreme threat of erosion (Kotoky et al., 2003; Holden, 2003; Mudur, 2003.). Kotoky et al. (2005) studied selected reach of Brahmaputra River with two sets of Survey of India toposheets (1914 and 1975) and a set of IRS satellite images covering the cloud-free period. Sankhua et al. (2005) reported that the area of the Majuli Island has decreased by 39.30 km2 over a period of 12 years. However, there is a lack of investigations looking into variability of stream power with braiding intensity and bank erosion for the Brahmaputra River. In view of the above, the objectives of the present work are set as follows. a) To identify a rational braiding indicator to adequately represent braiding phenomenon in the Brahmaputra River. b) To study the variability of stream power in relation to braiding process and bank erosion. c) To identify those reaches of the Brahmaputra River which exhibit less vulnerability and can be treated as control points for further detailed hydraulic investigations and mathematical modeling. To begin with, the paper first discusses relevant braiding indicators available in the literature and on the basis of this; an appropriate braiding indicator is identified for the present analysis. This is followed by data extraction, details of sites, computations of braiding indicators, assessment of bank erosion and stream power followed by analysis to identify the possible linkages between different aforementioned parameters. 1.1 Existing braiding indicators Several past studies had presented discrimination between the straight, meandering, and braided streams on the basis of discharge and channel slope. Lane (1957) suggested the following criterion for the occurrence of braiding. (1) S > 0.004 (Qm)-0.25 where Qm = mean annual discharge; and S = channel slope. Using bank full discharge Qb, Leopold and Wolman in 1957 (Richards, 1982) proposed the relationship for braiding to occur, which also predicts braids at higher slopes and discharges: (2) S > 0.013 Qb -0.44 where Qb = bank full discharge. Leopold and Wolman (1957) also indicated that braided and meandering streams can be separated by the relationship: (3) S = 0.06 Q 0.44 where S = channel; and Q = water discharge. However, these indicators have been revisited by Schumm and Khan (1972) and they concluded that none of these recognizes the importance of sediment transport. These results imply a higher power expenditure rate in braided streams, a conclusion reinforced by flume experiments (Schumm and Khan, 1972). Since bed material transport and bar formation are necessary in both meandering and braiding development processes, the threshold between the patterns should relate to the bed load. Henderson (1961) re-analyzed Leopold and Wolman's data to derive an expression including d50, median grain size (mm): (4) S > 0.002 d50 1.15 Qb -0.46 where, d50 = median grain size According to Eq. (4), a higher threshold slope is necessary for braiding to occur in coarse bed materials. Bank material resistance affects the rate of channel migration and should also influence the threshold, although its effect may be difficult to quantify and also be non-linear since greater stream power is required to erode clays and cobbles than sands. - 432 -
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Parker’s (1976) stability analysis indirectly illustrates the effects of bank material resistance by defining the meander - braid threshold as: (5) S/Fr = D/B where, D = mean depth of the flow; B = width of the stream, and Fr = flow Froude number. The depth, width and flow Froude number may be expressed in terms of discharge and bank silt-clay percentage, as suggested by Schumm (Richards, 1982). Meandering occurs when S/Fr d D/B, braiding occurs when S/Fr t D/B, and transition occurs for S/Fr is around equal to D/B. Ferguson (1981) suggested the following condition for braiding to occur, which predicts steeper threshold slopes for braiding in channels with resistant silty banks. (6) S>0.0028(Qb)-0.34Bc0.90 where, Bc = percentage of silty clay content in the bank material. Measures of the degree of braiding generally fall into two categories: (1) the mean number of active channels or braid bars per transect across the channel belt; and (2) the ratio of sum of channel lengths in a reach to a measure of reach- length (total sinuosity).Where sinuosity, P is defined as thalweg length / valley length. Sharma (2004) developed Plan Form Index (PFI), Flow Geometry Index (FGI), and Cross-Slope Ratio for identifying the degree of braiding of highly braided river. The PFI, FGI and Cross- Slope formulae have been given below: T u 100 PFI = B N
¦ FGI = [
d i xi
W uD
Cross-Slope =
] uN
BL 2 ( Bank level Av. bed level )
(7) (8) (9)
where, T = flow top width (m); B= overall width of the channel (m); BL = Transect length across river width; N = number of braided channel; di and xi are depth and top lateral distance of submerged sub-channel, respectively; and D = hydraulic mean depth.
Fig.1 Definition sketch of PFI
Plan Form Index (PFI) in Eq. (7) (Definition sketch as shown in Fig. 1) reflects the fluvial landform disposition with respect to a given water level and its lower value is indicative of higher degree of braiding.
Satellite /sensor IRS IA/LISS-I, IRS 1C and D/ LISS-III (Standard product)
IRS IA/LISS-I, IRS 1C and D/ LISS-III (Standard product)
Table 1 Characteristics of the remote sensing data used Acquisition Spatial Path/row Spectral bands and channels period resolution Visible band1990, 1997, 112/52 75m, 23.5m (Green channel) (0.52 -0.59μm) 2007 Visible bandRed channel (0.62-0.68μm) 1990, 1997, Near infrared (NIR) 112/53 75m, 23.5m 2007 (0.77 - 0.86μm)
In order to provide a broad range of classification of the braiding phenomenon, the following threshold values for PFI are proposed by Sharma (2004). Highly Braided: PFI < 4 International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444
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Moderately Braided: 19 > PFI > 4 Low Braided: PFI > 19 The newer braiding indicator PFI for the Brahmaputra River suggested by Sharma (2004) has been adopted in the present study to analyze the braiding behavior of the river. Attempt has been made to assess the temporal and spatial variation of braiding intensities along the whole stretch of the Brahmaputra River in Assam plains of Indian Territory based on the remote sensing image analyses. 2 Study area and extraction of channel forms Digital satellite images of IRS LISS-III sensor, consisting of scenes for the years 1990, 1997 and 2007 are used for the present study. In order to bring all the images under one geometric co-ordinate system, these are geo-referenced with respect to Survey of India (1:50,000 scale) topo-sheets using second order polynomial. IRS-P6 LISS images of 1990, 1997 and 2008 years are geometrically rectified with reference to the LANDSAT images of the same area. The UTM projection and WGS 84 datum has been taken for geo-referencing. Rectification of the images was done with a residual RMS (root mean square error) of less than 1. The bank line of the Brahmaputra River is demarcated from each set of imageries and the channel patterns are digitized using GIS software. The data used in the analysis have been presented in Table 1. Then satellite images of the other years were co-registered using an image-to-image registration technique. The study area of around 622.73 km from Dhubri to Kobo beyond Dibrugarh in Upper Assam is considered. 3 Methodology Appropriate GIS applications are done to precisely extract bank line information. Segment wise satellite-derived plan-form maps have been developed for the discrete years, i.e. 1990, 1997 and 2007. The stream power is estimated for the corresponding discharge data and measured width with available average bed gradients for the corresponding period and the latest flood-period for identified locations. 3.1 Data geo referencing and image processing One set of Survey of India topo-sheets (1965) and digital satellite images of IRS LISS-I and LISS-III sensors, comprising scenes for the years 1990, 1997 and 2007 are used in the present study. The geo-referencing was done by the hardcopy map on digitizing table using second order equation with root mean square error less than 1.0 and nearest neighbourhood re-sampling technique to create a geo-referenced image of pixel size of 23.5m × 23.5m. Subsequently, other images were also registered with the geo-referenced image using the image-to-image registration technique. The registered images for different dates pertaining to study area were used for further analysis. 3.2 Delineation of river bank line For convenience, the main river has been divided into 120 strips with 12 reaches, and reference cross sections were drawn at the boundary of each strip. Each ten cross sections were grouped as a reach with numbering from downstream to upstream of the river of equal base length (Table 2). Base line of Latitude 25.966oN and Longitude 90o E has been taken as permanent reference line. The derived data for each cross section from satellite images of years 1990, 1997 and 2008 were analyzed and the bank lines were also digitized for all the years. Intermediate channel widths, and total widths of channel at each predefined cross sections were measured using GIS software tools for computing Plan Form Indices for each cross sections for further analysis. The erosion depth in north and south banks of the river area during the study period was estimated by the GIS software tools through delineating the river bank lines and drawing polygons within bank line variations within the study period.
