Streamflow Lab
Feb. 10th, 2014
Objective: To measure velocity and cross-sectional area in a forested stream channel,
and compute volumetric flow rates.
Group Hypotheses:
Flow vs. distance downstream
Comparison of methods
NOTE: The data from today’s lab are due to the instructor by Wednesday at 5:00 pm.
Equation:
We’ll estimate discharge using two methods. First, we’ll measure velocity and crosssectional area of the channel, from which we can compute flow (in volume per time):
Flow = Velocityavg * AreaX-section (Qs = V * A)
[1]
Second, we’ll also use a conservative (that is, non-reactive) tracer called Rhodamine WT.
We’ll dose the creek with a known volume of solution of known concentration, and use
the mass breakthrough of dye (stream concentration over time) to estimate the flow:
Mass of Dye = Ctracer * Qtracer = Q * Σi = 1 – n CiΔt
[2]
Where stream flow (Q) is in units of volume per time, Ci is the tracer concentration
observed in the stream at each time, Qtracer is the volume of tracer delivered to the
stream, and Ctracer is the concentration of the tracer solution (20% by mass). The Δt in
this case is 1 minute (60 seconds). So we’re basically summing up the total mass of dye
based on each bar in the figure above, and assuming that the fluorometer sees ALL the
dye go by (100% mass recovery). That allows us to back calculate Q. Be careful of units!
Procedure (Part 1):
1. Divide into 4-5 groups. At least one person in each group will be in the water.
Take a flow kit, 1 metal stake, 3 pvc stakes, net and a 3’ or 4’ ruler to your stream
section.
2. Drive the metal stake at the waters’ edge and a PVC stake on opposite bank.
Attach the ruled fiberglass tape to the metal stake so that 0.00 is at the channel
edge and stretch to the opposite stake and wrap securely. The tape should be 3
to 6 inches above the water. Record channel width and divide by 6. Tie a flag at
1/6 (0.17), 3/6 (0.50), and 5/6 (0.83) of the channel width (middle of 3 equal
stream sections. For example if the channel width is 10.6 feet, tie flagging at
0.17 x 10.6’ = 1.82’, 0.50 x 10.6’ = 5.3’, and 0.83 x 10.6’ = 8.8’.
3. Record stream depth at each of the 3 flagged marks. The width of each section
will be 1/3 of the total stream width. Is this enough? How might this affect the
selection of the sampling station?
4. Use the ruler, 2 stakes, and string to establish a parallel line 3ft downstream
from the first tapeline.
5. Determine velocity for each stream section using the float method. Release the
float 1-2 feet upstream from the tape. Start the stopwatch when it reaches the
tape and stop when it reaches the string line; try to ensure that the float stays
within the 1/3 of the stream that you’re evaluating. Nets are provided to help
retrieve the floats. Mean velocity for the float is assumed to be = measured
velocity * 0.8 (why?). Repeat each measurement twice and take the average.
6. Fill the bottle containing “creamer” with water and shake. Pour 1/3 of the bottle
at a flag point and start the timer. When the middle of the tracer cloud reaches
the downstream string line, stop the time. Repeat for the other 2 flag points.
Note tracer behavior; did it go straight, was it slow in places and fast in others.
7. Note bank height of your stream reach. Estimate channel width and depth.
8. Pick another stream reach within 15 m of the first, and repeat.
Fig. 1 – Techniques for streamflow using velocity methods.
Procedure (Part 2):
9. Select an injection site. Place the fluorometer (a sensor that emits light of a
particular wavelength and collects light that fluoresces from the dye –
Rhodamine WT – at another wavelength) ~ 100 m downstream of the site.
10. Initiate the data collection for the fluorometer, and take a note of the time on a
third time piece (watch, laptop).
11. Dump the dye solution. Note the exact time that the dye is delivered to the
stream.
12. Estimate the mass of the dye (the solution that we’re adding is 20% by mass of
Rhodamine WT) added based on the volume of the solution.
