SEDIMENTATION: AN OPERATIONAL PROBLEM OF MITHRAO CANAL

trextemperMechanics

Feb 22, 2014 (3 years and 3 months ago)

62 views

SEDIMENTATION: AN OPERATIONAL PROBLEM OF
MITHRAO CANAL



---------------------------------------------------------------------------------------------------



1) INTRODUCTION


Mithrao Canal off
-
takes from Nara Canal, which off
-
takes from the left bank of S
ukkur
Barrage. The participation irrigation management was introduced in Sindh Province with the
establishment of Nara Canal Area Water Board in 1999. Mithrao Canal Division is divided into
four administrative sub
-
divisions (please refer Annexures


II & I
II). They are as following;

a. Sangahr Sub
-
Division.

b. Samaro Sub
-
Division.

c. Nabisar Sub
-
Division.

d. Naukot Sub
-
Division.


There are 55 minor & distributaries in Mithrao Canal and many direct outlets. The system is
considered to be efficient for its hi
gh cut
-
off since it helps in smooth gravitational flow of
canal water.



2) SEDIMENTATION PROBLEM


The irrigation channels in Indo
-
Pakistan were designed on regime theory after tedious
research by some renowned Irrigation Engineers like Mr. Inglis, Mr. Ken
nedy, Mr. Lacey. As
such, regime channels are supposed to be non
-
silting and non
-
scouring. However, some
channels undergo problems of erosion or deposition of sediment at some reaches. This
creates regulation and management problem and at times leads to br
eaches and
morphological changes with the channel. Currently Mithrao Canal is facing the problem of
sediment deposition at some reaches especially between RD
-
175 and RD
-
230.



3) OPERATIONAL ISSUE


There are three canals off
-
taking namely: Lower Nara Cana
l, Khipro Canal and Mithrao
Canal. The Lower Nara Canal has been provided with silt ejectors in the form of pipes at
glacis during the construction of new head regulator. This was constructed few years back by
the Chinese Engineers at about one thousand fe
et (i.e. 1 RD) downstream of the old head
regulator. The new head regulator was constructed to increase the discharge capacity of
Lower Nara. The new head regulator is provided with silt ejectors in the form of circulator
pipes. Since the silt ejectors on
the right side are not functioning well, as reported at site,
therefore, sediment is depositing on the left side just 350 ft downstream (as shown in photo
-
6) and it is likely to become a silt bar in due course of time. Khipro Canal has little cut
-
off
and a
s such does not carry any additional sediment load. Mithrao Canal has a huge cut
-
off of
08 ft, which means that velocity of incoming water would be very high at the intake. This
view is validated by the fact that a scour hole is developed just at the downs
tream of gates
(where friction blocks are erected to dissipate the energy) and, therefore, section has
widened (as shown in photos 1, 2&3). To control the situation, jhuck work has been done.
Due to high cut
-
off, Mithrao Canal carries more sediment load an
d it is further increased due
to erosion of the bed immediately downstream of the head regulator.


It has been reported by the operational staff that this year Mithrao Canal was immediately
opened after the end of closure period (every year irrigation cana
ls are closed for about 15
days to carry out necessary repair/maintenance works) while Lower Nara Canal and Khipro
Canal were opened after a week. Therefore, bulk of the sediment load entered into Mithrao
Canal’s system. Although formation of silt bar is a

slow process, however, this factor could
have also helped in aggravating the situation.


4) OBSERVATIONS


Following are the major observations;

• Jhuck work has been done on both sides of the canal to help in deposition of sediment
around canal banks. Th
is also reduces the size of section and thus increases the velocity and
sediment transport rate.

• Trees along the canals have been cut off to put floating groynes so as to supplement the
jhuck work.

• The numbers of cut trees are much more than the numb
er of floating groynes.

• The silt bars were not found above the water surface. Their presence can only be confirmed
through sounding and cross sectional survey.

• The velocity of canal water at some particular reaches was found to be very low. This could

be the result of deposition of sediment, called as silt bars. This is validated by the L
-
Section
and discharge data (attached as Annexures
-

IV & V) showing deposition of sediment along a
long reach and ultimately reduced velocities.

• The drag line was s
tanding on left bank of Mithrao Canal at RD
-
210. To remove the
deposited sediment, drag line was brought to excavate from canal bed the deposited
sediment of silt bars. This would supplement the earlier works of floated groynes and jhuck
work.

• The drag l
ine, employed through a contractor, was not functioning due to some problem in
its bucket. It was informed that the drag line would start functioning from the following day
(i.e. 01st June, 2007).

• Another drag line was found on road as it was in its way
towards RD
-
235. It was informed
that this drag line belonged to the Mechanical Workshop and it would also start functioning
from 01st June, 2007. This drag line had a much more capacity.

• It was informed by the field staff that previously Mithrao Canal wa
s regulated after heading
the water at upstream. Now it is no more practiced, resulting in large influx of sediment into
Mithrao system.

• The field staff informed that silt has deposited in a particular reach of RD
-
175 to RD
-
235.
Another factor responsib
le for deposition of sediment ion this particular reach could be the
non
-
installation of any off
-
taking channel though there are 14 direct outlets. Due to this
reason, water is not carrying away its share of sediment from the system, resulting in
dispositi
on of sediment within the main channel especially at reduced velocities.

• The process of rising of banks was found in process. This further validates the fact that
sediment is depositing within the system.



5) SUGGESTIONS / RECOMMENDATIONS



The deposit
ion of


silt control may be controlled through shortening of sections at original
design sections.


The deposited sediment may be ejected through mechanical means


to smoothen the flow
of water.


The raising of banks would lead to further


loss of water t
hrough seepage and consequently
leading to more waterlogging & salinity. Instead, long term measures may be adopted to
control deposition of sediment. This would require a detail study.


The cut
-
off is very high at


the head of Mithrao Canal. Avenues may b
e explored for the
generation of electricity with such a high available head.


The capacity building of staff


may be raised through training on modern lines and on issues
like regulation & management of gates, erosion & deposition of channels.



********
***********