In this article we will discuss about the layout of canal headwork with the help of diagrams.
1. Parts of a Weir:
It is very essential to know various parts of a weir before actually designing the weir. Fig. 15.6 gives common parts of a weir structure.
Generally weirs consist of the following parts:
1. Upstream boulder pitching for bed protection,
2. Upstream curtain wall,
3. Upstream or front apron,
4. Shutter on the crest of the weir,
5. Weir body wall,
6. Downstream or rear apron,
7. Downstream curtain wall, and
8. Downstream protection for channel bed.
2. Divide Wall:
As the name indicates it is a long solid wall or a groyne. It is constructed at right angles to the weir axis. It divides the river channel into two compartments. In the smaller compartment which is near to the head regulator a still pond is created. The reason for creation of a still pond is that the smaller compartment is partially cut off from the main river channel. The still pond is formed in front of the head regulator. The length of the divide wall extends a little beyond the length of the head regulator.
Generally divide wall is constructed with masonry (Fig. 15.7).
The top width of the wall varies from 1.5 to 2 m. The divide wall should always be founded on a strong foundation. Generally it is necessary to provide well foundation for at least 30 m. It is clear that the divide wall retains water on its both faces. Whereas on one side the water is still in the pocket on the other side there is river flow. The divide wall is designed after taking this point into consideration.
3. Under-Sluices or Scouring Sluices:
They are openings provided in the body of a weir or anicut at low levels. They are located in the smaller compartment in front of a still pond. These sluices are perfectly controlled by means of gates which are operated from top.
When a still pond is created in the pocket in front of the head regulator silting takes place in the pocket. It reduces capacity of the pocket. The sluices are used to remove or to scour the deposited silt. The sluices are located quite below the crest of the head regulator.
Thus the scouring sluices maintain the pond clear in front of the head regulator. The sluices are also opened during peak flood period to lower the discharge over the crest of the weir. The capacity of sluices is kept about two times the canal discharge. Then it ensures efficient scouring action.
4. Fish Ladder:
When a weir is constructed across a river with a view to obstructing the flow, the passage is fully closed. As a result fish is also obstructed from moving freely. The structure provided for allowing free passage to the fish is called fish ladder. Fig. 15.8 shows a plan and elevation of the ladder.
The fish ladder is generally provided on the other side of the divide wall in the bigger compartment. The baffles stagger the flow of water. But the fish can pass through the ladder.
5. Head Regulator:
It is a structure constructed at the entrance (head) of the canal where it takes off from the river (Fig. 15.9).
The regulator serves following purposes:
(i) It regulates the flow of irrigation water entering into the canal.
(ii) It can be used as a meter for measuring the canal discharge.
(iii) It regulates and prevents excessive silt entry into the canal.
The head regulator is generally constructed with masonry. It consists of a raised crest with abutments on both sides. The crest may be-subdivided in various bays by constructing piers on the crest. The piers support roadway and a platform for operating gates. The gates control the flow over the crest. They are housed and operated in grooves made in the abutments and piers. Sill of the regulator crest is kept high to prevent silt entry.
Sometimes the gates are provided in tiers. Then a lower tier may be kept closed to raise the sill of the regulator. It is flanked with adequate wing walls. The head regulator is given proper protection by providing aprons on the upstream and the down-stream side of the barrel. To prevent seepage cut-offs are also essential. To take irrigation water at low velocities the waterway of the head regulator is kept sufficiently big.
6. Silt Excluder and Other Silt Control Works at Headwork:
Excessive silt reduces capacity of the canal. It is true that the silt entry in the canal cannot be checked completely. Very fine silt always remains in suspension in the canal water. Actually there should be some amount of silt in the irrigation water as it has manurial property. Silt entry in the canal can be controlled in number of ways.
a. Still Pond System:
It is adopted at every headwork. It consists in creating a pocket by constructing a divide wall.
From Fig. 15.10, it is clear that the still pond is created in the pocket. The pocket is closed from three sides. The head regulator is on one side, weir on the other side and divide wall on the third side. Thus the velocity of water is destroyed in the pocket. The water drops down its load in the pond. Finally clear water enters the canal through the openings in the head regulator.
b. Regulator Gates:
Silt concentration is more in the bottom layers. Obviously only top layers are allowed to enter the canal to prevent the silt load from entering the canal. This is achieved by providing shutters on the regulator crest. Most convenient method is to provide the shutters in 2 or 3 tiers. It reduces the extraordinary height required for working the shutters. It also accomplishes the purpose of silt control very effectively.
The shutters may operate in the same or different grooves made in the piers and the abutment. The bottom tier is usually kept closed to allow only surface water to enter into the canal. Bottom shutter goes behind the sill when it is opened. Other shutters go up when vent-way is to be opened.
c. Silt Excluder:
This structure excludes the silt from irrigation water as the name implies. It separates the lower silt ladden portion of water from the upper silt free portion. It consists of a series of parallel tunnels of low height. The tunnels are constructed in the pocket parallel to the flow in the river. The height of the tunnels depends upon the silt distribution in the flow. Fig. 15.11 shows a plan and a section of the silt excluder.
The lower portion of the flow which contains heavy silt load enters the tunnels. The silt load is then driven towards the scouring sluices.
This water passes on the downstream side of the weir through the sluices. Thus only clear water is allowed to enter the canal.
7. River Training Works:
When the river is wide some training work is done before constructing the headwork. The aim of training is to induce the flow of water along the desired direction. Generally guide banks, marginal embankments and sometimes spurs are provided for the purpose.
Causes for Failure of Weirs on Permeable Foundations:
Many weirs designed and constructed especially on pervious foundations failed or damaged in course of time. The problem was analysed by many research workers.
