Contents preface (VII) introduction 1—37

Table 4.7 Recommended well diameters for different pumping rates (6)

Anticipated	Nominal size of	Optimum size of	Smallest size of
pumping rate	pump bowls	well casing	well casing
(m^3/day)	(mm)	(mm)	(mm)
Less than 540	102	152 ID	127 ID
410–	950	127	203 ID	152 ID
820–	1910	152	254 ID	203 ID
1640–	3820	203	305 ID	254 ID
2730–	5450	254	356 OD	305 OD
4360–	9810	305	406 OD	356 OD
6540–	16400	356	508 OD	406 OD
10900–	20700	406	610 OD	508 OD
16400–	32700	508	762 OD	610 OD

4.8.3. Design of Well Screen
The design of a well screen (i.e., its length, slot, open area, diameter, and material) is the most important aspect of a well design. The basic requirements of a well screen are as follows (16):
(i) It should be corrosion resistant,
(ii) It should be strong enough to prevent collapse,
(iii) It should prevent excessive movement of sand into the well, and
(iv) It should have minimum resistance to the flow of water into the well.
4.8.3.1. Length of Well Screen
The intake portion of a well must, obviously, be placed in the zones of the maximum hydraulic conductivity. Such zones are determined by interpreting the lithologic log, visual inspection and sieve analysis of the samples collected during drilling, laboratory tests for hydraulic conductivity and the results of pumping tests. The optimum length of the well screen depends primarily on the nature of the aquifer stratification and the permissible drawdown.
In the case of a homogeneous unconfined aquifer of thickness less than 45 m the screening of the bottom one-third to one-half of the aquifer is recommended (6). In thick and deep aquifers, however, as much as 80 per cent of the aquifer may be screened to obtain a higher specific capacity and greater efficiency even though the resulting yield may be less. These guidelines are applicable to non- homogeneous unconfined aquifers also. However, screen sections are positioned in the most permeable layers of the lower portions of the aquifer (leaving depth of about 0.3 m at the upper and lower ends of the screen to prevent finer material of the transition

148 IRRIGATION AND WATER RESOURCES ENGINEERING
zone from moving into the well) so that maximum drawdown is available. Wherever possible, the total screen length should be approximately one-third of aquifer thickness.
For homogeneous confined aquifers, the central 80 to 90 percent of the aquifer thickness should be screened assuming that the water level in the well would always be above the upper boundary of the aquifer. In case of non-homogeneous confined aquifer, 80 to 90 per cent of the most permeable aquifer layers should be screened.
If the effective size of two strata are the same, the stratum with lower uniformity coefficient (i.e., relatively poorly graded) is more permeable and should, therefore, be screened.