1. RIVER TRAINING AND DEVELOPMENT

River TrainingVarious measures are adopted on a river to stabilize the river channel along a certain alignment with a certain cross-section. These measures are required to be adopted because rivers in alluvial plains frequently alter their courses and cause damage to land and property adjacent to their banks.

1.1 Objectives and purposes of river training worksThe main objectives of river training are as follows.

1. To provide safe passage for flood discharge without over flowing the banks for protection of cultivated or inhabited areas.

2. To prevent out-flanking of works like a bridge, weir or aqueduct constructed across the river and to bring the river on to the work in a straight non tortuous approach

3. To protect the banks from erosion and to improve the alignment by stabilizing the river channel.

4. To deflect the river away from the bank which it might be attacking.5. To provide minimum depth of flow and a good course for navigation purposes.6. To transport efficiently the bed and suspended sediment load.

1.2 Classification of river training works:1. High water training 2. Low water training3. Medium water training

High water training: It is also called training for discharge. To provide for expeditious disposal of maximum flood and provide protection against damage due to floods. It is mostly concerned with the most suitable alignment and height of marginal embankments for disposal of floods. It also includes measures for channel improvement

Low water training: It is used for providing sufficient depth for navigation during low water season. This is achieved by contracting the width of the channel and is usually carried with the help of groynes. Thus it can also be called “training for depths”.

Medium water training: It is also called training for sediment. Medium water training is undertaken to provide efficient disposal of bed and suspended sediment load and thus to preserve river channel in good shape.

Out of the three types of river training Medium water training is most important. This is so because a river training work adopted to alter the river cross-section and alignment must obviously be designed in accordance with stage of river at which maximum movements of the sediment takes place during any period under consideration. Although there is maximum activity of the bed of the river at high stage of flow, such stage is maintained only for a short duration. On the other hand, there is a little movement of sediment at low stages which persists for a long duration. In between the two, there is stage at which combined effect of forces causing sediment

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movement and the time for which such forces are maintained is maximum. This is therefore the most important stage as it has considerable influence on the river configuration.

1.3 Different methods of river training worksVarious methods of river training including the bank protection are as follows

1. Marginal embankment or levees2. Guide banks or guide bunds3. Groynes or spurs4. Cut offs5. Pitching of banks and provision of Launching aprons6. Pitched island7. Miscellaneous methods such as sills, closing dykes, bundling, etc

1.4 Study of hydraulic structures such as levees, dykes, and erosion control structures

Marginal embankment or leveesThese are earthen embankments provided to confine the flood water of the river within the cross-section available in between the embankments. Thus spreading of flood water beyond the marginal embankment is prevented. The alignment should follow the normal path of meandering of the river. The retirement (spacing away from the main channel) of the embankments is governed by technical (B>4.75Q0.5) as well as human considerations, as the land falling on the river side of the embankments remain unprotected. The section of embankment is usually governed by considerations for design of earthen dams. If the marginal embankments are likely to come in contact with high velocity of flow then the water side of the embankment should be provided with pitching protection. Provide launching apron if the embankment is close to the main river channel. For retired embankments both of these are unnecessary.

Effects of marginal embankments on river flow during floodsThe effect of confining of the flood waters of a river between marginal embankments or levees is:1. To increase the rate at which the flood wave travels down the stream.2. To increase the water surface elevation at floods.3. To increase the maximum discharge at all point downstream.4. To produce the water surface slope of the stream on the upstream of the levee portion.5. To increase the velocity and scouring action through the levee sections.

Some of these effects will depend upon the embankment which is located too close or too far from each other.

Merits of river training by marginal embankments1. They prevent the spreading of flood water over large areas of flood plain. The spreading of

flood water cause considerable hardships to the inhabitants of the flood plain (damage their house and property). This is avoided by providing the embankments

2. Initial cost of the embankment is low, although when raised subsequently they become expensive.

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3. Construction of embankment is easy and presents no difficulty, as it can be done by utilizing local materials and unskilled labour.

4. Embankments may be constructed in parts, provided the ends are properly protected.

Demerits of embankments1. Embankments cause raising of high flood levels2. Embankments may fail by piping due to burrowing by small animals like crabs, worms, and

rats etc. As such they need to be supervised closely during flood and protected as soon as they are in danger.

3. In event of breach there is a sudden and considerable inflow of water which may cause damage to the neighbor-hood and may result in the deposition of considerable quantities of sand rendering vast areas unproductive. More over embankment breaches may result in flooding almost the entire areas, protected by the embankment system.

4. In a flood plain unprotected by embankments the flood water is spread over the plain during every flood season and leaves a deposit of fine silt behind them as the recede. Thus the land gets benefited by way of inundation irrigation as well as adding new fertile soil during every flood season. When protected the land in the flood plain would be deprived of these benefits.

Design of levees:Spacing between levees: B > 4.75*Q0.5

Level of top of levee = HFL + Free Board (= 1m to 1.25m)Top width of levee > 3m, width is decided as per useSide slope depends upon:

(1) Nature of material of which levee is composed(2) Method of construction(3) Length of the time the levee is likely to be subjected to the wave action.

Usual slope provided: (a) slope on the river side 3:1 to 5:1 (b) Land side 4:1 to 7:1Banquette: is a terrace of earth added to the base of high levee on land side to prevent danger of sloughing from seepage.

Groynes or Spurs:Groynes are stone, gravel, rock or pile structures built at an angle to a river bank to deflect flowing water away from critical zone to prevent erosion of the bank, to establish a more desirable channel for flood control, navigation & erosion control. They are used on wide, braided rivers to establish a well defined channel that neither aggrades nor degrades nor shifts its location from year to year. In this case the groynes may have long dikes at their outlets to help define the channel and many kilometers of a river are controlled by groynes. Groynes are also used on meandering rivers to control flow into or out of a bend or through a crossing.

Groynes do not restrict the regularized channel in a continuous way but produce new bank lines. Since groynes separate the spaces between the regulation line and the bank either making the flow difficult or impeding the velocity of water in these spaces, they trap bed load and built up new river banks. These are also called spur dikes or transverse dikes.

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Classification of groynes1. Classification according to the material of construction

(a) Permeable groynes(b) Solid impermeable groynes

2. Classification according to its height below flood level(a) Submerged groyne(b) Non submerged groyne

3. Classification according to the function it serves:(a) Attractive groyne (inclined d/s)(b) Deflecting groyne (set at right angles)(c) Repelling groyne

4. Special types of groynes(a) Denheys T – headed groyne(b) Hockey type groyne(c) Burma type groyne

Purposes of groynesThe purpose has already been detailed earlier, it is summarized as under:

(1) Contacts a river channel & improves its depth.(2) Protects the river bank.(3) Silts up the area in the vicinity by creating a stack flow

Permeable Groynes: They permit flow of water through them. They dampen the velocity and thus reduce the erosive action of the stream. In case river water carries sediment load, the groynes causes deposition of sediment load near it because of reduction of velocity. It could then be called SEDIMENT GROYNE.

Advantages of permeable groyne:1. Cheap Construction2. Small quantities of stone are required for its construction and hence it is useful where stone is

Cheap scarce.3. Better performance with respect to solid impermeable groynes4. Groyne does not change the flow abruptly; hence there is no serious formation of eddy and

scour.5. It is more suitable for deep and narrow rivers.6. Submerged permeable groynes do not create turbulence and eddy conditions as in case of

submerged solid groynes.

Disadvantages:Not strong enough to resist shock & debris & logs. They are more suitable for the upper reaches of the river.

Types of permeable groynes: (1) Tree Groyne (2) Pile GroyneTree Groyne: A tree groyne consists of trees held in position by thick wire ropes anchored firmly on the bank and tied to a heavy buoy. The trees used for the groynes should be very leafy with abundant branches. Branches, roots and twigs help trap sediment by reducing the flow

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velocity. A tree groyne should be completed before the early floods. Tree groynes have been successfully used:

(1) To divert or deflect the current which is directly threatening erosion of a bund.(2) For closing a river channel and opening the other.(3) To silt up a channel of the river at its source by checking the flow in it.

