Role of sediment transport in operation and maintenance of supply and demand based irrigation channels : application to Machai Maira Branch canals

Like in many emerging and least developed countries, agriculture is vital for Pakistan’s
national economy. It contributes 21% to the annual gross domestic product (GDP),
engages 44% of total labour force and contributes 60% to the national export. Pakistan
has a total area of 80 Mha (million hectares) with 22 Mha arable land, out of which 17
Mha is under irrigation, mostly under canal irrigation. Due to the arid to semi-arid
climate, the irrigation is predominantly necessary for successful crop husbandry in
Pakistan.
The development of modern irrigation in Indo-Pakistan started in 1859 with the
construction of the Upper Bari Doab Canal on Ravi River and with the passage of time
the irrigation system of Pakistan grew up to the world’s largest contiguous gravity flow
irrigation system, known as the Indus Basin Irrigation System (IBIS). In the IBIS almost
all irrigation canals are directly fed from rivers, while river flows carry heavy sediment
loads. Irrigation canals receiving such flows get massive amounts of sediments, which
are then deposited in the irrigation canals depending upon the hydrodynamic conditions
of the canals. Sediment deposition in irrigation canals causes serious operation and
maintenance problems. Studies reveal that silt reduces up to 40% of the available
discharge in irrigation canals.
Researchers have been striving since long to manage this problem in a sustainable
way and a number of approaches have been introduced in this connection. As a first step
sediments are controlled at river intakes by silt excluders and ejectors. Then a canal
design approach is adopted for keeping sediments in suspension and to distribute them
as much as possible on the irrigated fields. Even then sediments tend to deposit in
irrigation canals and become a serious problem in canal operation and maintenance,
which then requires frequent desilting campaigns to keep water in the canals running. It
causes a continuous burden on the national economy. In emerging and least developed
countries, adequate and timely availability of funds for operation and maintenance is
generally a problem. It causes delays in canal maintenance, which affects their hydraulic
performance. Water is then delivered inadequately and inequitably to the water users.
The story becomes further complicated when it comes to downstream controlled
demand based irrigation canals under flexible operation. In fixed supply based operation,
canals always run at full supply discharge and such operation, generally, does not allow
sediment deposition in the canal prism due to sufficient velocities. Whereas in demand
based flexible operation the canals cannot run always at full supply discharge but instead
the discharge is changing depending upon the crop water requirement in the canal
command area. Such type of canal operation is not always favourable to sediment
transport as under low discharges, flow velocities fall quite low and hence sediment
deposition may occur in the canal prism. The questions arise here what sort of
hydrodynamic relationships prevent sediment deposition in downstream controlled
irrigation canals and how these relationships can be adopted, while catering crop water
requirements of the command area? How the maintenance needs can be minimized by
managing sediment transport through better canal operation?
This study has been designed to investigate such type of relationships and practices
in order to manage sediment transport in downstream controlled demand based irrigation
canals and to attain maximum hydraulic efficiency with minimum maintenance needs.
The hypothesis of the study states that in demand based irrigation canals the volume of
silt deposition can be minimized and even the sediments which deposit during low crop
water requirement periods can be re-entrained during peak water requirement periods. In
this way a balance can be maintained in sediment deposition and re-entrainment by
adequate canal operation.
Two computer models have been used in this study, namely, Simulation of
Irrigation Canals (SIC) and SEdiment TRansport in Irrigation Canals (SETRIC). Both
models are one-dimensional and are capable of simulating steady and unsteady state
flows (SETRIC only steady state flows) and non equilibrium sediment transport in
irrigation canals. The SIC model has the capability to simulate sediment transport under
unsteady flow conditions and can assess the effect of sediment deposition on hydraulic
performance of irrigation canals. Whereas the SETRIC model has the advantage of
taking into account the development of bed forms and their effect on resistance to flow,
which is the critical factor in irrigation canal design and management. In the SETRIC
model, a new module regarding sediment transport simulations in downstream
controlled irrigation canals has been incorporated.
