Environmental Engineering Reference
In-Depth Information
Fig. 3.15 Calibration of a horizontal-axis current meter in a swimming pool. The meter is attached
to a cart that is pushed at a constant velocity (Photo by Masaki Hayashi)
where
A
i
(m
2
) is the cross sectional area of each rectangular subsection, and
x
i
(m) is
the distance along the line of measurements. This method of computation is called
the midsection method, and is known to provide more accurate values of
Q
than
many other methods (Dingman
2002
:611).
In Eq.
3.22
, total discharge is calculated using a number of data points measured
over the length of time required to quantify flow across the entire cross section.
In stream sections undergoing rapid flow transience (for example, during a storm
event or water extraction/release upstream), discharge may change substantially
during the measurement period. If these conditions are suspected, stream stage
should be monitored during the measurement period to assess the potential magni-
tude of error caused by the transience.
3.6.4.2 Tracer-Dilution Methods
Tracer-dilution methods are useful alternatives to the velocity-area method where it is
difficult or impossible to use a current meter due to high velocities, turbulence, debris,
rough channels, shallow water, or other physical reasons; or where the cross-sectional
area cannot be accurately measured (Kilpatrick and Cobb
1985
). However,
