In this article we will discuss about:- 1. History of Falls 2. Location of Falls 3. Types 4. Classification 5. Selection 6. Cistern 7. Roughening Devices Used 8. Off-Take Alignment 9. Design of Sarda Type Fall 10. Design Steps for Straight Glacis Fall.
Contents:
- History of Falls
- Location of Falls
- Types of Falls
- Classification of Falls
- Selection of the Type of Fall
- Cistern Design of Fall
- Roughening Devices Used in Falls
- Off-Take Alignment of Fall
- Design of Sarda Type Fall
- Design Steps for Straight Glacis Fall
1. History of Falls:
In old days, modern scientifically designed falls were not in existence. The excess fall of the general country was used to be adjusted or dispensed with by increasing the length of the canal by giving circuitous alignment to the canals. Eastern Yamuna canal in India, constructed during Mughal period, does not have any fall and canal alignment is very much circuitous. Development of falls started late in the 19th century when large irrigation projects like Ganga, Cauvery, Eastern and Western Yamuna canals were constructed.
A brief description of each fall which came into being one after the other is being given here:
1. Ogee Type Fall:
This fall was first constructed on Ganga canal. The crest of the fall was kept at level with the U/S bed of the canal. A smooth concave surface was provided on the D/S side, to provide smooth transition and to reduce impact and disturbance. The energy of water passing over the fall, remained preserved and thus deep scourings for very long lengths on D/S side were used to develop. Crest of the fall being at the U/S bed level, there used to be considerable draw down effect on the U/S side.
2. Rapids:
This fall consists of a sloping glacis. The slope is kept from 1 in 10 to 1 in 20. Because of long sloping glacis, the formation of hydraulic jump is assured. It is hydraulic jump formed on D/S side, which carries out the dissipation of excess energy. This fall worked very well but the major drawback of this fall was its high cost.
3. Stepped Fall:
As its name indicates, it consists of a number of steps. The total fall is broken into a number of steps of smaller falls. This fall also has the disadvantage of being very costly. As far as its performance is concerned, it is very satisfactory.
4. Notch Type Fall:
This fall consists of notches situated in a high crested wall, built across the channel. The fall may be having only one notch or a number of notches depending upon the discharge to be handled. All the notches have smooth entrance and flat circular lips projecting D/S side to disperse water.
This fall maintains the depth discharge relationship and does not cause any draw down on the U/S side. This fall continued to be in use for quite a long time. One of the major defects of these falls was that they cannot be used as regulators.
5. Vertical Drop Falls:
In such falls the nappe of falling water impinges vertically into the water cushion developed on the D/S side. Nappe does not remain in contact of the crest wall on D/S side. In such falls, excess energy of falling water is dissipated by turbulent diffusion in the pool of water. Sarda type fall and C.D.O. type fall, are the usual examples of vertical drop falls.
6. Glacis Type Fall:
In such falls a sloping ramp is provided D/S of the crest. Because of slope, formation of hydraulic jump is assured. The energy dissipation in such falls is carried out with the help of hydraulic jump.
The glacis type falls may be divided into two categories:
(i) Straight glacis type, and
(ii) Parabolic glacis type.
(i) Straight Glacis Type:
This fall consists of a straight sloping ramp at D/S. If a baffle platform and baffle wall are provided on the D/S side, it is known as Inglis fall. In such falls the formation of jump is a must on the baffle platform.
(ii) Parabolic Glacis Type:
The fall having parabolic D/S glacis is known as Montague type fall.
2. Location of Falls:
The location of the fall on a channel depends upon the general slope of the country through which channel passes.
Following points should however by considered while deciding the location of the fall:
1. In the case of main canals which do not do irrigation directly, the position of the fall should be located by considering economy in cost of excavation of the canal with regard to balancing depth and the cost of the fall itself. Main canal should not be allowed to run into much filling. The canal being very large, any breach in it may cause lot of damage to life and property.
