Environmental Engineering Reference
In-Depth Information
0
drawdown [m]
-5
-10
Drawdown in a half-confined aquifer
according to de Glee
-15
0
5
10
15
20
distance [m]
Fig. 12.5
Drawdown of groundwater piezometric head in a half-confined aquifer due to pumping,
according to de Glee (
1930
)
and calculate the vector of drawdowns according to:
s = (Q/(2*pi*T))*besselk(0,r/sqrt(T*c));
The correct variant of the Bessel function (here
besselk
) and the parameters
are found in MATLAB-help. The graphical representation of the results (Fig.
12.5
)
is shown by using the
plot
command:
plot (r,-s);
In Fig.
12.6
drawdowns in a confined, an unconfined and a half-confined aquifers
are compared. The drawdown for the half-confined situation lies between the
drawdown for the confined and the unconfined aquifers. The user may easily find
parameter values for which that reasonable result is not true. The reason for the
apparent incompatibility is that all three formulae are valid under different
conditions. The formula of de Glee is derived for the half-space below the half-
permeable layer, i.e. under the assumption that the aquifer is too extended, making
the value of its thickness irrelevant.
The complete code is included in the accompanying software under the name
'welldrawdown.m'
12.4 Unsteady Drawdown and Well Function
In a confined aquifer the drawdown of the piezometic head
s
is given by the formula
of Theis (
1983
)
2
.
s
is a function of the distance from the well
r
and the time
t
after
the start of pumping:
2
Charles Vernon Theis (1900-1987), US-American hydrogeologist.
