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Classroom activity Grade 11 Science

Ondry River Stream Table: Bank Erosion at an Intake

A grade 11 stream-table investigation where teams vary discharge, slope, and bank cover, then explain a migrating bend beside a fictional water intake.

Objectives

  1. Students will run controlled stream-table trials that change one variable at a time.
  2. Students will identify where a channel erodes and where it deposits, and explain the difference.
  3. Students will apply their results to predict bank movement at a water intake on a river bend.

Vocabulary

Key words: discharge, gradient, cut bank, point bar, meander migration, suspended sediment

Teacher overview

A river bend is not a fixed feature, because the water moving through it removes material from the outside of the curve and lays it down on the inside. In this investigation, teams of three build a single meander in a stream table and run four trials that each change one variable, then use their own results to explain why a bend on a fictional river is moving toward a drinking water intake. The dye trace in Trial 1 comes early because it shows where the fastest thread of flow travels through a curve, which is the outside of the bend rather than the inside. Results here depend on the sand each class uses, so the recording sheet asks for ranks and descriptions rather than a single correct measurement.

Activity Directions

  1. Before class. Mix fine sand with pea gravel at roughly three parts to one, and fill each stream table to a depth of about two inches. Set up one two-liter bottle per team with tubing and a pinch clamp so inflow can be held steady. Place a bucket or pan at the low end to catch outflow. Print one handout per student. Every place named in this activity is fictional.
  2. Framing (6 min). Show a photograph or sketch of a river bend and define discharge, gradient, cut bank, and point bar. State the guiding question written at the top of the handout: what makes a bend move, and what does that movement do to the water a treatment plant takes in?
  3. Channel build (7 min). Each team carves a channel with one clear S-curve down the length of the table, about two inches wide and one inch deep, and sets the table slope with blocks to a rise of one inch over the table length. Teams press the sand firm along both banks so every team starts from a similar surface.
  4. Trial 1, baseline (8 min). Open the clamp to a slow steady inflow and run the channel for three minutes. Halfway through, add two drops of dye at the upstream end and time how long the color takes to reach the downstream end. Mark on the handout diagram where sand was removed and where it built up, and collect a jar of outflow. Rank the jar's cloudiness against the other trials at the end.
  5. Trial 2, higher discharge (8 min). Rebuild the banks, keep the same slope, and open the clamp to roughly double the inflow. Run three minutes and record the same observations.
  6. Trial 3, steeper gradient (8 min). Rebuild the banks, return to the baseline inflow, and raise the upstream end so the drop across the table is about double. Run three minutes and record the same observations.
  7. Trial 4, bank cover (8 min). Rebuild the banks, return to baseline slope and inflow, and pin strips of moss or foam along the outside of both curves so they sit half in the water. Run three minutes and record the same observations.
  8. Application (6 min). Read the intake scenario aloud from the handout, then have teams answer the application items using their own trial results rather than a textbook answer.
  9. Debrief (4 min). Ask two questions of the whole class. First, which single change moved the most sand, and how do you know from your own data? Second, if the fictional utility can only afford to act on one of the three variables you tested, which should it choose and why? Take answers from three teams before closing.
Ondry River Stream Table: Bank Erosion at an Intake — Grade 11 Science
Name: ____________________________ Date: ______________

Student Handout

Guiding question: What makes a river bend move, and what does that movement do to the water a treatment plant takes in?

Every place in this handout is fictional. The Ondry River and the town of Pellbury are invented for this investigation. Record your own measurements; the numbers your team gets depend on your sand and your table, so there is no single correct value.

For each trial, sketch the channel, mark with a C where sand was removed and with a D where sand built up, record the dye travel time, and rank the outflow jar from 1 for clearest to 4 for cloudiest.

Trial What changed Dye travel time Where sand was removed Where sand built up Clarity rank
1 Baseline
2 Roughly double inflow
3 Roughly double slope
4 Bank cover added

The intake scenario. The fictional town of Pellbury draws its drinking water from the Ondry River at a bend. The intake pipe sits in the bank on the outside of the curve, and survey records show the bank there has moved 26 feet closer to the pipe in eleven years. Along the bend and the reach just upstream of it, a cattle operation removed the willows and grass along both banks about twelve years ago. The plant operators report that their filters clog fastest during spring high flow.

Answer the following using your own trial results.

