Geomorphological Manifestations of the Flood Hazard A Remote

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					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 - 208 016, India
E-mail: 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                                                              draining the interfan area between the Gandak and Kosi
                                                                          megafans in north Bihar alluvial plains, eastern India (Fig.
   The Indo-Gangetic Plains are drained by some of the                    1). The total basin area of the river system is about 8848 km2.
largest river systems in the world. These plains are also                 Flooding in the Baghmati River basin is a regular
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 1954-1990, 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.

Study Area Description

   The Baghmati River is a foothills-fed river (Sinha &                   Figure 1 Location map of the Baghmati River basin alongwith major
Friend, 1994) originating in Kathmandu area in Nepal and                           sub-surface faults (after GSI, 2000) .


Geocarto International, Vol. 18, No. 4, December 2003                     E-mail: geocarto@geocarto.com                                 51
Published by Geocarto International Centre, G.P.O. Box 4122, Hong Kong.   Website: http://www.geocarto.com
phenomenon, inundating about 2370 km2 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 (1970-1989) for
upstream station and for 34 years (1956-89) for downstream station,
whereas the sediment load data is available for a period of 11 years
(1979-1989) for upstream station and 29 years (1961-1989) 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          Figure 2 Flood damage trend in Baghmati River basin (a) Area
various image-processing techniques.                                              affected due to flooding (b) population affected due to
   Flood-prone windows in the Baghmati River basin lie in different               flooding, (c) total flood damage in the basin area. Even
                                                                                  after implementation of several remedial measures, the
satellite images acquired during different passes of the satellite. As            flood problem in the Baghmati River basin has not
a pre-processing step, these images were normalized through                       reduced in comparison to 1960s (Source: Ganga Flood
minimizing radiometric-striping effects (Malleswaran Rao and                      Control Commission, Patna).

52
Dookahatala, 1983). The right image was normalized with                   additional points and the root mean square error was obtained
respect to the left image using the equation,                             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%.
   g0′ = g0 ⋅ σ √σr + (ml - mr ⋅ σ √σr)                         (1)           The DEM of the Baghmati River basin after density
                                                                          slicing is shown in the Figure 4, which clearly depicts the
where
                                                                          changes in basin slope from upstream to downstream. In
  g′0 - normalized right image pixel grey value
                                                                          general, the upstream basin area shows a regional slope in S
  g0 = the right image input pixel grey value
                                                                          direction with relatively steep gradient, whereas downstream
  σl - standard deviation of the entire left image
                                                                          basin area is characterised by SE slope. The overlaying of
  σr - standard deviation of the entire right image
                                                                          the Baghmati stream on the DEM indicates the relationship
  ml - mean value of entire left image
                                                                          between slope and the river flow direction. Barring a few
  mr - mean value of the entire right image
                                                                          exceptions, the channel follows the basin topography.
    For normalization, the upper-left scene (22-49B1) was
taken as the base image for both 1989 and 2000 data and all               Channel Morphology and Hydrology
the other images were normalized with respect to this image.
After normalization, the images were registered with respect                  The Baghmati basin falls in the interfan area bounded by
to the 1:250,000 scale toposheets of 1986 and were glued to               the Gandak megafan in west and Kosi megafan in east. This
get a nearly complete basin of the Baghmati river (Fig. 3).               interfan area is characterised by slightly concave at the
    The present study has made use of band 4 (infrared)                   edges and gentler floodplain gradient (10 cm/yr) (Geddes,
image, standard False Colour Composite (FCC) and FCC                      1960). The channel slope is also gentle in the alluvial plain
generated from Principal Component (PC) bands. The                        area and ranges from 53 cm/km to 11 cm/km from
standard (FCC) was prepared by assigning band 2, 3 and 4                  Dhengbridge to Hayaghat (GFCC, 1991). Channel shifting
images to blue, green and red respectively and using linear               is an important geomorphological process in the basin area
with saturation (2.5%) stretching. The PCA images of                      (Sinha, 1996) and detailed analysis of fluvial dynamics of
Baghmati River basin were generated using the multi spectral              the Baghmati River using multi-date topographic sheets and
band 1, 2, 3 and 4 and a composite of PC-images was                       satellite images has indicated several major and minor
prepared by assigning the PC-1, PC-2 and PC-3 on red,                     avulsion events on a decadal scale (Jain & Sinha, 2003a, in
green and blue respectively. This PCA colour composite                    press). A summary of channel migration history for the
provides a better visualization of drainage network,                      period 1924-2000 shown in Figure 5 suggests eastward
abandoned channels and high moisture area.                                shifting of the river, which has also shifted the flood hazard
    The alluvial plains of the Baghmati River have very low               in the river basin from west to east. However, the enormity
relief and gentle slope. Therefore, the topographic contours              of flood is not same all along the present course of the river
are not available on the toposheets of this region. A Digital             and only few localities are severely affected.
Elevation Model (DEM) was prepared for the Baghmati                           Table 1 presents a summary of bankfull discharge, most
basin using 109 elevation points; following weighted-average              probable flood, mean annual flood and 50-yr return period
technique. The height of each cell was determined by the                  flood for the Baghmati River based on flood frequency
average of 6 nearest control points of known height. The                  analysis. Higher values of the most probable flood and the
accuracy analysis was carried out with elevation data from 15             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)
                                          (m3/sec)             (m3/sec)              (m3/sec)          (m3/sec)        (m3/sec)
       Dhengbridge       Gumbel’s          1100                 3033                  1155              1473            3298
                        Distribution
                       Log Pearson-                                                     1334             1334           3048
                      III Distribution
                        Log-Normal                                                      1342             1342           2986
                        Distribution
         Hayaghat        Gumbel’s            870                 2617                    834             1076           2606
                        Distribution
                       Log Pearson-                                                      956             956            2460
                      III Distribution
                        Log-Normal                                                       948             948            2525
                        Distribution