Pile groyne: These groynes are constructed with timber piles driven down in the river bed up to 9m depth. The groynes consist of main verticals in two or three rows braced together by transverses and diagonals. The timber main verticals are spaced 2 to 3m apart in 2 or three similar rows separated 1.5m to 2m apart. Between the main verticals there can be two intermediates embedded at least 1.5m below bed. These are filled with alternate layers of bush wood & stones. Sometimes they catch silt, become sand bound and start acting as solid groynes. To safeguard it against scour in such a case, it is protected by stone apron 1m thick having length of 3m along shank and 6m along the nose. Permeable groynes are suitable for silt laden rivers. If the river has to be confined in a defined channel generally impermeable groynes are used.

Non-submerged groynes & submerged groynes:

Non-submerged groynes: Height > HFL. For training rivers & protection of banks generally non submerged groynes are used.

Foe deep rivers, depth is large, HFL is too high and as such submerged groynes can be used where Height < HFL.

Submerged groynes could be either permeable groyne or solid groyne. Submerged permeable groynes are considered better than solid groynes since former do not create turbulent & eddy conditions as strong as with the latter. Where river bed is scoured submerged groynes can be used as a curative measure.

Submerged groynes = submerged sills = submerged dykes.

Classification according to function served:(1) Attracting groyne: A groyne pointing downstream tends to attract the river flow

towards the bank on which it is provided. Hence the name is attracting groyne. Such groyne causes the scour hole to form closer to the bank than the groyne inclined at right angles to the bank or inclined slightly upstream, and therefore it tends to maintain deep current close to the bank. Attracting groynes usually make an angle of 600 with the bank. However the angle of inclination of Attracting groyne may be in the range of 300 to 600. Further in this case the main attack is on the u/s face. Hence u/s face needs better protection compared to d/s.

The attracting groyne safeguards the opposite bank against the attack of the current as they attract the current towards the bank attached to them. Attracting groyne is not useful for bank protection and may sometimes even endanger the adjacent banks since silting between successive groynes is absent, it is not commonly used.

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(2) Repelling groyne: A groyne pointing u/s tends to repel the river flow away from the bank on which it is provided (hence the name). Angle of inclination of Repelling Groyne (R.G) with bank varies from 600 to 800. On the u/s side of R.G. a still water pocket is formed and suspended sediment carried by the river gets deposited in the pocket. The head of R.G. causes disturbance in the flow at its nose and heavy scour at the nose slightly d/s of it due to eddy formation. Head of R.G. needs a very long protection because it is subjected to direct attack of a swirling current. As compared to Attracting Groyne, Repelling Groyne are more effective and do not cause any trouble. As such R.G. are commonly used for purposes of bank protection and river training.

(3) Deflecting Groyne: A groyne either perpendicular to the bank or pointing slightly u/s and having relatively a short length tends to only deflect the flow without repelling it. A deflecting groyne only gives local protection.

Special types of groynesDenehy’s or T-Head groyne: Usually spaced 800m apart, its upstream cross groyne length > downstream cross groyne length.

Hockey Head groyne or Hockey groyne: These have curved heads such that its shape is like hockey stick.

Central considerations in the design of groynes

(1) Length of groyne: The length of groyne depends upon the position of original bank line and the designed normal line of the trained river channel. Too long groynes constructed out of easily erodible material are liable to damage and they may be extended gradually as silting between them proceeds.

(2) Spacing: 2 to 2.5 times the length of groyne at convex banks. Equal to the length of groyne on concave banks. Use larger spacing for wider rivers than narrow rivers. Also spacing between groynes should not exceed five groyne length. A spacing of about 2 groyne length results in well defined channel for navigation. Larger the ratio of groyne to river width, stronger the local acceleration and thus greater hindrance to shipping. Permeable groynes can be spaced wider apart than solid impermeable groynes. An approximate formula for spacing ‘L’ is suggested as [(2gL/c2h) < 1] where h is depth of mean discharge and c is Chezy’s coefficient (approximate value 40).

(3) Size and spacing best determined from model test

(4) Series of groynes more useful than single

(5) Number of groynes: If the reach of the river to be protected is long or if single groyne is not strong enough to deflect the current nor quite effective for silt deposition upstream or downstream of itself, groynes when constructed in series with proper spacings are quite effective as they create a pool of nearly still water between them which resists the current and gradually accumulates silt forming a permanent bank line in course of time. While a series of

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groynes is quite useful for protecting a bank, a single groyne suitably placed may be useful for holding the river to a fixed point such as under a bridge or preventing its outflanking.

In most cases a series of STREAM DEFLECTORS (VANES) constructed of wood panels or metal (i.e. floating drums with sheet metal vanes) placed at suitable angle (often almost parallel to the bank) and depth, can be used to either divert an eroding flood from river bank or on the other hand to induce bed erosion & local deepening of the flow.

Pitched Island:A pitched island is an artificially created island in the river bed. It is protected by some stone pitching on all sides. A pitched island is created with sand core and boulder lining. To protect it from scouring, a launching apron is also provided. The location, size and shape of pitched islands are usually decided on the basis of model studies. Pitched island serves following purposes:

1) Creating an oblique approach upstream of weirs, barrages and bridges by training the river bed to be axial.

2) Rectifying adverse curvature for effective sediment exclusion.3) Redistributing harmful concentration of flow for relieving attack on marginal bunds,

guide banks and river bends, etc.4) Improving the channel for navigation.

A pitched island causes scour around it and thus redistributes the discharge on its two sides. Pitched island upstream of weirs and barrages have been found to be quite effective.

Bank erosion protectionRiver bank protection may be carried out by

(i) Planting(ii) Faggoting (Faggots or Fascines are bundles of branches, usually willows)(iii) Thatching(iv) Mattresses, and articulated concrete mattresses(v) Rubble stone pitching(vi) Watt ling(vii) Gabions(viii) Bagged concrete and concrete slabs, Flexible Brick pitching(ix) Asphalt slabs, asphalt, asphalted concrete(x) Soil cement blocks(xi) Geotextiles: Woven or non woven fabrics, meshes, grids, strips, sheets and

composites of different shapes & constituents

It is important to appreciate that any protective facing of banks must be continued to the river bed and be provided with footing.Governing criterion of bank erosion protection is wave height. Preferred type of protection is ‘Dumped rock riprap’. Hand placed stone pitching (where labor cost is low), however, ‘Dump rock riprap’ is better. In the absence of availability of rock at a reasonable cost, alternatives of concrete or asphaltic facing or soil cement protection may be used.

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Dumped rock riprap (dumped stone riprap)

Riprap of hard angular rock fragments is laid on a thick layer of rubble or quarry chips. Experience has proved that dumped riprap is the preferred type of slope protection for earth dams. It must be well graded from rock spalls to the maximum size required, and be composed of dense, sound, durable rock fragments with acceptable shape factors. Except in rare instances the riprap must be underlain by bedding layers of finer materials designed to act as filters to prevent the embankment materials from being washed through the interstices in the riprap. The size of the rock in the riprap blanket, to achieve satisfactory performance, will depend upon the magnitude of the wave action upon the dam.

Most design specifications for the riprap relate the criteria for selection of rock size and thickness of riprap layer directly to the design of wave height. The following is an example of such a tabular type of specification.

General requirements(1) For embankment slopes 2:1 to 4:1 dumped riprap shall meet the following criteria

(recommended by US Army Corps of Engineers).

Maximum Wave height (m)

Average size D50

(m)Maximum rock size (kg)

Layer thickness (m)

0.00 – 0.30 0.20 45 0.310.30 – 0.60 0.25 91 0.380.60 – 1.20 0.31 227 0.461.20 – 1.80 0.38 680 0.611.80 – 2.40 0.46 1134 0.762.40 – 3.00 0.61 1814 0.91

(2) Riprap should be well graded from a maximum size of at least 1.5 times the average rock size to 2.5 cm spalls suitable to fill voids between rocks

(3) Riprap blanket shall extend to at least 2.4m below lowest water level, for river banks up to bed and 3 to 4m on river bed as toe trench.

(4) Rocks should be un-weathered, specific gravity greater than 2.60, should meet soundness and density requirements for concrete aggregate.

(5) Filter shall be provided between the riprap and embankment soils to meet the following criteria.

Maximum wave height (m) Minimum D35 size for filter (cm) Filter thickness (cm)0.00 – 1.20 2.50 – 3.50 15.101.20 – 3.00 3.80 – 5.001.20 – 2.40 - 22.502.40 – 3.60 - 30.00

(6) No filter is needed if embankment material meets the above requirements for D35 size. A filter may not be required if the embankment consists of CH (Plastic clay) or CL (clay, low plasticity) with liquid limit greater than 30, resistant to erosion. Filter helps to minimize the effect of frost heave or draw down pore pressure.

