Geomorphological Manifestations of the Flood Hazard: A Remote Sensing Based Approach

Transcription

1 Geomorphological Manifestations of the Flood Hazard: A Remote Sensing Based Approach Vikrant Jain and R. Sinha Engineering Geosciences Group, Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur , India rsinha@iitk.ac.in Abstract Flood hazard is one of the most severe problems in the Himalayan river basins. Although floods are essentially hydrological phenomenon, the uneven distribution of floods in the river basin highlights the control of geomorphological and geological factors. A proper understanding of these factors is critical for a successful flood management programme. Remote sensing data is of immense value in evaluating the geomorphological and geological controls in flooding. The present paper highlights the control of geomorphology and neotectonics on flood hazard in north Bihar Plains, eastern India. The Indian Remote Sensing (IRS) data has been used and a variety of image processing techniques have been employed. Introduction The Indo-Gangetic Plains are drained by some of the largest river systems in the world. These plains are also severely affected by frequently occurring disastrous floods and are presently regarded as the worst flood affected region in the world (Agarwal and Narain, 1996). Almost every year, monsoon floods in the Indo-Gangetic Plains cause countless misery to the inhabitants living on the floodplains. The plains of north Bihar in eastern India have recorded the highest number of floods in India in the last 30 years (Kale, 1997). In the last 5 decades, the flood management programs on the rivers of Indo-Gangetic Plains have largely failed. The available data suggest that during , more than 2700 billions of rupees were spent on the flood control measures in India, but the annual flood damage increased nearly 40 times and annual flood affected area increased 1.5 times in this period (Agarwal and Narain, 1996). These data emphasize the need for a better understanding of the flood hazard in these plains. The advent of satellite remote sensing data has facilitated a fuller appreciation of geomorphological and geological factors of floods apart from hydrological understanding available from meteorological and hydrological data. This paper demonstrates the use of satellite remote sensing data for assessment of flood hazard in the Baghmati river basin of north Bihar plains in eastern India and, coupled with hydrological data and field observations, provides some understanding of causative factors of flooding. draining the interfan area between the Gandak and Kosi megafans in north Bihar alluvial plains, eastern India (Fig. 1). The total basin area of the river system is about 8848 km 2. Flooding in the Baghmati River basin is a regular Study Area Description The Baghmati River is a foothills-fed river (Sinha & Friend, 1994) originating in Kathmandu area in Nepal and Figure 1 Location map of the Baghmati River basin alongwith major sub-surface faults (after GSI, 2000). Geocarto International, Vol. 18, No. 4, December 2003 Published by Geocarto International Centre, G.P.O. Box 4122, Hong Kong. geocarto@geocarto.com Website: 51

2 phenomenon, inundating about 2370 km 2 of area in the plains (GFCC, 1991), and is responsible for extensive damage to lives and property every year. Annual variation in flood hazard is quite variable and unpredictable; however 5-yr average flood shows a sharp increase from 1969 to 1973 (Fig. 2). Further, it is interesting to note that the flood affected regions are not uniformly distributed over the entire basin bur are localized to certain pockets (GFCC, 1991), which points to the control of geomorphological factors on flood hazard. On the basis of the gravity data, seismic surveys and deep drilling data from, some basement faults transverse to the Himalayan faults have been demarcated in the Gangetic Plains by various workers (Sastri, et al. 1971; Rao, 1973; Karunakaran and Rao, 1976; Valdiya, 1976; Agrawal, 1977; Dasgupta, et al., 1987; Dasgupta, 1993, GSI, 2000). In the north Bihar region, the important faults are Sitamarhi Fault (SF), West Patna Fault (WPF), East Patna Fault (EPF) and the Monghyr-Saharsa Ridge Fault (MSRF) (Fig. 1). The occurrence of recent earthquakes (1934, 1988) in this region demonstrates that these faults are active and responsible for neotectonic activity in the basin (Banghar, 1991). Several remote sensing, hydrological and channel morphological evidences in the Baghmati River basin also suggest that this basin is affected by neotectonic activity (Jain, 2001). A number of NE-SW and NW-SE trending faults have been mapped in the Baghmati basin (GSI, 