2. In case of branches and distributaries, which do direct irrigation, the falls are located considering command area. The F.S.L. of the canals should remain above the G.L. for most of the length. However the F.S.L. of channel D/S of fall may remain below G.L. for some length. The area D/S of the fall for which F.S.L. of channel remain below G.L., is commanded by taking outlets from U/S of the fall.
3. Location of the fall may be slightly varied so that its construction may be combined with a regulator or bridge.
4. The falls may be provided large in number but of smaller fall or they may be smaller in number but of larger fall. Both the alternatives should be weighed and adopted.
3. Types of Falls:
The falls may be divided into following two types:
1. Meter Falls:
The falls which can be used to measure the discharge flowing over them, are known as meter falls. Such falls must have broad crest so that the discharge coefficient remains constant under variable head. Glacis type falls are most suitable as meter falls. If section of the channel at the site of the fall has to be flumed, from economic considerations, smooth U/S transition should be provided to avoid turbulences and also to maintain accurate stage discharge relationship.
2. Non-Meter Falls:
The falls which cannot be used to measure the discharge passing through them, are known as non-meter falls. Vertical drop falls cannot be used as meter falls, as some vacuum is developed under the nappe of falling water, which causes non-uniform flow conditions.
4. Classification of Falls:
The falls can be classified into following four categories according to the approach conditions:
1. Falls in which D-Q Relationship is Maintained:
In such falls there is neither drawdown nor heading up of water on U/S side. The Depth-discharge relationship for the channel is maintained. Trapezoidal notch falls and low crested rectangular notch falls, fall under this category.
2. Falls which Maintain Fixed F.S.L. on the U/S Side of the Fall:
Siphon fall or Siphon spillway high crested weir fall are the falls which fulfill this condition. Such falls causes silting on U/S side.
Such falls are necessary under following circumstances:
(i) When a sub-channel has to take off at some distance U/S of the fall.
(ii) When Hydro-electricity is being generated using drop in the fall.
3. Combination of Fall and Regulator:
In such falls the level of water on U/S side, can be varied at will. The regulation is done with the help of vertical needles, horizontal stop logs and sluice gates. They are used as regulators also. They are installed at locations where some sub-channel is to take off from the parent channel and also fall is to be provided on the parent channel. The falls of this category are exclusively the rectangular notch falls fitted with sluice gates.
4. Miscellaneous Type Falls:
The falls of this category suit to some specific conditions. Well or cylindered fall is one such fall. It consists of a vertical well connected to D/S side with the help of a horizontal pipe. Water enters the vertical well from U/S side and escapes into Tail water on D/S side. The energy is dissipated by turbulences in the well. They are quite suitable for low discharges but large drops.
Fall may be inform of a steep sloped channel or pipe. The U/S water surface and D/S water surface are connected by steeply sloped open channel or closed pipe.
5. Selection of the Type of Fall:
The type of fall should be chosen considering following points:
(i) Vertical drop fall is found most suitable for discharges upto 15 cumecs and drops upto 1.5 m. It should not be flumed.
(ii) Glacis fall with baffle wall has been found suitable for all discharges and drops of more than 1.5 m. It can be flumed.
(iii) Straight glacis fall, without baffle wall, is found suitable for drops upto 1.5 m and discharges upto 60 cumec. They may be flumed on economic considerations.
Hence knowing the drop and discharge, the most appropriate fall may be selected for use.
Out of all the falls discussed earlier only vertical drop and straight glacis falls are found in most common use. Others have become obsolete and out of date.
6. Cistern Design
of Fall:
The depressed portion of the floor lying just D/S of the crest wall, of the fall is known as cistern. The main purpose of cistern is to dissipate the surplus energy of water falling or gliding from the crest of the fall.
Slope of the glacis (if any), the cistern, and roughening devices, are the elements which are used is one way or the other, to dissipate the surplus energy of water.
Cistern for Vertical Drop Fall:
In this case there is no sloping glacis and water falls vertically after passing over the crest. The falling water nappe impinges against the water filled in the cistern and its direction gets changed from vertical to horizontal. The energy is dissipated by means of impact and deflection of flow suddenly from vertical to horizontal direction.