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Fig. 2
Delineation of River for 1990
Fig. 3 Delineation of River in 2007 with bank lines in 1990 Table 2 Reach number 1 2 3 4 5 6 7 8 9 10 11 12
Identification of reaches in respect of the location Locations in vicinity Dhubri Goalpara Palasbari Guwahati Morigaon (Near Mangaldai) Morigaon (Near Dhing) Tezpur U/s of Tezpur (Near Gohpur) Majuli (Near Bessamora) U/s of Majuli (Near Sibsagar) Dibrugarh U/s of Dibrugarh
4 Variation of braiding and bank erosion The computed Plan Form Index for each reference cross-section totaling 120 in numbers across the study reaches are plotted against reach cross-section number in Fig. 4 for the three discrete years. From the plot, it can be readily inferred that for the period 1990-2007, the PFI values decreased significantly indicating the increase in braiding intensities in majority of cross sections. The analysis can further be extended by computing mean PFI and extreme values that are maximum PFI (indicating least braiding within the reach) and minimum PFI (indicating highest braiding within the reach) values for Reach 1 to Reach 12 comprising ten cross-sections and each is computed and shown in Table 3. The corresponding plot for the Mean, Minimum and Maximum PFI against reach numbers are plotted and shown in Figs. 5(a)-(c) respectively. The mean PFI enveloping with maximum and minimum cross-sectional PFI suggests the ranges of variation in braiding intensities within a reach. It has been further elaborated for clarity in Figs. 6(a)-(c) respectively through trend of PFI with time for the discrete International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444
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years studied. It can be easily figured out that higher values are predominant in the year 1990, whereas in 2007 lower values are predominant. All three statistically measured PFIs are registering little changes or similar trend in three to four identified reaches with rock-outcrops numbered 2, 4, 6-7 and 9, which are in the vicinity of Jogighopa, Guwahati, Tezpur and Bessamora in Majuli. It strongly suggests that irrespective of the time, the aforementioned four discrete reaches show little changes in braiding intensity and pattern. It confirms the existence of the aforesaid four geological control points which hold the river, and in between there are intermittent fanning out of the river with time.
Fig. 4 Cross-section wise PFI values of the River in discrete years Table 3 Comparison of Plan Form Index (PFI) for the year 1990, 1997 and 2007 for the Brahmaputra River Reach number 1 2 3 4 5 6 7 8 9 10 11 12
Mean 22.69 15.39 10.55 55.97 14.19 28.34 31.94 19.12 10.14 13.61 12.38 20.95
PFI (1990) Min. 9.90 5.23 3.50 8.46 6.18 9.98 18.23 3.28 5.25 6.09 7.65 4.27
Max. 45.16 48.67 31.15 113.89 20.92 108.61 82.47 83.19 22.71 34.31 27.93 87.05
Mean 8.94 8.60 7.71 33.62 13.55 14.74 17.21 10.77 10.69 7.87 6.81 4.89
PFI (1997) Min. 13.22 3.50 2.83 8.94 4.52 7.73 7.48 4.39 3.64 5.25 3.36 2.76
Max. 4.37 21.09 16.22 124.48 38.39 30.46 37.27 32.43 48.37 10.65 12.23 7.57
Mean 15.66 20.30 9.99 73.64 8.08 7.78 18.50 6.89 6.34 6.54 5.41 2.61
PFI (2007) Min. 5.74 4.42 3.75 16.47 4.88 3.40 4.37 3.84 2.00 3.77 3.52 1.63
Max. 38.65 98.05 24.79 136.85 19.67 31.30 100.00 14.95 17.55 12.57 10.91 3.70
As discussed, the graphical plots of Plan Form Index for the Brahmaputra River shows a decreasing trend thereby registering an increasing level of braiding with time and space in view of the threshold limits, as described in Section 1.1. These plots clearly demonstrate the rationality of using the Plan Form Index as a measure of braiding and closely conform to the actual physical situation of the occurrence of braiding vividly depicted in the satellite images. In light of the threshold values of Plan Form Index, it can be readily inferred from graphical plots showing maximum, minimum and mean values of PFIs of cross sections that have heavy with moderate and low braiding characteristics resulting in a very complex channel hydrodynamics. Using the GIS software, vector layers of the bank line and river flow domain have been worked upon and results have been presented in Table 4, which accounts for the reach wise erosion area for periods - 436 -
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1990-2007 and 1997- 2007. Erosion lengths within the reaches have been identified based on the exhibited bank line shifts in the duration of study area. Similarly, erosion area is estimated approximately through area estimation using GIS software tools for polygon areas with the shifting bank-lines in study period.
Fig. 5 Variation in PFI with extreme values along the river
Fig. 6 Variation in PFI values along the river in discrete years
Figure 7 depicts the area eroded in left and right banks of the study reach number 1-11 of the Brahmaputra River. In Reach 2 near Goalpara where right bank shows considerable shifting with narrowing down to incised channel at Pandu near Guwahati in Reach 4. Plot of reach-wise land loss and erosion prone bank length (Fig. 7) indicates that downstream of Guwahati, river tends to move towards right side whereas in upstream side it tends towards south side keeping the control point at Guwahati invariant. Similar pattern also repeats at other control points. It warrants in-depth study of interaction of geo-tectonic activities conjunctively with fluvial regime in the region to understand the complex physical processes completely for suggesting more practical result oriented river management interventions. The satellite image based an estimation of area eroded in the Brahmaputra River during periods 1990-2007 and 1997-2007 is presented in Table 4, which shows the eroding tendency along the river banks of the Brahmaputra River in the entire study area. For the period of 18 years, the total land loss per year excluding forest area is found out to be 62 km2/year. Table 4 indicates high eroded area in left bank of exhibiting serious avulsion phenomenon during this period. During this period, in the upstream of the Dibrugarh, river practically changed its dominant path. For more recent period of 1997-2007, the total land loss per year (excluding avulsion) is found out to be 72.5 km2/year, registering sharp increase in land lost due to river erosion in recent years. International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444
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Fig. 7 Bank erosion along the river in discrete years Table 4 Land lost in Main Brahmaputra River during the study period North bank Reach Number
1 (Dhubri) 2 (Goalpara) 3 (Palasbari) 4 (Guwahati) 5 (MorigaonMangaldai) 6 (MorigaonDhing) 7 (Tezpur) 8 (TezpurGohpur) 9 (MajuliBessamora) 10 (MajuliSibsagar) 11 (Dibrugarh) 12 (U/s of Dibrugarh) Total
Minimum PFI values
South bank
Total erosion length (km) 40.19 39.5 54.87 21.02
Land loss area 1990-2007 (km2) 124.461 79.046 48.668 7.92
Land loss area 1997-2007 ( km2) 94.129 40.902 42.914 1.654
Total erosion length (km) 7.05 4.85 14.02 24.38
Land loss area 1990-2007 (km2) 194.983 17.816 23.006 5.385
Land loss area 1997-2007 (km2) 10.791 5.052 15.859 12.079
1997
2007
13.22 3.50 2.83 8.94
5.74 4.42 3.75 16.4
6
35.606
2.138
47.91
96.979
103.7
4.52
4.88
24.86
29.057
7.275
47.8
10.795
56.72
7.73
3.40
8.58
38.758
4.733
52.95
16.628
44.774
7.48
4.37
8.85
31.187
5.794
44.16
26.098
71.227
4.39
3.84
24.69
25.562
12.327
47.17
32.788
28.998
3.64
2.00
16.93
60.657
16.878
54.95
44.018
42.118
5.25
3.77
37.86
37.506
43.529
43.89
46.595
6.066
3.52
70.5
20.376
55.454
57.54
399.529
333.416
3.36 Forest area excluded southern side
353.85
538.805
327.726
389.13
914.62
730.8
Moreover, the vulnerability of the stream bank erosion is significant as one can see from Table 4. Almost 750 km of bank line in both sides of the river has potential erosion tendency. The plot of potential erodible bank length versus reach number (Fig. 7) also shows that downstream of Guwahati (Reach number 4), erosion tendency is considerably high in the north bank line. Whereas in the upstream of Guwahati erosion tendency is considerably high in south bank-line, indicating that geological control point at Guwahati in respect to other control points has significant causative impact on the morphological behaviour of the Brahmaputra River. The bank erosion as well as eroded bank length of Morigaon Reach between Tezpur and Guwahati is relatively high. The erosion pattern of the left and right banks of the Brahmaputra River during the study period is depicted in Fig. 8. The extent of bank erosion in different location can be qualitatively assessed from the presented map, with the bank-line extracted for discrete - 438 -
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years through GIS tools.
