Test: Open Well and Dug Well - Civil Engineering (CE) MCQ

Specific capacity is the measure of the well performance indicating the rate of water percolation into a well. It is also defined as the yield of a well under the head of one meter. The specific capacity of the well is not constant but decreases as the discharge increases.

Yield of a well is the rate at which water percolates into the well under the safe maximum depression head or the critical depression head. It depends on the position of the water table, permeability, and porosity of the soil, rate of water withdrawal from the well, and amount of water storage in the well.

A shallow well is that well which does not encounter mota formation. A deep well is that well which goes below the water table and finally finds its foundation in the mota formation.

When the limit of critical velocity is not exceeded it may be rightly assumed that K/A is constant for a well. K/A is the specific yield of well per unit area of the well. The value for K/A given by Marriot for clay sub-soil is 0.25, for fine sand = 0.50 and for coarse sand, it is 1.0.

Given, Yield of well = 0.004 x 3600 = 14.4 m³/hour
Q or yield = K x H
14.4 = K x 4 or K = 3.6 m³/hour under head of 1 meter.
From Marriot’s table, the value of K/A for sandy sub-soil formation = 0.5
3.6/A = 0.5 or A = 7.2 m²
Diameter of well = (A x 4/3.14)^1/2 = 3.07 m.

Denkli is generally provided on the unlined wells which are supposed to irrigate only small fields. Churus is suitable for lined wells and can be conveniently used deep wells up to a maximum depth of 30 m. Persian wheel (also known as Rahat in some parts of India) is suitable when the depth of well is 12-30 m. The windmill is not very reliable.

Open wells have comparatively bigger diameter varying from 2-9 m and are suitable for low discharges in the order of 1-5 liters per second. The yield of the well is limited as the groundwater storage is limited. An open well irrigates small area and its cost per hectare is generally more than that of a tube well.

Specific yield (K) is calculated from the formula, K = 2.303 (A/T) log (H₁/H₂) where A is the area of the well, T is the total time of recuperation to bring the water level from depth H₁ to H₂.
Given values, A = 3.14/4 x 9 = 7.065 sq. m, T = 4 hours, H₁ = 7m, H₂ = 2m
K = 2.303 x 7.065 x 1/4 x log (7/2) = 2.213 m³/hr under a head of one meter.

Permanent lining or wells with impervious lining is sunk in regions with alluvial soil formation. Unlined wells or temporary wells are of very shallow depths and can be constructed only when the water table is very near to the ground. Wells with pervious lining is generally suitable in the strata of gravel or coarse sand.

The depression head is kept equal to one-third of the critical depression head. The maximum or critical yield corresponds to the critical depression head and the maximum safe yield corresponds to the working head.