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SPECIFIC YIELD (Sy) :
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, VWY = Volume of water yielded under gravity effect and V = Total volume of water.
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 Vwr = Volume of water retained under gravity effect
Sy +SR=n where, n = porostiy
COEFFICIENT OF TRANSMISSIBILITY:
H = Thickness
K = Coefficient of permeability
(a) Theims Theory
q = Rate of flow in m3/s
h1 = Height of water table of 1st observation well
h2 = Height of water table of 2nd observation well
s1 = Drawdown of 1st test well
s2 = Drawdown of 2nd test well.
h1 + s1 = h2 + s2
S1 = Drawdown of 1st test well
S2 = Drawdown of 2nd test well
r1 and r2 are radius of 1st and 2nd 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.
(i) Theims theory:
(ii) Dupits theory
SPHERICAL FLOW THROUGH WELL
qs=k2πrs, R = Radius of well
S = Drawdown
qs = Rate of flow through spherical well in m3/s
(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):
Volume = A. H
A = Area of well
C/A = specific yield or specific Capacity of an open well.
T = Time in ‘sec’
h1 = Position of water table of t = 0
h2 = position of water table of t = T
VALUES OF PERMEABILITY:
|SOIL||K (cm/sec)||Degree of|
|2.||Fine gravel-Fine sand||1 to 10-2||medium|
|3.||Silt-Sand admixtures,loose silts, rock flour and loess||10- 2 to 10-4||Low|
|4.||Dense slit, clay silt|
|10-4 to 10-6||Very low|
|5.||Homogenous clays||< 10-6||Impervious|