FIELD EXERCISE: STREAM FLOW DYNAMICS & SEDIMENTATION

 Daniel J. Bisaccio 18 School Street Troy, NH 03465 Donald L. Woodrow Department of Geoscience Hobart & William Smith Colleges Geneva, NY 14456

Level: Senior high

Anticipated Learning Outcomes

• Students will produce a vertical profile of a stream and be able to map as well as define thalweg, point bar, and cut bank.
• Students will measure flow velocity differences across a stream transect and equate flow velocity with size of substrate sediment collected.
• Students will be able to quantify discharge values for a stream.

Background

Streams are the great shapers of landscape as they work to erode, transport, and deposit sediment. In this exercise, we are going to examine how one kind of stream does some of its work.

A meandering stream is one which has a curving or sinuous path. Its curves are called meanders after the characteristic features of a stream made famous in classical times. That stream, the Menderes River, is located in the southwest part of modern Turkey. Meandering streams are often muddy because they transport a lot of fine sediment in suspension. They also transport sediment along the stream floor, mainly fine gravel and sand. Each meander displays a cut bank and point bar (see figure 1). At the cut bank the stream is eroding, at its deepest part (the thalweg) the stream is transporting sediment and it is depositing sediment on the point bar.

We are going to measure the profile of a stream, determine its current velocity at various points and depths and examine the sediment on the stream bottom. Our purposes are to determine the relationships between stream profile and flow velocity and between sediment grain-size and flow velocity.

Materials

• Meter stick
• Nylon rope marked at 0.5 meter intervals and long enough to span stream
• Cork float
• "Flower Garden Pinwheel" with one blade painted black to be used to determine stream velocities
• Dip container to collect substrate sediment and containers to store sediments collected
• Stop watch to be used to determine stream velocity

Procedure

1. Calibrate your pinwheel to determine stream velocity.
1. Determine the time it takes a cork float to drift 10 meters downstream WHILE a student counts the revolutions of the pinwheel which has been positioned below the stream surface. (See figure 2)
2. Calculate the stream flow velocity (meters/second) based on the number of pinwheel revolutions as shown in this example. Cork float travels 1 meter in one second and in that second the pinwheel revolves ten times. Therefore, 10 pinwheel revolutions/ second=1 meter/second stream flow (It is easier to count revolutions for 10 seconds and divide the number of revolutions by 10 to obtain a velocity in revolutions per second.)
2. Collect the stream data.
1. Set up a nylon rope across the stream and select at least four data-collecting points across the stream as shown in Figure 1 (points A, B, C, D).
2. At each data-collecting point, measure the stream depth and construct the cross-profile of the stream as shown in the sketch.
3. Students should identify the kind of sediment found on the stream bottom at each data-collection point. Look for mud, sand, gravel, or mixtures and note on the cross profile what you saw. If you plan to carry away sediment samples, put them in containers labelled with the proper location information.
4. Using the pinwheel, determine the stream flow velocity at each data-collecting point. At each point the velocity should be determined at the stream surface, near the bottom and at a middle depth of the stream.
5. Calculate the stream flow velocities and note them on the stream profile.
6. Calculate the stream discharge (Q) using this formula:

Discharge (Q) = width x depth x velocity

Q = (meters) x (meters) x (meters/second)

Results

1. One side of the stream profile is steep (cut bank) and the other side has a gentle slope (point bar). The deepest part of the stream is the thalweg. Label the thalweg.
2. The stream flow velocity varies both across the stream and with depth. The greatest velocities are near the thalweg.
3. The coarsest sediments are located on the stream floor where the stream flow velocity is greatest.