Environmental Engineering Reference
In-Depth Information
channel or stream. If the cross-sectional dimensions of the channel are
known and the depth of flow is measured, then flow area can be com-
puted. From the relationship
q
=
a
×
v
, discharge
q
can be estimated.
In preliminary fieldwork, this simple procedure is useful in obtaining
a ballpark estimate for the flow rate, but it is not suitable for routine
measurements.
Example 2.16
Problem:
A floating object is placed on the surface of water flowing in a
drainage ditch and is observed to travel a distance of 20 m downstream
in 30 seconds. The ditch is 2 m wide, and the average depth of flow is
estimated to be 0.5 m. Estimate the discharge under these conditions.
Solution:
The flow velocity is computed as distance over time, or:
v
=
d
/
T
= 20 m/30 s = 0.67 m/s
Channel area
a
= 2 m × 0.5 m = 1.0 m
2
, and discharge
q
=
a
×
v
= 1.0 m
2
× 0.66 m
2
= 0.66 m
3
/s.
2.13.2 basis of Traditional flow Measurement
Flow measurement can be based on flow rate or flow amount.
flow
rate
is measured in gallons per minute (gpm), million gallons per day
(MGD), or cubic feet per second (cfs). Water/wastewater operations need
flow rate meters to determine process variables within the treatment
plant, in wastewater collection, and in potable water distribution. The
flow rate meters normally used are pressure differential meters, mag-
netic meters, and ultrasonic meters. Flow rate meters are designed for
metering flow in closed pipe or open channel flow.
flow amount
is measured in either gallons (gal) or cubic feet (ft
3
).
Typically, a totalizer, which sums up the gallons or cubic feet that pass
through the meter, is used. Most service meters are of this type. They
are used in private, commercial, and industrial activities where the
total amount of flow measured is used to determine customer billing. In
wastewater treatment, where sampling operations are important, auto-
matic composite sampling units—flow proportioned to grab a sample
every so many gallons—are used. Totalizer meters can be of the veloc-
ity (propeller or turbine), positive displacement, or compound type. In
addition, weirs and flumes are used extensively for measuring flow in
wastewater treatment plants because they are not affected (to a degree)
by dirty water or floating solids.
2.13.3 flow Measuring devices
In recent decades, flow measurement technology has evolved rapidly
from the old-fashioned way of measuring flow discussed earlier to the
use of simple practical measuring devices to much more sophisticated
