measurement equipment have been developed in ACOP. A pumping sampler and large time point-integrated suspended sampling procedure for sand sizes has been developed in ACOP. This sampler has been extensively used in ACOP/ISRIP to define the shape of the suspended sediment concentration profile and to study the time and space variation of suspended sediment concentration. The errors of measurement for the suspended samples can be reduced by the use of an appropriate nozzle size for specific sediment and channel flow conditions, adequate time of immersion of the sampler a t a particular point, and also, most important is the experience and skill of the operator who is using a sediment sampler. Therefore, it is concluded that the results of sediment transport research studies depend on the skills and precautions taken by the organization dealing with sediment sampling in a alluvial channel system.

62

6. DESlLTlNG OF CHANNELS IN PUNJAB AND ITS SUBSEQUENT EFFECT ON CHANNEL BEHAVIOR

Introduction:

Pakistan's lndus Basin Irrigation System is based on run-of-the-river supplies, with rivers carrying quite a substantial amount of silt load. Although at barrages and headworks of canals, a number of silt excluding measures have been adopted, still considerable silt charge passes into irrigation channels, which ultimately gets deposited in the tail reaches of distributaries and minors where the velocity of flow is comparatively low. Siltation of channels affects the distribution of canal supplies in two ways: firstly, it reduces the carrying capacity of the channels; and secondly, the outlets located in the head reaches start drawing more water than their authorized share due to raised water levels. High withdrawals in the head reaches leave insufficient water for carrying the balance of the sediment, with the result that siltation problems in the middle and tail reaches is further accentuated . The canal water supplies to the tail portions of distributaries and minors are thus adversely affected, which necessitates remodelling of the channel by regrading the channel bed through a desilting operation. In this paper, the experience of desilting channels has been given as part of remodelling of channels both before independence of Pakistan, and after independence, as well as the various measures taken by the provincial Government of Punjab in the form of a desilting campaign from 1992 to 1995 on a "Self Help" basis.

Remodelling Distributaries (1901-1939). A review by W.M.Jasson:

A number of channels were remodelled in the Lower Jehlum Canal System, which included Mithalak Distributary, Mungi Distributary, Awagat Distributary, and Kheowala Distributary. These experiences were reviewed by Jasson (1 940). from which the following material has been taken.

During remodelling of these channels, desilting was carried out in selected reaches from time-to-time and the channels were monitored to observe the effectiveness of remodelling. Desilting only the channel bed may not solve the problem of channel silting without diagnosing its cause and taking comprehensive remedial measures to remove this cause. The causes of the silt problem can be classed under three different heads:

a) in the parent channel; b) c) in the distributary itself.

in the head regulator; and

63

In the case of Mithalak Distributary, the defective design of the head regulator was the main cause of the silting in the head reaches. With the proper design of the distributary head regulator. desilting of the channel in the head reaches, and adjusting the outlets for equitable water distribution and silt charge, the channel behaved well without showing any signs of silting tendency. In this channel, where the channel sections were exceptionally wide and the bed was shallow, hanging bushing spurs were used to facilitate berm formation and self eroding of the channel bed by running water proved successful.

In the case of Mungi Distributary, the channel showed a persistent problem of silting in the channel bed, even with the desilting operation taken from time-to-time and the provision of a skimming platform in front of the head regulator. To increase the sediment carrying capacity of the channel, the bed slope of the channel was increased along with the provision of kings‘ vanes in front of the head regulator to exclude coarse silt from entering the channel.

In the case of Mungi Distributary, Mr. Jasson remarks ” The experience of this channel shows how it becomes necessary to continuously watch a distributary after remodeling. It was a case that if at first you don’t succeed, try, try again and learn from your failures”.

In case of Awagat Distributary, the channel desilting was not found to be a successful operation. The channel silted up again and again. The outlets were adjusted and the crest of the head regulator was raised and an advanced sill was constructed to prevent excessive silt entry into the channel, but these remedial measures remained futile. Mr. Jasson attributed the main cause as shortage of irrigation water to the tail due to high absorption losses at the rate of 18% of the total flow. In the case of Awagat and Khewala distributaries, the main problem of channel silting was due to a defective head regulator and flatter slopes adopted during regrading of the channel bed, without considering the sediment carrying capacity of the channels.

Experience of Desilting of Channels: Pakistan‘s Scenario

Sedimentation of channels is a major problem in Pakistan due to the flat topography of the Indus Basin System with an average gradient of 0.25 ftll000 ft. In the vast irrigation network, it becomes obligatory to dispose of silt within the channel system by:

a) Sediment transport from main canals to secondary canals and into tertiary systems, then finally into the fields by equitable distribution and the sediment carrying capacity of the system.

Exclusion of silt from the head regulator of an offtake to such a degree that the regime of the parent channel is not disturbed and the sediment

b)

64

input from the channel’s head regulator into the offtake is in accordance with the sediment carrying capacity.

In case the sediment entry into the channel is not regulated, or the channel cannot attain its stable conditions, then the only option would be to desilt and regrade the channel bed to overcome the operational problems of inadequate freeboard in the head reaches, or shortage of irrigation water at the tail end.

c)

Management of Desilting Operation:

In the past, the practice of the operational staff in the Irrigation Department has been to revise the longitudinal section of irrigation channels every five years by redesigning the channel section using Lacey’s regime channel approach. Prior to a desilting operation, the existing hydraulic data for the channel is observed to determine the extent of sedimentation in the channel bed. The root cause of the phenomenon of channel silting is diagnosed and the channel’s full supply levels, bed levels and its geometric parameters are fixed, based on Lacey’s approach, by selecting an appropriate Lacey’s silt factor for redesign. The desilting operation is carried out during closure of canals from the middle of December to the middle of January. The following measures are taken by field staff to implement a desilting operation:

.

a) Fixing of benchmarks along the channel alignment and setting of vertical, as well horizontal, control points.

Regrading of channel bed by manual labor, and by machinery using tractors and dozers.

b)

c) Strengthening and raising of channel banks to provide the designed free- board.

d) For wider channels, killa bushing or hanging bushing spurs are applied to help in deposition of fine sediment particles along the sides of the channel for berm formation to attain the design top widths.

The channel is operated at design discharge and is monitored to observe it’s behavior in response to attainment of full supply levels. In the case of a reduction in supply of irrigation water to some of the outlets, as compared to design supply, temporary pipe outlets are installed. Within a running period of six months, the channel adopts its own stable condition within the given geometric parameters. Afterwards, the outlets are adjusted to draw their design irrigation water supply.

e )

65

Effect of Desilting Operation on Channel Behavior:

The channel behavior depends upon the adoption of channel prism parameters (i.e. regraded channel bed slope and measures for controlling the width of the channel). The measures for berm formation, such as ltilla bushing or hanging bushing spurs, may lead to constricting the channel and preventing sedimentation. In this case, the channel would behave as a self bed eroding channel, and subsequently. attains a stable condition. Supportive measures for berm formation and attainment of channel stability has been found successful in exceptionally wider and shallower channels. The main parameter which effects the subsequent behavior of the desilted channel is the regraded bed slope. The sediment transport relations developed by scientists and engineers indicate that the slope of the channel bed has a predominant effect on its

sediment transport capacity. In case the channel slope cannot be increased beyond a certain limit, as required for sediment transport, due to a command constraint or topographical limitations. then the regraded channel would again become silted. Therefore, in this case, the only remedy for channel stability lies i i i the preventiori of excessive silt entry into the channel by provision of silt excluding devices a t the head regulator of the channel.

, sediment transport capacity. The higher the channel bed gradient, the higher is its

The other parameter which is affected by the desilting of a channel is the full supply levels within a channel reach. There may occur some deviation in the actual water surface levels, as compared to designed fully supply levels, resulting in inequitable distribution of water to the outletslturnouts. If the outlets are not adjusted according to the remodelled channels new full supply levels, i t may happen that either the banks freeboard becomes inadequate, or flooding of tail reaches takes place. Therefore, any desilting operation and regrading of Ihe channel bed requires an adjustment of outlets with regard to:

a) equitable distribution of irrigation water and according to the culturable command area designed capacity of each outlet; and

Crest setting of outlets with respect to the regraded bed level of the channel so as to draw the proper share of silt from the channel.

b)

Desiiting campaign 011 Self Help Basis:

u The Punjab Provincial Government started a desilting campaign on a self help basis in the year 1992 and this program is still continuing during the annual closure of canals during the months of December and January. The Provincial Government, being aware of the vital importance of the canal irrigation system, attaches top priority to the efficient working of this system. About 50 to 70 % of the maintenance and repair budget is allocated for desilting of the channel bed and its regrading, along with berm cutting of branch canals, distributaries and minors. With the passage of time,

+:'

66

the Irrigation and Power Department input towards the desilting operation o f distributaries and minors has been reduced because of inadequate financial control, resulting in less budgetary allocation f rom the Provincial Government for this purpose. Also, the lack o f financial resources has been the main constraint in the way of proper upkeep and maintenance o f the irrigation facilities. For the last many years, farmers started on their own silt clearance of distributaries and minors, on a small scale, in their respective canal command areas on a self help basis. Therefore, for the Provincial Government, mobilizing the farming community for desilting of irrigation canals on a self help basis offers an attractive alternative t o partly overcome the resource constraints.