                                                                                                                                       53
that the river is flooded almost every year. Further, the peak             Benibad to Hayaghat (all marked on the satellite image, Fig.
discharge of Baghmati River is quite variable and                          3) and Badlaghat (outside the view of the satellite image,
unpredictable and ranges from 600 m3/sec to 3300 m3/sec                    shown in Fig. 1). These windows have been selected for
(Sinha and Jain, 1998). The Baghmati River also carries a                  detailed study following the reports of the Ganga Flood
significant sediment load, which varies from 10.4 million                  Control Commission (GFCC, 1991), our hydrological
tonnes/yr at upstream station (Dhengbridge) to 7.21 million                modeling (Jain and Sinha, 2003 b) and repeated field visits.
tonnes/yr at downstream station (Hayaghat) (Table 2). Most                 A detailed description of flooding behaviour in these windows
of the coarser sediment load comes from upstream hilly                     and our understanding of the causative factors are presented
catchment area during the monsoon period but a major                       next.
fraction of the wash load is picked up by the river in the
plains itself (Jain, 2001). Due to flat gradient and less stream           Window 1: Area Upstream of Bairgania Town
power (Table 2), extensive deposition occurs in the alluvial                  The left bank of the Lalbakeya River and the right bank of
plains area. The variation in total sediment load between                  the Baghmati River in the Indian territory spill frequently
upstream and downstream stations indicates deposition of                   during monsoon season upto Bairgania town (Fig. 6a), even
large amount of sediment load (≈ 3 mt/yr) in the midstream                 though both these rivers are embanked all along. Hydrological
basin area.                                                                data of the Baghmati River suggests that at Dhengbridge, the
                                                                                                                     3
                                                                           mean annual flood discharge (1473 m /sec) is higher than
                                                                                                            3
Window-wise Study of Flood Prone Area                                      the bankfull discharge (1100 m /sec), which clearly explains
                                                                           the frequent overspilling in the region. The satellite image of
   Major flood-prone windows in the Baghmati river basin
include the areas around Bairgania, Sheohar, Katra, and




                                                                           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
Figure 3   The Baghmati river basin as seen on satellite image (IRS LISS            height range shown in the legend is further sub-divided into 8
           II, band 4) of February 1989. Boxed area are flood-prone                 sub-classes with an interval of 2 m and is shown as different
           windows (see text for details).                                          shades.

