Geoscience Reference
In-Depth Information
1995 in Malda and West Dinajpur districts of West Bengal owing to flood-locking
in the Mahananda after heavy and widespread rainfall caused heavy damages. The
Yamuna, the Ganga's major right-bank tributary, threatened capital Delhi and inun-
dated large areas in Haryana and Uttar Pradesh. Among right-bank tributaries, the
rivers in the lowermost reaches, e.g., Mayurakshi, the Ajay and the Damodar inun-
date and cause acute drainage congestion. Called 'the Sorrow of Bengal' before a
number of dams and reservoirs were built on it and its tributaries the Damodar used
to flood south Bengal almost every year in the 1940s and 1950s; the Kangsavati, the
Rupnarayan and the Haldi did the same, simultaneously.
The important among flood-control measures, taken in the Ganga sub-basin,
include dams and storage reservoirs, barrages and marginal embankments, or flood-
levees, as they are called. While the reservoirs are many, embankments running
to over 5,000 km have been constructed along the banks of the Ganga and its tribu-
taries. These are not very high and were built above the levels of dominant discharge
of the rivers, leaving a sufficient margin beyond the water-edge. Embankments
normally prevent high floods in the basin; some of these have falling aprons and
protective slopes to control erosion and rotational slips during rains. Roads over
these facilitate inspection and public use in monsoon months. Ill effects of jacketing
a river by embankments are well-known; they aggrade river-beds, reduce bed-slopes
and raising water-level, create further flood hazards.
Sir William Wilcock, a British irrigation engineer, who visited India in 1930,
observed that embankments on the deltaic tributaries brought about adverse changes
in their condition. He attributed changes in the courses of the Ganga's big torrential
tributaries not to natural forces but to jacketing them by embankments. He added
that if the spill was not restricted by artificial constructions, it would spread all over
the land and leave very little silt on their beds. In such cases, the adjoining land
would not rise beyond a foot in 100 years. With this, the river-bed would rise and
no river would die. Very often, he said, engineers by obstructing the spread a spill,
accelerated silt deposition in its own bed, or on its immediate surrounding and thus
killed rivers.
Embankments, or levees, have been constructed on all rivers to control flood
since ages, throwing up widely different views on their effect on the stability of
rivers. One view is that rivers carrying high silt charge tend to lay their beds after
construction of embankments; so they are to be periodically raised to control ris-
ing flood-levels. Therefore, they can help prevent floods in regions where the silt
charge of the river is not too high, as in the Mahananda, the Godavari and the
Krishna but on streams like the Yellow River of China, the Kangsavati in West
Bengal, flood embankments have raised river-beds. Their heights are raised from
time to time and the process goes on. This is the view of two Indian experts -
S. L. Kumar and Kanwar Sain - but another expert, S. V. Chitale held that embank-
ments enhanced a river's sediment-carrying capacity by augmenting discharge and
hence did not aggrade it. If they are constructed with wide spacing in between,
along an aggrading river, any increase in sediment transportation cannot stop
aggradations and bed-levels would continue to rise; this cannot be due to embank-
ments. He also held that tidal rivers have an inherent tendency to aggrade and
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