During the years 1992 t o 1995, about 2476 secondary canals were desilted under the administrative control of the Deputy Commissioners o f districts, while the Irrigation Department was responsible for overall planning, technical assistance and supervision o f work. Human and material resources for the desilting operation were planned as per the following daily schedule:

a) b) c)

Manpower for 20 days @ 100 cft lmanlday. Tractors for 20 days @ 3000 cftltractorlday. Government machinery for 20 days @ 15000 cft/Dozer/day

By employing these resources, the quantum o f desilting included a total desilted channel length o f 10643 miles and the quantity of silt removed was about 509 million cf t .

The Government and farmers have observed that the desilting campaign on a self help basis has been instrumental in creating an environment o f community mobilization for the welfare o f less privileged sections of the rural population. This has also helped in projecting a sense of farmers participation in irrigation management. However, these desilted channels have not been monitored by the Irrigation and Power Department to evaluate the subsequent behavior o f the channel wirh regard to its effect on channel prism geometric parameters, equitable distribution of irrigation water, and crop yields.

The achievement data for the last four years of the desilting campaign ( f rom the year 1992 to 1995) shows that as compared with the beginning year of 1992, n o w the desilting campaign on a self help basis has lost its enthusiasm and the quantum of work has a substantial decreasing trend. The data of achievements for desilted channels substantiate this trend, which is shown in the following table.

.

67

Table 6.1. Achievements of last four desilting campaigns.

No. of channels Length of channels desilted (miles) silt removed (lac

EVALUATION OF DESILTATION OF LAGAR DISTRIBUTARY

The International Irrigation Management Institute (IIMI) carried out a study on Maintenance and Repair (M&R) activities of Lagar Distributary in relation to silt clearance during the closure period of 1992. The annual budget for the maintenance of distributaries and minors in the M & R Program consists of five components: silt clearance, berm cutting, jungle clearance, repairs to banks and repairs to masonry works. Out of these, silt clearance by itself, or sometimes combined with berm cutting, appeared to consume most of the budget. In analyzing the expenditures for the Upper Gugera Irrigation Division for the five year period, 1985-86 to 1989-90, it was found that on average about 52% of the budge1 was spent annually on silt clearance which is carried out during a short period of 3 to 4 weeks during canal closure (Figure 6.1 ). An assessment of the quality and quantity of the desiltation work on Lagar Distributary was attempted in two ways: (1) direct observation by IlMl staff; and (2) topographic surveys using leveling instruments before and after closure works. The results of these surveys are presented in Figure 6.2. From the longitudinal sections, it is observed that there is little difference in the bed levels before and after desiltation. Secondly, the existing bed level is almost everywhere higher than the design bed level. The difference ranges from a few inches to more than 1 foot.

The objective of channel desilting is to bring the channel banks to its design dimensions. This Process Documentation Research (PDR) study based on IlMl staff observations indicates that PID (Provincial Irrigation Department staff) appear to be at a loss about the depth'to which the bed should have been desilted. Another quality control problem is the mode of supervision. It is easier to check visually the shape of a cross-section than the bed level. For bed desiltation IlMl staff observed that one burgi (a reference point) at Lagar head of 21 inches height has been found not a t all representative. From direct observations, it is clear that generally the quality of the contractors work has been questionable. Apart from the fact that the bed is not properly desilted to the required or desired depth, the silt that is excavated, mainly

68

c

.

