Page 75 - Water and wastewater engineering

2-18 WATER AND WASTEWATER ENGINEERING

The seventh column ( S) is the difference between the third and fifth columns. For example,
for August and September 1994:
3
3
227 664 m 669 600 m 441 936 m 3
,
,
,
3

,
,
,
202 176 m 648 000 m 3 445 824 m 3
The last column( ( S )) is the sum of the last value in that column and the value in the sixth
3
column. For August 1994, it is 441,936 m since this is the first value.
For September 1994, it is

,
( 441 936 m 3 ) ( 445 824 m 3 ) 887 760 m 3
,
,
The following logic is used in interpreting the table. From August 1994 through March 1995,
the demand exceeds the flow, and storage must be provided. The maximum storage required
6
3
for this interval is 2.370 10 m . In April 1995, the storage (∆S ) exceeds the deficit (Σ(∆ S ))
from March 1995. If the deficit is viewed as the volume of water in a virtual reservoir with a
6
3
total capacity of 2.370 10 m , then in March 1995, the volume of water in the reservoir is
3
6
6
6
2.204 10 m (2.370 10 0.166 10 ). The April 1995 inflow exceeds the demand and
3
6
fills the reservoir deficit of 0.166 10 m .
3
Because the inflow ( Q in ) exceeds the demand (0.25 m /s) for the months of April and May
1995, no storage is required during this period. Therefore, no computations were performed.
From June 1995 through December 1996, the demand exceeds the inflow, and storage is
3
6
required. The maximum storage required is 3.695 10 m . Note that the computations for stor-
age did not stop in May 1996, even though the inflow exceeded the demand. This is because the
storage was not sufficient to fill the reservoir deficit. The storage was sufficient to fill the reser-
voir deficit in January 1997.
Comment. These tabulations are particularly well suited to spreadsheet programs.
The storage volume determined by the Rippl method must be increased to account for water
lost through evaporation and volume lost through the accumulation of sediment.
Groundwater
Unlike surface water supplies, groundwater is less subject to seasonal fluctuations and long-term
droughts. The design basis is the long term or “safe” yield. The safe yield of a ground water basin
is the amount of water which can be withdrawn from it annually without producing an undesired
result. (Todd, 1959) A yield analysis of the aquifer is performed because of the potential for over-
pumping the well with consequent failure to yield an adequate supply as well as the potential to
cause dramatic ground surface settlement, detrimental dewatering of nearby ponds or streams or,
in wells near the ocean, to cause salt water intrusion.
Confined Aquifer. The components of the evaluation of the aquifer as a water supply are:
(1) depth to the bottom of the aquiclude, (2) elevation of the existing piezometric surface,
(3) drawdown for sustained pumping at the design rate of demand, and (4) recharge and drought
implications.