Tanks irrigation system consist of the following components: 1. Tank Bund for Water Storage 2. Escape Weir 3. Tank Outlets or Supply Sluices.
Component # 1. Tank Bunds for Water Storage:
Tank bund is an embankment of low height. Generally it is made of earth. Since earth of various types is available tank bunds may be constructed using principles adopted for construction of earth dams.
Generally tank bunds of three types are constructed, they are:
i. Homogeneous Type (Tank Bund of Type A):
In construction of this type uniform and homogeneous material is used. It is constructed with relatively flat side slopes from consideration of stability. Most of the bunds belong to this type. (Fig. 10.1)
When the height of Tank bund is more than 5 metres the section is modified suitably with seepage checking trenches, blankets, toe drains. Design principles of earth dams are dealt with subsequently.
ii. Zoned Type (Tank Bund of Type B):
When earth of different types is locally available the bund may be constructed by dividing the section in different zones. (Fig. 10.2)
Outer zone is generally made of pervious material. The inner zone is made of impervious material.
iii. Diaphragm Type (Type C Tank Bund):
Many times zoning is done by providing a central core wall-called diaphragm. It is generally constructed with masonry or concrete. (Fig. 10.3)
In such types the diaphragm is taken quite deep in the foundation preferably upto impervious stratum.
Design of Tank Bund:
Although high bunds are designed on the principles for design of earth dam small tank bunds are constructed using empirical standards. Commonly adopted bund dimensions are given in Table 10.1.
The side slope of Tank bund is kept quite flat. 2 : 1 (Horizontal: Vertical) is a common slope. However for lesser heights steeper slopes may be adopted.
Like earth dams the upstream face of the tank bund is generally given stone pitching. It is also called revetment. Thickness of the pitching may vary from 0.3 to 0.6 metres. A toe is also provided to support the sloping face. General arrangement is shown in Fig. 10.4.
Storage Capacity of a Tank:
Storage capacity of a tank can be calculated using trapezoidal formula.
It is stated as:
Where V is volume of space enclosed between two adjacent contours.
A1 and A2 are the areas enclosed in two contours.
H is contour interval.
This method is useful in finding capacity in two successive contours only. But since tank bunds are of small heights the method is quite useful. The effective or utilisation storage in a tank is the volume between level of sill of the outlet or lowest sluice and full tank level.
Component # 2. Escape Weirs:
That tanks are small storage works constructed to meet local requirements. Obviously attempt is not made to contain full run-off coming down from the catchment area. It is therefore necessary to make suitable arrangement to pass down excess water beyond Full Tank level (F.T.L.) safely.
Structure constructed to provide passage to excess water is called escape weir. It is also called tank surplus weir. The water starts spilling over the weir as soon as tank is filled upto its crest. However, temporarily due to rush of incoming water the level in the tank rises above F.T.L.
The new level reached is called Maximum Water Level (M.W.L). It depends on the extent of flood. For design purposes M.W.L is calculated taking into account maximum flood discharge likely to occur and the waterway available at the site of the escape weir. The surplussing or spilling water is carried down through a channel which is generally a natural drainage and has enough capacity.
i. Selection of Site for a Tank Weir:
Following points may be taken into consideration while selecting a site for a tank weir:
(i) Tank weir performs the function of surplussing excess flow. Therefore it is preferable to locate the weir in a natural saddle away from the tank bund.
(ii) To carry surplus flows existence of a well-defined escape channel is very necessary at a site selected for construction of a weir.
(iii) The saddle, where natural surface level (N.S.L) is approximately same as full tank level (F.T.L.) should be given first preference.
(iv) Hard foundation if available at the site reduces the cost of construction of bed protection works.
(v) When a site away from tank bund is not available, as far as possible weir may be located on one end of the tank bund.
(vi) Surplus weir may be housed in the body of the tank bund only as a last resort.