The study has been conducted on the Upper Swat Canal – Pehure High Level Canal
(USC-PHLC) Irrigation System, which consists of three canals, Machai Branch Canal,
PHLC and Maira Branch Canal. The Machai Branch Canal has upstream controlled
supply based operation and the two other canals have downstream controlled demand
based operation respectively. These canals are interconnected. The PHLC and Machai
Branch canals feed Maira Branch Canal as well having their own irrigation systems.
PHLC receives water from Tarbela Reservoir and Machai Branch Canal from the Swat
River through USC. Water from Tarbela Reservoir, at present, is sediment free, whereas
the water from Swat River is sediment laden. However, various studies have indicated
that soon Tarbela Reservoir will be filled with sediments and will behave as run of the
river system. Then PHLC will also receive sediment laden flows. The design discharges
of Machai, PHLC and Maira Branch canals are 65, 28 and 27 m3/s respectively. The
command area of the USC-PHLC Irrigation System is 115,800 ha.
The USC-PHLC Irrigation System has been remodelled recently and water
allowance has been increased from 0.34 l/s/h to 0.67 l/s/h. The upper USC system, from
Machai Branch head to RD 242 (a control structure from where the downstream control
system starts), was remodelled in 1995, whereas the system downstream of RD 242 was
remodelled in 2003. The upper part of Machai Branch Canal up to an abscissa of about
74,000 m is under fixed supply based operation, whereas the lower part of Machai
Branch Canal, Maira Branch Canal and the PHLC are under semi-demand based flexible
operation. The semi-demand based system is operated according to crop water
requirements and follows a Crop Based Irrigation Operations (CBIO) schedule. When
the crop water demand falls below 80% of the full supply discharge, a rotation system is
introduced among the secondary offtakes. During very low crop water requirement
periods the supplies are not reduced beyond a minimum limit of 50% of the full supply
discharge because of the canal operation rule.
The study consisted of fieldwork of two years in which daily canal operation data,
monthly sediment inflow data in low sediment periods and weekly sediment data in peak
concentration periods were collected. Three mass balance studies were conducted in
which all the water and sediment inflows and outflows were measured with suspended
sediment sampling at selected locations along the canal and boil sampling at the
offtaking canals, immediately downstream of the head regulators. Further in the four
months during the peak sediment season June, July, August and September, mass
balance studies were conducted by boil sediment sampling in order to estimate water and
sediment inflow to and outflow from the system. To determine the effect of sediment
transport on the canals’ morphology, five cross-sectional surveys were conducted and
changes in bed levels were measured. On the basis of these field data the two computer
models, used in this study, were calibrated and validated for flow and sediment transport
simulations.
The downstream control component of the system is controlled automatically and
the PHLC has been equipped with the Supervisory Control and Data Acquisition
(SCADA) system at the headworks. Any discharge withdrawal or refusal by Water
Users Associations (WUA) through offtaking secondary canals, or any discharge
variation in the inflow from Machai Branch Canal is automatically adjusted by the
SCADA system at Gandaf Outlet, the PHLC headworks. The SCADA system has
Proportional Integral (PI) discharge controllers. The study found that the existing PI
coefficients led to delay in discharge releases and resulted in a long time to achieve flow
stability. The discharge releases showed an oscillatory behaviour which affected the
functioning of hydro-mechanically operated downstream control “Aval Orifice” (AVIO)
and “Aval Surface” (AVIS) gates. After calibration and validation of the model the PI
controllers were fine-tuned and proposed for improved canal operation, which would
help in system sustainability and in improved operational efficiency of the canals.
Field data show that during the study period sedimentation in the studied irrigation
canals remained within control limits. The incoming sediment loads were, generally,
lower than the sediment transport capacities of the studied irrigation canals. Hence this
incoming sediment load was transported by the main canals and distributed to the
offtaking canals. The sediment transport capacities of the studied irrigation canals were
computed at steady and unsteady state conditions. The canal operation data showed that
the system was operated on Supply Based Operation (SBO) approach rather than CBIO.