2000) (Fig.1). In the upstream region, the area around Sheohar and Sitamarhi is uplifting, but due to difference in the vertical movements along the SF and WPF, this area is tilting in south-east direction. In the midstream region, the area in between the WPF and EPF is tilting in NW direction due to uplift along EPF but subsidence along WPF. In the downstream region, the area in between EPF and MSRF falls in a graben area i.e. it is subsiding. Vertical movements along these faults have significantly affected the fluvial processes in general and flood hazard in particular (discussed later). Data used and approach Hydrological data for two stations namely, Dhengbridge (u/s) and Hayaghat (d/s) along the Baghmati River (see Fig. 1 for location) are available from Center Water Commission (CWC) and Ganga Flood Control Commission (GFCC), Government of India. Discharge data is available for 20 years ( ) for upstream station and for 34 years ( ) for downstream station, whereas the sediment load data is available for a period of 11 years ( ) for upstream station and 29 years ( ) for downstream station. Geomorphological study has been carried out with the help of remote sensing data of IRS LISS II for 1989 and 2000 period (22-49B1, 22-49B2, 21-49A1, 21-49A2), topographic sheets pertaining to the Baghmati River of 1:250,000 scale (72E, 72F, 72G and 72K) 1924 and 1986 and the corresponding toposheets of 1:50,000 scale. The major emphasis of the remote sensing data analysis has been to extract geomorphologic information through various image-processing techniques. Flood-prone windows in the Baghmati River basin lie in different satellite images acquired during different passes of the satellite. As a pre-processing step, these images were normalized through minimizing radiometric-striping effects (Malleswaran Rao and Figure 2 Flood damage trend in Baghmati River basin (a) Area affected due to flooding (b) population affected due to flooding, (c) total flood damage in the basin area. Even after implementation of several remedial measures, the flood problem in the Baghmati River basin has not reduced in comparison to 1960s (Source: Ganga Flood Control Commission, Patna). 52

3 Dookahatala, 1983). The right image was normalized with respect to the left image using the equation, g 0 = g 0 σ σ r + (m l - m r σ σ r ) (1) where g 0 - normalized right image pixel grey value g 0 = the right image input pixel grey value σ l - standard deviation of the entire left image σ r - standard deviation of the entire right image m l - mean value of entire left image m r - mean value of the entire right image For normalization, the upper-left scene (22-49B1) was taken as the base image for both 1989 and 2000 data and all the other images were normalized with respect to this image. After normalization, the images were registered with respect to the 1:250,000 scale toposheets of 1986 and were glued to get a nearly complete basin of the Baghmati river (Fig. 3). The present study has made use of band 4 (infrared) image, standard False Colour Composite (FCC) and FCC generated from Principal Component (PC) bands. The standard (FCC) was prepared by assigning band 2, 3 and 4 images to blue, green and red respectively and using linear with saturation (2.5%) stretching. The PCA images of Baghmati River basin were generated using the multi spectral band 1, 2, 3 and 4 and a composite of PC-images was prepared by assigning the PC-1, PC-2 and PC-3 on red, green and blue respectively. This PCA colour composite provides a better visualization of drainage network, abandoned channels and high moisture area. The alluvial plains of the Baghmati River have very low relief and gentle slope. Therefore, the topographic contours are not available on the toposheets of this region. A Digital Elevation Model (DEM) was prepared for the Baghmati basin using 109 elevation points; following weighted-average technique. The height of each cell was determined by the average of 6 nearest control points of known height. The accuracy analysis was carried out with elevation data from 15 additional points and the root mean square error was obtained as 1.34 m. For a total height difference of 46 meters (from 94 meters to 48 meters) in the basin area, it gives an error of 3%. The DEM of the Baghmati River basin after density slicing is shown in the Figure 4, which clearly depicts the changes in basin slope from upstream to downstream. In general, the upstream basin area shows a regional slope in S direction with relatively steep gradient, whereas downstream basin area is characterised by SE slope. The overlaying of the Baghmati stream on the DEM indicates the relationship between slope and the river flow