.

~~~~ . i i i ! I

I

i i

j a -

I : ' ; =

m '

"I

i d - ! i ~

from berm cutting, is thrown mostly on the inside slope of the banks, disregarding the likelihood that it will partly slide back into the canal.ln the “self help” portions of the Lagar Distributary, the desiltation work done by students and farmers was observed to be of better quality.

%

Lagar Distributary Contract Work

Length (Ft) 43000

Desilted Depth (Ft) 0.04

Desilted Volume Per 0.42 Unit Canal Length (Ft3/Ft)

Desilted Volume (Ft? 18100

Berm Cutting (Ft3) 86000

Total Excavation (Ft3) 1041 00

IlMl surveys before and after the closure works on Lagar Distributary indicate that the averagedepth of bed desiltation for the whole canal was 2.7 cm (1.06”). For the contractors stretch, this was only 1.3 cm (0.51 “), whereas the average depth of desiltation in the “self help” portions was 7.3 cm (2.87”). Table 2 given below summarizes the survey results.

Table 6.2. Results of Evaluation Surveys.

Self Help Basis

19000

0.24

1.73

32800

13000

45800

Figure 6.3 depicts the comparison of silt removal at Lagar Distributary by the contractor and that of work done through farmers and students on a ”self-help” basis. IlMl staff have observed that, considering the bulk of silt removed from the bed of the channel along with berm cutting, the available manpower. was used less efficiently in the self help campaign than by the contractors work force.

An evaluation of the effects of desiltation on canal performance, in this case Lagar Distributary, is indicated in Figure 6.4, which shows monthly averages of IlMl daily water level readings at Lagar head and tail for three years. Lagar tail was observed to be performing better than previous years after the 1992 desiltation campaign, while the water levels at the head remained about the same. This was the combined effect of desiltation and remodelling of outlets at the end of the closure period.

.

71

!

Figure 6.3. Results of the evaluation surveys of Lagar Distributary, silt removal per foot canal length for different types of work. Y

.

Figure 6.4. Lagar Distributary monthly average head and tail waterlevels for 3 years

72

,

Finally, it should be pointed out that desiltation alone cannot solve the problem of tail end shortage of water in the canals. Such efforts should necessarily be accompanied by other technical and institutional interventions to restore the appropriate hydraulic conditions in the canal system and to ensure equitable distribution of water, such as restoring outlets to their design dimensions and checking various types of illegal irrigation.

Concluding Remarks:

Desilting of channels, before the independence of Pakistan, had been carried out in a well planned manner as part of remodelling the channel system. The channels were monitored after the desilting operation and further remedial measures were taken in time to set the channels to its required design parameters and to control the excessive silt entry into the channel a t the head regulator. The main objective of the irrigation engineers was to diagnose the cause of sedimentation problems in the channel by monitoring of channel behavior on a continuous basis, execution of channel remodelling and its hydraulic structures in a proper manner by the application of their engineering skills to the best of their abilities.

After the independence of Pakistan, these good engineering practices continued and the channels were remodelled and their longitudinal sections were observed and revised after every five years. As part of these schemes, distributaries and minors were desilted by the Irrigation & Power Department in a well planned manner. With the passage of time, due to a lack of financial control and lack of financial resources, this practice did not continue. This has resulted in the silting of channels in the head reaches causing:

a) Inadequate freeboard in the upper reaches, thereby endangering the safety of canal banks;

b l Inequitable distribution of irrigation water within the channel system; and

c) Shortage of irrigation water to some tertiary canal systems (1.e. at the tails of distributaries and minors).

In view of these difficulties, the Provincial Government initiated a desilting campaign in 1992 on a "Self Help" basis, which has proven successful in terms of farmers and village population participation in irrigation management. To some extent, farmers at the tail end of distributaries and minors have been able to receive their due

operations by the Irrigation Department, keeping in view its glorious past experience of remodelling schemes, these efforts may prove beneficial in improving the channel behavior, such as equitable irrigation water supplies to the farmers for a longer duration. There is a need to monitor the channel conditions which have been desilted with regard to its channel geometric parameters, channel full supply levels and bed levels, and the quantum of irrigation water received by the farmers and its effect on crop yields.

. share of irrigation water supplies. It is felt that with the proper planning of desilting

73

7. RESEARCH STUDY RELATED WITH SILT SELECTIVE HEAD REGULATORS AND PERFORMANCE OF SILT EXCLUDERS AND SILT EJECTORS IN PAKISTAN

Introduction:

Prevention of excessive sediment entry into canals is an important problem for channel maintenance. All the difficulties which confront the Irrigation Canal Engineers are either directly or indirectly due to silt, which is brought from watersheds into the rivers and then enter the irrigation channels. The successful operation of an irrigation network depends upon the degree t o which the problems of silt distribution and sedimentation is tackled by the operational staff o f the irrigation system through the control or adjustment of the quantity of silt entering a channel, or the complete exclusion o f all silt liable to deposit on the channel bed. This paper is based on a literature survey of the work done by the Irrigation EngineerslScientists of the sub- continent on silt selective head regulators and the use of various devices for silt exclusion at the intake point, or silt ejection at the downstream of canal head regulators.

Silt Selective Distributary Head Regulator:

A distributary head regulator is an important hydraulic structure for distribution of irrigation water supplies, which acts as a regulator, a meter of supply, and a silt selective structure. The structure can be divided into three parts i.e. approach chamber, regulator in the form of Kurries (needles), and weir flume. The approach chamber selects the silt. The supply is regulated upstream of the weir flume, which meters the supply. A typical silt selective head regulator is shown in Figure 7.1.

Approach Chamber

Pitching upstream of the head regulator in the parent channel on the side accelerates the side velocity. and is further increased by the reduction of depth provided by raising of the bed of the parent channel in front of the head regulator. The difference between the mean velocity in the middle and the side velocity in the parent channel is reduced. Water on the side simply sweeps straight along without any disturbance. The floor of the approach is kept higher than the bed of the parent channel according to the requirements. The waterway at the entrance to the approach chamber is the chief factor which controls the silt selective power. -

74

.

Ln

0 z

m WLO + - _I

a a

0

V

m i z 0

Z s! t V W ln

Regulator

Maintaining the required supply level in the parent channel by regulation at the control point is convenient. A needle regulator is the most suitable device, which interferes the least with silt carriage in the parent channel and is easy and convenient to manipulate.

Meter Flume

The regulator is followed by a meter flume. The depth on the crest of the flume is kept about two-thirds the depth of the approach chamber. The length of the crest is kept equal to 2H where H is the head above the crest. The approach curve in the bed is laid with a radius 2H. The crest is followed by a convenient glacis. The distance of the gauge hole from the beginning of the crest is 3H. Water to the gauge well is admitted through a single hole, the area of which is 1/5000 of the area of the gauge well. The hole is provided in an iron plate secured flush with the wall and located an inch or two below the crest level at a distance of 3H from the beginning of the crest.

Design of Head-regulator with Required Silt Conductive Power:

The silt conductive power required for a head regulator depends upon the conditions in the parent channel and those which could be permitted in the offtaking channel as determined by available slope, permissible critical velocity ratio (C.V.R.), and the ratio of bed to depth.

R= C. V. R in an offtake C. K R in the Parent Channel

where

Silt Charge in an offtake - Sin Charge in the Parent Channel

r, .=Silt conduction power of head regulator

r -

76

Silt grade in omake r - 2- Silt grade in fhe pamnt channel

r2 = Silt selective power of head regulator with respect to its grade,

Depth in the parent channel Depth in the omke

A=

The attempt to predict r2 for a head regulator was not successful in experiments carried in model studies due to insufficient data on silt charge. Mr. K.R.Sharma in his study assumed r, = rz (silt selective power by weight), but exhaustive research work is needed in this respect.

The dimensions of the entrance of the approach chamber could be fixed from the following expressions:

Depth of f low in the approach channel, Ha, is given by;

H,=dJ:

where

d, = depth in the parent channel. Width of approach chamber, W, is calculated by the relation;

where

.

K = constant varying from 1.5 to 2; and S = set back of upstream wing.

Set back of upstream wing is given by the relation;

where 0, = discharge in the offtaking channel Q, = discharge in the parent channel b = bed width of the parent channel rn = side slope of the channel

77