54
         Table 2   Sediment transport characteristics of the Baghmati River           window draped over DEM (Fig. 6b) illustrates the
                                                                                      topographic influence on flooding. The area in
   Station     Bankfull Channel Stream     Annual     Sediment
                                                                                      between the Baghmati River and Lalbakeya River
               discharge Slope   Power Sediment load    yield
                  3                  -2                                               is clearly low-lying and this induces overspilling
               (m /sec.) (cm/km) (Wm )    (MT/yr)    (T/km2/yr)
                                                                                      along the left bank of Lalbakeya River and right
Dhengbridge        1100         53        12.98         10.407            2745        bank of Baghmati River. The Bairgania town
Runisaidpur         300         18         6.54            -                -         lying in the Lalbakeya-Baghmati interfluve region
Benibad             350         15         7.25            -                -         is therefore flooded frequently.
Hayaghat            870         11         6.36          7.21              854
                                                                                      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
Figure 5 Summary of channel migration history for the last 76 years (1924-2000); an   around Katra (in red) and yellow patches of fresh
         eastward trend is apparent.                                                  sand interpreted as crevasse deposits bordering
                                                                                      the main channel. From a site immediately
this area shows that the Bhakuwa Nala is joining the Lalbakeya River to               downstream of Katra (Nawada village), we have
its right bank and the Manusmaran River is joining the Baghmati River                 reported about 1m thick splay deposit, about 200
to its left bank upstream of Bairgania. Addition of discharge and                     m long and 100 m wide, invading an orchard
sediment load from these tributaries is a major reason of overspilling in             formed in one single flood of September 1999
this region. A spill channel is also observed, which is originating from              (Sinha et al, in press). Further, field observations
the Lalbakeya River and after flowing near the Bairgania town joining                 indicate that the Lakhandei channel is significantly
the Baghmati River in downstream reaches. The satellite image of this                 narrow and entrenched near Katra (Fig. 8b)

                                                                                                                                       55
                                                                            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 (200-
                                                                            600 m3/sec) to Hayaghat (500-2500 m3/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
     Low-lying area
                                                                            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
                                  Bairgania        Dhengbridge              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
     Spill channel                                                          channels, which act as the large lakes, e.g. Khabar Tal,
                                                                    (b)
                                                                            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
Figure 6    Window-1: Area around Bairgania (a) Stretched band 4 data,
                                                                            control in this part of the river basin. The area lies in a
            (b) Band 4 image draped over the DEM; note the low relief and
            tributary influence upstream of Bairgania.                      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
reflecting a very low bankfull capacity. It has also been                   and hence the river gradient is very low near the confluence
observed that the Lakhandei river receives a significant amount             of Kosi River. The occurrence of ponds/tals and low-lying
of backflow from the Baghmati river upstream of the                         areas are surface manifestation of the subsidence in this
confluence at Katra, the flood level of the Baghmati being                  region. Further, the base level of the Baghmati River is
higher, and this aggravates the flooding in the Lakhandei.                  controlled by the water level of the Kosi River, which does
                                                                            not allow the Baghmati River to erode its riverbed and
Window 4: Area from Benibad to Hayaghat                                     hence the pace with the subsidence can not be maintained,
   The reach from Benibad to Hayaghat is another flood-                     which causes extensive flooding in the area during monsoon
prone area in the Baghmati basin. The flood problem is                      season. This subsidence in this block causes river-damming
particularly acute near Hayaghat area, which remains inundated              effect, which in turn causes widespread flooding in this
for more than two months in a year. People living in this area              region. Even the Kosi River backflows in the Baghmati
leave their homes during floods and return after the flood                  channel for a considerable length during monsoon season
recedes.                                                                    (GFCC, 1991).
   Fig. 9a shows three tributaries namely Siari River, Baghmati                The area around Badlaghat is further influenced by
Nadi and Baghmati Nala joining the Baghmati River at                        embankments in the upstream reaches of the Baghmati
downstream of Benibad. Besides these tributaries, the Adhwara               River. The Baghmati River is embanked from Hayaghat to