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(7) Thickness of riprap layer(i) Greater than 1.5*D50 rock size of weight W30

(ii) t = [Wmax/γr]**(1/3)(8) Failures may occur if there is

(i) Segregation of large, over size stones in the pockets in the layer, allowing the bedding materials to be washed through the riprap.

(ii) Segregation of small, undersize stones in areas of riprap layer during placements, permitting holes to be formed in the layer.

(iii) No extension of primary riprap enough down the slope to below the line of attack at minimum pool level

Hand placed riprap Hand placed riprap consists of a single layer of a stones fitted together in a fashion similar to that for dry rubble masonry and laid on a layer of finer bedding material. Not better than dumped stone riprap.

Soil cement slope protection

Soil cement slope protection consists of a series of approximately horizontal layers of soil cement compacted to 15cm in stair step fashion up the embankment slope. The layers are usually 2 to 3m wide, compacted to 15cm vertical thickness and placed and compacted by standard soil handling construction equipment. Soil with a wide range of gradations may be successfully used for soil cement. Organic soils and those containing a high percentage of alkali-reactive minerals should be avoided. The cement content varies from 7 to 15% by volume of soil cement depending upon soil characteristics.

Other protection measuresPre cast concrete blockConcrete pavementBituminous Pavements

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Fig. 1.1 Section of a levee

Fig. 1.2 Attracting groyne

Fig. 1.3 Section of groyne

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Fig. 1.4 Pitced islandGuide bank:

Guide banks are made for guiding the stream near a structure so as to confine it in a reasonable width of the river. It was first designed by Bell in whose honor it is also sometimes known as Bell’s Bund. The design was further developed by Spring and is known as Guide bank.

The guide bank usually consists of a heavily built embankment in the shape of bell mouth on both sides of constricted channel. Usually only one embankment is required if the other side is a high and stable bank.

1.5. Design consideration in construction of guide banks. The design of a guide bank involves the following considerations:

The ultimate width to which an alluvial river can be constricted may be computed from the relation

L = 4.75 , where Q is the maximum discharge in cumecs, L is the width of the channel in meters. An extra allowance of 20% may be given for thickness of bridge piers etc. Length of upstream guide bank = 1.25 L to 1.5 L Length of downstream guide bank = 0.25 L

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Fig. 1.5 Guide bank

Fig. 1.6 Layout of Guide bank

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The two guide banks on both sides of stream can be made (a) parallel, (b) diverging, (c) converging at upper and (d) single guide bank depending upon local conditions. Both upstream and downstream ends of the banks are curved gently so as to lead the flow along smooth lines. The main parts of a guide bank are:

(i) u/s curved head or impregnable head(ii) d/s curved head(iii) Shank or a straight portion which join two curved heads(iv) Slope and bed protection

(i) U/S curved head:The u/s end curvature has a central angle of 1200 to 1400. It is suggested that the value of R can be calculated from the relation R = 0.45 L depending on river velocities.

(ii) D/s curved head: The curved d/s head ensures safety of approach embankment. The radius of curvature of the d/s head may be kept as half of the u/s radius with a central angle of 450 to 600.

(iii) Section of guide bund and material of construction

The material of construction of guide bank is the river sand available locally. Side slope is kept 2:1 to 3:1. It should have sufficient top width which should not be less than 4m so as to provide sufficient carriage way. The free board usually adopted for the design varies from 1.25 to 1.5m. The inside slope is protected by stone pitching, the total thickness of which varies from 0.4 m to 0.6 m. The stone pitching is 1 m above HFL. The thickness of stone pitching (T) as recommended by Inglis is given by

The thickness of the pitching should be 25% more at the impregnable head than for the rest of the bank.The rear side of the shank portion is not pitched, but is generally coated with 0.3 m to 0.6 m earth for encouraging vegetation growth so as to make it resistant against wind and rain.

(iv) Slope and toe protection

The toe of the slope is protected by the Launching apron. The quantity of stone in the apron should be sufficient to cover a scoured face fully after the apron fans out while launching at a slope of 2:1 with a thickness of 1.25T. It is generally laid in a width equal to 1.5 D where D is the depth of maximum anticipated scour below the bed, and has a thickness of 1.9 T. The maximum anticipated scour below HFL is taken as C.R where R is the Lacey’s normal scour depth given by the relation

,

where Q is the discharge and f is the silt factor. The value of C is generally assumed as 2.25 at the noses of the guide banks, 1.50 at the transition from the nose to the straight portion and 1.25

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in the straight reaches of the guide bund. Inglis and Joglekar gave the following values for maximum scour depth:

Maximum scour depth downstream of bridge:

Maximum scour depth around bridge piers:

Maximum scour depth at nose of guide banks of a large radius:

Artificial cutoff:To improve the flood capacity of a given reach and thereby reduce flood level in certain reaches, an artificial cutoff may be introduced as a river training measure. This is generally done when the loop length exceeds 1.5 to 2.5 times the chord length. Initially a pilot cut is provided to carry 8 to 10% of discharge and are permitted subsequently to develop the carrying capacity to about 40% to 50% of the total river discharge. The design alignment and the cross section are governed by the following considerations:

The direction of cut should be tangential to the main direction of flow. Alignment of cut should be much flatter than the curvature of river. Entrance to cutoff should be given bell mouth shape. The Lacey’s regime formula will be considered for cross-section of cutoff. The pilot section should be made very deep as deeper sections develop rapidly.

Example: Design a guide bank required for a bridge on a river having the following particulars:

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Design flood discharge = 50000 cumecsSilt factor = 1.10

Bed level of river = 130.00mHigh flood level = 140.00m

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Solution:Q = 50000 cumecs; f = 1.10Lacey’s water way = 4.75 = 1062 m.Provide 20% more length to account for thickness of piers and end contractions due to piers and abutments, = 212 mCross length between banks = L = 1274 m.

Upstream length of guide bank =

Downstream length of the guide bank =

Radius of the u/s curved head = 0.45 L = 573 m.The u/s end of the guide bank may, therefore, be curved at 1400 with the radius of 573m.Radius of the d/s curved head may be kept as 287m with an angle of 600 at the center.

Cross-section of the guide bankThe given HFL at the bridge site = 140.00mAssuming free board = 1.5 m,Level of top of guide bank equals 141.5 m. To be more safe and making an allowance for future settlement etc., the final level of top of guide bank is taken as 142.00 m.

Height of river bank above river bed = 142.0 – 130.0 = 12.00mKeep the top width of the bank as 4.0 m and side slope as 2.1 (as per requirement and site condition).

The stone pitching and the launching apron must be provided on the slope of the water side through the bank length. The rear side of the bank must also be pitched on the curve portion of the band.

Design of stone pitching and apron

The thickness of stone pitching is given by

This can be kept 1.0 m above HFL, i.e., up to level 141.00 m

Depth of scour, R =

For straight reach of guide bank, Maximum scour = C.R., where C = 1.25 for the straight portion.Maximum scour = 1.25 R = 1.25 x 16.77 = 20.97 m.R.L. at maximum anticipated scour = 140 – 20.97 = 119.03 m.Depth of maximum scour, D = 130 – 119.03 = 10.97 m.Length of apron = 1.5 D = 1.5 x 10.97 = 16.45 m.

For curvilinear transition portion of guide bank, Maximum scour = C.R., where C = 1.5 for the transition from the nose of the guide banks to the straight portion.

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Maximum scour = 1.5 R = 1.5 x 16.77 = 25.16 mR.L. of maximum scour = 140 – 25.16 = 114.84 mDepth of maximum scour D = 130 – 114.84 = 15.16 mLength of apron = 1.5 D = 1.5 x 15.16 = 22.74 m.Thickness of launching apron = 1.9T = 1.9 x 2.21 = 4.2 m.

2. RIVER NAVIGATION

2.1 Scope and definition

In the modern times, towing through water is not only required for transporting purposes, but is also required for recreational boating. However, boating and floating of ships through the natural rivers is not always safe. Rapids and sandbars may create problems, and may require considerable time to be passed. Isolated rocks, fallen trees, debris and other obstructions may create constant hazards, and may damage or even wreck the boats, streamers, or ships being towed through such waterways. It is, therefore, absolutely essential that all the waterways through which the boats or ships are to be towed, must be made completely safe.

