Clear the Flood: Design, Test and Combine a Model Bridge

Investigation question: Which bridge features keep a road open through a flood with floating logs, and can a bridge that combines the best features of several tested designs beat every one of them?

Scientific purpose

Groups use a simulated 40-year record of a model creek to work out the yearly chance of three flood sizes, choose the flood their bridge must survive based on what the road serves, and build a bridge across a sand channel in a stream tray under a 20-point budget. Every bridge faces the same three tests: a small flood, a design-size flood with ten floating 'logs', and a record flood with logs. Groups measure peak water, overtopping, logs trapped, scour at the footings and whether the bridge still holds 200 grams. In the second session the class scores every design in a decision matrix, each group borrows at least two features from other groups' bridges, and the combined version 2 is retested.

Why this helps students with this chapter

Most students imagine a flood knocking a bridge down with a wall of water. Real flood failures are usually quieter and more instructive: logs and branches pile up against piers until the current shoves the bridge sideways, as happened to the Highway 101 bridge over the Klamath River in December 1964, and fast water digs scour holes around the footings until they sink. A stream tray makes both visible in a few seconds. Choosing a design flood from a record forces students to treat flood risk as a chance, not a schedule, and the class decision matrix shows that no single bridge wins every test, which is why combining the best features works.

Standards connections

MS-ETS1-2 (practice, evaluating competing bridge designs systematically against criteria and constraints): Students score every tested design on the same criteria (overtopping, logs trapped, scour, load) and constraints (budget, deck height) in a class decision matrix built from shared test data. The criteria are supplied, so this practices the PE.

MS-ETS1-3 (practice, identifying the best characteristics of several designs and combining them into a better one): After the matrix, each group names the strongest feature of at least two other designs, combines them in version 2 and retests it, then reports whether the combination beat every original on each criterion, mirroring the three combined DART fixes in the lesson.

MS-ESS3-2 (practice, using flood-frequency data to set the design flood for a mitigation structure): Students turn a simulated record of yearly peak flows into yearly chances and expected numbers of floods over a bridge's life, and use them to choose which flood the bridge must survive. This uses hazard frequency data to inform a technology, but the record is simulated and short, so the role is practice.

Roles (rotate each session)

• Project manager: Keeps the budget sheet, checks that the design meets the constraints before building, and runs the 20-minute build clock.
• Builder: Leads construction from the sketch and places the bridge at the marked site with its footings set the same way for every test.
• Flood operator: Sets up the flood bottle, fills it to the 1.5 L line, peels the tape on the count, and drops the ten logs into the channel in the first 5 seconds.
• Gauge reader and recorder: Reads the peak water level on the upright ruler, measures scour at the footing pins and bridge movement, runs the load test and records every result.

Materials

Estimated cost: 40 to 80 per class (estimate); about 15 to 25 if the school has stream tables and reuses the Lab 1 sand (an estimate; Four under-bed storage boxes at about $10-15 each, a box of 1,000 craft sticks about $8, straws about $3, modeling clay about $5, a 2 lb bag of aquarium gravel about $5, plus one extra 50 lb bag of play sand about $7 if Lab 1 sand is not enough. Soda bottles, rulers, buckets, pennies and goggles come from the classroom or recycling. Typical U.S. retail prices.)

Safety

• Wear goggles while pouring floods and releasing logs; sandy water can splash into eyes.
• Water on the floor is a slip hazard. Keep a towel under each station, bail the lake into the bucket after every test, and wipe spills at once.
• A filled stream box weighs 20 kg or more. Never lift or carry a box with sand in it; set stations up where they will stay.
• Skewers have their points snipped off, and the cut bottle edges are taped, but handle both with care. Report any sharp edge to the teacher.
• Keep phones, laptops and electrical cords away from the water. Use a phone to film the peak only from outside the splash zone, in a plastic bag if possible.
• Do not pour sand or sandy water down sinks. Let the bucket settle, pour the clear water outside, and return the sand to the station. Wash hands after the lab.

