Measurement of River Flow

Jeremy Shannon, Michigan Technological University

Summary

The laboratory is an outdoor exercise where students visit a local stream and use two methods to calculate stream discharge. Tapes, Philadelphia rods,and a transit are employed for stream measurements. The students will learn how to measure the cross sectional area of a stream channel, calculate the wetted perimeter, calculate surface velocity, and estimate Manning's n value for channels.

Environmental Geology, Water Quality and Quantity, Environmental ...(more) | College Introductory, College Lower (13-14)

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Context

Audience

This exercise is used in an introductory environmental geology course that is populated by mostly non-geology majors, but they are usually engineering or science students.

Skills and concepts that students must have mastered

Students should know how to use graph paper and understand the concepts of stream velocity, discharge,and gradient(slope).

How the activity is situated in the course

This laboratory is administered as a stand-alone exercise toward the end of a spring course (after the snow has melted). Generally they have had an introduction to watersheds before they perform this lab.

Goals

Content/concepts goals for this activity

The content/concept goals for this laboratory are to understand what factors influence the velocity and discharge of a stream and the measurements and techniques required to calculate and/or estimate these values.

Higher order thinking skills goals for this activity

Higher order thinking goals for this activity include the comparison of two techniques for calculating stream discharge and evaluating the reasons why one method might be more accurate than the other. In addition, students are asked to think about how a stream like this should be monitored taking into account basin size, topography, vegetation, seasonal fluctuations, etc.

Other skills goals for this activity

Other skill goals associated with this lab involve the use of surveying equipment to collect accurate measurement of stream dimensions.

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.