Page 65 - Water and wastewater engineering

2-8 WATER AND WASTEWATER ENGINEERING

10 Peak flow
Ratio of minimum or peak to average daily sewage flow 1 Average daily flow

0.1 Minimum flow
1 10 100 1000
Population, in thousands
FIGURE 2-1
Ratio of extreme flows to average daily flow

pattern is that larger cities are supplied by large surface water bodies while many small communities
use groundwater.
Groundwater has many characteristics that make it preferable as a water supply. First,
groundwater is less subject to seasonal fluctuations and long-term droughts. Second, the aquifer
provides natural storage that eliminates the need for an impoundment. Third, because the ground-
water source is frequently available near the point of demand, the cost of transmission is reduced
significantly. Fourth, because natural geologic materials filter the water, groundwater is often
more aesthetically pleasing and to some extent protected from contamination.
Groundwater as a supply is not without drawbacks. It dissolves naturally occurring minerals
which may give the water undesirable characteristics such as hardness, red color from iron oxida-
tion, and toxic contaminants like arsenic.

Yield
One of the first considerations in selecting a water supply source is the ability of the source to
provide an adequate quantity of water. One measure of quantity is yield. Yield is the average flow
available over a long period of time.

Surface Water
When the proposed surface water supply is to be the sole source of water, the design basis is the
long-term or “safe” yield. The components of the design are: (1) determination of the allowable
withdrawal, (2) completion of a complete series analysis and, if the design drought duration exceeds
the recorded data interval, completion of a partial duration series analysis, and (3) completion of an
extreme-value analysis to determine the probable recurrence interval ( return period ) of a drought.
The allowable withdrawal is determined from regulatory constraints. Obviously, the municipality
desiring to use the surface water for supply cannot withdraw all of the available water. Enough
must be left for the ecological health of the river or stream as well as for downstream users.
In some cases, such as the Great Lakes, the water body is so large that the classic analysis of
drought conditions is not warranted. However, the fluctuation of the lake level does impact the
design of the intake structure, and it must be evaluated.