Teacher preparation and setup

1. Set up each station: fill the box with about 12 L of damp sand to about 4-5 cm deep over the upper 65 cm, leaving the lowest 15 cm empty as a 'lake' to catch flood water. Raise the upstream end about 4 cm on a book.
2. Pull the channel template down the middle of the sand to carve a channel about 8 cm wide and 3 cm deep, and mark the bridge site with tape on the box rim 30 cm from the upstream end. Mark a second site for the second group, 20 cm farther downstream, so two bridges can stand at one station without one feeding logs to the other during the same test.
3. Make the flood bottles: cut the bottoms off 2 L bottles, cover the cut edge with tape, and drill 2, 4 or 6 holes about 5 mm across in the caps. Test each one with 1.5 L held cap-down over the channel head: the Small flood should stay in the channel, the Design-size flood should just reach the tops of the banks, and the Record flood should spread across the floodplain. Adjust the hole count if your sand behaves differently.
4. Print the stream record, the importance cards (only road to a hospital, school bus route, farm road, park footbridge) and the cost sheet. Assign importance cards so every card is used at least once.
5. Bag 80 pennies per group, snip the points off the skewers, and cut or collect ten 4-6 cm twigs per group.
6. Practice one full test yourself (fill, release, logs, read, measure scour, load, bail, smooth the channel) so you can model it in two minutes.

Investigation design

Student procedure

1. Session 1. As a group, read the stream record and answer q1: the yearly chance of each flood size. Read your importance card and choose your design flood in q2, using the yearly chances and the number of floods expected in a 30-year bridge life.
2. Read the criteria and constraints on the recording sheet and write your group's targets in the criteria table. Then sketch your bridge in the drawing box, label every feature, and add up its cost. The project manager checks the budget (20 points or less), the deck height (underside no more than 7 cm above the channel bed) and the length (at least 16 cm).
3. Build version 1 in 20 minutes or less, using only kit materials.
4. At the station, smooth the channel with the template and set the bridge at your site. Push a toothpick into the sand beside each footing and mark the sand level on it. Push the clear ruler upright into the channel bed 5 cm upstream of the bridge as a water gauge. Tape a mark on the rim beside each end of the bridge.
5. Test 1, Small flood, no logs: tape the cap holes of the Small bottle, fill it to the 1.5 L line, hold it cap-down over the head of the channel, and peel the tape. The gauge reader reads the highest water level on the ruler (film it if you can) and watches for water touching the deck.
6. After the flow stops, record overtopping, scour (distance from each toothpick mark down to the new sand surface, in mm) and movement (distance each end moved from its rim mark). Place the 200 g bag at mid-span for 10 seconds: it passes if nothing breaks and the deck does not touch water or sand. Bail the lake, smooth the channel, refill any scour holes and reset the toothpicks.
7. Test 2, Design-size flood with logs: repeat with the Design-size bottle, and drop all ten logs into the channel about 10 cm upstream of the bridge in the first 5 seconds. Count the logs trapped against the bridge when the flow stops, then record everything else as in step 6.
8. Test 3, Record flood with logs: repeat with the Record bottle and the ten logs. Post your results on the class results chart.
9. Session 2. Copy the class results into the decision matrix. Score every design 0, 1 or 2 on each criterion (0 = misses the target, 1 = meets it, 2 = meets it with a clear margin) and total the scores. Write the best feature of each top design in the last column.
10. Answer q3: choose at least two features from other groups' designs to combine with your own. Redesign within the same constraints, sketch the changes, and build version 2.
11. Retest version 2 with Test 2 and Test 3 exactly as before and record the results in the version 2 table.
12. Post version 2 results, clean the station, and answer q4 through q10 individually.

Expected observations

• Expected, not yet tested: the Small flood stays in the channel and rarely harms any bridge; the Design-size flood reaches the bank tops and overtops low decks; the Record flood spreads onto the floodplain, overtops most decks under about 5 cm and pushes poorly anchored bridges sideways.
• Logs jam against piers in the channel and against low decks; open spans with high decks pass most logs.
• Scour holes form beside piers and at bridge ends that narrow the channel, often 5-15 mm deep in the Record flood; gravel armor around a footing visibly reduces the hole.
• No version 1 design meets every criterion within budget in most classes; version 2 designs that borrow an open span and footing armor usually improve on both debris and scour.

