Wells and about open well
Wells
A vertical hole dug into the ground to take water from the underground water storage reservoir is called a ‘well’.
Wells are classified as:
1. Open Well
2. Tube Well
When the size of the hole is much large then it is known as open well and when the size of the hole is very small, then it is known as tube well.
Open Wells
Wells which derive their supply from the first impervious strata are called open wells or dug wells.
Types of Open Wells
Open wells are classified as:
1. Shallow Well
2. Deep Well
Shallow Well
It is left in a pervious strata and does not meet any impervious layer below water table is called shallow well. It derives water from the underlying pervious layer.
Deep Well
These are rest on the impervious soil and get the supply from the underlying pervious layer through a small hole bored in impervious layer is called deep well.
Location of Wells
Wells should be constructed nearer to the areas to be irrigated and where favorable conditions exist.
Well should be constructed as such a level so that water flows by gravity from the well to the place where it is to be supplied for irrigation purposes.
Command Area
The command of well is the extent of the area to be irrigated by the well. This is also known as command area. The command of an open well varies from 2-10 hectares and that of a tube well varies from 4-22.5hectares.
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Construction of open wells
1. First of all, a circular pit is excavated up to the soft moist soil. The diameter of the pit should be more than the diameter of an open well.
2. Lay a well-kerb over the excavated surface in the pit.
3. Now construct the masonry lining over the kerb up to a few meters above the ground level.
4. Curing of lining is done up to seven days.
5. After this place a wooden platform over the masonry
6. Place earthen bags over the platform. A jham is used for removing the excavated soil from the well.
7. The well sinking is continued till the mota or pucca layer is reached. A smaller diameter hole is bored through the mota layer in the centre of the well.
8. To ensure vertical sinking of the well, 4 or 5 plum bob are suspended around the well steining ( lining).
9. Now construct more masonry lining over the previous lining up to the ground level.
10. Then construct parapet wall for providing water lifting arrangements.
Yield of an open well or dug well
Two methods are used to determine the yield of an open well:
1. Constant level pumping test
2. Recuperation test
1. Constant level pumping test:
In this method water is pumped out to create critical depression head.The rate of pumping is now so adjusted so that the rate of water entering into the well is kept equal to the rate of pumping keeping constant level.
Let Q is the quantity of water pumped in litres at this rate of pumping is time t1hours. Now pumping is stoped and the well is allowed to refil to the original level before pumping. Let time taken to refil is t2 hours.
Therefore, litres / hour
Recuperation Test
First of all the level of water is calculated in the well under test. Now water is pumped out until the water level is depressed to some safe point let it be h1. Now the pumping is stopped and well is allowed to recup. Let h2 be the depression head after T hours of recupering. Let A be the area of the well in m2. And K is the specific yield of well in cu.m.p.h/m. The specific yield or capacity of well may be defined as the amount of water in cu.m per h or litre per h under 1 m head. Then
Q=2.3log base 10 H1/ H2× A/T× h M3 /hour
Yield of the well Q = KX h = AXV
Here, T = time required to recup from h1
to h2
in t hours
h1 = Initial draw down in m
h2 = Final draw down in m
h = Average draw down in m
A = Area of the well in sq. m
K = Specific yield in cu.m / h under 1 m depression head
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