The chief requirement for navigating though a waterway is the availability of sufficient water depth in the waterway. A minimum water depth of about 2.7 meters is generally required for navigating safely and economically; although a depth of about 3.7 meters is generally aspired in the final developments of a navigable waterway. Availability of lesser depth in the rivers may completely eliminate the possibility of towing the ships through such rivers or may cause increased unit cost of transport.

2.2 Various requirements of navigable waterways

There is no rigidity about the requirements of a good navigable waterway, since it al depends upon the extent and type of traffic likely to pass through it. However, the various general requirements are enumerated as below:

(1) Sufficient water depth is available so as to pass the more heavily loaded barges cheaply and economically.

(2) The width of the waterway is sufficiently more than the width of the tow itself.

(3) The radii of the bends should not be sharp and should be high enough to allow the maximum length of the ship to pass through them.

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(4) The alignment of the waterway should be as straight as possible, because a highly irregular alignment increases the circuitry or length in excess of air line distance which the barge tow must travel. The existing channels generally have a length about 50 percent greater than the air-line distances.

(5) The flow velocities should not be high, as they may cause substantial reduction in the true speed for tows moving upstream and thereby increasing the time of transit and the cost of transport per kilometer. The speed of most of the barge tows in still water is of the order of 2.8m/sec. The flow velocities of the order of 1 m/s may, therefore, cause sufficient reduction in true speed (i.e. 2.8 – 1.0 = 1.8 m/s) and hence, should not exceed such a value.

(6) In order to minimize the transit time, the time required for the tow to pass through locks should be minimum. In certain cases, where the lock is not large enough to accept the entire tow, the tow is generally broken and taken through the lock in potions. This increases the time lost in locking and thereby increasing the transit time and the cost of transport. Hence, sufficient sized locks should be ensured for economic and better transport.

(7) Efficient and adequate terminal facilities for unloading the barges for transferring the cargo effectively must be ensured for economic and better navigation.

2.13 Various measures adopted for achieving navigability

There are three basic methods which are generally adopted for improving a river for navigation. These are:

(1) Open channel methods(2) Lock and dam arrangements(3) CanalizationThey are described below:

(1) Open channel methods. In the open channel methods, the existing waterway is improved to such an extent as to make navigation possible. This improvement natural waterway is possible only if the following conditions are satisfied:

(i) Sufficient discharge is available in the river throughout the year or at least for a reasonable portion of the year.

(ii) The existing river is having a satisfactory alignment without excessively sharp bends.

(iii) The river bed slope is reasonably flat so that the flow velocities are not excessive. (i.e. they are within 1 m/s or so).

(iv) The river width is not too small and is such that it can be improved economically for modern barge tows.

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(v) The material of the river bed and banks should permit satisfactory treatment by one or more of the open channel methods.

If the above requirements are approximately satisfied, the channel can be economically improved and made fit for navigation. But if the available conditions are far too short of requirements, open channel methods may prove to be highly uneconomical, and, therefore, should not be considered. However, these requirements and factors may be controlled to some extent by some suitable measures. Say for example, if the discharge in the river during lean periods is very low, while the average annual flow is adequate, reservoirs may be constructed so as to store water and augment the supplies during lean weather flows. Similarly, very sharp bends may be eliminated by cut off channels, provided the resulting channel slopes remain within limits.The various works and techniques that may by involved in improving the channel by the open channel methods are

(a) Constructing and Regulating the flow through storage reservoirs(b) Excavation and Dredging.(c) Contraction works(d) Bank stabilization.(e) Straightening the waterway by artificial cut offs.(f) Removal of snag, debris and other obstructions

These techniques are generally required together as one of them may rarely provide the necessary required improvement. These techniques are described below:

(a) Storage reservoirs. The storage reservoirs generally store water during high flows and can release the required amount of water during lean-flows, so as to make downstream navigation possible even during periods of low weather flows. However, the construction and planning of storage reservoirs for navigation alone is not generally justified economically. Hence, reservoirs are mostly planned under multipurpose projects, where navigation may be one purpose of that project. Moreover, the storage reservoirs can augment low supplies for navigation, only if the reservoir is situated at the head of a relatively short navigable reach.

This is because; as the distance from the reservoir to the navigable river-reach increases, reservoir-releases have to be increase so as to allow for transit losses due to seepage, evaporation, etc. The releases must also be made much in advance so as to allow for travel time to the navigable reach and their quantity has to be sufficient even after reduction due to channel storage.

(b) Excavation and dredging. Huge amounts of excavations are generally required for clearing sand bars and filled channel sections in order to make it fit for navigation. Besides the basic initial excavations, continuous desilting and proper maintenance is required in order to keep the waterway fit for navigation. These excavations from the bed and banks of the waterway are generally carried out by dredging by means of dredgers. Three types of dredgers are generally used. They are:

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(i) Dipper dredgers. They are merely floating power shovels and are used on small projects.

(ii) Ladder dredgers. They have an endless chain of buckets for bringing the excavated material up to the surface. The cuttings carried by buckets are discharged on a belt conveyor which is disposed of through a stacker conveyor at the rear of the dredger. Since the stacker conveyors (generally called spoil stackers) are limited in length to about 100 meters or so, ladder dredgers cannot be used when the excavated material (i.e. spoils) are to be discharged at a considerable distance from the dredge.

(iii) Suction dredgers. In these dredgers, the cuttings and water are collected in suction pipes, and the mixture is then discharged by pumping through a pipe supported by floats (called spoil pipe) into the desired spoil area. A line diagram of this operational process is shown in Fig. 2.1.

Fig. 2.1 Line plan of an ordinary suction dredge called Dust pan dredge.

A section dredge cannot operate in rocky or boulder river reaches. The suction head of these dredges is provided with jets or rotating blades so as to loosen the bed material and also with suction openings through which the soil and water mixture enters into the suction pipe. These dredgers can make cuts of about 10 m wide through sand bars, and various such parallel cuts can be made in order to achieve a wider channel.

(c) Contraction works. Contraction works are those engineering works which are constructed in order to change a wide shallow river into a narrow deep river; or to close off the river creeks (small branches) and thus to divert the entire water into the main river. When the bed and bank material of a river is course grained with little cohesion; a shallow wide channel, or at low water, a number of channels will develop. Such situations may be corrected with the help of spurs or groynes. Under the process, rivers

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carrying huge sediment loads can be corrected with the help of properly placed permeable spurs called sal Balli Dykes made of sal ballies, driven at some suitable distance center to center in rows across the river current and braced at top. The function of permeable sal balli dykes is to slow the current and thus promote silting in the dyked area. The concentration of flow in the narrower section also encourages deepening of channel. Several years are allowed for the effect of the structures to develop.

Similarly, the rivers carrying a little sediment load can be corrected by properly placed impermeable spurs or jetties which shall divert the flow, thereby confining the entire water in a smaller width and thus deepening the same.

(d) Bank stabilization. A good navigable channel must have stable banks. When the river banks are not stable and start caving, the river starts meandering, creating bends, which may obstruct the path of longer barge tows. Moreover, scouring at concave banks and silting at convex banks take place due to meandering. Hence at bends, sufficient depth will prevail at least near the concave side. But the targets, i.e. the crossings jointing the two successive bends, will definitely develop shallower channels with cross bars by the deposition of sediment scoured from the upstream bend. It is in these crossings that the controlling depths for navigation occur.

Spur or groynes, when suitably and intelligently placed, may prove to be useful in bank stabilization; because a spur placed along the concave bank shall promote silting. Banks may be protected more easily by pitching or by revetments. The entire concave bank is generally protected by pitching. The loosely dumped stone called apron or riprap is generally used, and it is extended from top of bank to beyond the toe of the underwater slope. This extension of revetment in the bed is essential so as to avoid the failure of revetment due to scour and consequent undermining of the underwater edge of the revetment.

The revetment must be flexible so as to adopt itself to the surface on which it is placed. Moreover the revetment must be relatively impervious so as to avoid, the washing of fines through it. It must also be strong enough to resist the flow currents. Various types of revetments are used. Concrete mattresses in the form of concrete blocks placed in wire meshes may sometimes be used, when ordinary stone dumping over a graded filter is not provided due to non-availability of stone in the nearby areas. Uncompacted asphalt paving is also finding a use in developed countries, and is under serious investigations. Compacted asphalt paving and monolithic concrete paving are not generally used, as they are liable to be cracked and damaged by uplift pressures.