Fig. 8
Land Lost in the Brahmaputra River during the period 1990-2007
The apparent fallacious temporal variability of land losses indicated in Table 4 can be easily understood from a perusal of Fig. 9 wherein the actual shift of bank lines as extracted from the satellite images are depicted with corresponding bank erosion presented in Table 4 during the study period in the 5th river reach.
Fig. 9 Temporal migration of river bank-line in discrete years
5 Variation of unit stream power and its linkage with braiding and bank erosion The hydraulic parameters of an open channel flow such as hydraulic mean depth, wetted perimeter, flow area, bed gradient, discharge and the physical properties, Ȗ(=ȡg) of the fluid are related to express the unit stream power of the flow at a point along the direction of flow (Yang, 1976; Chang, 1979). The unit stream power (N/m-s) can be expressed as; J uQu S : (10) w
where Ȗ is the specific weight of water and w is the channel width in meter. It is emphasized that a right combination of these parameters would give rise to the requisite stream power (SP) to erode the riverbed and produce incised channels. The unit stream power of a flow quantifies the potential energy expenditure of the flow encapsulated in the mass of the fluid. The International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444
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magnitude of unit stream power is one of the deterministic parameters of erosion of bed and banks of a stream. The unit stream power is one of the most dominant factors to influence channel sediment transport capacity. More recently, Shih and Yang (2009) estimated overland flow erosion capacity using the unit stream power. It has been found relevant to correlate the temporal changes in effective width of the Brahmaputra River in different reaches and estimate unit stream power to explain the braiding processes. In general, channel planforms, floodplain segments separated by the latter and alluvial channel bed-forms are shown to be geomorphic expressions of sediment transport process at different spatial and temporal scales (Alekseevskiy et al., 2008). To appreciate the linkage of stream power with braiding, certain considerations are vital. Assessment of braiding index essentially depends upon the corresponding stages of the river at measured locations. Care has been taken to measure this parameter at low flow periods to adequately represent the evolved channels of the river in a particular instance of time; however, it is essential to correlate stage-discharge of the river with corresponding measured braiding parameter to achieve meaningful and rational analysis of the fluvial channel process. The satellite data based study detailed in the previous section becomes of much relevance and fetched very significant and meaningful outcomes when it is correlated to hydraulic and hydrological parameters such as stage-discharge and sediment transport rates for the corresponding periods. In this study, three most vulnerable sites along the river are chosen, in view of the satellite data based study presented in previous section. These are Dhubri-Goalpara Reach (Reach 1-2), Guwahati-Palasbari Reach (Reach 3-4) and Morigaon–Tezpur Reach (Reach 6-7) which showed considerable increase in braiding and bank erosion in recent years. Each selected reach comprised of 20 cross-sections. The sediment inflow rates in selected reaches have been showed in Figs. 10 and 11 in discrete years. It is observed that in the Guwahati-Palasbari Reach, sediment flow rate increased, and considerable decease occurred in downstream reaches at Dhubri-Goalpara. It indicates that sediment feed between Tezpur and Guwahati was more, while there was deposition in downstream of Guwahati due to low stream power (Figs. 12 and 13). The process is in agreement with the fact that bank erosion in Morigaon near Tezpur was dominantly high while braiding considerably increased in downstream of Guwahati at Palasbari due to bed deposition (Figs. 5 and 6). The lowered stream power in downstream reaches can be clearly observed from Figs. 12 and 13.
Fig. 10 Temporal variation of sediment inflow in selected reaches
Variability of stream power at bankful and low flow periods with estimated braiding index values presented for selected river reaches namely Dhubri-Goalpara (Fig. 14), Guwahati-Palasbari (Fig. 15) and Tezpur- Morigaon (Fig. 16) suggest that wherever PFI goes up, the corresponding stream power becomes lowered indicating lesser potential energy expenditure per unit mass of water. This process triggers local river bed rise and formation of additional multiple channels. While the number of channels increases, concentration of flow becomes usual phenomenon along the bank-lines, which again instigates bank - 440 -
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erosion and thus further increases the sediment load into the river making river further prone to braiding. The process becomes complex as it hinders the incising of the channel at high stream power also as bank erosion overloads the sediment to the river creating a sediment transport rate greater than sediment carrying capacity of the channel.