Cistern performs following two functions (Fig. 21.9):
(i) It helps reduce intensity of impact of falling nappe of water and thus protects the D/S floor from being damaged.
(ii) The water in the cistern acts as a cushion against falling nappe of water and destroys its energy.
Following are some of the formulae which are used to fix the length and depth of the cistern:
(a) U.P. Irrigation Research Institute Formulae (Fig. 21.9):
(b) Etchevery Formulae:
These formulae were developed in U.S.A.
(c) Montagu’s Formula:
They were developed in Punjab.
(d) E.L. Glass Formulae:
They were developed for Bihar and Orissa.
In all the above formulae:
E = U/S Total energy above the crest
Lc = Length of the cistern
X = Depth of cistern below general floor level on D/S side
Ef2 = Energy of flow D/S for the discharge intensity q and the fall HL
HL = Fall in the U/S and D/S water levels. (Fig. 21.9)
Cistern for Glacis Type Falls:
In case of glacis type falls the energy is dissipated by hydraulic jump and not by impact as in case of vertical drop falls. For a given drop the energy line HL, and discharge intensity q, there will be a definite value of D/S specific energy or D/S depth required for the formation of jump.
In straight glacis falls, the glacis is sloped usually at 2:1 slope. Theoretically the cistern should be provided at the lowest level of the glacis where the jump may be formed. The position of the jump in a horizontal bed is not stable and there may be instances when jump is not formed on the bed but quite distant on D/S side.
This may cause bed and bank scours. To safeguard against such eventualities, the depth of the cistern is increased to 1.25 Ef2,. Thus level of the cistern is fixed by deducting 1.25 Ef2 from the total energy line on D/S side. If, however, the bed level on D/S is lower than the bed level of the cistern so obtained, the cistern should be provided at D/S bed level.
Length of the cistern is kept 5 Ef2 for normal soils, to 6 Ef2 for sandy soils. If bed level of cistern is lower than the D/S bed level of the channel, bed level of the cistern and that of channel should be connected by giving a slope of 5:1.
Cistern for Glacis Fall with Baffle Platform and Baffle Wall:
In this case dimensions of cistern, baffle platform and baffle wall are fixed as follows. In such falls baffle platform is formed just D/S of sloping glacis, then baffle wall, and lastly the cistern.
Discharge intensity (q), fall (HL) and depth of subcritical flow for unflumed fall are connected by-
Baffle Wall:
7. Roughening Devices
Used in Falls:
In case where hydraulic jump is not formed, and vertical fall conditions are also not there, the energy dissipation can be achieved by roughening devices, provided on the D/S side of the fall.
Following are some of the roughening devices mostly used:
1. Baffle Wall:
It is a weir type wall which is constructed across the channel at the D/S end of the cistern.
Its main purposes are two-fold:
(i) It keeps water head up on u/s side so as to ensure the formation of jump.
(ii) To withstand the thrust of high velocity which has not yet subsided.
2. Friction Blocks:
They are rectangular concrete blocks, adequately anchored with the floor. They are provided just D/S of the crest. They form a most simple and useful device of energy dissipation.
They act as follows:
(i) To divide the bottom high velocity of water laterally.
(ii) To reduce the velocity of water leaving the pucca floor of the fall.
(iii) To help in the formation of jump in case of glacis falls.
Recommendations about friction blocks for various falls are:
(a) Vertical Fall:
Two rows of friction blocks in staggered fashion are found adequate. They should be provided at 1.5 Dc from D/S toe of the crest where Dc is critical depth.
Length, breadth and height of the blocks should be 2Dc, Dc, Dc respectively.
Clear spacing between rows should be Dc and between blocks of the same row 2 Dc.
In addition to friction blocks, cube blocks may be provided at the end of the pucca floor. Size of cube blocks is kept about 1/8th of the depth of water on D/S side.
