Stream
Gaging
Instruments
Stream
Gaging is the common term for measuring the amount of water
flowing in an open channel and can range from a trickle in a
ditch to a flood on the Amazon River. The U.S. Geological
Survey (USGS) makes most streamflow measurements by current
meter. A
current meter is an
instrument used to measure the velocity of flowing water. The
principle of operation is based on the proportionality between
the velocity of the water and the resulting angular velocity
of the meter rotor. By placing a current meter at a point in a
stream and counting the number of revolutions of the rotor
during a measured interval of time, the velocity of water at
that point is determined.
Current meters
generally can be classified into two main types, those meters
having vertical-axis rotors and those having horizontal-axis
rotors. The comparative characteristics of these two types are
summarized below:
1. Vertical-axis rotor with
cups
Operates in lower velocities
than do horizontal-axis meters.
Bearings are well-protected
from silty water.
Rotor is repairable in the
field without adversely affecting the rating.
Single rotor serves for the
entire range of velocities.
2. Horizontal-axis rotor with
vanes
Rotor disturbs flow less
than do vertical-axis rotors because of axial symmetry with
flow direction.
Rotor is less likely to be
entangled by debris than are vertical-axis rotors.
The current
meters selected by the USGS for their standard use has been
the Price
Type AA and
pygmy vertical
axis current meters.
Through extensive testing and research, this style meter has
proven itself a highly accurate, durable, low maintenance and
economical choice for stream velocity measurements. The
equipment described here was designed to support current meter
measurements by wading off a bridge, boat or cableway. Support
equipment such as taglines, wading rods, and sounding reels
take measurements of the physical stream size thus allowing
the computation of total flow or discharge by Q=VxA where Q is
total flow in CFS or CMS, V is the velocity in FPS or MPS and
A the area in square feet or meters.
Also offered
are the Ott-type
horizontal axis current meters which were developed and are
primarily used in Europe. This horizontal axis style meter
offers the ability to measure oblique current velocities with
different pitch rotors available. The USGS does not use the
Ott-type meter because it is not as durable as the Price meter
under extreme conditions and requires more preparation and
care to operate than the Price-type meter.
Two different
ultrasonic Doppler or acoustic type velocity meters are
available for field use for portable and permanent field
installation. These systems are used in extremely low
reversing flow conditions or in extreme flood conditions where
a current meter cannot be put in the water.