(e) Straightening by artificial cut-off. Since the development of a cut-off eliminates sharp bends which are undesirable for navigation, artificial cut-offs may sometimes be used advantageously. A pilot cut is made and allowed to develop (Fig. 2.2). These cut offs have been used with success to avoid future caving and meandering.

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Fig. 2.2 Development of a cut off(f) Removal of snag, debris and other obstructions. Presence of debris, trees, isolated

rocks, and other obstructions, not only pose a direct hazard to the barge tows, but also promote the formation of sand bars. They must, therefore, be removed effectively in order to ensure safe and economical navigation. Different methods and equipments may be used in different cases, depending upon the circumstances of each case. Tractors, winches, derrick barges, explosives, etc. may be required in the process of clearing the waterway obstructions.

(2) Lock and dam arrangements. The arrangement consists of dams which create a series of slack water pools through which the traffic can move with locks to lift the vessels from one pool to the next. Lock and dam construction may be adopted where existing site conditions are not favourable for adopting open channel methods described earlier. This arrangement is a second choice to open channel methods. In this arrangement, water is required for lockages, sanitary releases, evaporation, percolation, etc. This requirement of water is much less than that required for open channel procedures. Hence, when the available water is less, these arrangements may have to be adopted.

The slack water pools behind the dams will submerge the rapids and channel bends and thus overcoming those problems. Further, because of their relatively large areas of cross-sections, the velocities in these pools shall be low enough as to cause lesser reduction in true speed of the barge tow moving upstream.

Lock and dam arrangements are suitable only on rivers bringing only a little sediment load. This is because; highly silt laden river water shall fill up the pools rapidly. Moreover, suitable sites for construction of small dams must be available for providing such arrangements.

(3) Canalization. A totally new channel cut is provided artificially around an otherwise impassable obstruction or between two navigable rivers. Such a cut is generally economical only when a short length of new channel opens a large length of existing waterways. Construction of a new channel connection between two existing waterways is also sometimes adopted, so as to ensure a continuous traffic way. However, canalization

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is a costly process, as the per kilometer cost of canal, capable of passing modern barge tows, is normally very high, and are adopted when very short lengths are required.

3. CANAL FALLS AND DIVERSION HEAD WORK STRUCTURES3.1. Canal falls

3.1.1. Definition. Whenever the available natural ground slope is steeper than the designed bed slope of the channel, the difference is adjusted by constructing vertical falls or drops in the canal bed at suitable intervals.

Fig. 3.1 A fall or drop

A drop in a natural canal bed will not be stable and, therefore, in order to retain this drop, a masonry structure is constructed. Such a structure is called a canal fall or a canal drop.

3.1.2. Proper location. The location of a fall in a canal depends upon the topography of the country through which the canal is passing. In case of the main canal, which does not directly irrigate any area, the site of a fall is determined by considerations of economy in cost of excavation and filling versus cost of falls.

The excavation and filling on two sides of a fall should be tried to be balanced, because the unbalanced earthwork is quite costly. By providing a larger drop in one step, the quantity of unbalanced earth work increases but at the same time the number of fall reduces.

An economy between these two factors has to be worked out before deciding the locations and extent of falls.

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In case of branch or distributary channels, the falls are located with consideration to commanded area. The procedure is to fix the FSL required at the head of the off-taking channels and outlets and mark them.

The FSL of the canal can then be marked, as to cover all the commanded points, thereby deciding suitable locations for falls in canal FSL, and hence, in canal beds.

The location of the falls may also be influenced by the possibility of combining it with a bridge, regulator, or some other masonry work; as such combinations often result in economy and better regulation. When a fall is combined with a regulator and bridge, it is called a fall-regulator with road bridge.

3.1.3. Types of falls

Various types of falls have been designed and tried since the inception of the idea of fall constructions came into being. The important types of such falls, which were used in olden days or are being used in modern days, are described below:

(1) Ogee falls.

The ogee type fall was constructed in olden days on projects like Ganga canal. The water was gradually led down by providing convex and concave cures, as shown in Fig. 3.2.

Fig. 3.2 Ogee fall

The performance of such a fall was found to have the following major defects:(i) There was heavy draw-down on the upstream side, resulting in lower depths, higher

velocities and consequent bed erosion. Draw-down may also affect the supply in a distributary situated just upstream of fall.

(ii) Due to smooth transition, the kinetic energy of the flow was not at all dissipated, causing erosion of downstream bed and banks.

Later, it was converted into a much better type called Vertical Impact type.

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(2) Rapids.

In some areas, long rapids at slopes of 1:15 to 1:20 (i.e. gently slopping glacis) with boulder facings, were provided. They worked quite satisfactorily, but were very expensive, and hence became obsolete.

(3) Trapezoidal notch falls.

The trapezoidal notch fall was designed by Ried in 1894. It consists of a number of trapezoidal notches constructed in a high crested wall across the channel with a smooth entrance and a flat circular lip projecting downstream from each notch to spread out the falling jet (Fig. 3.3).

Fig. 3.3 Trapezoidal notch fall

The notches could be designed to maintain the normal water depth in the upstream channel at any two discharges, as the variation at intermediate values is small.

Hence, the depth-discharge relationship of the channel remains practically unaffected by the introduction of the fall.

In other words, there would neither be drawdown nor heading up of water, as the channel approaches the fall.

These falls remained quite popular, till simpler, economical, and better modern falls were developed.

(4) Well type falls or cylinder falls or siphon well drops.

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This type of a fall consists of an inlet well with a pipe at its bottom, carrying water from the inlet well to a downstream well or a cistern.

The downstream well as shown in Fig. 3.4 is necessary in the case of falls greater than 1.8 m and for discharges greater than 0.29 cumecs.

The water falls into the inlet well, from where it emerges near the bottom, dissipating its energy in turbulence inside the well.

Fig. 3.4 Siphon well drop

These types of falls are very useful for affecting larger drops for smaller discharges. They are commonly used as tail escapes for small canals, or where high leveled smaller drains do outfall into low leveled bigger drains.

(5) Simple vertical drop type and Sarda Type falls.

A raised crest fall with vertical impact (Fig. 3.5) was first of all introduced on Sarda Canal System; owing to its economy and simplicity.

The necessity for economic falls arose because of the need of construction of large number of smaller falls on the Sarda Canal System.

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Fig. 3.5 Simple vertical drop fall

(6) Straight glacis falls.

In this type of a modern fall, a straight glacis generally slopping 2:1 is provided after a raised crest. The hydraulic jump is made to occur on the glacis, causing sufficient energy dissipation. They are suitable up to 60 cumecs discharge and 1.5 m drop.

(7) Montague falls.

The energy dissipation on a straight glacis remains incomplete due to vertical component of velocity remaining unaffected. The Montague profile is given by the equation,

(3.1)where X = The horizontal ordinate of any point of the profile measured from the d/s edge of crest.

Y = Vertical ordinate measured from the crest level. U = Initial velocity of water leaving the crest.

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Fig. 3.6 Montague type fall

(8) Englis falls or baffle falls.

A straight glacis type fall when added with a baffle platform and baffle wall as shown in Fig. 3.7, was developed by Englis. They are suitable for all discharge and for drops of more than 1.5 m. The baffle fall is provided at a certain height and distance from the toe of the glacis, so

as to ensure the formation of the jump on the baffle platform.

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Fig. 3.7 Baffle fall or Englis fall3.1.4 Design principles of various types of falls1. Design of a trapezoidal notch fall

A notch fall provides a proportionate fall. The whole width of the channel is divided into a number of notches. The crest may be kept higher than the bed level of the canal, but the weir openings should not exceed the bed width of the canal upstream.

Discharge formula. The discharge passing through one notch of a notch fall can be obtained by adding the discharge of a rectangular notch and a V – notch.

Fig. 3.8 A sketch showing a trapezoidal notch

The discharge passing through a trapezoidal notch such as given in Fig. 3.8 is given by

(3.2)

If is represented by n, then

(3.3)

where Cd = Coefficient of discharge ≈ 0.75

or (3.4)

The above discharge equation contains two unknowns l and n. For solving this equation, two values of Q and corresponding values of H must be determined. It is a common practice to

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design notches for full supply discharge (Q100) and half supply discharge (Q50) with values of H equal to the normal water depths in the channel in the respective cases. Let the normal water depths in the channel be represented by y100 and y50 respectively. Then H100 = y100, and H50 = y50.

