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Chapter 4:
Well Hydraulics
SPECIFIC YIELD (S_{y}) :
The specific yield of an unconfined aquifer is the ratio of volume of water which will flow under saturated condition due to gravity effects to the total volume of aquifer (V).
where, V_{WY} = Volume of water yielded under gravity effect and V = Total volume of water.
SPECIFIC RETENTION:
The specific retention of an unconfined aquifer is the ration of volume of water retained against gravity effect to the total volume of aquifer (V)
where V_{wr} = Volume of water retained under gravity effect
Remember:
S_{y }+S_{R}=n where, n = porostiy
COEFFICIENT OF TRANSMISSIBILITY:
T=kH
where,
H = Thickness
K = Coefficient of permeability
UNCONFINED AQUIFER:
(a) Theims Theory
Where,
q = Rate of flow in m^{3}/s
h_{1} = Height of water table of 1^{st} observation well
h_{2} = Height of water table of 2^{nd} observation well
s1 = Drawdown of 1st test well
s2 = Drawdown of 2nd test well.
h_{1} + s_{1} = h_{2 }+ s_{2}
S_{1} = Drawdown of 1^{st} test well
S_{2} = Drawdown of 2^{nd} test well
r_{1} and r_{2} are radius of 1^{st} and 2^{nd} observation wells respectively.
(b) Dupits Theory:
where, S = Drawdown in the well
K = Permeability coefficient in m/s.
R = Radius of influence in ‘m’
150m ≤ R ≤ 300m
r = Radius of test well in ‘m’.
Results of dupits theory are not accurate because ‘R’ is based on empirical relation.
CONFINED AQUIFER:
(i) Theims theory:
(ii) Dupits theory
SPHERICAL FLOW THROUGH WELL
q_{s}=k2πrs, R = Radius of well
S = Drawdown
q_{s} = Rate of flow through spherical well in m^{3}/s
PUMPINGINTEST
(a) Open end test
where, r = radius of pipe, h = head of water above the base of pipe, it may include gravity head and pressure head.
(b) Tacker test
where, L = Length of perforated section of pipe
r = Radius of pipe
h = head of which water is added
OPEN WELL (RCUPERATION TEST):
where,
Volume = A. H
A = Area of well
C/A = specific yield or specific Capacity of an open well.
T = Time in ‘sec’
h_{1} = Position of water table of t = 0
h_{2} = position of water table of t = T
VALUES OF PERMEABILITY:
SOIL  K (cm/sec)  Degree of permeability  
1.  Coarse Gravel  >1  High 
2.  Fine gravelFine sand  1 to 10^{2}  medium 
3.  SiltSand admixtures,loose silts, rock flour and loess  10^{ 2} to 10^{4}  Low 
4.  Dense slit, clay silt admixutures,nonhomogenous clays  10^{4} to 10^{6}  Very low 
5.  Homogenous clays  < 10^{6}  Impervious 
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