Environmental Engineering Reference
In-Depth Information
Fig. 3.18 (a) Example of a portable V-notch weir (Photo by Masaki Hayashi), (b)portable2.54-cm
Baski flume (Photo printed with kind permission of
Kirk Miller, U.S. Geological Survey 2012. All
©
Rights Reserved)
error can be introduced in estimated
Q
. It is strongly recommended that the methods
be tested in the field according to procedures described by Rantz (
1982
:216-220).
Other sources of error include the loss of tracer solution between the injection and
sampling points to groundwater or hyporheic exchange, photochemical and other
reactions, sorption to streambed materials, and interference of concentration moni-
toring devices by turbidity and other sensor-calibration issues. Errors due to
reaction and sorption, and to some extent sensor issues, can be minimized by the
choice of tracer. The ideal tracer has very low natural concentration in the stream, is
chemically and biologically conservative (no reaction, absorption, release or
uptake), is readily detectable at low concentration, and is harmless to the observer
and aquatic life. Sodium chloride and fluorescent dye are commonly used as tracers.
3.6.4.3 Other Methods of Discharge Measurement
For narrow streams with small discharge, it is often possible to divert the entire flow
to a container having a known volume, and measure the time it takes to fill the
container to calculate discharge (the bucket-stopwatch method). Examples of sites
presenting the opportunity for this method are a V-notch weir or a cross-section of
natural channel where a temporary earthen dam can be built over a small-diameter
pipe (Rantz
1982
:263). If a temporary structure is built for volumetric measurement,
the stage behind the structure (e.g., a dam) should be allowed to stabilize before the
measurement. The measurements should be repeated several times to obtain consis-
tent results. Where a portable V-notch weir or Parshall flume can be installed in the
stream (Fig.
3.18
), discharge is estimated from the stage measurements in these
devices using a laboratory-calibrated formula (see next section).
In situations where no current meter or tracer-dilution equipment are available,
or the condition does not permit the use of other methods, estimates of discharge
can be obtained using surface floats. Any distinguishable article can be used as a