The main causes of failures were found out to be:
1. Piping or Undetermining:
Undetermining or piping takes place due to excessive percolation of water below the foundation. The water percolates through the foundation under pressure. When it emerges out at the downstream end it may retain sufficient force to dislodge and to lift the soil particles. Thus progressively the foundation becomes weaker and the structure finally subsides in the hollow formed in the foundation.
2. Failure Due to Uplift:
When the percolating water exerts excessive upward pressure and when the apron is not sufficiently strong, it may fail by rupture of its part. The uplift pressure is not important for the upstream apron. The reason being the downward weight of stored water on the upstream apron is sufficient to neutralize the effect of uplift pressure.
Bligh’s Creep Theory:
Bligh assumed that the water which percolates into the foundation creeps through the joint between the profile of the base of the weir and the subsoil. He stated that this percolation water loses its head enroute. The seeping water finally comes out at the downstream end.
The total length covered by the percolating water till it emerges out at the downstream end is called a creep length. It is clear from the knowledge of hydraulics that the head of water lost in the path of percolation is the difference of water levels on the upstream and downstream ends. Also an imaginary line which joints the water levels on the upstream and the downstream end is called a hydraulic gradient line. Figs. 15.13 and 15.14 give the full explanation of Bligh’s theory.
In Fig. 15.13 arrows show the path followed by the creeping water.
B = L = total creep length and h is the head lost in creep.
Loss of head per unit creep length is h/L. It is a hydraulic gradient.
To increase the path of percolation vertical cut-offs or sheet piles can be provided (Fig. 15.14).
Bligh made no difference between vertical and horizontal path of percolation.
Hence total creep length, L = B + 2d1 + 2d2 + 2d3
When the water follows a vertical path the loss takes place in a vertical plane at the same section. This loss is proportional to the length of the path. For example, for a cut-off d1 loss is (h/L) × 2d1 and it takes place in its plane. Loss of head at other cut-offs may be calculated in the same way.
Bligh gave the criteria for the safety of weir against piping and uplift as follows:
The structure will be safe against piping when the percolating water retains negligible upward pressure when it emerges out at the downstream end of the weir. Obviously the path of percolation should be sufficiently long to provide safe hydraulic gradient. It depends on the soil type.
This condition is provided by equation L = CH
where L is creep length or path of percolation;
C is Bligh’s creep co-efficient for soil; and,
H is head of water against the weir.
Table 15.1 gives value of C for various types of soil.
To make the apron floor safe against the uplift pressure Bligh gave following criteria:
From Fig. 15.15, it is clear that the uplift pressure at any point is represented by an ordinate between the bottom of the apron floor and the hydraulic gradient line.
Hence uplift pressure = wH1
where w is density of water and H1 is the ordinate between the H.G. line and the bottom of apron floor.
Downward force exerted by the material in the apron is given by t.w. ρ. where t is the thickness of the apron floor and ρ is the sp. gr. of the material used in the apron.
The stable condition is attained when,
It is clear from Fig. 15.15 that Ht can only be known when t is known. Hence to determine ‘t’ following algebraical manipulation may be done.
where (H1 – t) is the ordinate between dotted H.G. line and top of the apron. It can be known easily. Hence the depth of apron can be calculated from equation (2). Now adding factor of safety of 4/3 to equation (2), the expression finally becomes
For economy greater apron length is provided on the upstream side which requires minimum practical thickness. Of course on the downstream side some minimum length of apron is required to protect the bed of the channel.
Dr. A.N. Khosla’s Theory:
Weirs designed and constructed on Bligh’s theory also failed due to undermining of the subsoil.
Dr. Khosla investigated the problem and drawn following conclusions:
1. The outer faces of the end sheet piles were much more effective than the inner ones and the horizontal length of the floor.
2. The intermediate sheet piles if smaller in length than the outer ones were ineffective except for local redistribution of pressures.
3. Undermining of the floor started from the tail end. If the hydraulic gradient (H.G.) at exist was more than the critical gradient for a particular sub-soil the soil particles would move with the flow of water thus causing progressive degradation of the subsoil, resulting in cavities and ultimate failure.
4. It was absolutely essential to have a reasonably deep vertical cut-off at the downstream end to prevent undermining.
Thus it was recognised that there was an urgent necessity for research on the problem. Research was done on the prototypes by inserting pressure pipes at suitable locations. A continuous and detailed record of the observations of pressures in pipes was maintained.
As a result following facts were established:
(a) The flow of water through the subsoil is in stream lines and therefore susceptible of mathematical treatment.
(b) The ratio (ф) of uplift pressure (P) at any point along the base of a particular weir founded on the permeable soil to the total head (H) is constant.
It is independent of:
(i) Head (H)
(ii) Nature of subsoil as long as it is homogeneous
(iii) Upstream and downstream water level
(iv) Temperature provided it is uniform throughout the subsoil.
However ratio (ф) varies with:
(i) Silt deposit or scour upstream or downstream of impervious floor,
(ii) Temperature which varies from point to point in the subsoil and in different seasons of the year.
(c) Law of loss under the floor was nearly a straight line law and for the sheet piles something like logarithmic.
Dr. Khosla broke the complicated weir profile into number of simple and common profiles.
He then studied each profile independently for finding out uplift pressures. The complete description of procedure of calculating the pressures will be rather out of scope of the book and hence it is not given here.
Thus he made it crystal clear that the loss of head does not take place uniformly in proportion to length of creep. It actually depends on the profile of the base of the weir.
Secondly he also proved that the safety against undermining is not obtained by flat hydraulic gradient but the exit gradient should be kept below the critical value.