The interfan area is bordered by mountain-fed rivers (Gandak and Kosi River)
and consists only of Foothills-fed, Plain-fed and Mix-fed rivers. To keep the
comparability the mountain-fed rivers (alluvial fan area) are also listed.
Kosi river
(mountain-fed)
Discharge
Data from only one hydrological station on the Kosi
are available, and that is at Baltara, very far down the main channel belt,
a few kilometer above the confluence with the Ganga. In most years the highest monthly average of the daily discharges
occurs in August. The monthly average starts rising in May, clearly
preceding the monsoonal rains and reflecting snow and ice melt in the upland
source areas.
Sediment
concentration
The suspended sediment measurements at Baltara are
low relative to the other rivers and are also remarkably constant with
discharge suggesting a relatively uniform availability of sand, silt and mud
grades of sediment. |
![](hydrology/dis_kosi.gif) |
Gandak river (mountain-fed)
Discharge
The discharge starts rising
in early June, well before the arrival of the monsoon. This must reflect the
melting of snow and ice in the high mountain source area that makes up about
three-quarters of the system. The average monthly discharge is very similar at
the two stations, which suggests that the alluvial course of the the gandak is a
very efficient conduit for the water supplied. Although the peak discharge
occurs in the month of August whereby the actual size of the peak discharges
also shows great variation from year to year. The estimates for the discharges of the mean annual
floods at the two stations show an increase downstream that presumably reflects
input from the plains. And it is also interesting to note that whereas the mean
annual flood at the upstream station is less than the bankfull discharge value,
the mean annual flood downstream is greater than the bankfull value, implying
spillage of floodwaters onto the floodplain there at least (on average) every
second year.
Sediment
concentration
The suspended sediment concentrations at various
discharges have higher peak concentrations of fine material (wash load) upstream
than downstream which probably reflects the generally somewhat greater frequency
of flood waves upstream. Otherwise the ratio of coarse suspended material to
fine is relativly stable at different discharges. The reeason for that might be
the erady availabilitiy of coarse, medium and fine material in the channel
system.
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![](hydrology/dis_gangak.gif) |
![](hydrology/sed_gandak.gif) |
Baghmati System
(Foothills-fed)
Discharge
The discharge
from the data (2 stations) shows a pronounced increase for June, when the main
monsoonal rain arrive, so the contribution of snow and ice melt is insignificant
for the Baghmati.
The mean annual
flood estimates for both are distinctly greater than the bankfull estimations
and this implies that overbank flooding (spilling) is a frequent occurrence. In
its middle reaches, it is clear that the Baghmati spills westwards into the
Burhi Gandak, and also eastwards in spill channels which take water into some of
its tributaries, for example the Lakhandei, before the water rejoins the main
channel belt. This spilling is a normal annual occurance, accorging to the
people who live near the channel belt, and this is confirmed by the fact that
the bankfull capacity of the river in the area is only one tenth of the peak
discharge.
Sediment
concentration
The monthly
average suspended sediment concentrations show distinctly higher values for the
wash-load at the relativly low discharges involved, compared with the
mountain-fed rivers. This may reflect the prevalence of muddy banks in the
channels of the Baghmati system. The total sediment load is very variable from
one year to another and this may be due to the variability of the monsoonal
rainfall in the rather small system area and the variability of erosion and
deposition in the avulsing channels.
(to get more details click here)
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![](hydrology/dis_baghm.gif)
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![](hydrology/sed_baghm.gif) |
Burhi Gandak River
(Plains-fed)
Discharge
Hydrological data from three stations in the Burhi
Gandak system are available. The most upstream of these (Chanpatia) has much
lower maximum of the monthly averages of the daily discharges because it is on a
tributary to the main channel reaches of the system. All stations show an
increase in the monthly averages of June, reflecting the onset of monsoonal
rainfall and an absence of upland snow belt influence. It is also notable that
peak discharges tend to be more variable from year to year at the downstream
stations. At the most downstream of the stations (Rosera) the bankfull discharge
measure is less than half of that at the midstream station (Sikanderpur) near
Muzaffarpur and bankfull flooding is much more common at the midstream station.
Sediment
concentration
Interestingly the downstream site has yielded higher
concentrations of wash-load material in its suspended-load samples, suggesting
the erosion of muddy material from the channel banks. Also the coarse/fine ratio
in the suspended load tends to decrease with higher discharge figures, perhaps
again reflecting the importance of local erosion of muddy banks. The total
suspended sediment load tends to increase significantly from the midstream
station to the downstream station and this further supports the importance of
bank erosion within the system.
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![](hydrology/dis_burigan.gif)
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![](hydrology/sed_burigan.gif) |
Kamla-Balan River
(Mixed-fed)
Discharge
The monthly average of the daily discharge data show
typical June in creases, due to the start of the monsoonal rains, but a special
feature in the data is that both stations show July an dSeptember discharge
maxima. Both rivers also show considerable variablilitiy of peak discharge from
year to year. At both stations, the mean annual flood is almost three times the
bankfull discharge, which indicates the prevalence of overbank flooding.
Sediment
concentration
The most striking feature of the Kamla-Balan system is
the very high suspended sediment concentration that occur, despite the rather
low discharge values even though the discharges of the mountain-fed systems. the
Kamla, at Jaynagar, shows its foothill origin by having higher total suspend
load concentrations and also higher ratios of course to fine suspended load,
than the Balan at Jhanjharpur.
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![](hydrology/dis_kamlabalan.gif)
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![](hydrology/sed_kamlabalan.gif) |
Additional morphological and hydrological characteristics of these rivers are
compared in the table below. The most interesting comparison involves the
sediment yield per unit
area of the catchment. The Kamla-Balan and Burhi Gandak rivers have smaller average
sediment load but their sediment yield is
much higher than that of the Kosi river. The high yields are characteristics of
the foothills- and plain-fed rivers. So not only the plains receive exceptional sediment influx from the high mountains, but once the sediment has
been deposited on the plains, it is remobilized in an exceptionally vigorous way
by the plains fed systems.
River |
Total catchment area
(103 km2) |
Total length (km) |
Average annual discharge
(in m3/s) |
Average sediment load
(Mt yr-1) |
Sediment yield
(Mt y-1 km-2) (x10-3) |
Kosi |
101 |
1216 |
2036 |
43 |
0.43 |
Gandak |
46 |
630 |
1529 |
79 |
1.73 |
Baghmati |
13 |
589 |
156 |
11 |
0.84 |
Burhi Gandak |
12 |
579 |
58 |
15 |
1.24 |
Kamla-Balan |
9 |
240 |
66 |
10 |
1.11 |
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