The Ganga Morphology - The Ganga: Water Use in the Indian Subcontinent - page 43

a braided or a meander reach at the ends. Though these reaches have straight chan-
nels between high banks, especially at bankful stages, the thalweg of the reaches
are actually wandering between the banks. The total length of the straight reaches
may be about 140 km. Three major and lengthy reaches are from Murshidabad to
Sonagai in the Bhagirathi and from Serampore to Garden Reach and Hooghly Point
to Diamond Harbour in the Hooghly river.
An important feature of straight reaches is that the river is confined to the high
banks in the same width and depth over a long period, while in braided reaches
the parameters changed widely in different years. Unlike in braided reaches, the
deposits on the bank are mostly of clay and silty clay in straight reaches, in which
many alluvial fans form on both sides. Erosion of banks in these reaches is also
less and spill-over in monsoon months are not common in straight reaches as in
braided or meander reaches of the Bhagirathi-Hooghly. Meander reaches abound in
the river, running to about 290 km. The bed-slope of these reaches is generally less
than those of braided or straight reaches, adjoining them. Extensive erosion occurs
in meander reaches as the banks are mostly made of silty fine sand, silty clay or
clayey silt deposits. In many of such reaches, their length is much less than that of
the straight length, raising tortuosity ratio. A 1986 survey showed that out of 59
meander loops, the ratio exceeded two in seven cases - six in the Bhagirathi and
one in the Hooghly, all forming acute bends. In the Bhagirathi, the loops near Diara
Balagachhi and Char-Chakundi in Murshidabad and near Purbasthali in Burdwan
have very high tortuosity ratio - from four to five; in these two ends of the loops they
try to join, forming cut-offs. In fact, two cut-offs occurred - one in Baidyanathpur
in Murshidabad, at about 95 km downstream in 1984 and the other in Purbasthali at
about 210 km downstream of the Bhagirathi off-take in 1990. The tortuosity ratio in
both exceeded four, but in some places in spite of that there has not been any cut-off,
owing perhaps to more erosive resistance of the bank materials.
The 1986 survey maps of the Bhagirathi-Hooghly showed that about 73% of the
Bhagirathi's total length is meandering, 19% straight and 8% braided. The Hooghly
meanders for about 56% of its length; it flows straight for about 43% and braids
for only 1%. Of the total length, the joint river meanders for 64%, goes straight
for about 31% and braids for only 5%. Thus, over all, the Bhagirathi-Hooghly is
predominantly a meandering river.
Bank Erosion and Flood Control
Floods in the Ganga in Uttar Pradesh occur in areas below its confluence with
the Yamuna at Allahabad. Downstream, its spills cause considerable damage dur-
ing high floods. In Bihar, where high flood synchronises with high discharges of
its tributaries, the river mouths are blocked by very high water-levels, causing
widespread flood in the sub-basins. The main Ganga from Rajmahal to Lalgolaghat
goes into occasional spates in vast areas, due mainly to drainage congestion and
flood occurring at the same time in the Ganga and its tributaries like the Ghagra,
the Gandak, the Kosi and the Mahananda, in which it is very severe. The September