Arrows may be used in place of friction blocks. Arrows are triangular in plan with tapper towards the top. Top is also sloped a little.
(b) Glacis Fall:
In case of flumed glacis fall, the friction blocks are generally provided in four rows, the first row being at a distance of 5 times the height of the blocks from toe of glacis. Height of the block is kept 1/8th depth of water on D/S side length 3h where h is the distance between rows.
Glacis Blocks:
Glacis blocks are just like friction blocks. They are provided just before the D/S toe of the glacis in one row. The effect of these blocks is to reduce the turbulences. The size of these blocks is Dc x 2Dc in plan.
Montagu gave following formula for finding the distance (L) upto which roughening of bed by friction or arrow blocks is required.
D1 = Depth of cistern
D2 = Depth on D/S side
HL = Drop or fall.
C = A constant whose value is 1 for vertical fall, 3 for horizontal impact, 4 to 6 for inclined impact and 8 for no impact falls.
3. Dentated Sill:
If high velocity still persists after cistern, a dentated sill should be provided at the end of the cistern. The object of such a sill is to deflect up the high velocity from near the bed and to break it.
4. Deflector:
It is a wall of height D3/10 provided at the D/S end of glacis falls. Its object is to deflect up the high velocity jet near the bed causing a reverse roller action.
5. Biff Wall:
It is also provided at the end of cistern. Its object is to deflect back the water from the cistern.
6. Cellular or Ribbed Pitching:
It is an arrangement of projecting bricks on the sides of the channel. This device roughens the perimeter of the channel to destroy surplus energy, D/S of the fall.
8. Off-Take Alignment of Fall:
When any distributary or branch takes-off from a parent canal, the alignment of the off take of both the canals should be carefully decided.
The requirements of a best off take alignment are as follows:
1. There should be minimum of disturbance water.
2. Both parent as well as off taking canals, should share the silt load according to their discharges.
3. There should be no scouring or silting on U/S of the off take.
The off-take alignment is considered best when it makes zero angle with the parent channel and then separates out form of transition curve. Transition curves, should be given to both off taking and parent canals.
If somehow it is not possible to made transition curves, then both the canals should make an angle with the U/S alignment of the parent channel.
If parent channel cannot be given any angle and has to be kept straight, the angle of off-take should be decided in relation to the edges of the parent channel rather than centre line. In this case section of the parent channel should be narrowed down from one edge only and not from both the edges equally.
When edge of the parent channel in which off-take head regulator is not located, is narrowed down, a silt jetty is formed just U/S of the cross-regulator, on that edge. This alignment should be avoided as far as possible.
9. Design of Sarda Type Fall:
This fall was used for the first time on Sarda canal in U.P. and hence named Sarda type fall. In the area where Sarda canal runs, a thin layer of sand-clay layer lies above the stratums of pure sand. In such conditions, falls of large drop are not feasible to be constructed. Large numbers of falls with small drops were constructed so as to avoid deep cuttings. A complete design of Sarda type fall is shown in Fig. 21.12.
In the design of Sarda type fall, following elements are required to be designed:
1. Design of Crest Wall:
Length of the crest wall is not flumed. Crest length may be equal to bed width of the canal. To allow for possible future increase in discharge, the length of crest wall may be kept equal to bed width plus depth. For discharges up to 14 cumecs the shape of the crest wall is kept rectangular with D/S side absolutely vertical. For more than this discharge shape of the crest wall is kept trapezoidal with D/S face batter of 1: 8 and U/S batter of 1:3.
Rectangular Crest:
Trapezoidal Crest:
Determining Crest Level:
Calculate the value of H from formula (1) or (2) as the case may be.
R.L. of crest = U/S F.S.L. — H
Height of crest above bed h = D — H.
For falls over 1.5 m the stability of the crest wall should be tested by actual analysis.