56
                    Figure 7   Window-2: Area in and around Sheohar (a) Standard FCC prepared from Feb. 1989 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,            hydrology and channel morphology with reference to
the area around Badlaghat receives the concentrated discharge              overbank flooding, and (b) incorporating effects of basin
from the upstream area, which causes extensive overspilling                geomorphology and neotectonics on fluvial processes in
in the region.                                                             general and flooding in particular. In the Baghmati basin,
                                                                           hydrological characteristics such as variable peak discharge,
Discussion                                                                 lower bankfull capacity than the mean annual flood discharge,
                                                                           high sediment load and extensive sediment deposition due to
   Hydrological response of a basin is governed by the basin               decrease in stream power are the key factors which make the
geomorphology and hence, an integrated approach to flood                   basin area prone to flood hazard. Furthermore, the dynamic
studies should involve the geomorphological understanding                  behaviour of the Baghmati river system, which often renders
of the river basin. Floods have long been considered as                    the flood management programmes ineffective in many
purely hydrological phenomenon, and therefore, flood                       instances, has been identified as a long-term causative factor
management programmes have essentially focused on                          of flooding (Jain and Sinha, 2000). Remote sensing data, due
hydrological variations. This has been one of the major                    to their synoptic view and temporal resolutions have been
reasons for the failure of flood management efforts across                 demonstrated to be effective means to understand flooding
the globe including India. Historical data reveal that even                behaviour and extent, in conjunction with topographic
after continuous efforts to control the floods, flood damages              analysis such as DEM, and supported by ground visits. The
and flood-affected areas in India have increased with time                 eastward shifting of the Baghmati river (Fig. 4) has often
(Agarwal and Narain, 1996). At present, flood is one of the                made the flood remedial measures useless along the old
most disastrous natural hazards in India and especially in                 channels and new areas prone to flooding have developed
north Bihar Plains. The failure of flood control measures in               along the new course. Further, the avulsion of an old channel
the country suggests an urgent need of an integrated flood                 into a new one causes a sudden increase in discharge in the
analysis including the hydrological, geomorphological and                  newly formed channel with a low bankfull capacity causing
geological understanding of the river basin. The UNDP                      extensive overbank flooding. Since people are normally not
flood policy study also called for greatly increased research              prepared for flooding along this newly formed channel, the
on river morphology, river training, mathematical modeling,                damage due to flood is quite severe in such cases. Flood
and land and water management (Brammer, 1990).                             damage trend (Fig. 2) shows a 16 times increase in the
   Our integrated approach to understand the flood hazard                  population affected by flood hazard after 1969 avulsion.
has followed two levels of analyses: (a) understanding of                  Therefore, the understanding of avulsion mechanism will

                                                                                                                                      57
Figure 8    Window-3: Area around Katra (a) FCC of Principal                Figure 9 Window - 4: Area from Benibad to Hayaghat (a) FCC of Principal
           Component bands 3,2 and 1 coded on RGB prepared from                      Component bands 3,2 and 1 coded on RGB prepared from image
           image of Feb. 2000; note large flood-prone areas around                   of Feb. 2000; note a large number of tributaries joining between
           Katra marked by water logged areas (blue) and high moisture               Benibad and Hayaghat and the angular drainage pattern. Area
           area (red); large areas of crevasse deposits (yellow) are also            upstream of confluence points show water-logging and high
           seen upstream of Katra (b) photograph showing the                         moisture zones (b) A small crevasse channel near Benibad
           entrenched, narrow (low w/d) channel of Lakhandei River;                  damaging the road during flood in September 1999.
           during flood season the water level rises upto the road level
           (seen on the upper right corner of the photograph).



have an important bearing on flood studies and management.                  1998). In recent years, large lengths (3,400 km.) of
   Further, basin-scale geomorphologic and neotectonic                      embankments have been constructed in Bihar particularly
studies aided by remote sensing data and coupled with                       after the devastating floods of 1954 (Agarwal and Narain,
DEM analysis have provided insight towards long-term                        1996). Even after the construction of embankments along all
causative factors of flooding to supplement the                             the north Bihar rivers, the inundation by spilling still continues
understanding of local factors through hydrological analysis.               in most parts through the gaps in the embankments and
Our studies indicate that the factors such as tributary                     breaching of embankments (GFCC, 1991). In the Baghmati
influences, topographic variations resulting through                        River basin, the flood control measures were initiated from
neotectonic movements, and tectonically-triggered                           1942. Since then, 466 kilometers of embankments have been
avulsions are important enough to be considered for any                     constructed/under construction. Initially, embankments in the
flood management program in the region.                                     downstream reaches worked effectively. However, after the
   We now turn our attention to flood control measures in                   construction of embankments in upstream region, the flood
the Baghmati river basin. Out of a range of flood control                   peak in the Baghmati River downstream of Hayaghat increased.
measures available such as embankments, reservoir dams,                     Hence, the embankments along Baghmati River just
watershed management through afforestation in the basin                     downstream of Hayaghat were frequently breached and did
area, channel improvement, upstream storage, detention                      not help much in quick drainage. In this way, the artificial
basins within flood plains, artificial drawdown of                          embankments have merely transferred the trouble from one
groundwater to absorb excess monsoon rainfall and                           place to another and have given a false security to the people
floodwater, diversion of flood water in an abandoned                        living in the area (Sinha, 1998). Moreover, these embankments
channel or in a canal and small scale irrigation strategies,                interfere with natural fluvial processes of the rivers. In areas
the embankments have been the most popular flood                            where the land is protected from spilling, water logging and
management strategy in India for several decades (Sinha,                    salinity problems have developed. The drainage congestion is