direction. Barring a few exceptions, the channel follows the basin topography. Channel Morphology and Hydrology The Baghmati basin falls in the interfan area bounded by the Gandak megafan in west and Kosi megafan in east. This interfan area is characterised by slightly concave at the edges and gentler floodplain gradient (10 cm/yr) (Geddes, 1960). The channel slope is also gentle in the alluvial plain area and ranges from 53 cm/km to 11 cm/km from Dhengbridge to Hayaghat (GFCC, 1991). Channel shifting is an important geomorphological process in the basin area (Sinha, 1996) and detailed analysis of fluvial dynamics of the Baghmati River using multi-date topographic sheets and satellite images has indicated several major and minor avulsion events on a decadal scale (Jain & Sinha, 2003a, in press). A summary of channel migration history for the period shown in Figure 5 suggests eastward shifting of the river, which has also shifted the flood hazard in the river basin from west to east. However, the enormity of flood is not same all along the present course of the river and only few localities are severely affected. Table 1 presents a summary of bankfull discharge, most probable flood, mean annual flood and 50-yr return period flood for the Baghmati River based on flood frequency analysis. Higher values of the most probable flood and the mean probable flood than the bankfull discharge indicate Table 1 Discharge characteristics of Baghmati River Station Probability Bankfull Maximum Most Mean 50-yr return Distribution discharge Observed Discharge probable flood Annual flood period flood (T=1.58 yrs) (T=2.33 yrs) (m 3 /sec) (m 3 /sec) (m 3 /sec) (m 3 /sec) (m 3 /sec) Dhengbridge Gumbel s Distribution Log Pearson III Distribution Log-Normal Distribution Hayaghat Gumbel s Distribution Log Pearson III Distribution Log-Normal Distribution 53

4 that the river is flooded almost every year. Further, the peak discharge of Baghmati River is quite variable and unpredictable and ranges from 600 m 3 /sec to 3300 m 3 /sec (Sinha and Jain, 1998). The Baghmati River also carries a significant sediment load, which varies from 10.4 million tonnes/yr at upstream station (Dhengbridge) to 7.21 million tonnes/yr at downstream station (Hayaghat) (Table 2). Most of the coarser sediment load comes from upstream hilly catchment area during the monsoon period but a major fraction of the wash load is picked up by the river in the plains itself (Jain, 2001). Due to flat gradient and less stream power (Table 2), extensive deposition occurs in the alluvial plains area. The variation in total sediment load between upstream and downstream stations indicates deposition of large amount of sediment load ( 3 mt/yr) in the midstream basin area. Window-wise Study of Flood Prone Area Major flood-prone windows in the Baghmati river basin include the areas around Bairgania, Sheohar, Katra, and Benibad to Hayaghat (all marked on the satellite image, Fig. 3) and Badlaghat (outside the view of the satellite image, shown in Fig. 1). These windows have been selected for detailed study following the reports of the Ganga Flood Control Commission (GFCC, 1991), our hydrological modeling (Jain and Sinha, 2003 b) and repeated field visits. A detailed description of flooding behaviour in these windows and our understanding of the causative factors are presented next. Window 1: Area Upstream of Bairgania Town The left bank of the Lalbakeya River and the right bank of the Baghmati River in the Indian territory spill frequently during monsoon season upto Bairgania town (Fig. 6a), even though both these rivers are embanked all along. Hydrological data of the Baghmati River suggests that at Dhengbridge, the mean annual flood discharge (1473 m 3 /sec) is higher than the bankfull discharge (1100 m 3 /sec), which clearly explains the frequent overspilling in the region. The satellite image of Figure 3 The Baghmati river basin as seen on satellite image (IRS LISS II, band 4) of February Boxed area are flood-prone windows (see text for details). Figure 4 Digital Elevation Model (DEM) of the Baghmati River basin prepared from point elevation data recorded from the toposheets. The relief of this alluvial plain of Baghmati River basin varies from 48 m to 94 m and the regional slope is in the S (in upstream region) to SE (in downstream region) direction. Each height range shown in the legend is further sub-divided into 8 sub-classes with an interval of 2 m and is shown as different shades. 54

