The Ganga Morphology - The Ganga: Water Use in the Indian Subcontinent - page 45

hence embankments cannot arrest it but here again the rising beds are not due to
embankments.
These are the adverse effects, caused by structures along, or across, the rivers -
barrages, dams, bridges etc. Generally, upstream of a barrage, a river aggrades and
downstream, it degrades but up to some distance, depending on the location of the
barrage site. The erosive tendency also changes after a barrage or a bridge comes
up. For instance, erosion of the right bank of the Ganga below Mokama developed
only after the construction of a bridge near it. The barrage at Farakka did the same
and changed the erosive pattern of the banks, both below and above it.
Bank erosion is associated sometimes with floods, particularly in alluvial rivers
and in unstable reaches in the sub-mountainous regions. Meandering of the Ganga
and of its tributaries changed their courses. When it did, it caused erosion in
Uttar Pradesh, Bihar, Jharkhand and West Bengal. It is markedly prominent in the
reach below Allahabad, from Mokama to Mansi to Narayanpur, from Manikchak to
Farakka to Aurangabad, from Lalgola to Akhriganj, from Purbasthali to Nabadweep,
from Howrah to Sankrail and from Diamond Harbour to Kulpi in West Bengal.
Erosion is acute in the reach between Mokama and Mansi.
The Ganga has a meandering-cum-braiding pattern in this reach for a length of
85 km in 1780 to 110 km in 1965. The active channel has been swinging over
a width of about 15 km, where alternate deep channels and alluvial fans formed.
Below Mokama the river swung southward in 1957 and eroded the south bank.
Below Surajgarh, it has been swinging between Mungher and Mansi; it flowed near
Mungher in 1936 but started moving north and by 1963, eroded the bank near the rail
tracks off Mansi station, to as high as 7.6 m, or 25 feet every day in rainy months and
threatened roads and rail-lines. The meanders are never static but move downstream
and cause cyclic changes once in 70 years or so, here and in other places on the
course too. In such a meandering river, efforts to prevent erosion by drastic measures
like long spurs obstruct the movement of meanders. The river would either damage
the spur heavily, or other repercussions would follow at either above or below the
spot which can cause sudden and considerable changes in the course by avulsions
and cut-offs. It was, therefore, decided to give local protection by short spurs. A new
technique of constructing spurs in large stone crates on the eroded bank was rather
successful.
Further below Mansi, up to Narayanpur, severe erosion in 1973 threatened the
National Highway No. 3 and the rail-line (see Fig. 5.2) and affected nearby vil-
lages. Simultaneously, the right bank also eroded too, below Mokama Bridge, near
Berhaiya and engulfed some villages and farmlands. A cut-off occurred in the reach
in 1965, reducing the river length from 17 to 9.70 km. In 1975 flood, the left bank
from Ganaul to Narayanpur rail station was heavily eroded. It was observed that ero-
sion here was by 119 m in 1962 and 1963. It increased to about 207 m, every year,
between 1969 and 1975. Before the 1976 floods, the river's edge was about 750 m
from the rail track and despite protective measures, it came closer to the line by
about 460 m after the flood. As recommended by the high-level Tripathi Committee
(1974) and the Ganga Erosion Committee (1977), spurs, bank revetment, bed bars,
tagging embankments etc. were constructed, which checked erosion and diverted
the main river to the right.