(vii) Care should be taken to see that escape channel carrying surplussing water is not likely to damage cultivated areas.
ii. Types of Weirs:
Escape weirs constructed in tank irrigation system is similar to a diversion weir or an anicut constructed across the river channel. It may be constructed either with masonry or rock-fill or concrete depending upon availability of construction material and site conditions.
iii. Masonry Weirs:
This type of weir is most commonly used in a tank irrigation system. Masonry weirs are generally constructed with vertical drop and are designed as gravity weirs. Self-weight of the body wall is the only restoring external force and it counteracts all dislodging forces like water pressure, uplift etc. On the body wall of the weir dam stones may be erected to enable extra storage.
Depending upon the site conditions masonry weirs may be constructed in three ways as given below:
Masonry Weir with Horizontal Floor:
In this type of weir vertical drop is given as shown in Fig. 10.5.
This type is suitable when on the downstream side hard rock is available in the foundation and the height of the weir is less than one metre or so.
Masonry Weir with Depressed Floor:
This type is similar to one explained above except that the downstream apron is depressed below the ground level (Fig. 10.6).
By depressing the apron below ground level a sort of stilling pond is formed. It helps in dissipating the energy of water spilling over the crest of the weir. This type of arrangement is generally used for weirs with greater heights say more than 2.5 metres. They are then designed like a fall.
Masonry Weir with Stepped Floor:
When the topography is such that there is no space for constructing horizontal or depressed horizontal apron, weir with stepped apron may be constructed as shown in Fig. 10.7. It is something like steps and is suitable for low heights of body wall.
Rockfill Weirs:
It is constructed with dry rock fill if such material is locally available in sufficient quantity. Fig. 10.8.
To support the rock fill masonry retaining walls are constructed as shown in the figure. Top surface of the weir is plastered. This type of weir requires a very big section because the slopes are quite flat.
Concrete Weirs:
Typical profile is shown in Fig. 10.9. The weir is constructed with reinforced steel to make the section monolithic. This type is constructed mostly where foundations are pervious.
In this type of weir a sloping glacis is provided on the downstream side. It helps in creating a hydraulic jump on the sloping face. When hydraulic jump is created the energy of flow is destroyed. Thus the bed is protected below the weir.
Component # 3. Tank Outlets or Supply Sluices:
For releasing stored water into irrigation channel opening is provided in a tank bund. It is constructed in the form of a culvert or a pipe line. It is laid at or close to ground level and runs through the tank bund.
Since the opening extends from upstream face of the bund to the downstream face, wing walls and other types of bank connections are necessary at the head as well as tail of the opening. In small tanks pipe outlets are constructed. For medium sized tanks masonry culverts are adopted. Also sometimes headwall type or sluice with tower head is used in medium sized tank bunds.
i. Pipe Outlets:
Either cast iron pipes or cement or earthenware pipes are used to construct pipe outlets or pipe sluices. Since the size of pipes is small they are not open to inspection once they are put in place through the bund. To avoid their bursting or leakage in pipes they are used in small tanks where depth of water stored in less than 2.5 metres approximately. The arrangement of pipe sluice is shown in Fig. 10.10. Any repair work in this type is possible only after bund is cut open.
ii. Culvert Type Sluice:
In this type masonry culvert of minimum size 0.6 metres wide and 0.75 metres high is constructed either with or without arch roof. The size of the culvert depends upon the water to be conveyed. The minimum size of 0.6 × 0.75 m permits, manual inspection and repairs, and cleaning from inside. The size of the barrel is so selected that with tank full and plug hole fully open velocity of flow through the barrel does not exceed 4.5 m/sec. The arrangement of culvert is shown in Fig. 10.11.
On the upstream, wing walls are appropriately constructed. On the downstream also proper bank connections are provided to ensure safety of the bund.
iii. Tower Head Type Sluice:
In this type heavy wing walls are not required as in the culvert type sluice. As a result there are no chances of failure due to cracking or bulging which occurs commonly in the wing walls. As shown in Fig. 10.12 tower head or a well is constructed in the upstream slope of the bund. Also the barrel extends well upto the upstream face of the bund.
iv. Flow Regulation in Tank Sluices:
It is very important to exercise full control on release of stored water. Different arrangements are made for flow regulation in dams and bunds depending upon their height, water pressure and quantity of water to be released. From reservoirs and big tanks large quantities of water are required to be released under great water pressures. In reservoirs with more than 9 metres head shutters moving on rollers or balanced valves are installed.