5 Figure 5 Table 2 Sediment transport characteristics of the Baghmati River Station Bankfull Channel Stream Annual Sediment discharge Slope Power Sediment load yield (m 3 /sec.) (cm/km) (Wm -2 ) (MT/yr) (T/km2/yr) Dhengbridge Runisaidpur Benibad Hayaghat Summary of channel migration history for the last 76 years ( ); an eastward trend is apparent. this area shows that the Bhakuwa Nala is joining the Lalbakeya River to its right bank and the Manusmaran River is joining the Baghmati River to its left bank upstream of Bairgania. Addition of discharge and sediment load from these tributaries is a major reason of overspilling in this region. A spill channel is also observed, which is originating from the Lalbakeya River and after flowing near the Bairgania town joining the Baghmati River in downstream reaches. The satellite image of this window draped over DEM (Fig. 6b) illustrates the topographic influence on flooding. The area in between the Baghmati River and Lalbakeya River is clearly low-lying and this induces overspilling along the left bank of Lalbakeya River and right bank of Baghmati River. The Bairgania town lying in the Lalbakeya-Baghmati interfluve region is therefore flooded frequently. Window 2: Area in and around Sheohar District The area around Sheohar district falls within the anabranched reach of the Baghmati river (Fig. 1) and is known for extensive flooding almost every year. The standard FCC of the area (Fig. 7a) shows white coloured patches of channel sand vouching for extensive overbank spilling in the area. Presently, the right anabranch of the channel lies abandoned (Fig. 7b) due to silting in the upstream region, but is activated during the monsoon period. Silting in the channel has reduced the bankfull channel capacity, and therefore, this channel is unable to carry increased discharges during the monsoon. Further, the drape of the satellite image over DEM (Fig. 7c) clearly depicts that the floodplain slope is towards SE, which accentuates the flooding effect in the area around Sheohar. The FCC of the principal component bands 3, 2, and 1 shows several other abandoned channels as well (in blue) (Fig. 7b). Most of these abandoned channels overspill during the monsoon period and flood the adjoining areas, marked by sandy patches on the satellite image. A photograph of this area taken in September 1999 shows the expanse of standing flood water (Fig. 7d) more than a month later than peak monsoon. Window 3: Area in and around Katra Town This area in between the Lakhandei and Baghmati rivers, shown in Fig. 8a, is one of the worst flood affected regions in the Baghmati basin. The Baghmati and Lakhandei rivers flow parallel to each other for almost 13 kms separated by only 2-3 kms before finally meeting at Katra. During monsoon season both the rivers carry very high discharges and overspill both the banks. Figure 8a is the FCC prepared from principal component bands and clearly marks the high moisture zone around Katra (in red) and yellow patches of fresh sand interpreted as crevasse deposits bordering the main channel. From a site immediately downstream of Katra (Nawada village), we have reported about 1m thick splay deposit, about 200 m long and 100 m wide, invading an orchard formed in one single flood of September 1999 (Sinha et al, in press). Further, field observations indicate that the Lakhandei channel is significantly narrow and entrenched near Katra (Fig. 8b) 55

6 Low-lying area Spill channel Figure 6 Bairgania Dhengbridge Window-1: Area around Bairgania (a) Stretched band 4 data, (b) Band 4 image draped over the DEM; note the low relief and tributary influence upstream of Bairgania. reflecting a very low bankfull capacity. It has also been observed that the Lakhandei river receives a significant amount of backflow from the Baghmati river upstream of the confluence at Katra, the flood level of the Baghmati being higher, and this aggravates the flooding in the Lakhandei. Window 4: Area from Benibad to Hayaghat The reach from Benibad to Hayaghat is another floodprone area in the Baghmati basin. The flood problem is particularly acute near Hayaghat area, which remains inundated for more than two months in a year. People living in this area leave their homes during floods and return after the flood recedes. Fig. 9a shows three tributaries namely Siari River, Baghmati Nadi and Baghmati Nala joining the Baghmati River at downstream of Benibad. Besides these tributaries, the Adhwara (b) River system also joins the Baghmati River upstream of Hayaghat. The channel plan form as seen on the satellite image is highly sinuous with very sharp bends making the channel prone to crevassing during high flows. The wide areas upstream of confluence points show clear water logging (in blue) and high moisture zones appear as red patches on the principal components FCC (Fig. 9a). The effect of tributaries is clearly reflected in peak