The dimensions of the silt selective head regulator can be obtained by the above relations for the given silt selective power of the head regulator. The experiments conducted by K.R.Sharma indicate that silt conductive power of a silt selective head regulator working under ideal conditions does not vary with the discharge of the offtake, so long as the depth in the approach chamber is not changed with respect to its ratio with the depth in the parent channel. Ideal conditions of working means that the width at the entrance to the approach chamber is suitably selected, so that the bottom water of the parent channel flows straight and has no tendency to rise into the approach chamber. It was observed that silt conducting power of these model studies on the silt selective head regulators were in the range of 79% to 83%. These studies are not applicable to the canal head regulators offtaking from a river at the side of a barrage or headworks as the conditions of flow in a river are quite different from those in a canal flowing steadily in a uniform section.

Origin of the Silt Excluders:

The idea of the silt excluders is credited to the late Mr. H.V. Elsden, whose paper (1.B Paper No.251 published in 1922 first brought the idea of silt regulators before the engineers of the subcontinent. The basic principle on which silt excluders are designed lies in the fact that in a flowing stream carrying silt in suspension, the concentration of silt in the lower layers is greater than in the upper ones. Consequently, if the lower water can be escaped without interfering with the silt distribution, the water remaining will have less silt per unit volume than the water upstream of the escape.

Elsden's Proposal:

Elsdens' design comprised of a regulator divided into two portions by means of a horizontal diaphragm over which the upper water passed into the canal, while the heavily silt laden lower water was passed through tunnels to waste. With some modifications in details of the structure, this form of construction is found in all silt excluders constructed to date.

Types of Silt Excluders Constructed:

(a) Khanki Type Silt Excluders:- . In 1934, a silt excluder a t the intake of Lower Chenab Canal was constructed which was located at Khanki Weir. Two undersluice bays adjacent .to the regulator were used for the construction of this excluder in which three tunnels running parallel to the regulator face were provided in each bay. The tunnel along the face of the regulator is the largest tunnel, with the upstream end just above the upstream abutment of the regulator and the extreme right tunnel in Bay no.2 is the shortest

78

J

in length. Each tunnel stepped out from the one next to it farther from the regulator face and has one opening at its upstream end and an opening facing the divide wall in the projection. The details are shown in Figures 7.2a and 7.2b. Following the Khanki pattern, Mr. Crump designed two excluders, one at Bong and the other a t Jaba Level crossings. The design features of Jabba are illustrated in Figure 7 . 2 ~ .

Haveli Canal Excluder a t Trimmu:

The left undersluices of Trimmu Barrage, which have eight bays, have been divided into two portions by a divide wall going far out to give a smooth approach. The four bays between the regulator face and divide wall are provided with a full slab excluder with the top of the slab at the crest level of the regulator and extending beyond the regulator face, which ensured the separation of the escape water before turning into the regulator. High efficiency of silt exclusion was claimed by the designer but considerable turbulence and surging action was noticed in the pocket wtten the canal and excluder were in operation simultaneously in the model.

(b)

(c) Kalabagh Type Silt Excluder:

This excluder is a modified Khanki type silt excluder covering only two bays of the undersluices without any additional divide wall. In the design, the length of the tunnels decreased in equal steps from left to right and the openings towards the pocket were extinct . The details of this type of excluder are given in Figure 7.3.

Remedial Measures for Improving Silt Excluder Performance:

To make the excluder work efficiently, all measures which increase bed shear stress and throw more sediment into suspension must be avoided and all measures which help in dropping of sediment in the lower layers should be used. These are:

a) Decrease the tractive force by flattening the river slope in the approach channel.

b) The canal discharge and excluder tunnel discharge should not be increased.beyond a permissible limit so as to create more bed shear than the normal.

79

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L

c

..-

.-- -I L W U 3 V X w Y C m .c x m

N

- .-

K E 3 0) U .-

R

x 5 al 3 h rn

o_

2

x

C a

.

m c m 7 L

m

m 3 0,

LL

L

._

82

I m

p1 . . 0 Z

z 0

V W v)

- t

c) All obstructions, proturbances and roughnesses that have the effect of throwing up towards the surface a certain quantity of water should be obviated as far as possible.

A n excluder in front of an intake located on the outside of a curve in a parent channel will not have maximum efficiency for consideration of discharoe.

The shape and placement of the openings of the excluder should be such that the f low is orthogonal to the intake of the excluder.

d)

e )

Efficiency o f Silt Excluder:-

F.F. Haigh. in the year 1938, advocated in his paper that the concept should not be accepted that increased escapage through the excluder created greater efficiency. He suggested a figure o f 20% extraction ratio as a reasonable figure, but suggested further research on this matter. He recommended that the velocity through the tunnels should not be less than 10 f t per second. The escapage through the tunnels, when less than full supply, was to be regulated by the gates at the end of the tunnel.

Haigh was the first to define the efficiencies o f an excluder, which he did as:

a) I, and lo are discharge and silt intensity, respectively. The suffixes f, c and x denote the approach flume, the canal, and the escape, respectively. The efficiency is given by;

or

b l If observations of the approach flume are not available, then the efficiency may be obtained from the canal data and escape observations by the formula:

04

The actual efficiencies of the excluder a t Jabba and Khanki, as recorded by him in 1937, vary from 13.4% to 48.10%.

Factors affecting Efficiencies:

In comparing the efficiency of excluders, it must be remembered that the greater the proportion of supply escaped, the greater will be the efficiency. However, the efficiency will not vary with the escapage. Since the silt intensity decreases rapidly with depth, additional escapage will increase the efficiency, but slowly. Another point to be considered is that the efficiency must be affected by the grade of the material carried by the water. Since the coarser silt has been found to cause trouble in practice, efforts are made to exclude silt particles larger than 0.2 mm diameter, and efficiencies are calculated on material of coarser grade than this. The proportion of the total silt which is greater than 0.2 mm and the relation of coarse silt carried in this grade will, however, vary a t different sites.

Comparison of Silt Excluders:

The orthogonal and the Khanki type excluders are almost equal in their performance in minimizing the silt entry into the canal intakes. The silt escaped by tunnels of these two types of excluders in undersluices is minimum as these are drawing from outside of the curve, and if these tunnels are eliminated, there will not be a marked reduction in the efficiency of the silt excluders. The percentage reduction of silt with an orthogonal type of excluder with an extraction ratio of 50% for the Hydel canal is 50.83%, whereas the overall percentage reduction of sediment, when all the canals are taken collectively, is only about 20%.

Further work on excluders at Trimmu and Kalabagh was done by Uppal in connection with the model studies of these excluders. From experiments for the Kalabagh Excluder, it was shown that the design using two bays was more efficient than the design using four bays. He found from his experiments that a smooth floor in front of tunnels in the pocket was not required.

- 0 At the Irrigation Research Institute, further research work on silt excluders was

done by Mushtaq Ahmad and Muhammad Ah. These studies showed that the openings of an excluder located orthogonal to the flow into the offtake channel gave the best efficiency. The maximum efficiency of an excluder is obtained when the approach is straight. If a channel approach is maintained by training works, using curved guide banks and spurs, the excluder is not as efficient. They showed that the best and cheapest way for sediment exclusion is still the establishment of a stable curvature and taking off the channel from outside a curve with a still pond in front of the regulator structure.

85

Studies on Silt Ejectors in Pakistan:

A silt ejector is placed at some distance down the canal and it extracts or ejects silt which has entered the canal. The studies on slit type ejectors were carried from 1944 to 1946 in the hydraulic laboratories of the Irrigation Research Institute, Lahore. A plain slit in the bed of a channel was placed at a distance of 10 (V,/V,)D downstream o f the hydraulic jump, where V, and V, are the critical and terminal velocities for the sand grade used and D is the depth of water in the channel. Slit type ejectors were, however, not used in this country and only the conventional ejector with a front opening had been designed and constructed upto 1947 when Pakistan came into existence.

In 1951, Messrs, Koonsman and Albertson of USA submitted a paper on the 'Design Characteristics of the Vortex Tube Sand Trap' in the fourth meeting of the International Association for Hydraulic Research. The Vortex Tube Sand Trap originated by Mr. Ralph L Parshall, was improved and perfected by Rohwer, Robinson and others who claimed 90% efficiency for sand size ranging from 0.4 to 1 .I mm for 9.5% extraction of the channel supply. In 1954, IRI took up a detailed study of the vortex tube silt ejector to check its performance for the sediment size and concentration as exist in Pakistan. This study, combined with the silt ejectors investigation for Taunsa Barrage, continued upto 1964. A series of experiments were conducted on conventional frontal type ejectors to study the possibility of utilizing the vortex type ejector in big canals and compare the efficiency of a vortex type ejector with the frontal ejector.

Various types of Silt Ejectors and their Comparison:

