Page 171 - Water and wastewater engineering

4-26 WATER AND WASTEWATER ENGINEERING

h 1 , h 2 height of piezometric surface above the confining layer, m
r 1 , r 2 radius from pumping well, m
2 2
The term h 2 h 1 may written as 2( h 2 h 1 )( h 2 h 1 )/2, where ( h 2 h 1 )/2 is the average
height of the aquifer between r 2 and r 1 . The product K ( h 2 h 1 )/2 represents the average
transmissivity between r 2 and r 1 (Bouwer, 1978).
The procedure then is to calculate the individual drawdowns ( h iw ) at r w at time t with Equa-
tion 4-8 and use this value as h 1 at r 1 in Equation 4-10 to determine the value of h 2 (and con-
sequently s 2 ) at another well located at r 2 . Then, as with the confined aquifer system discussed
above, determine the total drawdown by the method of superposition.
In general, finite-element numerical methods for estimating interference yield more satisfac-
tory results than the approximation technique presented here. These are left for more advanced
texts.

Evaluation of Interference Calculations. The first criteria in evaluating the results of the

interference calculation is to determine whether or not the operation of the wells results in failure
of the well.
• For a confined aquifer, this is a drawdown that lowers the resultant piezometric surface
below the bottom of the upper confining layer, that is, the top of the confined aquifer. If
it does, then the solution is unacceptable because the dewatering of the aquifer will cause
ground settlement and structural failure of the wells as well as buildings in or near the well
field.
• For an unconfined aquifer, failure occurs when the drawdown lowers the piezometric
surface below the pump. In effect, the well “drys up.”

These are “catastrophic” events. Prudent engineering design will ensure that the operation of the
well does not approach failure.
Because there is a need to provide a reliable groundwater source, it is unusual to pump all
the wells in the well field at the same time. Some wells must serve as a backup in case of pump
failures, downtime for maintenance, and emergency demand such as fires. Thus, the evaluation
of the interference calculations is guided by the need to assess the impact on the reserve wells,
that is, will the piezometric surface of the nonpumping wells be lowered to such an extent that
pumping from them is impractical or uneconomical?
A general operational technique then is to operate a fraction of the wells for shorter periods
of time and to rotate between wells to allow time for recovery.

Well Field Layout. The selected arrangement of wells and the number of wells is based on
the hydraulic analysis and the operational schedule that can be employed. The maximum day
demand must be satisfied with enough pumping reserve capacity to allow for pumps to be out of
service for repairs. The wells must be spaced to meet the hydraulic constraints of the aquifer as
well as property boundaries and any existing pipe network.

Pump Type
Vertical turbine pumps are frequently selected for municipal water supply. These are the same
type of pump that was discussed in Chapter 3. They may be either submerged pumps, where the