discharge data, which shows 3-4 times increase from Benibad ( m 3 /sec) to Hayaghat ( m 3 /sec). Further, the Baghmati river channel has a very low gradient (11 cm/km) in this reach (GFCC, 1991), which is interpreted as a neotectonic effect. The East Patna Fault (EPF) passes just south of Hayaghat with the upthrown block in the NW direction (Fig. 1). This uplift has reduced the channel gradient, making it prone to flooding and causes several crevasse channels in the region (Fig. 9b). Our systematic analysis of maps and satellite images has also demonstrated that the confluence points of rivers in this window have changed several times during the last ~ 76 years (Jain and Sinha, in press), mostly triggered by flood events. Window 5: Area around Badlaghat The area around Badlaghat lies in the downstream reaches of the Baghmati river, close to its confluence with the Kosi River. Besides extensive flooding, this region is severely affected by water logging problem. The available satellite imagery does not cover this area and therefore this window has been analysed through available maps and field visits. The downstream reach of Baghmati River is characterized by several low-lying areas (chaurs), ponds and some isolated channels, which act as the large lakes, e.g. Khabar Tal, Goal Dhanaliya Jhil and several marshy lands (Fig. 10). Several factors are responsible for flooding around Badlaghat area. The most important factor is the structural control in this part of the river basin. The area lies in a graben bounded by East Patna Fault (EPF) in the west and normal fault along Monghyr-Saharsa Ridge Fault (MSRF) in the east. Because of these faults, this area is subsiding and hence the river gradient is very low near the confluence of Kosi River. The occurrence of ponds/tals and low-lying areas are surface manifestation of the subsidence in this region. Further, the base level of the Baghmati River is controlled by the water level of the Kosi River, which does not allow the Baghmati River to erode its riverbed and hence the pace with the subsidence can not be maintained, which causes extensive flooding in the area during monsoon season. This subsidence in this block causes river-damming effect, which in turn causes widespread flooding in this region. Even the Kosi River backflows in the Baghmati channel for a considerable length during monsoon season (GFCC, 1991). The area around Badlaghat is further influenced by embankments in the upstream reaches of the Baghmati River. The Baghmati River is embanked from Hayaghat to 56

7 Figure 7 Window-2: Area in and around Sheohar (a) Standard FCC prepared from Feb image (b) FCC of Principal Component bands 3,2 and 1 coded on RGB prepared from image of Feb. 2000; several abandoned channels (marked in blue colour) in the region are observed (c) DEM of the area draped with standard FCC; the SE direction of floodplain slope is responsible for overbank flooding around Sheohar area from the S flowing channel (d) Photograph of the area in September1999; it is showing the effect of flood even after one month of the monsoon season. Phuhiya at its left and upto Badlaghat at its right bank. Thus, the area around Badlaghat receives the concentrated discharge from the upstream area, which causes extensive overspilling in the region. Discussion Hydrological response of a basin is governed by the basin geomorphology and hence, an integrated approach to flood studies should involve the geomorphological understanding of the river basin. Floods have long been considered as purely hydrological phenomenon, and therefore, flood management programmes have essentially focused on hydrological variations. This has been one of the major reasons for the failure of flood management efforts across the globe including India. Historical data reveal that even after continuous efforts to control the floods, flood damages and flood-affected areas in India have increased with time (Agarwal and Narain, 1996). At present, flood is one of the most disastrous natural hazards in India and especially in north Bihar Plains. The failure of flood control measures in the country suggests an urgent need of an integrated flood analysis including the hydrological, geomorphological and geological understanding of the river basin. The UNDP flood policy study also called for greatly increased research on river morphology, river training, mathematical modeling, and land and water management (Brammer, 1990). Our integrated approach to understand the flood hazard has followed two levels of analyses: (a) understanding of hydrology and channel morphology with reference to overbank flooding, and (b) incorporating effects of basin geomorphology and neotectonics