The depth of water in the channel at 50% discharge (i.e. y50) can be approximately evaluated in terms of full supply depth (y100) as follows:

Let (Kennedy’s Eq. for vel. in channels)

Now Using

We have

or

and

or

or

(3.5)

Number of notches. The number of notches should be so adjusted by hit and trial method that

the top width of the notch lies between to full water depth above the sill of the notch.

Notch piers.

The thickness of notch piers should not be less than half the water depth and may be kept more if they have to carry a heavy super structure. The top length of piers should not be less than their thickness.

Example 3.1. Design the size and number of notches required for a canal drop with the following particulars:

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Full supply discharge = 4 cumecsBed width = 6.0 mF.S. depth = 1.5 mHalf supply depth = 1.0 mAssume any other required data if required.

Solution. The bed width of the canal is 6 m. Each notch at top should be roughly equal to F.S. depth i.e. 1.5 m. So let us, in the first trial, provide 3 notches.

Full supply discharge through each notch = cumecs.

From equation (3.4) we have

Usingwhere

We haveoror (i)Now, using

where

we haveor (ii)

Subtracting (ii) from (i) we get

Putting the value of n in (ii) we get

or say, 0.25 m.

By this trial, we find the top width

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In other words, the values of l and n will become 3 times, when number of notches are reduced 3 times. Thus, when we provide only one notch instead of 3 notches, the values of n and l will triple.

Similarly, when we use 2 notches against 3, i.e., 2/3 times, the values n and l will become 1.5 times of those obtained for 3 notches.

Hence when we use 2 notches, values will be

and Top widthSince the top width is still quite low, we may use only one notch.

When we use only one notch, the values will ben = 3 x 0.13 = 0.39

l = 3 x 0.25 = 0.75 mTop width = 3 x 0.45 = 1.35 ≈ FSD (O.K.)

Since this condition gives us top width = 1.35 m, which is o.k., we may provide one notch, centrally placed in the given channel of 6 m width. The section of notch to be adopted is shown in Fig. 3.9.

Fig. 3.9 Section of notch (Example 3.1).

2. Design of a siphon well drop

A siphon well drop is adopted for smaller discharges and larger drops. The main features of the design involve determining the size of the inlet well and that of the pipe. A suitable size for the outer well, a proper provision of water cushion at the bottom of the inlet well, the bed and side slope pitching in the canal upstream as well as downstream, for suitable lengths, are also

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provided. The size of the inlet well and that of the siphon pipe are determined on the following considerations w.r. to Fig. 3.10.

Fig. 3.10 Considerations in siphon well drop.

First of all, the size of the trapezoidal notch is determined so as to pass the designed discharge by using eqn. (3.4) in the same way. Then let V1 be the velocity over the notch, V2 be the velocity of entry in the pipe, and V3 be the velocity through the pipe. All these values of velocities can be determined easily as below:

The head loss between the inlet well and the d/s FSL is then given by HL1 as

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Knowing all the above values, HL1 can be determined, and thus the R.L. of water surface level in the inlet well (i.e. d/s FSL + H1) can be determined.

Now approximate R.L. of center of pressure (C.P.) of the trapezoidal waterway through the notch= u/s canal bed level + ⅓ FSD (which can be calculated).

Then the height (Y) of the center of pressure above the water level in the inlet well= R.L. OF C.P. – R.L. of water in inlet well= (Known)

Now using the equation

(3.7)

where X and Y are the coordinates of the jet (issuing from center of pressure) w.r.t. the water surface level in the inlet well.

* Eqn (3.7) can be derived as below:

X = V1 * t (after a time t)

Y = gt2 [ using S = ut + gt2 and u = 0, we have S = gt2]

Y = g.

The value of X can be determined. Finally the diameter of the inlet well may be kept as about 1.5 times the value of X.

Example 3.2. Design the salient dimensions of a siphon well drop given the following particulars:

X

Y

V1

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Fall = 3.8 mGeneral ground level = + 163.36 mFull supply depth = 75 cmBed level upstream = + 162.83 mDischarge = 1 cumecBed width upstream and downstream = 2.4 m

Solution. For a trapezoidal notch, we have the discharge equation (3.4) as

At full supply discharge, we have

= F.S.D. = 0.75 m = F.S.Q. = 1 cumec

or 0.69 = l + 0.3n (i)At 50% full discharge, we have

(Equation 3.5) = 0.66 x 0.75 = 0.5 m.

0.64 = l + 0.2 n (ii)

Subtracting (ii) from (i) we get0.05 = 0.1 nn = 0.5

Substituting this value of n in (ii) we getl = 0.64 – 0.2 x 0.5l = 0.64 – 0.10 = 0.54

Hence, provide a trapezoidal notch in the steining of the inlet well, with 0.54 m bottom width and each side inclined to an angle of 14.040 with the vertical.

Now the width of water (at FSL) flowing over the notch

= 0.54 + 0.5 x (0.75)= 0.54+ 0.375= 0.915 m.

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Velocity (V1) over the notch

Now assume that the diameter of the pipe used be 1 m.

The velocity V3 through the pipe

= 1.27 m/sec.

Also assume that the diameter of the opening at the inlet of pipe be 0.5 m.

The velocity of entry into the pipe (V2)

= 5.1 m/sec

Now, loss of head between the inlet well and the d/s FSL is given by Equation (3.6).

Assume that the length of the pipe is kept as 12 m and = Darcy’s coefficient of friction be taken as equal to 0.012, we then have

= 1.50 m

R.L. of water surface in the inlet well = d/s FSL + 1.50

[D/s FSL = u/s FSL - fall = (162.83 + 0.75) – 3.8) = 159.78)]

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= 159. 78 + 1.50 = 161.28.

Approximate R.L. of the center of pressure (C.P.) of the trapezoidal waterway through notch= u/s canal bed level + ⅓ FSD

=162.83 +

= 162.83 + 0.25= 163.08

Height Y of C.P. above water level in the inlet well= 163.08 – 161.28= 1.80 m.

Now using Eqn. (3.7), we have

or

=

= 1.11 m.

Now the diameter of the inlet well may be kept as about 1.5 X, i.e. 1.5 x 1.11 = 1.665 m, say 1.7 m. Keep the diameter of the d/s outlet well as say 1.2 m.

Also provide a water cushion at the bottom of the inlet well. The complete details are shown in Fig. 3.11.

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Fig 3.11 Details of Example 3.2.

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4. Design of a Sarda type fall

Length of the crest. The length of the crest is kept equal to the bed width of the canal. Sometimes for future expansion, the crest length may be kept equal to (bed width + depth).

Fig. 3.12 Rectangular crest for Sarda type fall.

Fig. 3.13 Trapezoidal crest for Sarda type fall

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Shape of the crest. A rectangular crest with both faces vertical has been suggested for discharges under 14 cumecs. The top width is kept equal to and the minimum base

width is kept equal to (Take G = 2 for masonry) where d is the height of the crest

above the downstream bed level and h is the head over the crest.

For discharge over 14 cumecs, a trapezoidal crest with top width equal to with upstream side slope of 1:3 and downstream side slope of 1:8 is adopted.

Crest level. The following discharge formula is used to determine the height of the crest.

(3.8)

where Cd = 0.415 for rectangular crest = 0.45 for trapezoidal crestL = Length of crestBt = Top width of crest.

Height of the crest above bed = y – H(assuming h = H i.e. neglecting velocity of approach)where y is the normal depth of channel (upstream).

Upstream wing wall. For trapezoidal crest, the upstream wing walls are kept segmental with radius equal to 5 to 6 times H and subtending an angle of 600 at center, and then carried tangential into the berm as shown in Fig. 3.14.

Fig. 3.14 Upstream wing walls for Trapezoidal crest of Sarda Type fall

Upstream protection. Brick pitching in a length equal to upstream water depth may be laid on the upstream bed, slopping towards the crest at a slope of 1:10. Drain pipes should also be

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provided at the bed level in the crest so as to drain out the u/s bed during the closer of the channel.

Upstream curtain wall. brick thick upstream curtain wall is provided, having a depth

equal to ⅓rd of water depth.

Impervious concrete floor. The total length of impervious floor can be determined by Bligh’s theory for small works and by Khosla’s theory for large works. The minimum length of floor on d/s of the toe of the crest wall should be = [2(water depth + 1.2m) + drop].

The floor thickness required on the downstream side can be worked out for uplift pressures (using minimum thickness of 0.4 m to 0.6 meter) and only a nominal thickness of 0.3 meter is provided on upstream side.

