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Classroom activity Grade 7 Engineering & Design

Emberlynn Creek Silt Fence: A Design-Build Erosion Challenge

Grade 7 teams build a model silt fence, test it against a no-fence control with a measured water pour, then redesign and retest to reduce sediment reaching a model creek.

Objectives

  1. Students will build a silt-fence prototype using stakes and fabric to slow simulated stormwater runoff.
  2. Students will measure and compare sediment reaching a model creek with and without a silt fence, then redesign to improve performance.

Vocabulary

Erosion
Erosion is the wearing away and carrying off of soil and rock by moving water, wind, or ice. Fast-moving stormwater can strip soil from bare ground and stream banks.

Also: silt fence, sediment, control, iterate

Teacher overview

This is a small-team design-build activity for groups of three or four students. Near a fictional construction site along Emberlynn Creek, loose soil can wash into the water during a storm unless something slows the runoff down first, and this activity has teams build a small model of the erosion-control barrier used on real construction sites, a silt fence. Each team builds a soil slope inside a tray, pours a measured amount of water to represent a storm, and measures how much sediment reaches a collection cup standing in for the creek. A silt fence works by slowing the speed of runoff long enough for suspended soil particles to settle out before the water continues downstream, rather than by blocking the water completely, and teams test that idea directly against a fenceless control before redesigning their own prototype.

Activity Directions

The build. Each team receives a tray, soil, a strip of fabric, four stakes, and a collection cup. Before building, explain that the silt fence's job is not to stop water entirely, which would just cause it to pool and overflow all at once, but to slow it down enough that soil settles out first. Remind teams to wipe up spills and wash their hands after handling soil and muddy water.

  1. Prop the tray at a slight incline, about 2 inches higher at one end, and spread 2 cups of loose soil evenly across the raised half, leaving the lower half bare to represent the creek bed.
  2. Place the collection cup under the drain hole at the low end of the tray (or under a small notch cut in the low-end rim), so runoff flows off the soil and out into the cup.
  3. Push the 4 stakes into the soil in a line, about one-third of the way down the slope, spaced evenly across the width of the tray.
  4. Weave the fabric between and behind the stakes so it forms a low barrier angled slightly upslope, like a real silt fence.
  5. Before pouring any water, have the team predict in writing how much soil they expect to reach the collection cup.
  6. Slowly pour 2 cups of water across the top of the soil over about 30 seconds, representing steady rainfall.
  7. Let the water finish draining into the collection cup, then let any settled sediment sit undisturbed for two minutes.
  8. Carefully pour off the clear water from the collection cup and measure the leftover wet sediment using the measuring cup, recording the amount to the nearest eighth of a cup.
  9. Compare your team's sediment measurement with a shared "no fence" control tray, built and tested the same way but without stakes or fabric, and record both numbers on the class data chart.
  10. Redesign one part of your silt fence (the stake spacing, the fabric's angle, or how low the fabric sits), rebuild it, and repeat the same 2-cup pour test, recording a second sediment measurement.

The activity ends once every team has recorded sediment measurements for the control, the original design, and the redesign, and the class gathers to compare results and complete the debrief.

The debrief. After every team has posted its numbers, work through the following sample class data together as a model, then have each team run the same four calculations on its own chart. In the sample data, the no-fence control let 0.75 cup of sediment reach the collection cup, the original silt fence let 0.25 cup through, and the redesign let 0.125 cup (1/8 cup) through.

  1. Using the sample data, how much less sediment reached the collection cup with the original silt fence than with the no-fence control?
  2. Using the sample data, what percent less sediment reached the collection cup with the original silt fence than with the no-fence control, rounded to the nearest whole percent?
  3. Using the sample data, how much less sediment reached the collection cup after the redesign than with the original silt fence?
  4. Using the sample data, what percent less sediment reached the collection cup after the redesign than with the original silt fence?
  5. Why does slowing water down work better than blocking it completely?

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

Answer Key

The following sample class data is used to teach the debrief calculations; actual team results will vary.

  1. Answer: 0.5 cup less. Using sample data of 0.75 cup for the no-fence control and 0.25 cup for the original silt fence, 0.75 − 0.25 = 0.5 cup less sediment reached the collection cup with the fence in place.
  2. Answer: About 67%. Percent reduction from the control: (0.75 − 0.25) ÷ 0.75 × 100 ≈ 66.67%, which rounds to about 67%.
  3. Answer: 0.125 cup (1/8 cup) less. Using sample data of 0.25 cup for the original design and 0.125 cup for the redesign, 0.25 − 0.125 = 0.125 cup less sediment after the redesign.
  4. Answer: 50%. Percent improvement from the redesign: (0.25 − 0.125) ÷ 0.25 × 100 = 50%.
  5. Debrief question: "Why does slowing water down work better than blocking it completely?" Answer: A solid barrier would force water to pool behind it and eventually overflow or wash around it all at once, carrying trapped sediment with it, while a permeable fabric barrier lets water pass through slowly, giving suspended soil particles time to settle out before the water continues downstream.

Differentiation

Support

Pre-build the soil slope for teams (step 1 completed), so students begin directly at step 2 with placing the collection cup and building the fence itself. Provide the sediment measurements already recorded in eighths of a cup on a printed data chart, so students only compute the differences.

Extension

Have teams test a third variation, angling the fabric barrier the opposite direction (downslope instead of upslope), and compare its sediment measurement to both the original design and the redesign, explaining which angle worked best and why.

ELL supports

Demonstrate the difference between "settling" and "blocking" using a clear cup of muddy water left to sit for a minute before the activity begins, so the vocabulary is tied to something students can see. Provide picture cards showing the fence's fabric and stakes labeled in both English and students' home languages where possible.