The morphological data revealed that there was no significant deposition in the studied
canals. Therefore there was no particular effect on the canal operation and the hydraulic
efficiencies, attributed to sediment transport.
As mentioned earlier, the Tarbela Reservoir will soon be filled with sediments and
consequently PHLC will get sediment laden flows from the reservoir. Various studies
have been taken into account to project the time when sediment laden flows will flow
into the PHLC and what will be the characteristics and concentrations of the incoming
sediments to the PHLC from the reservoir. The studies project that the sediment inflow
from the Tarbela Reservoir will be much higher than the sediment transport capacities of
the PHLC and Maira Branch Canal under full supply discharge conditions. This scenario
will create sediment transport problems in downstream controlled canals, particularly
when they will be operated under CBIO.
Various management options have been simulated and are presented in order to
better manage sediments in the studied canals under the scenario of sediment inflow
from Tarbela Reservoir. The hydraulic performance of downstream controlled canals
will be affected under this scenario and frequent maintenance and repair will be required
to maintain the canals. Various options have been analysed to deal with the problem.
The study presents a sediment management plan for downstream controlled irrigation
canals by improvements in canal design and operation in combination with the need of
settling ponds at the canal headworks.
Currently sedimentation in the irrigation canals under study is not a big issue for
canal operation and maintenance (O&M). However, it would emerge as a major problem
when sediment discharge from the Tarbela Reservoir starts. The canals’ maintenance
costs will soar and the hydrodynamic performance of these canals will also be affected.
In this study, a number of ways have been evaluated and proposed to deal with the
approaching problem of sediment transport in these irrigation canals in order to keep
their hydraulic performance at desired levels and to minimize the maintenance costs.
The first and the foremost effect of sediment deposition will be reduction in canals’ flow
conveyance capacities, which will result in raise of water levels. The raise of water
levels will cause a reduction in water supply to the canals due to automatic flow releases.
It can be dealt with by a temporary and limited raise in target water levels depending
upon the maximum headloss at the downstream AVIS/AVIO cross regulator. Further, to
minimize the effect of water level raise on discharge through the AVIS/AVIO gates, the
decrement in such canals can be kept relatively small, in order to make the gates less
sensitive to water level changes. Further, for efficient withdrawal of sediment to the
secondary canals, it is needed to locate the secondary offtakes close to AVIS/AVIO
cross regulators on the downstream side. More sediment will be discharged because the
turbulent mixing of sediment at the downstream side of the control structures keeps
more sediment in suspension. In addition, during the peak sediment concentration
periods, the canals need to be operated at supply based operations, in order to minimize
the deposition.
Sediment transport in general and in irrigation canals in particular, is one of the
most studied and discussed topic in the field of fluid mechanics all over the world. It
also has been studied extensively in Indus Basin in order to design and manage irrigation
canals receiving sediment laden flows. The outcome of Lacey’s regime theory and the
subsequent work are the result of these studies. In addition to regime method various
other methods like permissible velocity method, tractive force method and the rational
methods, etc., have been developed for stable canal design. Anyhow, as a matter of fact,
the management of sediment transport in irrigation canals is still a challenging task even
after all these investigations and studies. Because most of the knowledge on sediment
transport is empirical in nature, most sediment transport formulae have inbuilt
randomness, which makes predictions difficult, when conditions are changed. It needs a
lot of care while applying a sediment transport formula, developed under one set of
conditions, to other situations. Therefore, it becomes extremely important to understand
the origin of the development of the formulae and the limitations associated with them
before applying some sediment transport formulae to different conditions and
circumstances. The introduction of numerical modelling made it comparatively easy to
test and shape the sediment transport relationships to some local conditions by running a
variety of simulations and calibrating the formula in light of the field measurements. The
sediment transport predictions can be made reliable in this way and can be used for
further analysis.