Project Grade 11 Engineering & Design
Thistlecomb Road Culvert: A Fish Passage and Flood Design
A six-session grade 11 project in which teams size a stream crossing that passes the design storm, restores trout passage, and fits a fixed budget and closure window.
Objectives
- Students will convert a design storm flow into a required structure opening area.
- Students will score four crossing alternatives against weighted criteria.
- Students will produce a dimensioned design that meets a budget and a closure limit.
Vocabulary
Key words: bankfull width, perched outlet, design storm, headwater, stream simulation
Teacher overview
Teams of three act as a design consultant to a fictional township that must replace a failing stream crossing under a rural road. The driving question is how to build one structure that carries the design storm without flooding the road, lets trout move upstream again, fits the money the township has, and closes the road for no longer than the school bus route can tolerate. The final deliverable is a design memorandum with a dimensioned cross-section, an alternatives matrix, a construction sequence, and a cost estimate, presented to a fictional township board. The misconception this project is built to correct is the belief that a culvert is adequate as long as it carries the flow, since the perched outlet that blocks fish is usually created by a pipe that passes water perfectly well but sits above the streambed, and a smooth pipe can run too fast for fish at the very flows that look successful on paper.
Project Brief
Client: the fictional Denholt Township Board of Public Works. Site: the fictional Thistlecomb Road crossing of the fictional Laddon Creek. Every figure below is a hypothetical value written for this project.
The existing structure is a 36-inch corrugated metal pipe installed decades ago. Its outlet now sits 2 feet above the pool below it, so trout can no longer move upstream past the road. The road overtops during large storms, and the township has been called out twice in three years to clear debris from the inlet.
Design requirements:
- The structure must pass a 25-year peak flow of 148 cfs without water rising above the road shoulder, which sits 9 feet above the existing streambed at the inlet.
- Through-structure velocity at the design flow must not exceed 4 feet per second, which is the township's adopted limit for trout passage.
- The clear span must be at least 1.2 times the bankfull width of the creek, which is 11 feet at this site.
- The natural streambed must continue through the structure, so the invert is set below the existing bed rather than at it.
- Construction cost must not exceed $310,000, including stream diversion and pavement restoration.
- The road may be closed for no more than 21 consecutive days.
Alternatives to evaluate: a larger round pipe, a concrete box culvert with an embedded streambed, an open-bottom concrete arch on footings, and a short-span bridge.
Planning-level costs for the structure itself, including excavation and backfill, are $150,000 to $190,000 for the larger round pipe, $220,000 to $270,000 for the box culvert, $240,000 to $290,000 for the open-bottom arch, and $330,000 to $400,000 for the short-span bridge. Stream diversion adds about $20,000 and pavement restoration adds about $15,000 to any alternative.
The final memorandum must contain a statement of criteria and constraints, the hydraulic sizing calculation with units shown, the alternatives matrix and its weights, a dimensioned cross-section and profile of the recommended structure, a construction sequence with a day-by-day closure schedule, a line-item cost estimate, and a one-paragraph recommendation written for board members who are not engineers.
Milestones
- Session 1, criteria and constraints. Each team produces a one-page brief that restates the problem in its own words and lists every requirement as a measurable criterion or a fixed constraint, ranked by which one the team believes will govern the design. Output: the ranked one-page brief.
- Session 2, hydraulic sizing. Teams compute the required flow area from the design flow and the velocity limit, compute the minimum span from the bankfull rule, and draw a trial cross-section showing the embedded streambed depth and the clear opening above it. Output: a sizing sheet with units shown at every step and one dimensioned trial section.
- Session 3, alternatives matrix. Teams weight four criteria, cost, closure duration, habitat continuity, and long-term maintenance, then score all four alternatives against them and record the reasoning behind each score. Output: a completed matrix with stated weights and a shortlist of two alternatives.
- Session 4, preferred alternative. Teams develop the chosen structure into a dimensioned cross-section and profile, add the streambed detail showing bed material and its depth, and check that the headwater at the design flow stays below the road shoulder. Output: two drawings and a headwater check.
- Session 5, construction and cost. Teams write a construction sequence covering stream diversion, excavation, structure placement, backfill, and paving, build a day-by-day closure schedule against the 21-day limit, and prepare a line-item cost estimate against the fictional budget. Output: a sequence, a schedule, and a cost estimate.
- Session 6, board presentation. Each team presents for six minutes to the fictional board, played by classmates and the teacher, and answers three questions, one of which must be about a constraint the team did not fully meet. Output: the presentation and the completed memorandum.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Rubric
| Criterion | 4, exemplary | 3, proficient | 2, developing | 1, beginning |
|---|---|---|---|---|
| Hydraulic sizing and evidence | Required area and minimum span are computed correctly with units shown throughout, and the trial section is checked against both independently | Both calculations are correct with minor unit or rounding lapses, and the section satisfies both requirements | One calculation is correct while the other is missing, assumed, or applied to the wrong dimension | Sizing is asserted without calculation, or the section does not satisfy either requirement |
| Fish passage and habitat continuity | The design removes the perched outlet, embeds the bed at a stated depth, keeps velocity within the limit, and explains how each choice serves passage | The design removes the perch and embeds the bed, and velocity is within the limit, with explanation given for most choices | Passage is addressed in general terms, with the perch removed but bed continuity or velocity left unresolved | The design carries flow but leaves the outlet perched or the bed interrupted |
| Constraint management | Cost and closure schedule are both within limits, and the memorandum states what was traded away to hold them | One limit is met and the other is exceeded by a stated amount with a documented reason | Both limits are addressed but at least one is exceeded without acknowledgment | Cost or closure is not estimated at all |
| Communication to the board | Drawings are dimensioned and readable, the matrix weights are justified, and the closing recommendation is clear to a non-engineer | Drawings and matrix are complete and mostly clear, with some technical language left unexplained | Drawings or matrix are incomplete, and the recommendation relies on jargon | Deliverables are fragmentary and the recommendation is absent or unsupported |
Differentiation
Support
Give these teams the required flow area already computed for the fictional site, with the calculation shown, so their Session 2 work begins at the trial section rather than at the arithmetic. Provide a partially weighted alternatives matrix with two criteria filled in, and supply a labeled example cross-section from a different crossing to model what a dimensioned drawing looks like.
Extension
Ask these teams to add a fifth alternative of their own to the fictional matrix and to defend its inclusion, then to test their recommended design against a storm larger than the one the township specified and report at what flow the road would overtop. A second task is to estimate the twenty-year maintenance cost of each shortlisted alternative and argue whether the cheapest structure to build is the cheapest to own.
ELL supports
Provide a labeled site diagram of the fictional crossing naming inlet, outlet, invert, streambed, headwater, and road shoulder, and post the same vocabulary with icons on the wall for the whole project. Allow the Session 1 brief to be submitted as a labeled table rather than prose, and offer presentation sentence frames such as "We recommend the ___ because it meets ___ while the others do not."
Extensions
Have students, under adult supervision and staying off the road and out of the water, locate a road-stream crossing near the school, photograph it, and classify it as passable or perched using the criteria from the fictional project. A second step is to compare their design against a published stream-simulation design guide from a state transportation or fisheries agency and to note which requirement in the guide the class project left out. Teams that want a physical model can build a scale flume from a plastic gutter, run water through a perched outlet and an embedded outlet, and record the difference in the drop a fish would face.