Graphic organizer Grade 12 Engineering & Design
Trawdenmoor Mill Dam: A Failure Mode and Control Organizer
A grade 12 failure mode organizer for a fictional aging low-head dam, tracing each component from observed condition to mechanism, control, and risk score.
Objectives
- Students will trace each dam component from an observed condition to its failure mechanism.
- Students will name a design or operational control and a measurable inspection trigger for each component.
- Students will rank components by a severity and likelihood risk score.
Vocabulary
Key words: failure mechanism, internal erosion, piezometer, inspection trigger, risk score, emergency action plan
Teacher overview
This organizer is a six-column failure chain paired with a risk-ranking table. For each component of the fictional Trawdenmoor Mill Dam, a 22-foot low-head dam built in 1928 on the fictional Cordwell River, students move left to right from what an inspector observed, to the mechanism that turns that observation into a failure, to the consequence, the control, and the measurable trigger that tells an operator to act. The misconception this organizer is built to correct is that a dam fails when its concrete breaks. Many dam failures begin somewhere less visible, such as seepage carrying soil out of an embankment, and an outlet gate that has quietly seized shut leaves the owner unable to lower the reservoir in an emergency, which is why the embankment and gate rows usually earn higher risk scores than the spillway.
Organizer
Fill one row for each component of the fictional dam. The last column must state a number an inspector could actually read from an instrument or a tape.
| Component | Observed condition | Failure mechanism | Consequence if unaddressed | Design or operational control | Measurable inspection trigger |
|---|---|---|---|---|---|
| Concrete spillway crest | |||||
| Left abutment embankment | |||||
| Low-level outlet gate | |||||
| Penstock and trash rack | |||||
| Piezometers and seepage weir | |||||
| Fish ladder and downstream sediment | |||||
| Emergency action plan and siren |
Rank the same components. Severity and likelihood each run from 1 to 5, and the risk score is severity times likelihood.
| Component | Severity (1 to 5) | Likelihood (1 to 5) | Risk score | Rank |
|---|---|---|---|---|
| Concrete spillway crest | ||||
| Left abutment embankment | ||||
| Low-level outlet gate | ||||
| Penstock and trash rack | ||||
| Piezometers and seepage weir | ||||
| Fish ladder and downstream sediment | ||||
| Emergency action plan and siren |
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Completed Exemplar
The rows below use the fictional Trawdenmoor inspection findings.
| Component | Observed condition | Failure mechanism | Consequence if unaddressed | Design or operational control | Measurable inspection trigger |
|---|---|---|---|---|---|
| Concrete spillway crest | Spalled concrete and exposed reinforcing steel along about 40 feet of the crest | Freeze and thaw cycles widen surface cracks until crest sections loosen and are plucked out by flow | Crest sections break away during a high flow, and the damage can progress into the dam body when flows are highest | Shotcrete overlay with a sacrificial wear surface, plus an inspection after every flow above the design threshold | Any crack wider than 0.25 inch, or exposed reinforcing steel longer than 6 inches |
| Left abutment embankment | A damp patch and cattail growth on the downstream slope | Seepage carries fine soil out of the embankment, opening an internal erosion path back toward the reservoir | Internal erosion progresses to a breach with very little visible warning | Filter drain at the toe with a measuring weir, plus weekly piezometer readings | Seepage that turns cloudy, or weir flow rising above 5 gallons per minute |
| Low-level outlet gate | The gate has not been fully cycled in four years | Corrosion and sediment bind the stem so the gate cannot travel its full stroke | The reservoir cannot be lowered for an emergency drawdown or a repair | Quarterly full-stroke exercise with the travel time logged each cycle | Stem travel time longer than 150 percent of the recorded baseline |
| Penstock and trash rack | A debris mat covers roughly a third of the rack after each storm | A blocked rack raises the differential head across it and starves the turbine of flow | Rack members bend, debris passes into the penstock, and the generating unit trips offline | Automatic raking system with a differential head alarm at the control panel | Differential head across the rack above 1.5 feet |
| Piezometers and seepage weir | Two of six piezometers read flat no matter how the reservoir level changes | Clogged tips make the instrument insensitive, so rising pore pressure goes unrecorded | The dam loses its earliest warning that the embankment is in distress | Annual response testing and redevelopment of every piezometer | No measurable response after a 2-foot change in reservoir level |
| Fish ladder and downstream sediment | Fine sediment has filled the plunge pool and buried the ladder entrance | Deposits at the entrance hide the attraction flow, so migrating fish cannot locate the ladder | Passage fails its permit condition and the reach above the dam loses spawning access | Spring sediment survey with a scheduled flushing release | Water depth at the ladder entrance below 1.5 feet |
| Emergency action plan and siren | No documented siren test since the plan was last revised | An untested notification chain fails at the moment it is first needed | Downstream residents receive no warning during a large release or a breach | Quarterly siren test and an annual tabletop exercise with county emergency management | Any failed siren test, or any unreachable contact on the notification list |
| Component | Severity (1 to 5) | Likelihood (1 to 5) | Risk score | Rank |
|---|---|---|---|---|
| Low-level outlet gate | 5 | 4 | 20 | 1 |
| Left abutment embankment | 5 | 3 | 15 | 2 |
| Concrete spillway crest | 4 | 3 | 12 | 3 |
| Piezometers and seepage weir | 4 | 3 | 12 | 3 |
| Penstock and trash rack | 3 | 4 | 12 | 3 |
| Fish ladder and downstream sediment | 3 | 4 | 12 | 3 |
| Emergency action plan and siren | 5 | 2 | 10 | 7 |
Differentiation
Support
Pre-fill the observed condition column for all seven rows of the fictional dam so students begin at the mechanism step, which is the reasoning this organizer is really practicing. Provide a two-row worked example on a different structure, such as a raw water intake screen, before students start.
Extension
Ask students to add an eighth row for a component the inspection report did not cover at the fictional dam, such as the spillway training walls or the access road that emergency crews would use, and to justify its severity and likelihood ratings against the seven rows already scored.
ELL supports
Provide a labeled cross-section sketch of a low-head dam with the crest, embankment, outlet gate, penstock, and plunge pool marked, so each row name points at a visible part. Offer sentence frames for the mechanism column: "Water moves through ___, which carries away ___," and "The part cannot ___, so the operator cannot ___."
Extensions
Suppose the fictional Trawdenmoor owner has enough money this year to address only the two highest-scoring rows. Ask students to write a one-paragraph recommendation naming those two components, the cost driver behind each, and the specific inspection trigger the owner should be watching on the five components that must wait.