13. Watch the dye transport downstream. The fluorometer will be logging the
concentration.
14. Remove the fluorometer after an hour (the approximate time over which dye in
the creek will work its way out of the system).
15. Examine the breakthrough curve to compute:
a. Time for dye to first reach each of the fluorometers
b. Time for dye to leave the system.
c. Flow (based on mass input = mass output)
Fig. 1 – Example breakthough curve for Rhodamine WT in Alexander Springs. Mean residence
time = 4.9 hours (upstream) and 10.1 hours (downstream). Flow ~ 120 cfs.
Report:
Abstract: Summarize the experimental findings. What were the measured flow rates?
How did they differ between methods? How did they differ between groups? How did
they differ from the tracer experiment? What else about flows is clear from the tracer
experiment?
Introduction: Why is the lab relevant? What did you expect to observe given the things
that you measured…that is, what were your hypotheses? You should introduce both
comparison of methods (the technical portion of the lab) AND the hydrologic portion of
the lab (i.e., what did you physically expect to observe).
Methods: Describe your methods. Use the following table to make your calculations.
Be sure to convert raw data to requested units.
Group # ______
Float Method
(repeat measurements twice for each section)
Reach #1
Width (ft)
Depth (ft)
Area (m2)
Float time (sec)
Section 1
Section 2
Section 3
1: ____
2: ____
3: ____
1: ____
2: ____
3: ____
1: ____
2: ____
3: ____
Section 1
Section 2
Section 3
1: ____
2: ____
3: ____
1: ____
2: ____
3: ____
1: ____
2: ____
3: ____
Total
Float Mean Velocity (m/sec)
Stream Mean Velocity (m/sec) (v * 0.8)
Estimated Flow (m3/sec)
Reach #2
Width (ft)
Depth (ft)
Area (m2)
Float time (sec)
Float Mean Velocity (m/sec)
Stream Mean Velocity (m/sec) (v * 0.8)
Estimated Flow (m3/sec)
Total
Tracer Method (creamer)
(repeat measurements only once for each section)
Reach #1
Tracer time (sec)
Tracer velocity (m/sec)
Estimated Flow (m3/sec)
Reach #2
Tracer time (sec)
Tracer velocity (m/sec)
Estimated Flow (m3/sec)
Fluorometric Tracer Method
(repeat measurements only once for each section)
Dosing Volume 20 mL
Dosing Concentration 20% ( =200,000,000 ppb)
Time
0
1
2
3
4…
Time for initial breakthrough (sec)
Time to leave the system (sec)
Estimated Discharge (m3/s, Eq. 2)
Concentration
C0
C1
C2
C3
C4
Ci * ΔT
Q=
(Ctr* Qtr)/ Σi = 1,n CiΔt
Results: Share your results, and summarize the group results. We will make the
fluorometer data available to all. Consider both the measurement and hydrologic
hypotheses. For example, you might make a table of flows by different measurement
technique at each station, and a graph of flow with distance downstream.
Discussion: This is where you consider your findings and summarize them. For
example:
- How did total flow for the velocity methods compare? Which do think is more
accurate?
- Each group sampled the same stream in different locations. When you’ve
computed your streamflow, send it to the other groups. Are they the same? Are
they different in the way that you expected? How important is measurement
uncertainty?
- How did the velocity methods compare with the tracer method? Given your
measured mean stream velocity and the distance between the dosing location
and the fluorometers, what is the expected travel time? What is the travel time
estimated from the shape of the dye breakthrough curve?
You should also present answers to the following questions:
- If your total channel flow (m3/s) measured using the float method is
representative of the annual mean flow, what is the annual flow (your total flow
m3/s * s/yr)?
- Assume the watershed area contributing to your measurement point is 60 km2.
What is the depth of annual flow (m/yr)? Compare this number with ET (~ 0.9
m/yr). Careful with your units.
- Assume a constant mean velocity for the channel. Given the cross sectional area
of the whole channel (bank-to-bank), what would the volume discharge have to
be for flood (bank overflow) conditions?