The conventional silt ejector (Figures 7.4a and 7.4b) known as a Frontal Silt Ejector comprise a series of small rectangular tunnels embodied in the canal bed with their openings facing the flow and the tail end converging towards the river side bank and taken through the banks to discharge the flow into an escape channel connected with the river. The main tunnels are subdivided into smaller tunnels to ensure even flow distribution at the entrance of the ejector. The top of the roof slab is generally level, or a little above the canal bed, and its upstream end extends by 1-3 feet beyond the tunnel entrance to minimize the flow turbulence a t the entrance of the tunnels. At the emergence, each tunnel is provided with a control gate for discharge regulation. These ejectors are designed for a wide range of discharge extraction ratios varying from 2 to 40% of the canal flow.

. *

Another type of silt ejector, which is gaining increasing usage in the United States and European countries, is the vortex tube ejector. The vortex tube consists of an open top tube placed across the bottom of the canal (Figure 7.51. The tube is placed in a transverse direction to the flow, either normal to it, or at some other angle down to O30. To control the rate of flow out of the tube, its downstream end is

86

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regulated by a valve or a control gate. The top o f the tube is cut away forming a slit in order t o receive sediment to be trapped.

Optimal Extraction Ratio and Efficiency of Silt Ejectors:

Some o f the earlier designers of silt ejectors were of the v iew that the greater the escapage through the ejector, the higher the efficiency of the ejector. The designed extraction ratio for some of the ejectors has been fixed as high as 1 5 % to 39% of the designed canal discharge. To optimize the extraction ratio f o r the silt ejector, a series of model tests were conducted. The efficiency of the ejector is plotted against the extraction ratio of the ejector in Figure 7.6., whereas the extraction ratio and efficiency are expressed as :

ejector discharge, oo canal discharge

Extraction Ratio=

where I u = silt intensity in canal upstream of ejector; and I, = silt intensity downstream of ejector.

The results of experiments indicate that with an increase of extraction ratio f rom 2.5% t o 12.5%. there is a proportional increase in efficiency of the silt ejector, but beyond 12.5% the increase is negligible. The optimum extraction ratio is a function o f sediment grade, sediment concentration, the discharge in the approach channel, and the elevation o f the separation point.

The efficiency of the frontal type silt ejector is plotted against a non- dimensional parameter d/D in Figure 7.7, where d is the depth of water over the diaphragm and D is the depth o f water in the canal. Maximum efficiency is attained at d/D =0.9 and Froude number F,=O.2. A t higher Froude numbers, or d/D ratios, the f low becomes rough, disturbs the normal sediment distribution at the entrance of the ejector, and creates a scour problem below the ejector flume as well.

*

.. The studies on the vortex tube ejector showed that the efficiency was

maximum when:

90

~ . ....... .. .. .. . . . .. .

O 0 N r-

91

0

d . 0 U In 3 v1

01 > L

-0 -I t

92

a) The Froude Nu-mber was equal to one (i.e. critical f low depth over the top of the tube);

b)

c)

d)

The diameter of the tube was the same as the critical depth;

The two lips of the tube were at the same elevation; and

The tube was a t an angle with the line of flow.

The investigations claimed 90% efficiency for the sediment sizes coarser than 0.65mm for a 9.5% extraction ratio.

The vortex tube silt ejector has not been found to be successful for the larger canals in Pakistan having discharges between 1,000 to 15,000 cusecs. The Froude number for these channels is of the order of 0.195. The maximum efficiency of the vortex tube is attained at a Froude Number equal to 0.8 but the normal Froude number of Lacey’s channel varies rom 0.19 to 0.21. At this Froude Number, due to low velocity in the channel, the vortex is not formed within the tube and it acts as a sluiceway with very low efficiency. The vortex tube type ejector can only be used efficiently on canals in Pakistan if the Froude Number can somehow be increased to 0.8 at the ejector site and the full length of the tube could be made equally active, thereby drawing a due share of discharge at all points along its entire length.

Concluding Remarks:

Irrigation Engineers have to take care of sediment imbalances within the irrigation channel network. For this purpose, silt excluding and silt ejecting devices are installed at the upstream of intake structures (head regulators) or downstream to control the sediment entry into the offtake channel. In this context, Mr. K.R.Sharma made experimental studies on silt selective head regulators so as to draw the desired concentration of sediment from the parent channel into the offtake channel. There is a need for further research work to determine, in a rational way, the dimensions of the silt selective head regulator based on sediment transport concepts for design purposes.

* In Pakistan, tunnel type silt excluders have been designed and constructed at

headworks, especially at Khanki headwork and the Upper Jehlum canal system. While comparing different types of excluders, it has been found that the orthogonal and the Khanki type silt excluders are equal in their performance. For the Kalabagh type silt excluder, it was observed that the design using two bays was more efficient then the design using four bays. Mr. Uppal found from his experiments that no smooth floor in front of the tunnels in the stilling pond pocket was required. The efficiency of a silt excluder depends on the extraction ratio through the escape channel/tunnels. In this connection, F.F. Haigh advocated that the concept should not be accepted that, the

93

i

more the escapage through-the excluder, the greater the efficiency. He suggested a figure of 20% extraction ratio as a reasonable figure and suggested further research work on this matter. In Pakistan, conventional frontal silt ejectors have been constructed and a number of model studies have been conducted at the Irrigation Research Institute. On the other hand, the vortex tube silt ejector has not been found to be successful for larger canals in Pakistan having discharges in the range of 1,000 cfs to 15,000 cfs. The results of experiments on the frontal type silt ejector indicates that with an increase of extraction ratio from 2.5% to 12.5%. there is a proportional increase in efficiency of the silt ejector, but beyond the value of 12.5%, the increase is negligible. Also, it has been found that maximum efficiency is attained in this type of ejector when the ratio of water depth over the diaphragm and depth of water in the canal is 0.90, and the Froude Number is less than or equal to 0.2. There is a need for further research work for evaluating the adequacy of using silt excluders or silt ejectors at a particular site to control the sediment flow into the channel.

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8. SILT DRAWING CAPACITY OF CANAL OUTLETS

Introduction:

In the Pakistan Irrigation System, the most commonly used types of outlets (turnouts) are Adjustable Proportional Module (APM), Adjustable Orifice Semi Module (AOSM), Open Flume and Pipe outlets. These outlets are so designed that they work semi-modular, that is their discharge is independent of downstream water level fluctuations in the watercourse (FREE FLOW). Also, their crest level is set in such a way that these modules preferably act as proportional semi-module. These outlets play a very important role in the successful working of irrigation channels. If these outlets do not take their due share of silt charge carried in the parent channels, the silt in the channels will go on accumulating to an extent that the channel would not be able to carry its design discharge, leading to a shortage of irrigation water a t the tail ends. Under these conditions, the remedy would be to remodeVadjust or use those outlets which have a comparatively higher power of conducting/drawing silt from the parent channel.

Hydraulic and Geometric Characteristics of Outlets Used in Punjab Irrigation System:

The simplest and the oldest known type of outlets is the pipe outlet. To start with, it was an earthenware pipe placed in the bank of the distributary a t the bed level. Later on, it was provided with a face and tail walls of masonry and embedded in concrete. These pipe outlets were gradually replaced by rectangular wooden or masonry barrels. The steel or cast iron pipe (Figure 8.1) was brought into the field at a subsequent stage. The discharge of the pipe, or barrel, outlet is given by the formula

where q = discharge of outlet; C = Coefficient of discharge; A, = Cross sectional area of pipe outlet; and H = Hydraulic head.

If the outlets have a free fall, H is generally measured from the center of the pipe or barrel to the full supply level in the distributary. If it is drowned, then H is the difference in the water level in the watercourse and the distributary. If the outlet has a free fall, the discharge through it is independent of the water level in the watercourse; under these conditions, it is semi-modular. The coefficient of discharge of the pipe outlet varies with different diameters of pipe, different pipe material, and lengths. Moreover, the coefficient of discharge varies with the change of downstream flow condition from free flow into drowned flow. The coefficient of discharge varies

95

+ 5 - 0-8 SECTION FOR ONE PIPE OUTLET

Figure 8.1. Section for Pipe Outlet.

from 0.6 to 0.78 depending upon the flow conditions and the dimensions of the pipe outlet. lnglis has found that under drowned conditions, the coefficient of discharge for an ordinary cast iron pipe of six inches diameter is almost constant and averages 0.74. The pipe outlet has the advantage over other types of outlet that it can draw the design discharge at a very low working head of 0.1 ft.

In an attempt to fix the pipe a t the bed level of the distributary, and yet obtain free fall conditions, pipes have been sometimes laid with their upstream end at the bed level of the distributary, and sloping upwards through the bank, until the lower lip of the pipe is raised 6 inches above the highest water level in the watercourse. There are, however, practical limits to the amount of slope that can be given, and it should not generally be more than 1 in 12.

