Spillway Science: Comparing Capacity, Discharge, and Safe Release

Materials Needed

• Plastic storage bin, baking tray, or shallow dish
• Water and a small cup or measuring jug with volume markings
• Sand, soil, or modeling clay
• Cardboard, craft sticks, plastic spoons, foil, or foam pieces
• Small rocks, gravel, or pebbles
• Ruler, timer or stopwatch, and paper or a digital spreadsheet
• Pencil and calculator
• Optional: food coloring, tape, and a phone or camera for documenting tests

Lesson Snapshot

• Learner: Fisher, age 14
• Suggested time: 60–90 minutes, with sections that can be split across sessions
• Big question: How can engineers determine whether a spillway can carry enough water safely?
• Primary new concept: Spillway capacity—how much water a spillway can carry in a given time, examined through measured discharge.

Connection to the Previous Lesson

In the previous lesson, Fisher designed, tested, and improved a model spillway, observing how slope, channel shape, and rough surfaces affected water speed and erosion. Building on that work, this lesson adds a measurable question: how much water can the spillway carry over a set period? Fisher will keep the earlier design goals—protect the dam, control erosion, and direct water safely downstream—while comparing flow capacity using repeated, fair tests.

Learning Objectives

By the end of the lesson, Fisher will be able to:

• Recall how slope and channel features affect water movement and erosion.
• Define discharge as the volume of water flowing past a point per unit of time.
• Measure volume and time, then calculate approximate discharge using discharge = volume ÷ time.
• Compare two spillway designs using consistent test conditions and recorded evidence.
• Explain how engineers balance carrying capacity with safe downstream flow and erosion control.

Success Criteria

• Fisher correctly identifies the reservoir, dam, spillway, and downstream area in the model.
• Fisher measures the same volume of water in repeated tests and records the time it takes to flow through the spillway.
• Fisher calculates or explains approximate discharge for at least two tests.
• Fisher uses results to compare designs and proposes an evidence-based improvement.
• Fisher explains why the design with the greatest discharge is not automatically the safest design.

Safety Notes

• Use small amounts of water on a washable, level surface and clean spills immediately.
• Keep electrical devices away from water; use them only for timing or recording from a dry location.
• Wash hands after handling soil, sand, or outdoor materials.
• Never approach or enter real dams, reservoirs, or spillways. Observe water infrastructure only from a safe, approved location.

Review: Reconnect to Prior Learning

Time: 5–10 minutes

Invite Fisher to sketch or point out a reservoir, dam, spillway, and downstream area. Then ask:

• What problem does a spillway solve?
• What happened in the previous model when water moved quickly over bare soil?
• Which change helped guide water or reduce erosion?
• What evidence did you use to decide whether an improvement worked?

Formative check: Listen for the connection between water speed, erosion, and design features. If needed, revisit the model or sketch before introducing the new measurement.

Introduction: Hook and Objectives

Hook — “Two spillways, one storm”: Imagine two spillway designs. One moves a small amount of water quickly and causes erosion at its outlet. The other moves a larger amount more steadily but takes a different channel shape. Which is safer during heavy rain? Explain that engineers need both observations and measurements to make a sound comparison.

Tell Fisher: “Last time, you designed and improved a spillway by observing its effects. Today, you will measure how much water it carries over time and use that information to judge capacity alongside safety.”

Key Vocabulary

• Discharge: The volume of water flowing past a point during a specified time.
• Capacity: The amount of water a channel or structure can carry under stated conditions.
• Variable: A factor that can change in a test, such as channel width or slope.
• Fair test: A comparison in which relevant conditions are kept the same so a design change can be evaluated.
• Downstream: The direction water flows away from its source.

Use consistent units. For example, if volume is measured in milliliters and time in seconds, discharge is recorded in milliliters per second (mL/s).

Body: Learning and Practice

Part 1: I Do — Model the Measurement

Time: 10–15 minutes

1. Set up a simple tray model with a reservoir, a small dam, a defined spillway channel, and a downstream collection area.
2. Choose a marked point near the outlet. Explain that the same point will be used for each measurement.
3. Measure a small, consistent amount of water, such as 100 mL. Keep the pour method and model position consistent.
4. Time how long the water takes to pass through the spillway or collect in the downstream area. State clearly which timing method is being used and use it the same way each time.
5. Calculate approximate discharge: volume ÷ time. For example, 100 mL flowing in 20 seconds gives 5 mL/s.

Think aloud: “A time by itself does not tell us how much water moved. A volume by itself does not tell us how quickly it moved. Together, volume and time give us an estimate of discharge. This is a model measurement, not a full-scale engineering rating.”

Quick check: If 120 mL flows in 30 seconds, what is the approximate discharge? (4 mL/s.) What must stay the same to compare a second design fairly?

Part 2: We Do — Plan a Fair Comparison

Time: 10–15 minutes

Together, select two designs or two versions of one design. For example, compare a wider channel with a narrower one while keeping the slope and lining as similar as possible. Alternatively, compare a smooth channel with a roughened channel while keeping the width and slope similar.