  1. In Trial 1, on which side of each curve was sand removed, and on which side did it build up? Name the two landforms.
  2. Compare your dye travel time in Trial 1 with Trial 2. What does the difference tell you about how discharge affects the speed of the water?
  3. Compare where sand was removed in Trial 2 with Trial 1. Which trial moved more material, and what evidence in your table supports that?
  4. Compare Trial 3 with Trial 1. Explain how a steeper gradient changed the pattern of removal and buildup.
  5. Compare Trial 4 with Trial 1. State what the bank cover did to the amount of material removed from the outside of the curve, and give the mechanism.
  6. Rank your four outflow jars from clearest to cloudiest and explain what that ranking measures about the river.
  7. Using your results, explain why the bank at Pellbury's intake has moved 26 feet toward the pipe, and name the change along the river that most likely sped it up.
  8. Pellbury's filters clog fastest during spring high flow. Which of your trials best models spring high flow, and what does that trial predict about the sediment reaching the intake?
  9. The utility can fund one of three actions: replanting the stripped banks, moving the intake to the inside of the curve, or adding a settling basin at the plant. Choose one, and defend it with evidence from your trials.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Answers 1 through 6 depend on each team's own sand, slope, and inflow, so accept any answer that matches the team's recorded data and states a mechanism. The patterns below are what a well-run trial usually produces.

  1. Answer: Removal on the outside of each curve, buildup on the inside. Accept any answer that correctly reads the team's own sketch. The eroded outer bank is the cut bank and the deposit on the inner bank is the point bar. Water moving around a curve carries the most momentum to the outside, so the fastest thread of flow strikes the outer bank while slower water on the inside drops its load.
  2. Answer: Shorter travel time at higher discharge. Accept the team's recorded times. More water in the same channel raises both depth and velocity, and faster water exerts more force on the bed and banks.
  3. Answer: Trial 2 moved more material. Accept any answer supported by a wider eroded zone, a larger point bar, or a cloudier jar in the team's own record. Once the water's force passes the threshold needed to move grains, each added bit of force moves many more grains, so sediment transport rises faster than the flow itself and a modest increase in discharge produces a large increase in sediment transport.
  4. Answer: Steeper gradient increased removal and pushed deposition farther downstream. Accept the team's own sketch. A steeper slope converts more potential energy into flow energy over the same distance, so the water can carry material farther before dropping it.
  5. Answer: Bank cover reduced the material removed from the outer bank. Accept the team's own comparison. Roots and stems slow the water at the bank face and hold grains in place, so the same discharge removes less material.
  6. Answer: Cloudiness usually ranks Trial 3 or Trial 2 highest and Trial 4 lowest. Accept the team's own ranking. The jar measures suspended sediment, which is the material the water is carrying rather than the material it has already dropped.
  7. Answer: The intake sits on a cut bank, so the bend is migrating toward it. The trials show that the outer bank of a curve is where material is removed, and Trial 4 shows that removing bank cover speeds that removal. The cattle operation stripped the willows and grass along the bend about twelve years ago, which matches the eleven years of recorded bank movement.
  8. Answer: Trial 2, the higher discharge trial, best models spring high flow. It predicts more suspended sediment reaching the intake, which is what loads the filters and shortens filter runs at the plant.
  9. Answer: Any of the three choices is acceptable when defended with trial evidence. Replanting is supported by Trial 4, which showed less removal with bank cover, and it treats the cause rather than the symptom. Moving the intake is supported by the observation that the inside of a curve is where material is deposited, though a point bar can bury an intake as it grows. A settling basin is supported by the jar ranking, since it removes suspended sediment after it arrives, but it does nothing to stop the bank from reaching the pipe.

Differentiation

Support

Give these students a prepared channel already carved into the sand so the build step does not consume their time, and a handout with the trial sketches drawn and only the C and D marks left to add. Provide a word bank containing cut bank, point bar, discharge, and gradient, and pair each student with a partner for the application items.

Extension

Ask these students to design and run a fifth trial that tests a variable the class did not try, such as a bend with a tighter radius or a channel with coarse gravel added to one bank. They must write the variable they changed, the variable they held constant, and what result would disprove their prediction.

ELL supports

Pre-teach discharge, gradient, cut bank, and point bar with a labeled overhead-view diagram of a bend and arrows for the fastest flow. Provide the frames "In Trial ___, the sand was removed from ___ because ___" and "This trial models ___ at the river." Allow teams to discuss in their home languages while recording in English.

Extensions

Have students find an aerial image of a river bend near their community from two different years and describe in a paragraph what changed and which side of the curve it changed on. Students can also write a short memo to the fictional Pellbury utility recommending a monitoring schedule for the bank beside the intake, naming what would be measured and how often.