Pipe outlets are generally good to use as it is cheap, is immune from tampering, and can also draw a fair share of silt if set at the bed level. The main drawback of this outlet is the variation in the coefficient of discharge with varying downstream flow conditions.

(b) The Scratchlev Outlets:

This type of outlet differs from the pipe outlet only at its downstream end. In the Scratchley Outlet (Figure 8.21 the pipe opens into a Cistern 2 or 3 f t square, a t the other end of which is fixed a cast iron or stone orifice of the correct dimensions required for the authorized discharge of the outlet. While the pipe is fixed at bed level, the orifice can be fixed at a higher level to ensure semi-modularity (free flow). If, however, the orifice is submerged, it functions in the same manner as the non- modular pipe outlet.

The formula for calculating the discharge of this type of outlet is the same as for the pipe outlet, but the coefficient C = 0.82 is used for drowned conditions.

The main objection to this type of outlet is that if the orifice were not built in stone or cost iron, the farmers would try to knock it about and enlarge it. By merely rounding the lips, they can increase the discharge considerably. Another disadvantage is the possibility of farmers making a hole in the Cistern wall and thus taking additional unauthorized discharge into the watercourse. Since, however, the outlet is easily open to inspection a t all times, these objections are not of great importance.

(c) The Open Flume Outlet:

The open flume outlet is simply a smooth weir with a throat constricted sufficiently to ensure a velocity above the critical and long enough to ensure that the controlling section remains within the parallel throat at all discharges upto the

97

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98

maximum. A gradually expanding flume is provided a t the outfall, to obtain the maximum recovery of head. The entire work is built in brick masonry, but the controlling section is generally provided with cast iron or steal bed and check plates.

Crump's open flume outlet was introduced on the Lower Bari Doab Canal in 1922 and was subsequently standardized throughout the Punjab Irrigation Branch (Figure 8.3). The length of the crest for this type of outlet is kept as 2.5 G, where G is the head above the crest. The length of the throat is fixed as 2G - .25 feet. The discharge of the open flume outlet is given by

q= CBtG'3'2'

where q = Discharge of outlet in cfs; C = Coefficient of discharge;

- - Bt G -

Width of throat; and Head above the crest. -

As the width of the open flume outlet is comparatively small, the value of the coefficient C is assumed to be less than the theoretical value of 3.09 on account of additional frictional losses in narrow flumes. The coefficient remains constant for varying heads over the crest so Long as the minimum modular head required is available. A visual test of the outlet taking its authorized discharge under full supply conditions is provided by the presence of a standing wave below the outlet. So long as a steady standing wave forms, the discharge through the outlet is independent of water level in the watercourse.

The coefficient of discharge C of this outlet varies with throat width B, and the following values of C are adopted;

For B, 0.3 to 0.2 f t , C = 2.90 For B, 0.4 to 0.3 f t , C = 2.95 For B, > 0.4 ft C = 3.00

The minimum modular head (MMH) for an open flume outlet is 0.15 to 0.2G.

The main disadvantage of an open flume outlet is that, in many cases, it is either deep or narrow, in which case it is easily blocked, or is shallow or wide, in which it fails to draw its fair share of silt.

99

100

(d) Open Flume With Roof Block:

A roof block is fitted in the gullet of an open flume at the vena contracta, but clear o f the water surface when the outlet is drawing its full supply discharge. This clearance is generally set a t 0.05 f t in the head reaches and 0.1 f t in the tail reaches of distributaries. This device enables the open flume t o start working as an orifice, as soon as the roof block comes into action wi th a resulting reduction in discharge. The discharge of this type o f outlet is given by

q= C A P

where C = Coefficient of discharge A - H - - Head above the orifice

Cross sectional area -

The value of the coefficient o f discharge is 5. With a curved shape o f roof block, the coefficient o f discharge would increase further.

The roof block device is not entirely fool proof because the reduction in discharge caused by its use can be counteracted by converting the effluent either into a jet form by an orifice with a slanting roof or into a jet f rom a narrow crested weir, provided that in each case, the jet f lows free of the block. This form of tampering can easily be done by suitably placing a slanting brick or bundle o f grass in the aperture o f the flume.

(e) Adjustable Proportional Module:

An adjustable proportional module (APM) is one of the forms o f semi-modular orifice. It consists essentially o f an orifice provided wi th a gradually expanding flume on the downstream side. The critical velocity is exceeded in the orifice and the discharge is thus independent of water level in the watercourse. A n APM outlet, introduced by Mr. Crump (Figure 8.4) in 1922, may be regarded as a long throated flume with a roof block, capable of vertical adjustment. The upstream and downstream approaches of the APM are exactly similar t o those o f Crump’s open flume outlet. The length of the crest is H + 1 ft. comprising a cast iron block wi th a base plate, two check plates, and a roof block which slides between the check plates and can be f i t ted in any position.

101

h

The discharge of an adjustable proportional module is given by

q = 7.3B,Yps

where 9 = Discharge of outlet in cusecs; Bt = Width of throat; Y = Opening of orifice; and H, = The head measured from the distributary Full and Supply

level and the tip of roof block.

Modularity in the APM is ensured by the formation of a standing wave, and so long as the wave is steady and remains clear of the exit of the orifice, the discharge coefficient 7.3 remains constant.

The minimum modular head is given by 0.82 H, - Bt/2

The experience gained with Crump’s APM outlet having its crest at 6/10 setting (0.6 times water depth in the channel) was that channels fitted with an APM outlet silted up badly. The APM as evolved by Mr. Crump has been replaced by an Adjustable Orifice Semi-Module (AOSM).

( f l Adjustable Orifice Semi-Module:

The adjustable orifice semi-module (AOSMI outlet is a modification in the structure of the APM outlet. At first, the only modification made was by lowering the setting of the outlet, which improved the silt draw. As a result of some experiments carried out on Lower Jehlum Canal, Mr. K.R. Sharma suggested certain changes in the shape of the upstream and downstream approaches. In the upstream approach, the upstream wing wall is given a sharp curve and a flare as shown in Figure 8.5. The length of the gullet is made 2 ft. The horizontal portion of the throat is omitted and a sloping glacis of 1 in 15 is substituted. Instead of the roof block having a horizontal base, a lemniscate curve with a tilt of 1 in 7.5 was proposed. This ensured the convergence of filaments, whereas in the original design, these were divergent.

The discharge formula for the AOSM outlet is the same as that of the APM outlet.

The silt drawing capacity of the AOSM outlet is fairly improved with its crest at 8/10th setting and 10/10th setting relative to the depth of water in the channel bed.

The AOSM outlet and APM outlets have a strong and substantial structure, but cases of tampering with it are not infrequent. The roof block is sometimes raised bodily and re-fixed by the village mason; however, tampering is easily detected. A

103

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104

wooden plank is sometimes inserted at the downstream side of the roof block and covered with earth and grass, thus forming an air tight roof in continuation of the roof block. This increases the discharge due to imperfect aeration of the jet.

(g] Pipe-cum Semi-module:

Pipe-cum semi-module (Figure 8.6) comprises a pipe taking off a channel and opening into a tank about 3 ft square on the other side of the bank. In the downstream wall of the tank is fitted a semi-module, which may be an open flume with the crest at a suitable level, or an adjustable orifice semi-module with any setting.

The discharge of the outlet will be equal to the discharge of the semi-module fixed at its lower end. But the head will be measured from the water level in the cistern below the pipe. There is bound to be some loss of head through the pipe , but the size of the pipe or barrel can be made sufficiently large to reduce this loss of head to a bare minimum, subject to such a velocity in the barrel as is enough to pick up the silt on the bed of the distributary.

This type of outlet has the same advantage of silt draw as that of a pipe outlet. Complete control of silt induction can be obtained by setting the pipe end at the bed level or below the bed level.

Essential Geometric Features of Outlets to Act as a Silt Extracting Device:

The essential geometric features of outlets which affect the silt drawing capacity of outlets are:

i) Position of wings; ii) Approach curves; iii) Glacis downstream; iv) v) Crest setting of outlets.

Position of roof block in case of APM or AOSM type outlets; and

i) Position of Wings;

The projection of the downstream wing, or set back of the upstream wing (Figure 8.4), are designed in such a manner that the whole mass of water moves towards the outlet upstream approach with the velocity close to the average velocity of flow in the channel. The projection of the downstream wing is essential to allow for reduction of the waterway in accordance with the discharge of the offtaking outlet and to guide the proportionate share of the irrigation water supply and

105

I I

I

I

I I

I

I I

I

106

the silt o f the parent channel into the outlet. The projection of the downstream wing, w, is given by:

w=CT(b+--) d 0 2

where

W = projection of downstream wing or width o f approach

q = discharge of outlet in cfs; Q = discharge of parent channel; d = depth of the parent channel; b = bed width of the parent channel; and C

The value o f the constant C is 2 in the case of major distributaries and 1.1 5 for minors. A suitable curvature of 1 in 5 for the design of the downstream projected wing improves the silt drawing capacity of an APM outlet.