on fluvial processes in general and flooding in particular. In the Baghmati basin, hydrological characteristics such as variable peak discharge, lower bankfull capacity than the mean annual flood discharge, high sediment load and extensive sediment deposition due to decrease in stream power are the key factors which make the basin area prone to flood hazard. Furthermore, the dynamic behaviour of the Baghmati river system, which often renders the flood management programmes ineffective in many instances, has been identified as a long-term causative factor of flooding (Jain and Sinha, 2000). Remote sensing data, due to their synoptic view and temporal resolutions have been demonstrated to be effective means to understand flooding behaviour and extent, in conjunction with topographic analysis such as DEM, and supported by ground visits. The eastward shifting of the Baghmati river (Fig. 4) has often made the flood remedial measures useless along the old channels and new areas prone to flooding have developed along the new course. Further, the avulsion of an old channel into a new one causes a sudden increase in discharge in the newly formed channel with a low bankfull capacity causing extensive overbank flooding. Since people are normally not prepared for flooding along this newly formed channel, the damage due to flood is quite severe in such cases. Flood damage trend (Fig. 2) shows a 16 times increase in the population affected by flood hazard after 1969 avulsion. Therefore, the understanding of avulsion mechanism will 57

8 Figure 8 Window-3: Area around Katra (a) FCC of Principal Component bands 3,2 and 1 coded on RGB prepared from image of Feb. 2000; note large flood-prone areas around Katra marked by water logged areas (blue) and high moisture area (red); large areas of crevasse deposits (yellow) are also seen upstream of Katra (b) photograph showing the entrenched, narrow (low w/d) channel of Lakhandei River; during flood season the water level rises upto the road level (seen on the upper right corner of the photograph). Figure 9 Window - 4: Area from Benibad to Hayaghat (a) FCC of Principal Component bands 3,2 and 1 coded on RGB prepared from image of Feb. 2000; note a large number of tributaries joining between Benibad and Hayaghat and the angular drainage pattern. Area upstream of confluence points show water-logging and high moisture zones (b) A small crevasse channel near Benibad damaging the road during flood in September have an important bearing on flood studies and management. Further, basin-scale geomorphologic and neotectonic studies aided by remote sensing data and coupled with DEM analysis have provided insight towards long-term causative factors of flooding to supplement the understanding of local factors through hydrological analysis. Our studies indicate that the factors such as tributary influences, topographic variations resulting through neotectonic movements, and tectonically-triggered avulsions are important enough to be considered for any flood management program in the region. We now turn our attention to flood control measures in the Baghmati river basin. Out of a range of flood control measures available such as embankments, reservoir dams, watershed management through afforestation in the basin area, channel improvement, upstream storage, detention basins within flood plains, artificial drawdown of groundwater to absorb excess monsoon rainfall and floodwater, diversion of flood water in an abandoned channel or in a canal and small scale irrigation strategies, the embankments have been the most popular flood management strategy in India for several decades (Sinha, 1998). In recent years, large lengths (3,400 km.) of embankments have been constructed in Bihar particularly after the devastating floods of 1954 (Agarwal and Narain, 1996). Even after the construction of embankments along all the north Bihar rivers, the inundation by spilling still continues in most parts through the gaps in the embankments and breaching of embankments (GFCC, 1991). In the Baghmati River basin, the flood control measures were initiated from Since then, 466 kilometers of embankments have been constructed/under construction. Initially, embankments in the downstream reaches worked effectively. However, after the construction of embankments in upstream region, the flood peak in the Baghmati River downstream of Hayaghat increased. Hence, the embankments along Baghmati River just downstream of Hayaghat were frequently breached and did not help much in quick drainage. In this way, the artificial embankments have merely transferred the trouble from one place to another and have given a false security to the people living in the area (Sinha, 1998). Moreover, these embankments interfere with natural fluvial processes of the rivers. In areas where the land is protected from spilling, water logging and salinity problems have developed. The drainage congestion is 58