In large tanks shutters which move in grooves and regulated by screw spears are adopted. In minor and medium size tanks where engagement of operation and maintenance staff is not economical plug hole system is traditionally used in south India. While arrangement for big works is dealt with sluices and crest gates for dams, the plug hole system is described hereunder.
v. Plug-Hole System:
In this system, entrance of a culvert or a pipe is controlled by constructing masonry regulation chamber of low height.
(Refer Fig. 10.10 & 10.11) On the top it is covered by a stone slab. It has one or more circular orifices or holes of suitable size. On the vertical side of the chamber facing the water storage a rectangular vent is provided with its sill at the base level of the culvert.
The vent way is closed by a vertically placed stone slab and to stop leakages from sides it is plastered with mud. Wooden shutters are also sometimes used to close the opening. This Vent way is used to draw water when water level in the tank is less than 1.2 metres high over the floor of the sluice. The size of the rectangular vent is so kept that even at 0.15 metres head it draws full supply.
The orifices or the plug-holes (as they are popularly called) made in the stone slab are kept closed by wooden cone shaped cylindrical plugs, Fig. 10.13.
The diameter of the plug hole varies from 10 cm to 30 cm depending upon the requirement of water. Similarly the plugs are made to fit smugly into the holes. The cone shaped plug is made of hard wood and is given a taper of 1: 4.
At point where diameter of the cone is slightly less (6 mm less), than the diameter of the orifice, a seating of iron strap and leather washer is provided. Width of the seating is kept 2.5 to 3.0 cm. The countersunk iron strap prevents the plug from splitting. A lifting rod round in shape passes through the centre of cylindrical cone. The rod is secured at both ends of the cone plug by split cotter pins.
When the height of a tank bund is more the rod is taken through holes made in guide beams fixed at intervals. The top end of rod is made flat as it emerges through a hole in a regulation platform. In the iron flat holes are made at 7.5 cm interval through which an iron pin is inserted to keep the plug raised to a desired level.
To open an orifice the plug is raised by means of raising rod. When the plug is raised clear of the plug hole by an amount not less than diameter of the hole the orifice takes its full discharge. The size of the orifice is so calculated that it takes full discharge with 0.3 metres water standing on the plug stone platform. When the computed size of the hole or orifice works out to be more than 30 cm in diameter more than one holes are made in the plug stone platform.
When the full tank level (F.T.L.) is more than 7.5 m two or more plug stones with independent plug holes are constructed at different levels to facilitate better regulation. When the water level above the floor of the barrel falls below 1.2 m or so, the vertical vent way is brought into operation.
They are the outlets provided in the dams to supply water from the reservoir to the area below the dam. They extend from the upstream face of a dam to the downstream face. They are also used to remove the sediment from the reservoir. The sluices are fitted with the gates. Gates are operated from a platform. The platform is generally provided at F.R.L. or above.
(i) Sluice:
In case of gravity dams the sluice may be provided as shown in Fig. 14.36.
Generally upstream end of the sluice has bell-mouth to provide smooth water entry. Portion of the sluice upto the gate is of bell-mouth shape. The section of the sluice may be kept the same from the gate to the downstream face.
(ii) Belgaum Type Outlet:
In this type masonry or a concrete culvert is constructed under the dam as shown in Fig. 14.37. The head wall is provided at the upstream toe of the dam.
This type is known as Belgaum type. It is used for earth dams. At the upstream end of the outlet regulating arrangement is provided. The culvert is generally protected by a lining. A bridge is also constructed for access upto the regulating platform.
Sometimes the head wall is constructed in the centre of a dam. It is than called a head wall type outlet. It is also provided in earth dams. When the head wall is constructed at a point where F.R.L. meets the upstream sloping face the sluice is called Dharwar type outlet.
(iii) Pipe Outlet:
Sometimes a pipe may be used in place of a culvert. Then an intake well is constructed on the upstream side. It is similar in construction to the Belgaum type.