section;

= value of a constant varying f rom 2 to 1.1 5

ii) Approach Curves:

The w id tho f the approach should be equal t o the projection o f the downstream wing so that water enters the mouth o f the outlet w i th the same velocity as it’s approach. This serves to ensure that no induced velocities are set in before water enters the mouth o f the outlet. The entry of water into the outlet should be smooth to eliminate the creation o f eddies.

The convex approach curve in the bed represents a sudden rise above the distributary bed, which is naturally diff icult for the rolling silt to jump over. The approach curve in the bed should be concave to begin with and convex as it reaches the crest. The coarse bot tom silt, when it once happens t o have an easy entry into the mouth o f the outlet, can easily be drawn over the convex part of an approach curve, as very high velocities are established just upstream o f the orifice o f AOSM and APM outlets.

iii) Glacis Downstream:

In order that an outlet may draw the maximum silt, the filaments of water leaving near the crest along the downstream glacis should have

107

higher velocities and should have free exit. Sharma proposed that the downstream glacis should be circular upto a distance of 0.4 H and then sloping 1 in 5 for a distance of 2.5 ft. tangentially to this circle. The radius of the circle is worked as 2H, where H is the head of water above the crest of the APM, AOSM and open flume outlets.

iv) Position of Roof Block:

The APM or AOSM roof block should come close, as much as possible, towards the parent channel so that the high velocities generated under the roof block exercise their effect in extracting water from near the bed of the channel to improve the silt draw.

v) Crest Setting of Outlets:

The observations regarding outlets for silt conducting power shows that it increases with a comparatively low setting of all types of outlets. The silt observation experiments on the APM outlet indicate that the average silt conducting power of the outlets with a setting of 0.60, 0.80 and at full supply depth of the channel is 100%. 110%, and 122%, respectively, based on the ratio of observed sediment concentration in the outlet section downstream and sediment concentration in the parent channel.

Performance of Various-Types of Outlets For Silt Drawing Capacity:

The outlets working in a canal network must be capable of disposing of the entire silt which enters the system. Irrigation water in a network of an alluvial canal system is continually percolating through the wetted channel perimeter along the entire length of the distribution system. The steady reduction of the total discharge on this account is not accompanied by a proportional removal of silt charge, with the result that on a tertiary canal system, the silt load if not deposited on the head reach actually increases as the water progresses further down the system, unless the outlets in the head reaches are so designed that they draw a large proportion of silt charge from the channel. In a distributary system, the absorption losses vary from 10 to 15%. Therefore, the excessive silt charge remaining in the channel must be removed by offtaking outlets and the silt conducting power of outlets should be 110% to 11 5% to enable them to draw their fair share of silt.

Various types of outlets used in the Punjab irrigation system have their peculiar performance with regard to silt extracting power with different f low conditions in the channel. It may be taken as axiomatic that the lower the setting of the outlet, compared with the bed level of the distributary, the higher is its silt drawing capacity. Therefore, pipe outlets have their advantage over other types of outlets because they

108

D (ft) 1 .o 1.5 2.0 2.5

q (cfs) 0.58 1.07 1.64 2.29

The Adjustable Proportional Module (APM) outlet's crest is set at 0.6D, which has a higher setting relative to the bed of the channel. The experience with this type of outlet shows that as the outlets on a channel were remodelled from pipe to APM outlets, the silt equilibrium of the channel was disturbed and the channel generally became silted. Subsequently, the APM outlet was replaced by the Adjustable Orifice Semi-Module and the modifications proposed by K.R Sharma, in the shape of the upstream approach and with a deeper setting, there was an appreciable increase in the silt draw of this type of outlet. The experiments by Sharma on AOSM outlets with different settings indicated the following improvements in silt drawing capacity:

3.0 3.5 4.0

3.01 3.8 4.64

109

II 11 Setting of AOSM Outlet I Silt Conducting Power

6/10th Setting

8/10th Setting

99.5 %

109.7 Yo

1011 0th Setting 1121.9 %

Pipe-cum semi-module has a great advantage over other devices. By placing the upstream end of the pipe a t the bed level, or any height above the bed level, complete control can be obtained over the silt induction by the outlet, at the expanse of head loss in the pipe, which can be reduced to about 0.1 ft for most cases. In case of deep channels, the pipe can be placed slanting, with its upstream end at or below the bed level of the distributary as required, with a slope of 1 in 12.

For semi-modularity of outlets, the available working head should be more than the minimum modular head of an outlet. The condition of minimum modular head for modularity of outlets on large distributaries for small available working heads requires higher outlet settings relative to the bed of the channel. It follows that it is difficult to satisfy the opposing condition of small head loss in the outlet and efficient silt conduction in the case of an outlet on large distributaries. To overcome this problem, the Bend Outlet was designed in 1925. The Bend Outlet is a form of orifice semi- module which can be set a t a higher setting, and with the provision of a bend pipe intake, it can draw a fair share of silt (Figure 8.7). The bend serves to change the direction of velocity from horizontal to upward vertical, which induces a direct sucking action on the bed silt, thus improving the silt draw. The experiments on this outlet show that a wooden bend with a mouth a t the bed level having 6/1Oth depth setting has an average silt drawing capacity of 108%.

Concluding Remarks:

The geometric features of an outlet have a significant effect on the silt drawing capacity of outlets. Various types of outlets have their own role in extracting/drawing silt from the parent channel. Their proper selection, depending upon the site hydraulic conditions, would have a positive impact on the channel behavior with regard to reducing sedimentation in the channel. Based on experience by the Irrigation Engineers on the silt conducting power of various types of outlets, it has been observed that an outlet having a silt conducting power of 110% to 115% would provide the means for equitable distribution of silt charge carried in a channel. Preferably, outlets having high silt conducting power should be used where high working heads are available, the watercourses have good slopes, and the silt is carried to the fields. On the other hand, when watercourses are not in good shape, the cultivators are no doubt upset, when

110

0 .- +

9 w

W -

:y: ::-:; x

-5 .Proposed extension

Longitudinal sect ion

Figure 8.7. Bend Outlet.

highly silt charged water is put into their watercourses and they tend to neglect them. Therefore, there should be a balance between the silt carrying capacity of the channel, the silt conducting power of outlets, and the silt and water carrying capacity of the watercourses to achieve the objective of equitable distribution of water and silt within an irrigation network of outlets, watercourses and subsequently of irrigation water to the fields.

\ In doing a literature survey on the subject of silt drawing capacity of outlets

used in the Pakistan canal system, it has been noted that there is a need for further research work on quantitative estimation of silt drawing capacity of various types of outlets by correlating their geometric and hydraulic parameters and the silt concentration within the parent channels. In computer simulation models and computer aided design modules, the concept of a concentration multiplier is used for distribution of sediment from the parent channel to the offtakes and outlets. Generally, assuming equitable distribution of silt from the parent channel to the offtakes and outlets, a concentration multiplier is taken as unity, whereas each outlet has its own characteristic silt conducting power depending upon its geometry and its setting relative to the bed of the channel. Therefore, it becomes necessary to extend the research work on silt drawing capacity of outlets in quantitative terms for computer simulation of channel hydraulics, including its topography.

112

BIBLIOGRAPHY

ChaDter 1:

1. A Manual of Irrigation Practice, Public Works Department, Irrigation Branch publication, 1961.

Chang, Howard H. "Fluvial Processes in River Engineering" 1988, John Wiley & Sons, U.S.A.

2.

3. Ahmad Chaudhri, Maqbool Ahmad Malik and Tariq Masood, "Instrumentation & Measurement Procedures in ACOP". Proceedings of third US-Pakistan Binational Symposium on Alluvial River Mechanics, National Science Foundation and Water & Power Development Authority, 1985.

Terms and Definitions in Sediment Transport, UNESCO Publication (1 982) 4.

Chapter 2:

1.

2.

3.

4.

5.

6.

7.

Lindley, E.S., "Regime Channels", Punjab Engineering Congress, Vol. 7,191 9.

Bose, M.K., "Silt Movement and Design of Canals", Punjab Engineering Congress, Paper No. 192, 1936.

Inglis, Claude, Sir, "The Effect of Variations in Charge and Grade on the Slopes and Shapes of Channels", Proc. International Association for Hydraulic Research, 1949.

Lane, E.W., "Stable Channels in Erodible Material", Trans. of the A.S.C.E., Vol. 103, (1937).

LeoDold and Maddock, "The Hvdraulic Geometrv of Stream Channels And Some Physiographic Implica.tions", U.S. Geological Survey.Professiona1 Paper 252, 1953.

Einstein, H.A., and Babarossa N.L., "River Channel Roughness", Trans. of A.S.C.E., Vol. 117, 1952.

Lane, E.W., "Development of Bed Roughness in Alluvial Channels" Proc. of A.S.C.E., Vol. 88, May'62.

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8.

9.

10.

11.

12.

13.

14.

Lane, E.W., "Design o f Stable Channels" Trans. o f A.S.C.E. Vol. 120, 1955.