Fig. 11
Fig. 12
Reach wise sediment flow in discrete years
Variation of stream power at bankful discharge during latest flood period
Fig. 13
Variation of stream power at low flow period
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Fig. 14
Variability of stream power with PFI index value at bankful and low flow period for Dhubri-Goalpara site
Fig. 15 Variability of stream power with PFI values during bankful and low flow period for Guwahati-Palasbari site
With the onset of the flood season, the volume of sediment transported in the Brahmaputra River increases, the thalweg starts to change position and the geometry and location of bars changes. As the flow begins to recede, deposition over the bed takes place in the form of bars and islands. The river then flows in several sinuous channels in between these sand bars. With the onset of the next flood season, the sequence of events is repeated. - 442 -
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Fig. 16 Variability of stream power with PFI values during bankful and low flow period for Tezpur-Morigaon site
The formation of bars and islands is so complex that an understanding the fluvial landform pattern is often difficult. The highly braided nature of the Brahmaputra River has led to its unpredictability, leading to many channels being active all at once. The process is in close conformity with the findings by Leopold and Wolman (1957) on flume experiments, which suggest that a bar of coarse sand diverts flow to cause channel widening, which then accentuates bar development, resulting in braided channel pattern. The braided pattern in an alluvial stream develops after local deposition of coarse material, which cannot be transported under local conditions of flow existing within the reach. This coarse material becomes the nucleus for a bar formation, and subsequently grows into an island made up of coarse as well as fine material. The formation of the bar deflects the main stream towards the banks and may cause bank erosion. Sediment transport takes place over the bar surface while incision in the lateral channels lowers the water surface to expose the bar which then becomes dissected. The complex of islands is stabilized by vegetation in natural streams and experiences further high stage sedimentation. However, the variability of flow behavior and channel pattern of Brahmaputra River is obvious in different reaches as we look at three selected reaches. The observation is in agreement with the findings of Wang et al. (2010) who conducted a numerical simulation of channel pattern changes and observed that the upper and lower parts of the same channel may have different planforms because the sediment transport conditions of the two parts differ greatly. The geological control point as discussed earlier at Guwahati, Jogighopa, Bessamora and Tezpur are due to obvious rock outcrops otherwise the tendency of the river on the rest stretches is highly dynamic, with this being an example of fluvial hydrodynamics with complex morphology. 6 Conclusions Based on the present study, the following conclusions can be inferred. 1) The braiding indicator, namely Plan Form Index (PFI), with high resolution remote sensing satellite data is found useful to analyze and monitor the complex braiding behaviour of a large river like the Brahmaputra River. 2) The study identifies three to four major geological channel control points present along the Brahmaputra River in Assam flood plains. These control points are located in the vicinity of Jogighopa near Goalpara, Pandu near Guwahati, Tezpur and Bessemora in Majuli. These channel control points International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444
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usually have well defined and stable hydrographical profiles. Intermittent fanning out, behaviour is displayed between these control points which are being temporally severed in the geological time scale. 3) The variability of stream power with bank erosion and braiding processes is investigated and a distinct behavioural pattern between these processes is observed. For example, with a low stream power, braiding appears to intensify which in turn may indicate a higher possibility of bank erosion. Acknowledgements The present study is sponsored and funded by National Disaster Management Authority of India (NDMA, Govt. of India), New Delhi, which is gratefully acknowledged here. References Alekseevskiy N. I., Berkovich K. M., and Chalov R. S. 2008, Erosion, sediment transportation and accumulation in rivers. International Journal of Sediment Research, Vol. 23, No. 2, pp. 93–105. Chang H. H. 1979, Stream power and river channel patterns. ASCE Journal of Hydrologic Engineering, Vol. 41, pp. 303–311. Ferguson R. I. 1981, Channel form and channel changes, J. Lewin (Ed), In British rivers, pp. 90–125 (London: Allen and Unwin). Henderson F. M. 1961, Stability of alluvial channels. Journal of Hydraulic Division, American Society for Civil Engineering, Vol. 87, pp. 109–138. Holden C. 2003, Historic island threatened, random samples. Science, Vol. 300, p.1368. Kotoky P., Bezbaruah D., Baruah J., and Sarma J. N. 2003, Erosion activity on Majuli – the largest river island of the world. Current science, Vol. 84, pp. 929–932. Kotoky P., Bezbaruah D., Baruah J., and Sarma J. N. 2005, Nature of bank erosion along the Brahmaputra River channel, Assam, India. Current science, Vol. 88, No. 4, pp. 634–640. Lane E. W. 1957, A study of the shape of channels formed by the natural streams flowing in erodible material, U.S Army Crops Engg. Div., Missouri River, M.R.D. Sediment, Ser. No. 9, p. 106. Leopold L. B. and Wolman M. G. 1957, River channel patterns: Braided, meandering and straight, United States, Geological Survey Professional Paper 282-B, pp. 35–85. Mudur G. S. 2003, Famous island set to go under, The Telegraph, 1May 2003. Parker G. 1976, On the cause and characteristics scale of meandering and braided in rivers. Journal of Fluid Mechanics, Vol. 76, pp. 459–80. Richards K. 1982, Rivers: forms and process in alluvial channels, London: Methuen and Co. Ltd.. Sankhua R. N., Sharma N., Garg P. K., and Pandey A. D. 2005, Use of remote sensing and ANN in assessment of erosion activities in Majuli, the world's largest river island, International Journal of Remote Sensing, 26:20, pp. 4445–4454. Sarma J. N. 2002, A Study on pattern of erosion and bankline migration of the River Brahmaputra River in Assam using GIS, Report Disaster Management in North-Eastern region, Dept. of Revenue, Govt. of Assam, pp. 50–53. Schumm S. A. and Khan H. R. 1972, Experimental study of channel patterns. Bull. of Geological Society of America, Vol. 83, pp. 1755–1770. Sharma Nayan 2004, Mathematical Modelling and Braid Indicators, In The Brahmaputra River Basin Water Resources, V.P.Singh (Ed.), Dordrecht: Kluwer Academic Publishers, Vol. 47, pp. 229–260. Shih Hui-Ming and Yang Chih Ted 2009, Estimating overland flow erosion capacity using unit stream power. International Journal of Sediment Research, Vol. 24, No.1, pp. 46–62. Wang Hong, Zhou Gang, and Shao Xuejun 2010, Numerical simulation of channel pattern changes. Part II: Application in a conceptual channel, International Journal of Sediment Research, Vol. 25 No. 4, pp. 366–379. Yang C. T. 1976, Minimum Unit stream power and fluvial hydraulics. ASCE Journal of Hydraulic Division, Vol. 102, No. HY7, pp. 919–933.
Notation g = gravity acceleration; GIS = geographic Information System; IRS = Indian Remote Sensing; LANDSAT = Land Remote-Sensing Satellite; LISS = Linear Imaging Self Scanner; PFI = Plan Form Index; UTM = Universal Transverse Mercator; WGS 84 = World Geodetic System 84. - 444 -
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Braiding process and bank erosion in the Brahmaputra River M. P. AKHTAR1, NAYAN SHARMA2, and C. S. P. OJHA3
