The divide wall is normally constructed of solid masonry, it is necessary to provide well foundations for at least 30 m length from the extreme and taking the well below the deepest possible scour. The divide wall should be designed for the following conditions:
1. Silt pressure upto the full tank level on the pocket side when the river level is low. At this time there is no water on the weir side.
2. During floods or high river stage, when the under sluices arc discharging a difference of water level of about 1 m, with higher water level on the weir side be taken for design purpose.
(3) The top width of the wall Is kept 1 i to 25 m, while the bottom width i worked out so u to contain the resultant within the middle third of the base Diversion Headworks Functions of the divide well (1) The floor level of the under sluice or the pocket portion “I. generally kept lower than the floor level of the main welt, Hence, a divide wall Ii essential to separate The two” floors This prevents the turbulent action.
2) If divide wall is not proved currents approach the scouring sluices from directions and their effectiveness is reduced The divide wall helps concentrating scouring action of the under sluices for washing out the deposited In the pocket by ensuring a straight approach through the pocket
3) The divide wall prevent cross current arid flow parallel to the weir. The cross currants end the flow parallel to weir cause formation of vortices and result in deep scour To prevent this, sometimes, divide walls are also provided at in interval along the main portion of a lone weir or at points of change of floor level both main portion of a long welt or at points of change of level both on upstream and downstream side.
4) The divide wall provides a comparatively ‘still pocket’ in front of canal hoed regulator This helps In more silt deposit n the pocket and entry of comparatively clear water into the canal
5) Divide wall Incidentally serves as one of the side walls of the fish ladders The Fish Ladder In big rivers, migrating fishes move from upstream to down stream in search of relatively warm water in the beginning of winter season. Before the monsoons (in may and June) they travel upstream in search of clear water. It is, therefore, essential to make the provision of some space in the construction of the weir for the uninterrupted movements of such migratory fish.
Shows the plan and cross-section of a fish ladder. The difference of water levels on the upstream and downstream sides on the weir is split up into water steps by means of the baffle walls constructed across the inclined chute of fish ladder. Most fish can travel upstream in water flowing with a velocity of 3 m/second. The design of fish ladder should be such that velocity of current in the chute is not more than this value. To have effective control, a grooved gate is provided at the extreme u/s and d/s end walls of the fish ladder. The fish ladder is situated near the under sluices where there is always some water in the river section below them.
(2) They control the silt entry into the canal.
(3) They scour the silt deposited in the river bed above the approach channel.
(4) They pass the low floods without dropping the shutter of the main weir, the raising of which entails goods deal of labour and time.
(5) They provide greater waterway for floods, thus lowering the flood levels.
(6) They help in impounding fair amount of receding flood so as to secure full storage. Capacity of Under Sluices A head regulator is a structure constructed at the head of a canal taking off from a reservoir behind a weir or a dam. A head regulator may consist of a number of spans separated by piers and separated by gates similar to that provided in a barrage. The Modern tendency is to use bigger spans of 6 to 18 m controlled by stoney gates or sector gates. Functions of head regulator (1) To make the regulation of supply in the canal
(2) To control the silt entry in the canal.
(3) To shut out river floods.
Q = 1.7 (L.KhH)H3/2
where Q = total discharge in cumecs
L = length of water way in metres H = head causing flow n = number of end contraction. k constant, depending upon the shape of the nose of the pier: varies from 001 to. 0.03.
(2) The regulators are normally aligned at 90° to the weir, but greater angles, upto 110 are considered preferable for providing smooth entry.
(3) The crest level of the head regulator should be higher than the crest of the under sluices by a minimum of Ito 1.2 metres If silt excluder is not provided and greater than 1,1 to 2 m If silt excluding device Ii provided.
(4) The hydraulic Jump calculation over the sloping glacis are done in the same way as for sloping glade weir The length of cistern below the end of the glad should at least be equal to 5(L2 DI). The level of the cistern bed should be well below the minimum level of the formation of hydraulic Jump under various discharge conditions.
(5) The design of the Impervious floor should be done on the basis of uplift pressure In the manner similar to that of sloping glascis weir. The worst condition will be during the high floods when the canal Is shut and maximum static head acts. In case the thickness becomes excessive, a reinforced concrete mat should be provided to resist the uplift pressure by bending action.