Cistern. The length and depth of cistern can be worked out from the following equations.

Lc = (3.9)

X = (3.10)

where Lc = The length of the cistern in metersX = Cistern depression below the downstream bed in metersH = Head of water over the crest, including velocity head, in meters, i.e. = (u/s TEL – Crest level).

Downstream protection. The d/s bed can be protected with dry brick pitching. The length of the d/s pitching is given by the values of Table 3.1 or 3 times the depth of downstream water, whichever is more. The pitching may be provided between two or three curtain walls. The

curtain walls may be brick thick and of depth equal to ½ the downstream depth or as given in

Table 3.1. (minimum = 0.5 m).Table 3.1 Size of curtain walls

Head over the crest H (meters)

Total length of d/s pitching (m) Remarks

Curtain wallsNo Depth in meters

Up to 0.3 m 3.0 All slopping down at 1 in 10 10.3 to 0.45 3.0 + Twice HL Horizontal up to end of

masonry wings and thenslopping down at 1:10

10.45 to 0.60 4.5 + ” 10.60 to 0.75 6.0 + ” 10.75 to 0.90 9.0 + ” ” 10.90 to 1.05 13.5 + ” ” 21.05 to 1.20 18.0 + ” ”1.20 to 1.50 22.5 + ” ”Slope pitching: After the return wing, the sides of the channel are pitched with one brick on

edge. The pitching should rest on a toe wall brick thick and of depth equal to half the

downstream water depth.

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Downstream wings: Downstream wings are kept straight for a length of 5 to 8 time and may then be gradually warped. All wing walls must be designed as retaining walls. Such a wall has a base width equal to ⅓rd its height.

Example 3.3 Design a 1.5 meter Sarda type fall for a canal having a discharge of 12 cumecs, with the following data:

Bed level upstream = 103.0 mSide slopes of channel = 1:1 mBed level downstream = 101.5 mFull supply level upstream = 104.5 mBed width u/s and d/s =10 mSoil = Good loamAssume Bligh’s Coefficient = 6

SolutionLength of crest: Same as d/s bed width = 10 m.Crest level. A rectangular crest is provided since the discharge is less than 14 cumecs. The discharge formula is given by

Q = 1.84.L.H3/2[H/Bt]1/6

Assume top width of the crest as 0.8 m.12 = 1.84 x 10 x H3/2 x H1/6/(0.8)1/6

H5/3 = 0.628H = 0.76 m.

Velocity of approachVa =

= 0.696 m/sec.

Velocity head =

= 0.025 m.u/s TEL = u/s FSL + Velocity head = 104.5 + 0.025 = 104.525 m

Reduced level of the crest = (u/s TEL – H) = 104.525 – 0.755 = 103.77 m.

Use crest level of 103.77 metersHeight of the crest above u/s floor

= 103.77 – 103.0= 0.77 m.

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Shape of crestWidth of the crest (Bt) = 0.55. where d = Height of the crest above d/s bed

= 103.77 – 101.5= 2.27 m

Bt = 0.55. = 0.55.

= 0.829 m.Keep 0.85 m width of the crest.

The thickness at base = = = = 1.5 m.

The top shall be capped with 20 cm thick C.C. 1:2:4.

Upstream wing wall. It shall be splayed straight at an angle of 450 from the u/s edge of the crest and shall be embedded by 1.0 m into the berm. On the d/s side, wing walls are kept straight and parallel up to the end of the floor and joined to return walls.

Upstream protection 1.5 m long brick pitching (equal to u/s water depth) is laid on the u/s bed, slopping down towards the crest at 1:10, and three drain pipes of 15 cm diameter at the u/s bed level should be provided in the crest so as to drain out the u/s bed during the closure of the canal.

Upstream curtain wall. Maximum depth of u/s curtain wall

= = = 0.5 m

Provide 0.4 m x 0.8 m deep curtain wall on the u/s.Cistern Depth of cistern,

R.L. of cistern = 101.5 – 0.3 = 101.2 m.

3.2 Weir Drops/ Drop Structures

Drop structures and weirs are small dams placed across a waterway to provide for changes in gradient, slow water velocities and reduce erosion by. Water flow is directed through the weir into a stilling basin where the energy of the flow is dissipated.

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Critical factors in the success of such structures are the proper engineering of the drop structure itself to withstand hydraulic pressure and to prevent outflanking. Many weir structures will require a stilling basin.

Structures may vary from low gabion walls to very large earth dams lined with mattresses. They are classified according to the shape of the downstream face at the center of the flow.

The most common weirs are vertical structures. The downstream face of a vertical weir is flush. These structures are often used on small streams, usually in a system of weirs. High vertical weirs require a stilling basin which may be created by constructing a scour apron and counter weir from gabion mattresses.

Stepped weirs differ slightly from vertical weirs. The addition of stepped downstream faces provides for some energy to dissipation at each level. Stepped weirs are appropriate for small structures in waters without heavy sediment loads. Stepped structures are often constructed with some degree of batter.

Where larger structures are required or bearing capacity of soils is limited, sloped weirs are most appropriate. Sloped weirs are ramped on both the upstream and downstream faces. As with vertical structures, sloped weirs may require a stilling basin.

Fig. 3.15 A photo showing a weir drop/Drop structureThe drop is located so that the fillings and cuttings of the canal are equalized as much as possible. Wherever possible, the drop structure may also be combined with a regulator or a bridge. The location of an offtake from the canal also influences the fall site, with offtakes located upstream of the fall structure.

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3.3. Head regulators

A canal head regulator is provided at the head of the offtaking canal and serves the following functions:

i. It regulates the supply of water entering the canalii. It controls the entry of silt in the canal

iii. It prevents the river floods from entering the canal

The regulator is aligned at right angle to the weir, but slightly larger angles are now considered preferable for providing smooth entry of water into the regulator as shown in Fig. 3.15.

Fig. 3.15 Alignment of Canal Head Regulator

The water from the under sluice pocket is made to enter the regulator bays so as to pass the full supply discharge into the canal. The maximum height of the gated openings called Head Sluices will be equal to the difference of pond level and crest level of the regulator. The entry of silt into the canal is controlled by keeping the crest of the head regulator by about 1.2 to 1.5 meters higher than the crest of the under-sluices. If silt excluder is provided, the regulator crest is further raised by about 0.6 to 0.7 meter. Silt gets deposited in the pocket, and only clean water enters the regulator bays. The deposited silt can easily be scoured out periodically and removed through the under sluice openings.

The crest level of the regulator called sill level, is not only governed by silt considerations, but is also governed by the discharge considerations. The full supply discharge has to pass through the regulator openings, the heights of which will be equal to the difference of pond level and sill level. The smaller the height of the openings, the larger will be the width of the openings.

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A regulator is provided with a very wide and a shallow waterway. Therefore, a drowned weir formula is used to calculate the discharge.

Fig. 3.16 Drowned weir discharge formula

(Free weir equation) (3.11)

(Drowned weir equation) (3.12)Total discharge Q = Q1 + Q2

+ (3.13)

The values of Cd1 and Cd2 may be taken as 0.577 and 0.80, respectively.where h = difference of the upstream and downstream water level, i.e., (pond level – maximum anticipated FSL of canal after making due allowance for future silting up of canal). h is called utilized head or working head or designed head for the regulator, and is taken as half of available head, where available head is equal to the difference fof pond level and actual FSL of canal. hv = head due to velocity of approach B = clear width of waterway h1 = depth of downstream water level above the crest i.e., (maximum anticipated FSL of canal minus sill level)

When all other variables are fixed and known, value of clear waterway width (B) can be calculated.

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The gate controlled opening is provided from the sill level to the pond level. During high floods, the water level in the river pocket will be much higher than the pond level. To avoid spilling of this water over the gates, a R.C.C. wall is provided from the pond level up to river HFL. This wall spans for the entire length of the regulator and will rest over the piers of the regulator bays. This wall is known as Breast wall, and will be subjected to vertical self-weight and horizontal water pressure acting against it from the upstream side.

Example 3.1: The head regulator of a canal has 3 openings each 3 m wide. The water is flowing between the upper and lower gates. The vertical opening on the gate is 10 m. The head on the regulator is 0.45 m (Afflux). If the upstream water level rises by 0.20 m, find how much the upper gates must be lowered to maintain the canal discharge unaltered.

Fig. 3.17: (Example 3.1)

Solution: The width of regulator openings = 3 spans of 3 m each = 9 mWhen the gate opening is 1 m, the discharge can be calculated by submerged orifice formula.