58
             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                         and in the different part of sub-basin area will reduce
River basin particularly in the low-lying areas, locally called                the influence of tributaries and hence will reduce the
‘chaurs’.                                                                      flooding problem at the confluence point of the channel
   The embankment strategy has lately been questioned at                       with main river. Because of their small size, these
the international level citing the failure in Mississippi and                  check dams will be environment friendly and also cost-
three major Chinese rivers (Rogers et al., 1989; Shu and                       effective. Further, as most of the sediment load comes
Finlayson, 1993) and alternative methods such as small                         from the upstream basin area, and not from the
scale irrigation strategies are now favored flood control                      downstream tributaries, these check dams will not be
measures in many flood-prone countries such as Bangladesh                      affected by the high silt load in the main Baghmati
(Brammer, 1990). In the Baghmati basin as well some                            River.
alternative methods besides embankments, have been                         (c) The proposal for diverting the floodwater at Belwa
considered by GFCC (1991) e.g. construction of dam in the                      village to the south flowing anabranch of the Baghmati
upstream reach flood regulator at Belwa village, channel                       river needs closer scrutiny keeping in view the
improvement, watershed management and underground                              neotectonic situation in the region. The south flowing
storage reservoir. Computations show that there will be no                     channel crosses the Sitamarhi Fault at Sheohar and
appreciable flood moderation in the middle and lower reaches                   uplift of the NE block will continuously decrease the
of the river even after the construction of dam and there will                 channel gradient. Therefore, regular channel
be very little effect (0.1-0.2 m) on the stage at Hayaghat                     maintenance may be required to maintain the flow.
(GFCC, 1991). Further, the large amount of silt load from                      Further, the eastward trend of channel migration will
the upstream basin area will also reduce the age of the                        require continuous monitoring of the left bank for
proposed dam. Based on our study, some suggestions for                         crevassing.
improving the flood management programs can be made.
(a) High sediment load is a major problem in Baghmati                      Conclusions
     River and has been cited as a major cause of avulsion
     and flooding due to rapid channel aggradation and                        An integrated approach employing remote sensing data
     reduction in bankfull capacity. Our analysis suggests                 coupled with DEM, hydrological study and field observations
     that a major proportion of the sediment load comes                    is recommended to understand the causative factors of
     from the upstream basin area. An extensive afforestation              flooding using the example of the Baghmati river basin in
     work in the upstream basin area will help to reduce the               eastern India. The study has shown that the tributary influence,
     sediment load in the Baghmati River                                   topographic control, abandonment of channels and
(b) Small check dams at the outlet of different tributaries                neotectonics in the basin area are the major reasons for the


                                                                                                                                        59
flooding in the study area. The remedial measures such as                  Jain, V. (2001) Fluvial Geomorphological analysis with special
small check dams and afforestation work in the upstream                       reference to flood hazard, Baghmati River basin, north Bihar,
                                                                              India, Unpublished Ph.D. Thesis, Indian Institute of Technology,
basin area may be more effective in reducing the flood
                                                                              Kanpur, India.
hazard in the region.
                                                                           Jain V. and Sinha, R. (2003a) Hyperavulsive-anabranching Baghmati
                                                                              river system, north Bihar plains, eastern India, Zeitschrift für
Acknowledgement                                                               Geomorphologie (Annals of Geomorphology), 47 (1):101-116.
                                                                           Jain, V. and Sinha, R. (2003b) Derivation of Unit hydrograph from
   The work presented in this paper is a part of the doctoral                 GIUH analysis for a Himalayan river, Water Resources
thesis of the senior author (VJ) at the Indian Institute of                   Management, 17: 355-375.
Technology Kanpur and the Institute Fellowship for the
                                                                           Jain, V. and Sinha, R. (in press) Fluvial dynamics of an anabranching
same is thankfully acknowledged. The Central Water                            river system in Himalayan foreland basin, north Bihar plains,
Commission and the Ganga Flood Control Commission,                            India, Geomorphology.
both Government of India organizations, are thanked for                    Kale, V.S. (1997) Flood studies in India: A brief review. Journal of the
providing the hydrological data. VJ was supported through a                   Geological Society of India, 49: 359-370.
sponsored project from Dalhousie University, Canada and
                                                                           Karunakaran, C. and Rao, A. R. (1976) Status of exploration for
Research Associate Fellowship from CSIR, India when this                     hydrocarbons in the Himalayan region-contributions to stratigraphy
paper was written. We thank Prof. Martin Gibling for the                     and structure. Geological Survey of India Miscellaneous Publication,
financial assistance as well as his valuable suggestions.                    41 (5): 1-66.
                                                                           Malleswaran Rao, T. Ch. and B. I. Dookahatala (1983) Techniques for
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