Abstract The present work explores relations between stream power, braiding intensities and bank erosion in certain stretches of the Brahmaputra River. In this paper, an objective approach is presented to enable quantitative assessment of spatio-temporal behaviour of channel braiding process of the Brahmaputra River by using the Plan Form Index and corresponding estimation of stream power to establish a behavioural pattern of variability of potential energy expenditure. The braiding index is compared for discrete years to understand the morphological behaviour. Subsequently, a real time estimation of stream power for certain stretches of Brahmaputra River is done in order to analyse its variability in braiding intensity and bank erosion. The paper presents the dynamic behaviour of the channel pattern of the Brahmaputra River System in Assam valley of India over a time span of 18 years. The procedure addresses the selection of input parameters from digital satellite images, comprising scenes for the years 1990, 1997 and 2007 with specific dates, from Dhubri near Indo-Bangladesh Border to Upper Assam. Deployment of GIS technique has been made to extract the required parameters to derive Plan Form Indices for the entire study reach. Stream power estimation is done for corresponding latest floods and for corresponding dates of image scenes. The study indicated that due to consistent aggradation of riverbed inducing temporal declination of stream power, there is an occurrence of wide spread braiding. This in turn incurs substantial yearly land loss due to bank erosion, caused by flow concentrations due to temporal evolution of multiple channels in the Brahmaputra River. Key Words: River plan form, Braiding indices, Bank erosion, Stream power
1 Introduction The Brahmaputra River is the largest river in the Indian subcontinent and ranks fifth in the world in terms of discharge. The specific yield from its catchment area is one of the highest in the world due to incidence of very high rainfall on a narrow drainage basin. Significant areas of prime inhabited land are lost every year to river erosion in the Brahmaputra River basin. Furthermore, unrelenting bank erosion process has caused channel widening which created navigation bottle-neck zones in the Brahmaputra River due to inadequate draught during non-monsoon. For efficient management of bank erosion problem spanning over hundreds of kilometre length along the Brahmaputra River, the need has arisen for a convenient scientific methodology which can aid systematic monitoring of bank-line changes, help prioritization of erosion zones, and facilitate maintenance of navigation for all-weather fairway. The Brahmaputra River flows through the Indian state of Arunachal Pradesh for 278 km, mostly across the Himalayas, where it is called Dihang or Siang. The Dihang debouches onto the plains at Pasighat (elevation 155 m). Near Kobo in Assam, 52 km downstream from Pasighat, the Dihang is joined by two large rivers - the Lohit and the Dibang River, and from hereafter the river is known as the Brahmaputra 1
Ph.D Student, Dept. of WRD and M, I.I.T. Roorkee, India, E-mail: [email protected] Prof., Dept. of WRD and M, I.I.T. Roorkee, India, E-mail: [email protected] 3 Prof., Dept. of Civil Engg., I.I.T. Roorkee, India, E-mail: [email protected] Note: The original manuscript of this paper was received in Feb. 2011. The revised version was received in Nov. 2011. Discussion open until Dec. 2012. International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444 - 431 2
River. The Brahmaputra River flows for about 670 km through the province of Assam, where it receives 103 tributaries - 65 on the right (north) bank and 38 on the left (south) bank (Sarma et al., 2002). The mighty Brahmaputra River along with the well-knit network of its tributaries controls the geomorphic regime of the entire region, especially in the Brahmaputra River valley. The erosion phenomenon is evident on the Brahmaputra River channel for a stretch of 270 km from Panidihing Reserve Forest to Haloukonda Bil, as studied with the help of Survey of India toposheets (1914 and 1975) and Indian Remote Sensing (IRS) satellite imagery (1998). The stretch covers the world’s largest river island, “Majuli” under extreme threat of erosion (Kotoky et al., 2003; Holden, 2003; Mudur, 2003.). Kotoky et al. (2005) studied selected reach of Brahmaputra River with two sets of Survey of India toposheets (1914 and 1975) and a set of IRS satellite images covering the cloud-free period. Sankhua et al. (2005) reported that the area of the Majuli Island has decreased by 39.30 km2 over a period of 12 years. However, there is a lack of investigations looking into variability of stream power with braiding intensity and bank erosion for the Brahmaputra River. In view of the above, the objectives of the present work are set as follows. a) To identify a rational braiding indicator to adequately represent braiding phenomenon in the Brahmaputra River. b) To study the variability of stream power in relation to braiding process and bank erosion. c) To identify those reaches of the Brahmaputra River which exhibit less vulnerability and can be treated as control points for further detailed hydraulic investigations and mathematical modeling. To begin with, the paper first discusses relevant braiding indicators available in the literature and on the basis of this; an appropriate braiding indicator is identified for the present analysis. This is followed by data extraction, details of sites, computations of braiding indicators, assessment of bank erosion and stream power followed by analysis to identify the possible linkages between different aforementioned parameters. 1.1 Existing braiding indicators Several past studies had presented discrimination between the straight, meandering, and braided streams on the basis of discharge and channel slope. Lane (1957) suggested the following criterion for the occurrence of braiding. (1) S > 0.004 (Qm)-0.25 where Qm = mean annual discharge; and S = channel slope. Using bank full discharge Qb, Leopold and Wolman in 1957 (Richards, 1982) proposed the relationship for braiding to occur, which also predicts braids at higher slopes and discharges: (2) S > 0.013 Qb -0.44 where Qb = bank full discharge. Leopold and Wolman (1957) also indicated that braided and meandering streams can be separated by the relationship: (3) S = 0.06 Q 0.44 where S = channel; and Q = water discharge. However, these indicators have been revisited by Schumm and Khan (1972) and they concluded that none of these recognizes the importance of sediment transport. These results imply a higher power expenditure rate in braided streams, a conclusion reinforced by flume experiments (Schumm and Khan, 1972). Since bed material transport and bar formation are necessary in both meandering and braiding development processes, the threshold between the patterns should relate to the bed load. Henderson (1961) re-analyzed Leopold and Wolman's data to derive an expression including d50, median grain size (mm): (4) S > 0.002 d50 1.15 Qb -0.46 where, d50 = median grain size According to Eq. (4), a higher threshold slope is necessary for braiding to occur in coarse bed materials. Bank material resistance affects the rate of channel migration and should also influence the threshold, although its effect may be difficult to quantify and also be non-linear since greater stream power is required to erode clays and cobbles than sands. - 432 -
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Parker’s (1976) stability analysis indirectly illustrates the effects of bank material resistance by defining the meander - braid threshold as: (5) S/Fr = D/B where, D = mean depth of the flow; B = width of the stream, and Fr = flow Froude number. The depth, width and flow Froude number may be expressed in terms of discharge and bank silt-clay percentage, as suggested by Schumm (Richards, 1982). Meandering occurs when S/Fr d D/B, braiding occurs when S/Fr t D/B, and transition occurs for S/Fr is around equal to D/B. Ferguson (1981) suggested the following condition for braiding to occur, which predicts steeper threshold slopes for braiding in channels with resistant silty banks. (6) S>0.0028(Qb)-0.34Bc0.90 where, Bc = percentage of silty clay content in the bank material. Measures of the degree of braiding generally fall into two categories: (1) the mean number of active channels or braid bars per transect across the channel belt; and (2) the ratio of sum of channel lengths in a reach to a measure of reach- length (total sinuosity).Where sinuosity, P is defined as thalweg length / valley length. Sharma (2004) developed Plan Form Index (PFI), Flow Geometry Index (FGI), and Cross-Slope Ratio for identifying the degree of braiding of highly braided river. The PFI, FGI and Cross- Slope formulae have been given below: T u 100 PFI = B N
¦ FGI = [
d i xi
W uD
Cross-Slope =
] uN
BL 2 ( Bank level Av. bed level )
(7) (8) (9)
where, T = flow top width (m); B= overall width of the channel (m); BL = Transect length across river width; N = number of braided channel; di and xi are depth and top lateral distance of submerged sub-channel, respectively; and D = hydraulic mean depth.
Fig.1 Definition sketch of PFI
Plan Form Index (PFI) in Eq. (7) (Definition sketch as shown in Fig. 1) reflects the fluvial landform disposition with respect to a given water level and its lower value is indicative of higher degree of braiding.