Before testing, agree on:

• Which single feature will change?
• What will remain the same (water volume, pour location, model slope, timing point, and materials where possible)?
• How will you record erosion, splashing, leaks, or water escaping the planned route?
• What result would indicate greater discharge? What result would indicate better control or less erosion?

Have Fisher write a prediction: “I predict design ___ will have a higher measured discharge because ___. I also expect its erosion/control result to be ___ because ___.”

Part 3: You Do — Spillway Capacity Challenge

Time: 25–35 minutes

Challenge: Compare two small spillway designs to determine how their capacity and safety differ. Use the previous lesson’s design knowledge, but focus this time on collecting consistent measurements.

Design requirements:

• Each model includes a reservoir, dam, spillway, and downstream area.
• Both designs are tested with the same measured volume of water.
• Each design is tested at least twice, if time and materials allow.
• Fisher records time, calculates approximate discharge, and notes whether water escaped or caused visible erosion.
• Fisher identifies a trade-off or recommends a next improvement based on evidence.

Procedure:

1. Build or adapt the first spillway design. Mark the measurement point and check that the tray is stable.
2. Measure the chosen water volume. Use the same volume for every trial.
3. Pour the water using the same starting position and method each time. Start and stop the timer using the agreed method.
4. Record the time, calculate discharge, and note water direction, splashing, leaks, and erosion.
5. Reset the model as consistently as possible. Test a second version that changes one chosen feature.
6. Repeat trials if possible. Compare results and explain any variation between repeated tests.
7. Use the evidence to recommend one change, such as adjusting width, slope, lining, or outlet protection. Do not assume that maximizing speed or discharge alone makes a design safer.

Data Table

Part 4: Apply — Make an Engineering Recommendation

Time: 10 minutes

Ask Fisher to imagine recommending a design for a farm pond after several days of rain or a community stormwater basin. Discuss how an engineer would consider the volume of incoming water, the spillway’s capacity, erosion protection, and where the water goes downstream. Explain that this tabletop activity demonstrates comparison and measurement, but it cannot establish a safe capacity for a real dam.

Prompt: “If one design carried water at a higher measured discharge but caused more erosion, what additional feature or test would you recommend before choosing it?”

Assessment

Formative Assessment

• Review questions about the prior lesson’s spillway purpose, water speed, and erosion.
• Observe whether Fisher identifies and controls variables during test planning.
• Check the discharge calculation and units.
• Use questions during testing: “What stayed the same?” “What changed?” “What evidence supports that comparison?”
• Review whether notes distinguish measured results from observations or estimates.

Summative Assessment: Spillway Comparison Brief

Fisher presents a short written, verbal, or recorded recommendation using this structure:

• Claim: Design ___ was more suitable for the stated goal because…
• Evidence: With the same water volume, the measured discharge was…; I also observed…
• Reasoning: This result suggests… because…
• Trade-off: The design’s capacity and erosion/control results differed in this way…
• Next step: I would test or change ___ to improve confidence or safety because…

Simple Rubric

Differentiation and Learning-Format Options

• Scaffolds: Provide a labeled model diagram, a prefilled calculation example, a calculator, and a data table with units already listed. Test one design at a time and reduce the number of trials if needed.
• Sentence starters: “We kept ___ the same and changed ___.” “The discharge was ___ because ___.” “This design carried water ___, but it also ___.”
• Multiple response formats: Fisher may speak findings, dictate notes, record audio, draw a comparison diagram, or complete a digital table.
• Advanced extension: Calculate the mean discharge for repeated trials, graph discharge against a changed feature, and discuss measurement uncertainty. Add a fixed time limit or a second water volume and consider whether the design behaves consistently.
• Digital option: Enter trial data into a spreadsheet and create a simple bar graph.
• Outdoor or everyday option: Safely observe how rainwater is guided along a driveway, garden edge, or drain from a dry, stable location. Do not test near real dams, floodwater, or fast-moving water.

Conclusion: Closure and Recap

Three-minute recap: Ask Fisher to complete these statements verbally or in writing:

• Building on the previous lesson, one feature that can reduce erosion is…
• Discharge means…
• To compare designs fairly, we kept ___ the same and changed…
• The evidence that helped me compare capacity was…
• A spillway with a greater discharge is not automatically safer because…

Reinforce the learning progression: the previous lesson focused on designing, observing, and improving a spillway; this lesson added measured discharge and fair comparisons. Effective spillway decisions consider both how much water can be carried and how safely that water is controlled downstream.

Exit ticket: In three to five sentences, answer: “How did measuring discharge improve your spillway comparison? Use one result from your test and explain one safety factor that the number alone cannot show.”

Next logical lesson: Investigate how a spillway outlet or stilling basin can reduce water energy downstream, using the same measurement-and-testing approach to compare outlet protection designs.