(6) A concrete cut off at the end of the Impervious floor should always be provided to keep the exit gradient well within the limits.
(7) The pier separating the various spans of the regulator should be extended beyond the sloping gladls so that they may provide support to the upward banding reinforced concrete mat of the cistern floor
(8) in order to prevent spilling of water towards the canal during high floods, a reinforced concrete breast wall should be provided Prom pond level to well above the high flood level. The breast wall is supported between the piers and is designed for its self weight as well as the water pressure from upstream. They may also be designed for additional weight of gate lifting arrangements. It supported on them.
(9) For the proper operations of the regulator, a bridge is provided, spanning over the piers.
(10) The stability of the piers of the head regulator should be tested to withstand the overturning moment caused by the high pressure head during floods.
(11) There ii no empirical formula to determine tbe length of the talus of downstream of head regulator. Generally a talus of 4 to 5 times the depth of canal and 08 to 1 m thick in concrete blocks or stones are considered to be sufficient. Type of Regulation There are two methods of regulation adopted at a head regulator to control the entry of silt Into the canal:
(i) Still pond regulation.
(ii) Open flow regulation (i) Still pond regulation. In this method. the pocket sluices arc entirely clod and, the canal draws water from the still pond in the pocket. The water in excess of the canal requirements Is thus not allowed to escape under the sluice ,sates The velocity water in the pocket Is very much reduced on account of excessive water way since only the supply required for the canal enters the pocket The silt is thus deposit in the pocket and clear water enters the canal When the silt deposited has a level about to 1 meter below the crest level of the regulator, the supply In the canal Is shut off for about 24 hours and the sluice gates ar. opened to scour the deposited silt end discharge it downstream The process Is repeated. (ii) Open flow regulation Still pond repletion. In this method. the pocket sluices arc entirely clod and, the canal draws water from the still pond in the pocket. The water in excess of the canal requirements Is thus not allowed to escape under the sluice ,sates The velocity water in the pocket Is very much reduced on account of excessive water way since only the supply required for the canal enters the pocket The silt is thus deposit in the pocket and clear water enters the canal When the silt deposited has a level about to 1 meter below the crest level of the regulator, the supply In the canal Is shut off for about 24 hours and the sluice gates ar. opened to scour the deposited silt end discharge it downstream The process Is repeated.
Open flow regulation, In this system, the under sluices may be kept open so that the river supply in excess of the canal requirements Is escaped. Top water passes Into the canal while the bottom water maintains a certain velocity in the pocket to keep the silt to remain In suspension. The advantage of this system Is that the canal Is not to be closed for scouring the silt. However, the method is vary treacherous on account of the uncertain approach channel conditions in the river. Silt Control at Head Work The entry of silt into the canal can be controlled by:
(1) Providing a divide wall in the river at the canal side so as
(a) to create a trap or pocket
(b) to create the scouring capacity of under sluices by concentrating the current towards them.
(2) Paving the bottom of the approach channel to reduce disturbance.
(3) Installing a silt excluder.
(4) Making entry of clear top water in the canal by
(a) Providing raised sill in the canal.
(b) lowering sill level of scouring sluices.
(5) Reducing the velocity of water at the intake by providing wider head regulator.
(6) Avoiding unsteady flow by making the entry smooth
(7) Handling carefully the regulation of weir . Type of weir Weirs are classified into two heads, depending upon the criterion of the design of their floors.
1. Gravity weirs
2. Non gravity weirs
A gravity weir is the one in which the uplift pressure due to the seepage of water below the floor is resisted entirely by the weight of floor. In the non gravity type, the floor thickness is kept relatively less, and the uplift pressure is largely resisted by the bending action of the reinforced concrete floor.