Satellite /sensor IRS IA/LISS-I, IRS 1C and D/ LISS-III (Standard product)
IRS IA/LISS-I, IRS 1C and D/ LISS-III (Standard product)
Table 1 Characteristics of the remote sensing data used Acquisition Spatial Path/row Spectral bands and channels period resolution Visible band1990, 1997, 112/52 75m, 23.5m (Green channel) (0.52 -0.59μm) 2007 Visible bandRed channel (0.62-0.68μm) 1990, 1997, Near infrared (NIR) 112/53 75m, 23.5m 2007 (0.77 - 0.86μm)
In order to provide a broad range of classification of the braiding phenomenon, the following threshold values for PFI are proposed by Sharma (2004). Highly Braided: PFI < 4 International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444
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Moderately Braided: 19 > PFI > 4 Low Braided: PFI > 19 The newer braiding indicator PFI for the Brahmaputra River suggested by Sharma (2004) has been adopted in the present study to analyze the braiding behavior of the river. Attempt has been made to assess the temporal and spatial variation of braiding intensities along the whole stretch of the Brahmaputra River in Assam plains of Indian Territory based on the remote sensing image analyses. 2 Study area and extraction of channel forms Digital satellite images of IRS LISS-III sensor, consisting of scenes for the years 1990, 1997 and 2007 are used for the present study. In order to bring all the images under one geometric co-ordinate system, these are geo-referenced with respect to Survey of India (1:50,000 scale) topo-sheets using second order polynomial. IRS-P6 LISS images of 1990, 1997 and 2008 years are geometrically rectified with reference to the LANDSAT images of the same area. The UTM projection and WGS 84 datum has been taken for geo-referencing. Rectification of the images was done with a residual RMS (root mean square error) of less than 1. The bank line of the Brahmaputra River is demarcated from each set of imageries and the channel patterns are digitized using GIS software. The data used in the analysis have been presented in Table 1. Then satellite images of the other years were co-registered using an image-to-image registration technique. The study area of around 622.73 km from Dhubri to Kobo beyond Dibrugarh in Upper Assam is considered. 3 Methodology Appropriate GIS applications are done to precisely extract bank line information. Segment wise satellite-derived plan-form maps have been developed for the discrete years, i.e. 1990, 1997 and 2007. The stream power is estimated for the corresponding discharge data and measured width with available average bed gradients for the corresponding period and the latest flood-period for identified locations. 3.1 Data geo referencing and image processing One set of Survey of India topo-sheets (1965) and digital satellite images of IRS LISS-I and LISS-III sensors, comprising scenes for the years 1990, 1997 and 2007 are used in the present study. The geo-referencing was done by the hardcopy map on digitizing table using second order equation with root mean square error less than 1.0 and nearest neighbourhood re-sampling technique to create a geo-referenced image of pixel size of 23.5m × 23.5m. Subsequently, other images were also registered with the geo-referenced image using the image-to-image registration technique. The registered images for different dates pertaining to study area were used for further analysis. 3.2 Delineation of river bank line For convenience, the main river has been divided into 120 strips with 12 reaches, and reference cross sections were drawn at the boundary of each strip. Each ten cross sections were grouped as a reach with numbering from downstream to upstream of the river of equal base length (Table 2). Base line of Latitude 25.966oN and Longitude 90o E has been taken as permanent reference line. The derived data for each cross section from satellite images of years 1990, 1997 and 2008 were analyzed and the bank lines were also digitized for all the years. Intermediate channel widths, and total widths of channel at each predefined cross sections were measured using GIS software tools for computing Plan Form Indices for each cross sections for further analysis. The erosion depth in north and south banks of the river area during the study period was estimated by the GIS software tools through delineating the river bank lines and drawing polygons within bank line variations within the study period.
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Fig. 2
Delineation of River for 1990
Fig. 3 Delineation of River in 2007 with bank lines in 1990 Table 2 Reach number 1 2 3 4 5 6 7 8 9 10 11 12
Identification of reaches in respect of the location Locations in vicinity Dhubri Goalpara Palasbari Guwahati Morigaon (Near Mangaldai) Morigaon (Near Dhing) Tezpur U/s of Tezpur (Near Gohpur) Majuli (Near Bessamora) U/s of Majuli (Near Sibsagar) Dibrugarh U/s of Dibrugarh
4 Variation of braiding and bank erosion The computed Plan Form Index for each reference cross-section totaling 120 in numbers across the study reaches are plotted against reach cross-section number in Fig. 4 for the three discrete years. From the plot, it can be readily inferred that for the period 1990-2007, the PFI values decreased significantly indicating the increase in braiding intensities in majority of cross sections. The analysis can further be extended by computing mean PFI and extreme values that are maximum PFI (indicating least braiding within the reach) and minimum PFI (indicating highest braiding within the reach) values for Reach 1 to Reach 12 comprising ten cross-sections and each is computed and shown in Table 3. The corresponding plot for the Mean, Minimum and Maximum PFI against reach numbers are plotted and shown in Figs. 5(a)-(c) respectively. The mean PFI enveloping with maximum and minimum cross-sectional PFI suggests the ranges of variation in braiding intensities within a reach. It has been further elaborated for clarity in Figs. 6(a)-(c) respectively through trend of PFI with time for the discrete International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444
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years studied. It can be easily figured out that higher values are predominant in the year 1990, whereas in 2007 lower values are predominant. All three statistically measured PFIs are registering little changes or similar trend in three to four identified reaches with rock-outcrops numbered 2, 4, 6-7 and 9, which are in the vicinity of Jogighopa, Guwahati, Tezpur and Bessamora in Majuli. It strongly suggests that irrespective of the time, the aforementioned four discrete reaches show little changes in braiding intensity and pattern. It confirms the existence of the aforesaid four geological control points which hold the river, and in between there are intermittent fanning out of the river with time.
Fig. 4 Cross-section wise PFI values of the River in discrete years Table 3 Comparison of Plan Form Index (PFI) for the year 1990, 1997 and 2007 for the Brahmaputra River Reach number 1 2 3 4 5 6 7 8 9 10 11 12
Mean 22.69 15.39 10.55 55.97 14.19 28.34 31.94 19.12 10.14 13.61 12.38 20.95
PFI (1990) Min. 9.90 5.23 3.50 8.46 6.18 9.98 18.23 3.28 5.25 6.09 7.65 4.27
Max. 45.16 48.67 31.15 113.89 20.92 108.61 82.47 83.19 22.71 34.31 27.93 87.05
Mean 8.94 8.60 7.71 33.62 13.55 14.74 17.21 10.77 10.69 7.87 6.81 4.89
PFI (1997) Min. 13.22 3.50 2.83 8.94 4.52 7.73 7.48 4.39 3.64 5.25 3.36 2.76
Max. 4.37 21.09 16.22 124.48 38.39 30.46 37.27 32.43 48.37 10.65 12.23 7.57
Mean 15.66 20.30 9.99 73.64 8.08 7.78 18.50 6.89 6.34 6.54 5.41 2.61
PFI (2007) Min. 5.74 4.42 3.75 16.47 4.88 3.40 4.37 3.84 2.00 3.77 3.52 1.63
Max. 38.65 98.05 24.79 136.85 19.67 31.30 100.00 14.95 17.55 12.57 10.91 3.70
As discussed, the graphical plots of Plan Form Index for the Brahmaputra River shows a decreasing trend thereby registering an increasing level of braiding with time and space in view of the threshold limits, as described in Section 1.1. These plots clearly demonstrate the rationality of using the Plan Form Index as a measure of braiding and closely conform to the actual physical situation of the occurrence of braiding vividly depicted in the satellite images. In light of the threshold values of Plan Form Index, it can be readily inferred from graphical plots showing maximum, minimum and mean values of PFIs of cross sections that have heavy with moderate and low braiding characteristics resulting in a very complex channel hydrodynamics. Using the GIS software, vector layers of the bank line and river flow domain have been worked upon and results have been presented in Table 4, which accounts for the reach wise erosion area for periods - 436 -
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1990-2007 and 1997- 2007. Erosion lengths within the reaches have been identified based on the exhibited bank line shifts in the duration of study area. Similarly, erosion area is estimated approximately through area estimation using GIS software tools for polygon areas with the shifting bank-lines in study period.