(i)In the second case, when upstream water level rises by 0.2 m, let the gate opening be x meter to keep the discharge unaltered.

(ii)Equating (i) and (ii),

. Hence, the gate must be lowered by an amount 1 – 0.83 = 0.17 m3.4. Under sluices

A comparatively less turbulent pocket of water is created near the canal head regulator by constructing under-sluice portion of the weir. A divide wall separates the main weir portion from

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the under-sluice portion of the weir. The crest of the under-sluice portion of the weir is kept at a lower level than the crest of the normal portion of the weir.

The crest level of the head regulator is also kept higher than the crest level of the under-sluices, so that only silt free water is admitted into the canal through the head sluices. Silt gets deposited over the under-sluice floor, and may be periodically removed over the crest of under-sluice and towards downstream side of the river by opening these gate-controlled openings (under-sluices). Since the under-sluices help in removing the silt from near the regulator, they are also called scouring sluices.

Sill of the under-sluice pocket is kept at or slightly above the deepest river bed and about 0.9 to 1.8 meters below the sill of the canal head regulator. The length of the under-sluice pocket between the divide wall and the head regulator may be taken as 1.5 m the upstream length of the divide wall.

Fig. 3.18 Under-sluices lengh, l

However, this length is governed by the dischargeing capacity of the under-sluices, which should be sufficient to enable them to serve their main functions. The discharging capacity of under-sluices may be selected as follows:

(iii) They should be able to ensure sufficient scouring capacity, for which the discharging capacity should be alteast twice the full supply discharge of the main canal at its head.

(iv) They should be able to pass the dry weather-flow and low floods during the months excluding the rainy season, without the necessity of dropping the weir shutters.

(v) They should be able to dispose of 10 t0 15% of the high flood discharge during severe floods.

The spans of the under-sluices should be wide enough (usually 10 to 20 m) in order to be sufficient in scouring action.

Divide Wall

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The divide wall is a masonry or a concrete wall constructed at right angle to the axis of the weir, and separates the weir proper from the under-sluices. The divide wall extends on the upstream side beyond the beginning of the canal head regulator; and on the downstream side, it extends up to the end of loose protection of the under-sluices.

The main functions served by the divide wall are:i) It separates the under-sluices from the weir proper. Since the crest level of the under-

sluices is lower than that of the weir proper, the two must be separated, and this is done by the divide wall.

ii) It helps in providing a comparatively less turbulent pocket near the canal head regulator, resulting in deposition of silt in this pocket and, thus, to help in the entry of silt-free water into the canal.

iii) Divide wall may keep the cross-currents, if at all they are formed, away from the weir. Cross currents lead to vortices and deep scours, and therefore, prove hazardous to weirs.

3.5. Silt excluders

Silt excluders are those works which are constructed on the bed of the river, upstream of the head regulator. The clearer water enters the head regulator and the silted water enters the silt excluder. A silt excluder consists of a number of rectangular tunnels running parallel to the axis. The bottom layer of water which is highly charged with silt and sediment will pass down the tunnels and escape over the floor of the under-sluice way, since the gates of the under-sluice ways shall be kept open upto the top of tunnels.

Fig. 3.19 Silt excluder

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4. HYDROPOWER PLANT STRUCTURES

Faraday had shown that when a coil is rotated in a magnetic field, electricity is generated. Thus, in order to produce electrical energy, it is necessary that we should produce mechanical energy, which can be used to rotate the coil. The mechanical energy is produced by running a prime mover (known as turbine) by the energy of fuels or flowing water. This mechanical power is converted into electrical power by electric generator which is directly coupled to the shaft of the turbine, and is run by the turbine. The electrical power, which is consequently obtained at the terminals of the generator, is then transmitted to the area where it is to be used for doing work.

The plant or machinery which is required to produce electricity, i.e., the prime mover and electric generator, is collectively known as the power plant. The building, in which the entire machinery along with other auxiliary units is installed, is called the power house.

4.1. Types and classification of hydropower structures

4.1.1 Classification of Hydropower Plants on the Basis of Hydraulic Characteristics

On the basis of this classification, the hydro plants may be divided into the following types:i) Run-off river plantsii) Storage plantsiii) Pumped storage plantsiv) Tidal plants

i) Run-off river plantsThese plants are those which utilize the minimum flow in a river having no appreciable pondage on its upstream side. A weir or a barrage is sometimes constructed across a river simply to raise and maintain the water level. Such a scheme is essentially a low head scheme and may be suitable only on a perennial river having sufficient dry weather flow.

Run-off river plants generally have a very limited storage capacity, to supplement the normal flow. Therefore, a small storage capacity, called pondage, is provided for meeting the hour to hour fluctuations of load or of stream flow over a day. When the available discharge at site is more than the demand, i.e., during off-peak hours, the excess water is temporarily stored in the pond on the upstream side of the barrage, which is then utilized during the peak hours.

ii) Storage plantsA storage plants have an upstream storage reservoir of sufficient size, so as to permit sufficient carry-over storage from the rainy season to the dry season, and thus to develop a firm flow. In this scheme, a dam is constructed across the river, and the power house may be located at the foot of the dam. The power house may sometimes be located much away from the dam. In such

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cases, the power house is located at the end of tunnels which carry water from the reservoir. The tunnels are connected to the power house machines by means of pressure penstocks.

iii) Pumped storage plantsA pumped storage plant generates power during peak hours, but during the off-peak hours, water is pumped back from the tail water pool to the head water pool for future use. The pumps are run by some secondary power from some other plant in the system. The plant is thus primarily meant for assisting an existing thermal plant or some other hydel plant.

Fig. 4.1 Typical section through pumped storage plant

During peak hours, the water flows from the reservoir to the turbine and electricity is generated. During off-peak hours, the excess power available from some other plant is utilized for pumping water back from the tail pool to the head pool. This minor plant thus supplements the power of another major plant. In such a scheme, the same water is utilized again and again and no water is wasted.

For heads varying between 15 to 90 m, reversible pump turbines have been deviced, which can function both as turbine as well as a pump. Such reversible turbines can work at relatively high efficiencies and can help in reducing the cost of such a plant. Similarly, the same electrical machine can be used both as a generator as well as a motor by reversing the poles.

iv) Tidal plantsTidal plants for generation of electric power are the recent and modern advancements, and essentially work on the principle that there is a rise in sea water during high tide period and a fall during the low ebb period. The water rises and falls twice a day; each fall cycle occupying about 12 hours and 25 minutes. The advantage of this rise and fall of water is taken in a tidal plant. In other words, the tidal range, i.e., the difference between high and low tide levels is utilized to generated power. This is accomplished by constructing a basin separated from the ocean by a partition wall and installing turbines in openings through this wall.

Water passes from the ocean to the basin during high tides, and thus running the turbines and generating electric power. During low tide, the water from the basin runs back to ocean, which can also be utilized to generate electric power, provided, special turbines which can generate power for either direction of flow are installed. Such plants are useful at places where tidal range is high. The tidal range at this place is of the order of 11 meters.

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4.1.2 Classification of Hydropower Plants on the Basis of Operating Head on Turbines

On this basis, the plants may be divided into the following types:i) Low head scheme (head < 15 m).ii) Medium head scheme (head varies between 15m to 60 m)iii) High head scheme (head > 60 m).

i) Low head schemeA low head scheme is one which uses water head of less than 15 meters or so. A run-off river plant is essentially a low head scheme. In this scheme, a weir or a barrage is constructed to raise the water level, and the power house is constructed either in continuation with barrage or at some distance downstream of the barrage, where water is taken to the power house through an intake canal.

ii) Medium head schemeA medium head scheme is one which uses water head varying between 15 to 60 meters. This scheme is thus essentially a dam reservoir scheme. It has features somewhere between low head and high head scheme.

iii) High head scheme

Fig. 4.2 Run-off river plant

Fig. 4.3 Diversion canal plant

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A high head scheme is the one which uses water head of more than 60 m. A dam of sufficient height is, therefore, required to be constructed, so as to store water on the upstream side and to utilize this water throughout the year. High head schemes up to heights of 1800 meters have been developed in some countries of the world.

4.2. Different accessories

The different accessories of hydropower plants are used along with the major structures. They are essential for functioning of the hydro electric power.

Fig. 4.4 Plan of a high head scheme Fig. 4.5 Section through a high head scheme

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