Simon D.B. and Richardson E.V., "Resistance t o Flow in Alluvial Channels" Proc. o f A.S.C.E., Vol. 86, May 1960.

Simons D.B. and Richardson E.V., "Forms of Bed Roughness in Alluvial Channels" Proc. of A.S.C.E., Vol. 87, May 1961.

Proceedings o f American Society o f Civil Engineers, "Uniform Water Conveyance Channels in Alluvial Material" Paper No.2484, Vo1.86, Journal of Hydraulic Division, May, 1960.

S.S. Kirmani, "Design o f Silt Stable Canals", IBP Publication No.92, WAPDA, 1963.

Khalid Mahmood, M.I.Haque and M. Hamid Mehrdod, "Measurement and Analysis o f Bedforms", published in Mechanics o f Alluvial Channels, a Water Resources Publication, 1990.

Khalid Mahmood and W.L. Dorough, "Bedload Measurement Through Sequential Bed Profiles", published in Mechanics of Alluvial Channels, a Water Resources Publication, 1990.

ChaDter 3:

1.

2.

3.

4.

5.

S.L. Malhotra and P.R. Ahuja, "A review of the progress on Theory and Design o f Stable Channels in Alluvium" Proceedings o f the Regional Technical Conference on Flood Control in Asia and the Far East, F.C. Series No.3.

E.S. Lindley, XEN Punjab Irrigation, "Regime channels" Punjab Engg. Congress Proceedings-1919.

Gerald Lacey, "Regime Flow in Incohesive Alluvium" Central Board of Irrigation, Publication No. 20-1939.

Professor T.Blench, "Regime Behavior of Canals and Rivers" Butterworths Scientific Publications. 1957.

E.W. Lane, "Stable Channels in Erodible Material" first published in the proceedings o f American Society of Civil Engineers in Nov. 1935 and later as Paper No.1957, A.S.C.E. Transactions, Vol: 102 (19371, pp. 123.194.

114

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

E.W. Lane, "Design o f Stable Channels" First published as Proceeding Paper No.280 (Sept: 1953) o f A.S.C.E. and later with discussions as Paper No.2776, A.S.C.E. Transactions, Vol: 120, 1935 pp: 1234-1 279.

Discussion by G.Lacey etc. "Uniform Water Conveyance in Alluvial Material" Journal of the Hydraulic Division o f A.S.C.E. Vol: 87 HYI, Jan. 1961, pp. 187- 194.

Irrigation Branch, Punjab Paper No.10, dated 25th August, 1904.

E.S. Lindley, Executive Engineer, "Regime Channels" Punjab Irrigation Branch, Paper No.49, Punjab Engineering Congress Proceedings - 191 9, (Repeated..51.

Dr. N.K. Bose, "Silt Movement and Design o f Channels" Punjab Engineering Congress, Paper No.192-1936.

N.K. Bose & K.R. Erry, "Design of Channels in Alluvium" Punjab Engineering Congress, Paper No. 252-1 942.

G. Lacey, "Shock in Regime Channels" Paper No.233, Proceedings of the Punjab Engg. Congress - 1940.

Thomas Blench, "Scientific Irrigation Channel Design", Paper No. 187, Punjab Engg. Congress Proceedings - 1936.

Thomas Blench, "The Lacey Slope Formula" Paper No. 184, Punjab Engineering Congress Proceedings - 1935.

C.C. Englis. "Historical Note on Empirical Equations Developed by Engineers in India for Flow o f Water and Sand In Alluvial Channels" International Association o f Hydraulic Research - 1948.

Daryl B. Simons. "Theory and Design of Stable Channels in Alluvial Materials", Thesis submitted in partial fulfilment the requirements for the Degree of Doctor o f Philosphy, Colorado State University, Fort Collins, Colorado, U.S.A., May, 1957.

Dr. Mushtaq Ahmad and Ch. Abdul Rahman, "Silt Carrying Capacity of a Channel and Criteria for Canal Closure against Excessive Silt Entry" Paper No.350, Proceedings o f West Pakistan Engineering Congress, 1961.

Dr. Mushtaq Ahmad and Abdul Rahman, "Design of Alluvial Channels as Influenced by Sediment Charge" Paper No.351, Proceedings o f West Pakistan Engineering Congress, 1962.

115

19.

20.

21.

22.

Khalid Mahmood, "Design of Channels in Alluvial Soils" Issue of Indus, Feb- Mar, 1970.

PRC, CHEECHI., Study to Establish Hydraulic Design Criteria for Pakistan Irrigation System", 1986.

Naimatullah Cheema, M. Hasnain Khan and Tahir Hameed, "Alluvial Channel Redesign Procedure" Pakistan Engineering Congress Vol: 64, 1992.

Bagh Ali Shahid and F.J. Watts, "A New Approach for the Design of Alluvial Canal System in Pakistan" Pakistan Engineering Congress, Vol: 65, 1994.

ChaDter 4:

1.

2.

3.

4.

5.

6.

Khalid Mahmood,"Design of Channels in Alluvial Soils", Issue of Indus, Feb- Mar, 1970.

Dr.Mushtaq and Ch.Abdul Rehman, "Silt Carrying Capacity of a Channel and Criteria for Canal Closure against Excessive Silt Entry " Proceedings of West Pakistan Engineering Congress, Paper No.350,1960.

Report of the Technical Sub-committee for Evaluation of Alternative Hydraulic Design Procedures, March 1987.

Naimetullah Cheema, M.Hasnain and Tahir Hameed, "Alluvial Channel Redesign Procedure",Proceedings of Pakistan Engineering Congress,Vol:64,1992.

Howard H. Chang, "Fluvial Processes in River Engineering" John Wiley and Sons, 1988.

Khalid Mahmood, "Stable Sandbed Canals" The World Bank, Agriculture Production and Services Division, Washington DC, 1990.

ChaDter 5:

1. A Manual of Irrigation Practice, Public Works Department, Irrigation Branch Publication, 1961.

2. Alluvial Channel Observation Project "Procedure for Observations and Transmittal of Field Hydrological Data" (ACOP Publication-1 2), June 1978.

116

3. Alluvial Channel Observation Project," Guide t o Hydrological Practices in ACOP", (ACOP Publication-48), June 1978.

4. Ahmad Masud Choudri, Maqbool Ahmad Malik and Tariq Masood, "Instrumentation and Measurement Procedures in ACOP". Third US-Pakistan Binational Symposium on Alluvial River Mechanics, National Science Foundation and WAPDA, 1985.

Khalid Mahmood, "Stable Sand Bed Canals" World Bank Publication, 1990. 5.

ChaDter 6:

1. K.R. Sharrna, " Remodelling of Mithalak Distributary", Proceedings of Punjab Engineering Congress, Paper No. 154, 1932.

2. A.W.M. Jasson, "Remodelling Distributaries Irrigated by Colony Canals", Proceedings of Punjab Engineering Congress, Paper No.230, 1940.

A Manual of Irrigation Practice, Public Works Department, Irrigation Branch Publication, 1961.

Desilting Campaign on Self Help Basis, Government of the Punjab, lrrigationand Power Departrnent,l994-95.

Report of a Process Documentation Study,Discussion Paper 7 "The Punjab Desiltation Campaign during 1992 Canal Closure Period",IIMI Pakistan,Sept 1992.

3.

4.

5.

ChaDter 7:

1. K.R.Sharrna, "A Silt Selective Distributary Head Regulator" Proceedings of Punjab Engineering Congress, Paper No.189, 1936.

F.F.Haigh, "Silt Excluders", Proceedings of Punjab Engineering Congress, Paper

I

, 2. No.211, 1938

3. Mushtaq Ahrnad, Moharnrnad Ali & Abdul Khaliq, "Sediment Exclusion Methods and Devices at the Intake of Canals", Proceedings of West Pakistan Engineering Congress, Paper No.341, 1960.

117

4. Mushtaq Ahmad, "Design of Silt Excluders and Silt Ejectors", Proceedings of West Pakistan Engineering Congress, Golden Jubilee Publication, 1963.

Abdul Shakoor and Saeed Ahmad, "Silt Ejectors or Sand Traps, Hydraulic Model Study with Special Reference to Taunsa Barrage Project", Publication of Irrigation Research Institute, 1990.

5.

ChaDter 8:

1. K . R . Sharma, "Silt Conduction by Irrigation Outlets", Proceedings of Punjab Engineering Congress,Paper No. 168,1933-34.

S.K Sharma, "Design Of Irrigation Structures", 1988 2.

3. S.I.Mahbub and N.D Gulhati,"lrrigation Outlets", Proceedings of Punjab Engineering Congress, Paper No.264, 1944.

A Manual of Irrigation Practice, Public Works Department, Irrigation Branch Publication, 1961.

4.

118

IIMI-PAKISTAN PUBLICATIONS

RESEARCH REPORTS

Report #

R - 1

3-2

Title

Crop-Based Irrigation Operations Study in the North West Frontier Province of Pakistan (Volume I: Synthesis of Findings and Recommendations

Crop-Based Irrigation Operations Study in the North West Frontier Province of Pakistan (Volume 11: Research Approach and Interpretation)

Crop-Based Irrigation Operations Study in the North West Frontier Province of Pakistan (Volume 111: Data Collection Procedures and Data Sets)

Salinity and Sodicity Research in Pakistan - Proceedinos of a one-dav Workshoo

Author

Carlos Garces-R D.J. Bandaragode Pierre Strosser

Carlos Garces-R Ms. Zaigham Habib Pierre Strosser Tissa Bandaragoda Rana M. Afaq Saeed ur Rehman Abdul Hakim Khan

Rana M. Afaq Pierre Strosser Saeed ur Rehman Abdul Hakim Khan Carlos Garces-R

IIMI-Pakistan

3-3 Farmers' Perceptions on Salinity and Sodicity: A case study into farmers' knowledge of salinity and sodicity, and their strategies and practices to deal with salinity and sodicitv in their farmino svstems

Neeltje Kielen

3-4

3-5

1-6

Modelling the Effects of Irrigation Management on Soil Salinity and Crop Transpiration at the Field Level (M.Sc Thesis pulished as Research Report)

Water Distribution at the Secondary Level in the Chishtian Sub-division

Farmers Ability to Cope with Salinity and Sodicity: Farmers' perceptions, strategies and practices for dealing with salinity and sodicity in their farming systems

S.M.P. Srnets

M. Amin K. Tareen Khalid Mahmood Anwar lqbal Mushtaq Khan Marcel Kuper

Neeltje Kielen

Year

June 1994

-

- June 1994

- June 1994

Mar 1995 - May 1996

June 1996

July 1996

- Aug 1996

119

Report #

R-7

Title

Salinity and Sodicity Effects on Soils and Crops in the Chishtian Sub-Division: Documentation of a Restitution Process

Author

Neeltje Kielen Muhammad Aslam Rafique Khan Marcel Kuoer

- Year

Sept 1996

-

R-12

R-13

R-14

Khalid Riaz I Sept I Robina Wahai 1996 R-8 I Tertiary Sub-system Management:

(Workshoo oroceedinos)

Modeling of Sediment Transport in Irrigation Canals of Pakistan: Examples of Application (M.Sc Thesis published as Research Report)

Methodologies for Design, Operation and Maintenance of Irrigation Canals subject to Sediment Problems: Application to Pakistan (M.Sc Thesis published as Research Report)

Government Interventions in Social Organization for Water Resource Management: Experience of a Command water Management Project in the Punjab, Pakistan

A-9

R-10

R-11

Mobilizing Social Organization Volunteers: An Initial Methodological Step Towards Establishing Effective Water Users Organization

Canal Water Distribution at the Secondary Level in the Punjab, Pakistan (M.Sc Thesis published as Research Report)

Development of Sediment Transport Technology in Pakistan (An Annotated Biblioaraohv)

Mehmoodul Hassan Zafar lqbal Mirza D.J. Bandaragoda

Oct 1996

Steven.Visser

M. Hasnain Khan 1996

Gilles Belaud

Alexandre Vabre

Waheed uz Zaman D.J.Bandaragoda

- Oct 1996

- Oct 1996

-

h

120