2. Design of Cistern:
Find out length (Lc) and depth (x) of the cistern by following formulae-
3. Design of Impervious Floor:
Total length of impervious floor is determined by Bligh’s theory or by Khosla.s theory. The critical condition of seepage head occurs when water is filled upto crest level and there is no water on D/S side. Out of total impervious length, a minimum length (ld) determined as follows must be provided on D/S side of the crest-
ld = 2(D + 1.3) + HL in metres.
HL = Drop in metres and D is water depth on D/S side.
Some of the remaining impervious floor automatically comes under crest wall, and rest on U/S side of the crest.
Thickness of Floor:
The thickness of floor is found out on the basis of uplift pressure. U/S impervious floor does not theoretically require any thickness as uplift is counterbalanced by weight of water. However a minimum thickness of 30 cm to 40 cm is provided from practical considerations.
Thickness of the D/S floor is worked out on the basis of actual uplift pressure. If thickness worked out on the basis of uplift pressure is very small, minimum thickness of 40 cm for small falls and 60 cm for large falls must be provided.
Full uplift pressure should be allowed for working out the thickness of impervious floors in case of falls resting on permeable soils, while 75% may be taken in case of ordinary or black cotton soils.
The top of cistern and D/S impervious floor should in addition he lined with bricks on edge either in lime or cement mortar. A vertical cut off, at least 1.5 m deep, must be provided at the D/S end of the impervious floor.
4. Up Stream Protection:
Brick pitching on the U/S side of the crest is laid on a slope of 1 in 10 upwards on U/S side. The length of this protection may be 2 to 4 m. The minimum length should be equal to U/S water depth. A drain hole should be provided in the crest wall at level with the U/S protection. This hole drains off water from U/S side to D/S side during canal closures.
5. U/S Approaches:
For falls with less than 15 cumec discharge, the U/S approach wings may be splayed straight at an angle of 45°. For larger discharges, the wings are kept segmental. The radius of the segment is kept 5 to 6 times H and its angle at the centre should be 60°. After this, the wing walls are carried into the berm tangentially and embedded in it for a length of about 1 m. The foundation of the wings is kept on the impervious floor itself.
6. D/S Protection:
The D/S protection consists of following elements:
(i) Bed Protection:
It is a protection to the bed, provided by dry brick pitching 20 cm thick laid on 10 cm thick ballast. The lengths of bed protection for different heads along with curtain wall details are given in Table 21.1.
(ii) Side Protection.
After the wing walls, the side slopes of the channel are pitched with one brick on edge in the length equal to three times the D/S water depth. The D/S masonry wing walls are warped from vertical to a slope of 1:1. If banks are sloped at 1 ½ : 1 the side pitching has to be warped from a slope of 1:1 to 1 ½ : 1. The pitching is supported on toe wall ½ brick thick. The depth of the toe wall is kept half of the depth of water on D/S side.
(iii) D/S Wing Walls:
They are kept vertical for a length of 5 to 8 times the √EHL from the crest. Thereafter they are gradually warped or flared to a slope of 1 :1 or 1 ½ : 1. Average splay of 1 in 2.5 to 1 in 4 for achieving the required slope is given to the top of the wings. The wings generally follow the circular path starting tangentially from the starting point of warp in plan.
Wing walls should be designed as retaining walls. For heavy structure they should actually be designed, but for normal conditions the thickness at any depth should not be less than 1/3, the height of the wing wall above that depth.
10. Design Steps for Straight Glacis Fall:
1. Design of Crest:
(i) Crest Length:
Glacis type falls may be flumed when they are to be combined with bridge so as to cause economy.
The fluming may however be limited to the following percentage of bed width:
(ii) Crest Width:
If fall is to be used as a meter a broad crest should be adopted. The minimum width of the crest should be kept 2.5 E.
For non-meter falls the crest width should be 2/3 E.
2. U/S Approach:
(i) Curved Side Walls:
Glacis falls may be used as meters or non-meters.
For meter glacis fall, wings are curved and curvature of the wings is kept 5 to 6 time E. The segment of the curve forms an angle of 60° at the centre and thereafter carried straight into the embankment.