9 Figure 10 Flood affected area-5: Area around Badlaghat; this region falls in a graben area bounded by East Patna Fault and Monghyr-Saharsa Ridge (see Fig. 1). Presence of several muddy areas and lakes in the region also indicate the subsidence in the region. During monsoon season, this area receives concentrated flow of the Baghmati River from embanked upstream channels and backflow from the Kosi River. a major problem in the downstream part of the Baghmati River basin particularly in the low-lying areas, locally called chaurs. The embankment strategy has lately been questioned at the international level citing the failure in Mississippi and three major Chinese rivers (Rogers et al., 1989; Shu and Finlayson, 1993) and alternative methods such as small scale irrigation strategies are now favored flood control measures in many flood-prone countries such as Bangladesh (Brammer, 1990). In the Baghmati basin as well some alternative methods besides embankments, have been considered by GFCC (1991) e.g. construction of dam in the upstream reach flood regulator at Belwa village, channel improvement, watershed management and underground storage reservoir. Computations show that there will be no appreciable flood moderation in the middle and lower reaches of the river even after the construction of dam and there will be very little effect ( m) on the stage at Hayaghat (GFCC, 1991). Further, the large amount of silt load from the upstream basin area will also reduce the age of the proposed dam. Based on our study, some suggestions for improving the flood management programs can be made. (a) High sediment load is a major problem in Baghmati River and has been cited as a major cause of avulsion and flooding due to rapid channel aggradation and reduction in bankfull capacity. Our analysis suggests that a major proportion of the sediment load comes from the upstream basin area. An extensive afforestation work in the upstream basin area will help to reduce the sediment load in the Baghmati River (b) Small check dams at the outlet of different tributaries and in the different part of sub-basin area will reduce the influence of tributaries and hence will reduce the flooding problem at the confluence point of the channel with main river. Because of their small size, these check dams will be environment friendly and also costeffective. Further, as most of the sediment load comes from the upstream basin area, and not from the downstream tributaries, these check dams will not be affected by the high silt load in the main Baghmati River. (c) The proposal for diverting the floodwater at Belwa village to the south flowing anabranch of the Baghmati river needs closer scrutiny keeping in view the neotectonic situation in the region. The south flowing channel crosses the Sitamarhi Fault at Sheohar and uplift of the NE block will continuously decrease the channel gradient. Therefore, regular channel maintenance may be required to maintain the flow. Further, the eastward trend of channel migration will require continuous monitoring of the left bank for crevassing. Conclusions An integrated approach employing remote sensing data coupled with DEM, hydrological study and field observations is recommended to understand the causative factors of flooding using the example of the Baghmati river basin in eastern India. The study has shown that the tributary influence, topographic control, abandonment of channels and neotectonics in the basin area are the major reasons for the 59

10 flooding in the study area. The remedial measures such as small check dams and afforestation work in the upstream basin area may be more effective in reducing the flood hazard in the region. Acknowledgement The work presented in this paper is a part of the doctoral thesis of the senior author (VJ) at the Indian Institute of Technology Kanpur and the Institute Fellowship for the same is thankfully acknowledged. The Central Water Commission and the Ganga Flood Control Commission, both Government of India organizations, are thanked for providing the hydrological data. VJ was supported through a sponsored project from Dalhousie University, Canada and Research Associate Fellowship from CSIR, India when this paper was written. We thank Prof. Martin Gibling for the financial assistance as well as his valuable suggestions. References Agarwal, A. and Narain, S. (1996) Floods, Floodplains and Environmental Myths. 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