Fig. 5 Variation in PFI with extreme values along the river
Fig. 6 Variation in PFI values along the river in discrete years
Figure 7 depicts the area eroded in left and right banks of the study reach number 1-11 of the Brahmaputra River. In Reach 2 near Goalpara where right bank shows considerable shifting with narrowing down to incised channel at Pandu near Guwahati in Reach 4. Plot of reach-wise land loss and erosion prone bank length (Fig. 7) indicates that downstream of Guwahati, river tends to move towards right side whereas in upstream side it tends towards south side keeping the control point at Guwahati invariant. Similar pattern also repeats at other control points. It warrants in-depth study of interaction of geo-tectonic activities conjunctively with fluvial regime in the region to understand the complex physical processes completely for suggesting more practical result oriented river management interventions. The satellite image based an estimation of area eroded in the Brahmaputra River during periods 1990-2007 and 1997-2007 is presented in Table 4, which shows the eroding tendency along the river banks of the Brahmaputra River in the entire study area. For the period of 18 years, the total land loss per year excluding forest area is found out to be 62 km2/year. Table 4 indicates high eroded area in left bank of exhibiting serious avulsion phenomenon during this period. During this period, in the upstream of the Dibrugarh, river practically changed its dominant path. For more recent period of 1997-2007, the total land loss per year (excluding avulsion) is found out to be 72.5 km2/year, registering sharp increase in land lost due to river erosion in recent years. International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444
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Fig. 7 Bank erosion along the river in discrete years Table 4 Land lost in Main Brahmaputra River during the study period North bank Reach Number
1 (Dhubri) 2 (Goalpara) 3 (Palasbari) 4 (Guwahati) 5 (MorigaonMangaldai) 6 (MorigaonDhing) 7 (Tezpur) 8 (TezpurGohpur) 9 (MajuliBessamora) 10 (MajuliSibsagar) 11 (Dibrugarh) 12 (U/s of Dibrugarh) Total
Minimum PFI values
South bank
Total erosion length (km) 40.19 39.5 54.87 21.02
Land loss area 1990-2007 (km2) 124.461 79.046 48.668 7.92
Land loss area 1997-2007 ( km2) 94.129 40.902 42.914 1.654
Total erosion length (km) 7.05 4.85 14.02 24.38
Land loss area 1990-2007 (km2) 194.983 17.816 23.006 5.385
Land loss area 1997-2007 (km2) 10.791 5.052 15.859 12.079
1997
2007
13.22 3.50 2.83 8.94
5.74 4.42 3.75 16.4
6
35.606
2.138
47.91
96.979
103.7
4.52
4.88
24.86
29.057
7.275
47.8
10.795
56.72
7.73
3.40
8.58
38.758
4.733
52.95
16.628
44.774
7.48
4.37
8.85
31.187
5.794
44.16
26.098
71.227
4.39
3.84
24.69
25.562
12.327
47.17
32.788
28.998
3.64
2.00
16.93
60.657
16.878
54.95
44.018
42.118
5.25
3.77
37.86
37.506
43.529
43.89
46.595
6.066
3.52
70.5
20.376
55.454
57.54
399.529
333.416
3.36 Forest area excluded southern side
353.85
538.805
327.726
389.13
914.62
730.8
Moreover, the vulnerability of the stream bank erosion is significant as one can see from Table 4. Almost 750 km of bank line in both sides of the river has potential erosion tendency. The plot of potential erodible bank length versus reach number (Fig. 7) also shows that downstream of Guwahati (Reach number 4), erosion tendency is considerably high in the north bank line. Whereas in the upstream of Guwahati erosion tendency is considerably high in south bank-line, indicating that geological control point at Guwahati in respect to other control points has significant causative impact on the morphological behaviour of the Brahmaputra River. The bank erosion as well as eroded bank length of Morigaon Reach between Tezpur and Guwahati is relatively high. The erosion pattern of the left and right banks of the Brahmaputra River during the study period is depicted in Fig. 8. The extent of bank erosion in different location can be qualitatively assessed from the presented map, with the bank-line extracted for discrete - 438 -
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years through GIS tools.
Fig. 8
Land Lost in the Brahmaputra River during the period 1990-2007
The apparent fallacious temporal variability of land losses indicated in Table 4 can be easily understood from a perusal of Fig. 9 wherein the actual shift of bank lines as extracted from the satellite images are depicted with corresponding bank erosion presented in Table 4 during the study period in the 5th river reach.
Fig. 9 Temporal migration of river bank-line in discrete years
5 Variation of unit stream power and its linkage with braiding and bank erosion The hydraulic parameters of an open channel flow such as hydraulic mean depth, wetted perimeter, flow area, bed gradient, discharge and the physical properties, Ȗ(=ȡg) of the fluid are related to express the unit stream power of the flow at a point along the direction of flow (Yang, 1976; Chang, 1979). The unit stream power (N/m-s) can be expressed as; J uQu S : (10) w
where Ȗ is the specific weight of water and w is the channel width in meter. It is emphasized that a right combination of these parameters would give rise to the requisite stream power (SP) to erode the riverbed and produce incised channels. The unit stream power of a flow quantifies the potential energy expenditure of the flow encapsulated in the mass of the fluid. The International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444
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magnitude of unit stream power is one of the deterministic parameters of erosion of bed and banks of a stream. The unit stream power is one of the most dominant factors to influence channel sediment transport capacity. More recently, Shih and Yang (2009) estimated overland flow erosion capacity using the unit stream power. It has been found relevant to correlate the temporal changes in effective width of the Brahmaputra River in different reaches and estimate unit stream power to explain the braiding processes. In general, channel planforms, floodplain segments separated by the latter and alluvial channel bed-forms are shown to be geomorphic expressions of sediment transport process at different spatial and temporal scales (Alekseevskiy et al., 2008). To appreciate the linkage of stream power with braiding, certain considerations are vital. Assessment of braiding index essentially depends upon the corresponding stages of the river at measured locations. Care has been taken to measure this parameter at low flow periods to adequately represent the evolved channels of the river in a particular instance of time; however, it is essential to correlate stage-discharge of the river with corresponding measured braiding parameter to achieve meaningful and rational analysis of the fluvial channel process. The satellite data based study detailed in the previous section becomes of much relevance and fetched very significant and meaningful outcomes when it is correlated to hydraulic and hydrological parameters such as stage-discharge and sediment transport rates for the corresponding periods. In this study, three most vulnerable sites along the river are chosen, in view of the satellite data based study presented in previous section. These are Dhubri-Goalpara Reach (Reach 1-2), Guwahati-Palasbari Reach (Reach 3-4) and Morigaon–Tezpur Reach (Reach 6-7) which showed considerable increase in braiding and bank erosion in recent years. Each selected reach comprised of 20 cross-sections. The sediment inflow rates in selected reaches have been showed in Figs. 10 and 11 in discrete years. It is observed that in the Guwahati-Palasbari Reach, sediment flow rate increased, and considerable decease occurred in downstream reaches at Dhubri-Goalpara. It indicates that sediment feed between Tezpur and Guwahati was more, while there was deposition in downstream of Guwahati due to low stream power (Figs. 12 and 13). The process is in agreement with the fact that bank erosion in Morigaon near Tezpur was dominantly high while braiding considerably increased in downstream of Guwahati at Palasbari due to bed deposition (Figs. 5 and 6). The lowered stream power in downstream reaches can be clearly observed from Figs. 12 and 13.