Likely difficulties

• Floods vary if bottles are filled to different levels or held at different heights. Mark the 1.5 L line and hold the cap 2-3 cm above the channel head every time.
• Groups forget to smooth the channel and reset scour pins between tests, so scour adds up across tests. Make the reset part of the gauge reader's checklist.
• Reading the peak water level is hard in a few seconds. Film the gauge on a phone and pause the replay, or have two students read and average.
• Bridges that are simply heavy (lots of clay) survive by weight, which hides design differences. Count clay in the budget and remind groups that heavier is not always better: real foundations sink in scour holes.
• Two groups per station can mean waiting. Have the waiting group measure and record for the testing group, then swap; plan about 3 minutes per test.

Structured debrief

1. Build the decision matrix on the board from the posted results and circle the top two or three totals. Ask what the winners share. In most classes the answer is fewer or no piers in the channel and a higher deck: debris passes under an open span, and water does not speed up around a pier.
2. Ask each group to read one version 2 result against its version 1. Collect which combined features helped most. Make the ETS1-3 point explicit: the best version 2 designs usually borrowed one feature for debris (open span), one for scour (gravel armor around the footings) and kept their own cheapest deck, the same logic as the three combined DART fixes.
3. Return to the stream record. Ask why the hospital-road groups chose the record flood even though it happened in only 1 of 40 years: over 30 years the expected number is close to one, and failure would cut off emergency care. Tell students real highway bridges are designed for rare floods, often one with about a 1 percent yearly chance, and engineers check what an even rarer flood would do to the foundations.
4. Connect to California. The 1964 Klamath bridge failed with logs piled against it; the replacement crosses the river in a different place. Scour around foundations is the most common cause of bridge failures in the United States, which is why inspectors check bridges after big floods. Close by asking what data a county would want before rebuilding a bridge: a long flow record, debris loads, and the scour depth reached in past floods.

Recording sheet: what students fill in

Lab 2 recording sheet: Clear the Flood

Your bridge carries a road across Model Creek. Floods in the tray are made with flood bottles: more holes in the cap means a faster, higher flood. Logs are twigs released upstream. The tables below come from an imagined record of the creek.

Analysis questions: answer key

Individual evidence of learning

Each student submits the recording sheet (q1, q2, the criteria table, the version 1 sketch, results tables, the class matrix and q3) and answers to q4 through q10. The matrix and q3 and q7 are the evidence for systematic evaluation and for combining the best features; q1, q2, q6 and q10 show flood data informing the design flood.

Assessment criteria

Accessibility adaptations

• Provide a parts kit pre-counted to 20 points and a picture glossary of bridge parts (deck, pier, footing, abutment, span, armor).
• Students with limited fine-motor control can be project manager or gauge reader; partners build to their directions.
• Offer a printed side-view grid for the sketch, and accept a photo of the built bridge with labels added as the drawing.
• For students who struggle with fractions and percents, give a 'divide by 40' strip (1/40 = 2.5 percent) for q1 and a worked example for q6.
• Keep stations at table height with clear space around them for students using wheelchairs or walkers.

Low-cost alternative

If storage boxes are not available, use one shared stream tray at the front: each group builds and the class runs the tests as a demonstration, two groups per period while the others score designs from the posted data. A 1 m length of vinyl rain gutter with an end cap, filled with sand and tilted, makes a narrower channel for about $10; scale the bridge length and deck height to fit it.

Model limitations

This activity has not been physically tested by the authors; the expected observations are predictions, and the flood bottles must be tuned to each tray. The tray is not to scale: its sand is uniform and loose, so scour happens in seconds rather than over hours, and real riverbeds include gravel, clay and bedrock. Real floods last hours or days, carry whole trees and sediment, and rise and fall more gradually than a 1.5 L bottle. Model bridges are light and rest on the sand, while real bridges sit on deep foundations. The 40-year record is invented and short; real design floods come from much longer records and statistical estimates.

Teacher notes

Tune the three flood bottles before class so the Small, Design-size and Record floods behave as described; this matters more than any other prep step. Score the matrix from posted data only, so groups cannot argue their own results upward. If time is short in session 1, skip Test 1 for all groups; Tests 2 and 3 carry the analysis. For q6, the expected-number method is the target; the 'at least one' calculation is an extension. Real bridge design uses much longer records and statistical estimates, and foundations reach below the deepest expected scour; say so in the debrief.