GEOG 2043
GEOG 2043BACKGROUND: The velocity-area method of stream discharge measurement introduced in the previous lab becomes more difficult and less accurate if streams are highly turbulent and/or have rough irregular channels. As you saw with the last lab, current meters don't work very well in stream reaches with large rocks and shallow flow because of the high turbulence produced there. A better measurement technique for these streams is the dilution gauging method. The basic procedure for dilution gauging involves introducing a tracer into the flow at an upstream location and measuring the arrival of a tracer at a downstream location. The concentration of the tracer changes with time as a function of discharge. There are two primary methods of dilution gauging: 1) constant injection, and 2) slug injection.
The constant injection method is probably the most precise method of measuring stream discharge as it overcomes problems with flow variations caused by eddies and instream storage. This method involves injecting a tracer solution at a constant rate for a period of time sufficient for the downstream concentration to reach a steady state equilibrium value. Discharge is calculated using the following equation:
where Q is the stream discharge, Qt is the injection rate, Ct is the tracer concentration of the solution being injected, Ceq is the tracer concentration in the stream at equilibrium, and Cb is the background concentration of the tracer in the stream. Despite the fact that this method may be the most precise way of measuring discharge, it does have some logistical problems. First, you need a storage reservoir large enough to allow the tracer concentration to reach an equilibrium state in the stream. Second, you need a pump and some power source to maintain a constant injection rate of tracer into the stream. Third, some traces are toxic to the stream ecosystem.
As mentioned above, the second dilution method is the slug injection
method. This discharge measuring technique involves dumping
a tracer of know volume and concentration into a stream and measuring the
downstream concentrations over time, until the concentration of the tracer
reaches the background level in the stream. Calculating the discharge
from the slug injection method involves integration, or calculating the
area under the curve of concentration vs. time,
where Vt is the volume of tracer solution, Cd(t) is the tracer concentration at the downstream site as a function of time. This equation simplifies to:
The slug injection method has the benefits of not requiring a large
reservoir for tracer and this makes it suitable for remote sites.
It is also fairly quick (approximately half an hour from start to finnish),
and inexpensive. Some things to keep in mind for this method are
that the sampling interval at the downstream end has to be short enough
to catch the peak, and the tail of the curve may be fairly long due to
eddies and instream storage. We will use the slug injection method
for this lab.
TRACER: The tracer must be readily soluble and dissolve completely and quickly in the stream, and it must be easily detectable at low concentrations. Background concentrations of the tracer should be low or undetectable. The tracer must be chemically, physically and biologically un-reactive, or conservative. That is, it must not undergo chemical reactions that cause it to change into another form, it must not adsorb onto sediments, and algae can't eat it. The tracer must also be harmless to the observer and to stream biota. This requirement is often controversial as what some people consider harmless may be considered harmful by others. You think about it and decide. For this lab we are going to use NaCl, or salt. For comparison, you will also be gauging the stream using the velocity-area method following last week's procedure.
SITE REQUIREMENTS:
VELOCITY-AREA METHOD:
See Lab 2 for necessary equipment and procedures.
TASKS AND QUESTIONS:
1) SITE MAP
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