Fig. 10 Temporal variation of sediment inflow in selected reaches
Variability of stream power at bankful and low flow periods with estimated braiding index values presented for selected river reaches namely Dhubri-Goalpara (Fig. 14), Guwahati-Palasbari (Fig. 15) and Tezpur- Morigaon (Fig. 16) suggest that wherever PFI goes up, the corresponding stream power becomes lowered indicating lesser potential energy expenditure per unit mass of water. This process triggers local river bed rise and formation of additional multiple channels. While the number of channels increases, concentration of flow becomes usual phenomenon along the bank-lines, which again instigates bank - 440 -
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erosion and thus further increases the sediment load into the river making river further prone to braiding. The process becomes complex as it hinders the incising of the channel at high stream power also as bank erosion overloads the sediment to the river creating a sediment transport rate greater than sediment carrying capacity of the channel.
Fig. 11
Fig. 12
Reach wise sediment flow in discrete years
Variation of stream power at bankful discharge during latest flood period
Fig. 13
Variation of stream power at low flow period
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Fig. 14
Variability of stream power with PFI index value at bankful and low flow period for Dhubri-Goalpara site
Fig. 15 Variability of stream power with PFI values during bankful and low flow period for Guwahati-Palasbari site
With the onset of the flood season, the volume of sediment transported in the Brahmaputra River increases, the thalweg starts to change position and the geometry and location of bars changes. As the flow begins to recede, deposition over the bed takes place in the form of bars and islands. The river then flows in several sinuous channels in between these sand bars. With the onset of the next flood season, the sequence of events is repeated. - 442 -
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Fig. 16 Variability of stream power with PFI values during bankful and low flow period for Tezpur-Morigaon site
The formation of bars and islands is so complex that an understanding the fluvial landform pattern is often difficult. The highly braided nature of the Brahmaputra River has led to its unpredictability, leading to many channels being active all at once. The process is in close conformity with the findings by Leopold and Wolman (1957) on flume experiments, which suggest that a bar of coarse sand diverts flow to cause channel widening, which then accentuates bar development, resulting in braided channel pattern. The braided pattern in an alluvial stream develops after local deposition of coarse material, which cannot be transported under local conditions of flow existing within the reach. This coarse material becomes the nucleus for a bar formation, and subsequently grows into an island made up of coarse as well as fine material. The formation of the bar deflects the main stream towards the banks and may cause bank erosion. Sediment transport takes place over the bar surface while incision in the lateral channels lowers the water surface to expose the bar which then becomes dissected. The complex of islands is stabilized by vegetation in natural streams and experiences further high stage sedimentation. However, the variability of flow behavior and channel pattern of Brahmaputra River is obvious in different reaches as we look at three selected reaches. The observation is in agreement with the findings of Wang et al. (2010) who conducted a numerical simulation of channel pattern changes and observed that the upper and lower parts of the same channel may have different planforms because the sediment transport conditions of the two parts differ greatly. The geological control point as discussed earlier at Guwahati, Jogighopa, Bessamora and Tezpur are due to obvious rock outcrops otherwise the tendency of the river on the rest stretches is highly dynamic, with this being an example of fluvial hydrodynamics with complex morphology. 6 Conclusions Based on the present study, the following conclusions can be inferred. 1) The braiding indicator, namely Plan Form Index (PFI), with high resolution remote sensing satellite data is found useful to analyze and monitor the complex braiding behaviour of a large river like the Brahmaputra River. 2) The study identifies three to four major geological channel control points present along the Brahmaputra River in Assam flood plains. These control points are located in the vicinity of Jogighopa near Goalpara, Pandu near Guwahati, Tezpur and Bessemora in Majuli. These channel control points International Journal of Sediment Research, Vol. 26, No. 4, 2011, pp. 431–444
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usually have well defined and stable hydrographical profiles. Intermittent fanning out, behaviour is displayed between these control points which are being temporally severed in the geological time scale. 3) The variability of stream power with bank erosion and braiding processes is investigated and a distinct behavioural pattern between these processes is observed. For example, with a low stream power, braiding appears to intensify which in turn may indicate a higher possibility of bank erosion. Acknowledgements The present study is sponsored and funded by National Disaster Management Authority of India (NDMA, Govt. of India), New Delhi, which is gratefully acknowledged here. References Alekseevskiy N. I., Berkovich K. M., and Chalov R. S. 2008, Erosion, sediment transportation and accumulation in rivers. International Journal of Sediment Research, Vol. 23, No. 2, pp. 93–105. Chang H. H. 1979, Stream power and river channel patterns. ASCE Journal of Hydrologic Engineering, Vol. 41, pp. 303–311. Ferguson R. I. 1981, Channel form and channel changes, J. Lewin (Ed), In British rivers, pp. 90–125 (London: Allen and Unwin). Henderson F. M. 1961, Stability of alluvial channels. Journal of Hydraulic Division, American Society for Civil Engineering, Vol. 87, pp. 109–138. Holden C. 2003, Historic island threatened, random samples. Science, Vol. 300, p.1368. Kotoky P., Bezbaruah D., Baruah J., and Sarma J. N. 2003, Erosion activity on Majuli – the largest river island of the world. Current science, Vol. 84, pp. 929–932. Kotoky P., Bezbaruah D., Baruah J., and Sarma J. N. 2005, Nature of bank erosion along the Brahmaputra River channel, Assam, India. Current science, Vol. 88, No. 4, pp. 634–640. Lane E. W. 1957, A study of the shape of channels formed by the natural streams flowing in erodible material, U.S Army Crops Engg. Div., Missouri River, M.R.D. Sediment, Ser. No. 9, p. 106. Leopold L. B. and Wolman M. G. 1957, River channel patterns: Braided, meandering and straight, United States, Geological Survey Professional Paper 282-B, pp. 35–85. Mudur G. S. 2003, Famous island set to go under, The Telegraph, 1May 2003. Parker G. 1976, On the cause and characteristics scale of meandering and braided in rivers. Journal of Fluid Mechanics, Vol. 76, pp. 459–80. Richards K. 1982, Rivers: forms and process in alluvial channels, London: Methuen and Co. Ltd.. Sankhua R. N., Sharma N., Garg P. K., and Pandey A. D. 2005, Use of remote sensing and ANN in assessment of erosion activities in Majuli, the world's largest river island, International Journal of Remote Sensing, 26:20, pp. 4445–4454. Sarma J. N. 2002, A Study on pattern of erosion and bankline migration of the River Brahmaputra River in Assam using GIS, Report Disaster Management in North-Eastern region, Dept. of Revenue, Govt. of Assam, pp. 50–53. Schumm S. A. and Khan H. R. 1972, Experimental study of channel patterns. Bull. of Geological Society of America, Vol. 83, pp. 1755–1770. Sharma Nayan 2004, Mathematical Modelling and Braid Indicators, In The Brahmaputra River Basin Water Resources, V.P.Singh (Ed.), Dordrecht: Kluwer Academic Publishers, Vol. 47, pp. 229–260. Shih Hui-Ming and Yang Chih Ted 2009, Estimating overland flow erosion capacity using unit stream power. International Journal of Sediment Research, Vol. 24, No.1, pp. 46–62. Wang Hong, Zhou Gang, and Shao Xuejun 2010, Numerical simulation of channel pattern changes. Part II: Application in a conceptual channel, International Journal of Sediment Research, Vol. 25 No. 4, pp. 366–379. Yang C. T. 1976, Minimum Unit stream power and fluvial hydraulics. ASCE Journal of Hydraulic Division, Vol. 102, No. HY7, pp. 919–933.
Notation g = gravity acceleration; GIS = geographic Information System; IRS = Indian Remote Sensing; LANDSAT = Land Remote-Sensing Satellite; LISS = Linear Imaging Self Scanner; PFI = Plan Form Index; UTM = Universal Transverse Mercator; WGS 84 = World Geodetic System 84. - 444 -
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