An Ocean in Colorado?

Background

Medano Creek exhibits a unique phenomenon, in that it flows in periodic surges like ocean waves coming onto the beach. For surge flow (also known as pulsating flow) to occur, three conditions must be met: swift water velocity, shallow water depth, and a sandy streambed. Medano Creek meets all three criteria, as does Sand Creek on the other side of the dunefield. Medano Creek has a steep slope, as streams go, and as a result, the stream has a high velocity. It is also shallow and flows over a bed of almost pure sand. As the water flows, ridges and troughs (antidunes) of sand form on the floor of the creek. The troughs act as tiny dams holding the water back until the flow breaks over the top, causing a surge. New antidunes continually form and break.

Surge size and frequency depend on how much water is flowing in Medano Creek. In the spring and early summer, the average frequency occurs approximately every 15 seconds. The students will test to see if the surges really do come every 15 seconds.

In normal precipitation years, these experiments are best conducted during spring and early summer, when Medano Creek is flowing. This is a seasonal creek that typically stops flowing past the main dunes access area from midsummer through early spring. Call the Visitor Center (719) 378-6399 for current conditions. Note that in the morning, water temperatures can be quite chilly. Plan this activity for late morning or afternoon, if possible.

Procedure

Prior to the field trip, have students view the Pulsating Flow animation and read about Medano Creek on this web site.

Before beginning the experiments, question students about what they learned about Medano Creek, such as where the water comes from, where it goes, and that it exhibits the rare phenomenon of surge flow. Point out that the waves are formed by the tiny sand dams, or antidunes, on the stream bed. Have students locate some antidunes. Watch the water pool and then the surge.

For the following hydrology experiments, print out one copy of the Surge Flow Graphing Worksheet for each group.

Experiment 1 - Surge Frequency

Describe the purpose of the first experiment to students: to measure how often the surges happen on Medano Creek. As a class, spend a few minutes considering and discussing possible methods and procedures for conducting this experiment.

After experiment ideas have been discussed, divide the class into groups of four students each. Give each group a stopwatch and a worksheet. Space groups along the creek, with each group about 30 feet apart (so they are out of immediate earshot of one another.) Have each group decide how to measure surge frequency and write out their procedure. Each member of the group should practice timing a surge and recording the result. Have the groups average the results of the three measurements. Gather back as a class and compare results. Did measurements differ in places where the creek is deeper or shallower? Why or why not? If there was a difference, describe it.

Experiment 2 - Antidune Development

The groups will now measure how much the water around an antidune changes during the surges. Give each student an assignment: one student will record data, the second will manage the stopwatch, the third will take measurements (on the upstream side of the antidune), and the fourth will observe the antidune development in the area of measurement.

Demonstrate how to conduct the experiment, placing a yardstick on the upstream side of the antidune in such a way that measurements are consistent. Measurers will read the depth of the water at their location every five seconds. Remind them to be sure that the bottom of the yardstick does not become buried by new sand. If it does the yardstick will need to be repositioned. The student with the stopwatch will call out "time" when it is time for students to take a reading. The Measurer will call out the reading, which will be written down by the Recorder. Timers should also call out to the Recorder the time when a surge breaks. Recorders will place a special mark on the graph when a surge breaks on the yard stick. Continue measuring and recording for 60 seconds.

Repeat up to four times at the same location, recording data with different colored pencil on the same graph at the same location.

After the activity, have the students discuss the results. Was there any consistency in the difference in water levels just before the antidune broke? Discuss the antidune's role in helping to create the surge flow phenomena. Discuss what might happen if there was no sand at the base of the dunes. How would this affect Medano Creek?

Critical Thinking

Has anyone ever been to a beach by an ocean or lake? How are the waves and surrounding environment similar and/or different from what students are seeing along Medano Creek?

Adapted from Educator's Guide to Great Sand Dunes, by Lori Cooper, Friends of the Dunes.