Measurement of River Flow
Skills and concepts that students must have mastered
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Description and Teaching Materials
This lab involves a visit to a local stream where students will use two methods to calculate stream discharge and compare the results. This lab is meant as a follow-up to a previous lab where students assessed the characteristics of the watershed in terms of size, monthly precipitation inputs vs. monthly stream discharge, and evaluation of flooding. The following steps describe the procedures and equipment used:
Step 1: Have the students visually estimate (guess!) stream discharge.
Step 2: Pick a section of stream suitable for wading. Three sets of hip boots are provided for volunteers.
Step 3: Using tapes and Philadelphia rods, measure stream depths every ~0.5 meters across the channel. Students should then graph their results and calculate the cross sectional area of the river flow. Using the graph, calculate wetted perimeter and then calculate hydraulic radius.
Step 4: Using transit or autolevel and a Philadelphia rod, measure the stream gradient over a particular distance.
Step 5: Have the students assess this section of stream bottom and estimate Manning's n value (handout provided). Calculate average stream velocity (equation provided in uploaded material) and then stream discharge.
Step 6: Now have the students pick two locations, one upstream and one downstream. Using the tapes and Philadelphia rods, once again measure the cross sectional area at both locations and then calculate the average cross sectional area. Finally, measure the distance between the two locations.
Step 7: Using sticks and stop watches, measure the stream velocity using the upstream location as the starting point and the downstream location as the end point. Perform this several times at various distances from the bank. Convert the surface velocity to an effective stream velocity by multiplying by 0.85 (explain how stream velocity changes with depth). Now calculate discharges for the various velocities by multiplying by the average cross sectional area. Finally, calculate an average discharge.
Step 8: Compare the discharges from the two methods (survey method vs. float method) and also compare to their initial visual estimate. If possible, use a flow meter and collect a measurement for comparison.
Step 9: Have students turn in data collection tables, graphs, and a small write-up about the practicality and accuracy of their methods.