Depending upon the material and certain design features gravity weir (or ‘simply weirs) can further be sub-divided into the following types:
1. Vertical drop weir.
2. Sloping weir: Masonry or concrete slope weir Dry stone slope weir.
3. Parabolic weir. 1. Vertical drop weir A vertical drop weir consists of a vertical drop wall or crest wall, with or without crest gates. At the upstream and downstream ends of the impervious floor, cutoff piles are provided. To safeguard against scouting action, launching aprons are provided both at upstream and downstream of the floor. A graded inverted filter is provided immediately at the downstream end of the impervious floor to relieve the uplift pressure. Vertical drop weirs are suitable for any type of foundation. 2. Masonry or Concrete Slope Weir Weirs of this type arc of recent origin. They are suitable for soft sandy foundations, and are generally used where the difference in weir crest and downstream river bed is limited to 3 metres, When water passes over such a weir, hydraulic jump is formed on the sloping glacis. 3. Dry stone slope weir A dry stone weir or rockfill weir consists of a body wall (or weir wall) and upstream and downstream rockfills laid in the form of glacis, with few intervening core wall Okhla weir on Yamuna river, near Delhi, is the example of such weir. 4. Parabolic Weir A parabolic weir is similar to the spillway section of a dam The body wall for such, a weir is designed at a low dam. A cistern is provided at the downstream side to dissipate the energy. The upstream and down stream protection works are similar to that of a vertical drop or sloping glacis weir Causes of Failure of Weirs And Their Remedies 1. Piping Water seeps under the base of the weirs founded on permeable soils When the flow lines emerge out at the d/s end of the impervious floor of the weir, the hydraulic gradient or the exit gradient may exceed a certain critical value for the soil. In that case, the surface soil starts boiling and is washed away by percolating water. With the removal of the surface soil, there is further concentration of flow lines into the resulting depression and still more soil is removed. This process of erosion thus progressively works backwards towards the upstream and results in the formation of a channel or a pipe underneath the floor of the weir, causing its failure. Remedies Piping failures can be prevented by: (1) Providing sufficient length of the impervious floor so that pith of percolation is increased and the gradient is decreased.
(ii) Providing pile at downstream ends
(2) Rupture of Floor Due to Uplift If the weight of floor is insufficient to resist the uplift pressure, the floor may burst and effective length of impervious floor is thereby reduced. The final, failure, however, is due to the reduction of the effective length with Design of Impervious Floor for Sub Surface Flow We have already seen that the subsurface flow or the foundation seepage may cause harm in two ways (i) piping, and (ii) uplift. Following the damage to Khanki weir In 1895. a group of Anglo Indian Engineers under the leadership of Colonel 3. Clibborn, Dean of the Roorkee College and J.S. Beresfored, Inspector General of irrigation, India, carried out experiments and confirmed the Darcs law for seepage through granular soils, Later, in 1912, Bligh advanced a theory for the sub surface flow and published it in his book, ‘The practical design of Irrigation Works’, In 1932, after analysing about 200 dams ill over the wand, Lane evolved his weighted creep theory Scientific study of the sub surface flow wu, however, made by Pavlovaky (1922), and Khosla and his associates (1936). Bllgh’s Creep Theory The design of the Impervious floor, or the apron is directly dependent on the possibilities of percolation in the porous soil on which the apron Is built. Bligh assumes as an approximation that the hydraulic slop. or gradient Is constant throughout the Impervious length of the apron. He further assumed the percolating water to creep along the contact of the base profile of the apron with the subsoil, losing head enrouts proportional to the length of its travel He designated the length of the travel as the creep length.
which is the sum of horizontal u well as vertical length of creep. Bligh asserted that no amount of sheet pilling or another cut-off could ever stop the percolation unless the cutoff extends upto the Impermeable sub-soil strata, Thus according to Bligh’s theory, the total creep lengths L for the case.
This means that in calculating the length of creep, the depth f every cutoff (i.e. vertical creep) is multiplied by the coefficient 2
CH is the total loss of head, the Mass of head per unit length of the creep (c) would be
C = h/2d1 + 1 + 2d1 = H/L
He called the loss of head per unit length of creep as percolation coefficient. The reciprocal of this (i.e. is called the coefficient of creep (C) and Bligh assigned its safe values for Different Soils, as given in table.
| Main Heading | ||
|---|---|---|
| 1. | Light sand and mud (as in nile) | 18 |
| 2. | Fine micaceous sand (as in Nothern India rivers) | 25 |
| 3. | Coarse grained sand ( as is central and south India) | 12 |
| 4. | Boulders or shingle, gravel and sand mixed | 5 to 9 |
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