For non-metre fall, the U/S side walls may be splayed at an angle of 45° from U/S edge of the crest and carried into the embankment for about 1 m.
(ii) Hump:
The bed curve of the approach may be curved or straight sloped.
In case of meter fall, bed curve should start from th same cross-section as the side curves start. The radius of hump or bed curve should be equal to-
For non-meter falls the bed approach in sloped at ½ : 1 slope and joined tangentially to the U/S edge of the crest with radius E/2.
3. U/S Protection:
If glacis falls are not flumed, no protection is generally required on the U/S side. If fall is flumed, both bed and sides should be protected for a length equal to U/S water depth. It may be in form of a dry brick on edge laid at a slope of 1 in 10 in the bed.
4. Discharge Formula:
The discharge passing over the crest is worked out from the following formula.
Q = CLtE3/2
where Q is the discharge in cumecs, Lt is the effective length of the crest. If there are piers on the crest, the effective length of the crest is worked out by deducting 0.2 n from the total length of the crest.
Lt = L — 0.2 in, where L is the total length of the crest and n is the number of piers.
Value of C for broad crested fall is Liken as 1.70 and with narrow crest i.e. equal to 2/3 E, its value is taken as 1.84.
Using equation Q = 1.84 Lt E3/2 value of E can be found out.
5. Crest Level:
Crest level = Total U/S T.E.L. – E
Total U/S T.E.L. = U/S F.S.L. + velocity head
If crest level works to be too high, the fall may be flumed or if it is already flumed, the fluming ratio should be further increased so that crest is not higher than 0.4 times the U/S water depth.
6. D/S Glacis:
D/S glacis may be curved or straight sloping. The main purpose of glacis is to ensure formation of hydraulic jump at its toe. Theoretically the glacis slope should be such that it imparts maximum horizontal acceleration and thus ensures maximum dissipation of energy. Montague gave following equation for curved profile-
Where, x = horizontal ordinate measured from the D/S edge of the crest
y = vertical ordinate measured below crest level
where q = intensity of discharge per metre length of the fall.
It is very difficult and costly to construct a curved profile and as such is not used much. A straight glacis with 2 : 1 slope may be used in place of curved profile.
If baffle platform with baffle wall is to be used, the slope of glacis may be increased to 2/3 : 1 to ensure the formation of jump in the baffle platform.
For meter falls glacis with 2 : 1 slope should be used even if baffle wall is introduced. The sloping glacis should be joined to D/S edge of the crest with curve of radius E. The same curve should be used for joining D/S end of glacis with the horizontal floor.
7. Cistern Design:
Length of cistern = 5 to 6 Ef/2
R.L. of cistern = D/S T.E.L. – 125 Ef2
Ef2 is the energy of flow.
8. Length and Thickness of Impervious Floor:
It is designed according to safe exit gradient and uplift pressure on the basis of Khosla’s theory.
9. D/S Protection:
(i) Bed Protection:
No bed protection is required on D/S side as deflector wall is generally provided at the end of the floor. The height of deflector wall above D/S bed should be D3/10 where D3 is the depth of water on D/S side channel.
(ii) Side Protection:
Dry brick pitching should be done on sides for a length of 3D3. The pitching should be supported on a toe wall 40 cm wide and D3/2 deep subjected to a minimum depth of 50 cm.
The end should be protected with curtain or profile wall 40 cm thick and D3/2 depth.
10. D/S Expansion:
Vertical sides are carried straight to the toe of glacis or end of baffle platform if provided. Thereafter the expansion of the walls starts. The most preferred expansion is rectangular hyperbolic. The bed width Bx at any distance x from the D/S toe of the glacis or from the beginning of the expansion,
where Le = total length in which expansion is to be done. However in case of small falls, straight 1 in 3 expansion may be provided.
The side walls in expansion may be splayed out from vertical to 1 : 1 and extended into the berm.
11. Energy Dissipators:
Friction blocks, glacis blocks and deflector walls should be used to dissipate the surplus energy.