Lesson plan Grade 6 Engineering & Design

Where to Build Post-and-Brush Dams on Sedge Run

Students weigh three stream reaches against a post supply and a one-day work limit, then choose where a fictional crew should build low post-and-brush dams.

Grade: Grade 6
Subject: Engineering & Design
Material: Lesson plan
Time: 55 min
Prep: Low prep
Materials: One printed copy of the student handout per student, Chart paper or a whiteboard for the class criteria list

Standards

MS-ETS1-1 NGSS

Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions.

How this material aligns: Students write the post supply and the work-time limit as constraints and the treated stream length as the criterion before any reach is chosen, which is the precision this standard requires.

All materials for MS-ETS1-1 →

MS-ESS3-3 NGSS

Apply scientific principles to design a method for monitoring and minimizing a human impact on the environment.

How this material aligns: The lesson designs a method for reducing the impact of a channel cut too deep by past land use, and its closing letter asks students to name the measurement the crew should take the next spring to check whether it worked.

All materials for MS-ESS3-3 →

6.RP.A.3 Common Core Math

Use ratio and rate reasoning to solve real-world and mathematical problems, e.g., by reasoning about tables of equivalent ratios, tape diagrams, double number line diagrams, or equations, including finding a percent of a quantity and converting measurement units.

How this material aligns: Items 3, 7, and 8 apply rate reasoning, finding the hours a pace of 5 dams per hour needs for a reach and the percent of the post supply one reach would take.

All materials for 6.RP.A.3 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-09-23.

Objectives

Students will separate the design criterion from the two constraints in a written site brief.
Students will use rate and percent reasoning to find the posts and hours each reach would take.
Students will recommend one reach and justify it against the stated criterion and constraints.

Vocabulary

Floodplain: A floodplain is the flat land next to a river that the river spills onto when it floods. Floodplains are natural storage areas that give floodwater a safe place to spread out.
Water table: The water table is the underground level below which the ground is completely soaked with water. Dig a hole deeper than the water table and it will start filling with water.

Also: reach, criterion, constraint

Teacher overview

Sedge Run is a fictional stream near the town of Felwick whose channel has cut down well below its old floodplain, so that even a wet spring leaves the ground beside it dry. A restoration crew proposes low structures made of driven posts woven with brush, which slow the water, spread it across the floodplain, and let more of it soak in. Students receive a site brief with three candidate reaches, then use rate and percent reasoning to decide where a single crew, a single post delivery, and a single work day should go. The misconception to address directly is that students picture these structures as small concrete dams meant to hold back a pond, and they therefore judge a design by how little water leaks through it. These structures are built to leak: water spilling around and through them is what raises the shallow water table beside the stream, and a structure that sealed the channel completely would simply force the water to cut a new path around it.

Procedure

Framing the problem (8 min). Sketch a stream channel on the board with the water surface far below the flat ground on either side. Ask what happens to a heavy rain in that shape of channel, and collect answers. Name the design goal in the crew's own words: raise the water level in the channel a little so that high flows spread onto the floodplain instead of racing downstream.
Reading the site brief (10 min). Distribute the handout and read the site brief aloud. Working in pairs, students underline the one criterion the town is trying to maximize, the treated stream length, and circle the two constraints, the post supply and the work time. Build a class list on chart paper. If students list the spacing rule as a constraint, accept it, and point out that it is a fixed design standard, while the post supply and work time are the two limited resources the plan must fit.
Modeling one calculation (7 min). Work a practice reach at the board that does not appear on the handout. A practice reach 300 meters long at one dam every 30 meters needs 10 dams, and at 9 posts each that is 90 posts. Show the division and the multiplication in full, and stop to ask what the 10 and the 90 each count.
Student handout (20 min). Students complete items 1 through 10 independently, then compare answers with a partner for items 7 through 10. Circulate and watch for students who divide the post supply by the reach length rather than by the posts per dam.
Design decision and exit check (10 min). Poll the class on which reach they recommended and ask two students who chose differently to state the criterion each was serving. Close with a written exit ticket: "Name the reach you chose and the one constraint that ruled out a larger plan." Collect the tickets, and if fewer than 80 percent name a constraint rather than a preference, reteach the criterion and constraint sort at the start of the next class.

Where to Build Post-and-Brush Dams on Sedge Run — Grade 6 Engineering & Design

Name: ____________________________ Date: ______________

Student Handout

Site brief, Sedge Run (a fictional stream)

The town of Felwick has hired one crew for one work day on Sedge Run. The crew's rules are below. Answer every item using only the numbers given in that item.

The design rule is one post-and-brush dam for every 30 meters of stream.
Each dam uses 9 posts.
The crew has 360 posts delivered to the site.
The crew builds 5 dams in one hour and has 6 hours of work time.
Reach A is 600 meters long. Reach B is 450 meters long. Reach C is 900 meters long.
The town wants the greatest length of stream treated that the posts and the work time allow.

Reach A is 600 meters long. The rule is one dam every 30 meters. How many dams does Reach A need?
Each dam uses 9 posts. How many posts does the 20-dam plan for Reach A need?
The crew has 360 posts. What percent of those posts would the 180 posts for Reach A use?
Reach B is 450 meters long. At one dam every 30 meters, how many dams does Reach B need?
Reach C is 900 meters long. At one dam every 30 meters, how many dams does Reach C need?
Reach C needs 30 dams. Each dam uses 9 posts. How many posts is that?
The crew builds 5 dams in one hour. How many hours would the 30 dams in Reach C take?
Reach A needs 20 dams and Reach B needs 15 dams. At 5 dams per hour, how many hours would building both reaches take?
Reach C needs 30 dams and Reach B needs 15 dams. Write the ratio of Reach C dams to Reach B dams in simplest form.
The crew has 360 posts and 6 hours. Reach A is 600 meters and needs 180 posts and 4 hours. Reach B is 450 meters and needs 135 posts and 3 hours. Reach C is 900 meters and needs 270 posts and 6 hours. The town wants the greatest treated length. Which single reach should the crew build?

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

Answer Key

Answer: 20 dams.
600 meters divided by 30 meters per dam gives 20 dams.
Answer: 180 posts.
20 dams times 9 posts per dam gives 180 posts.
Answer: 50 percent.
180 posts out of 360 posts is 180 divided by 360, which equals 0.5.
Answer: 15 dams.
450 meters divided by 30 meters per dam gives 15 dams.
Answer: 30 dams.
900 meters divided by 30 meters per dam gives 30 dams.
Answer: 270 posts.
30 dams times 9 posts per dam gives 270 posts.
Answer: 6 hours.
30 dams divided by 5 dams per hour gives 6 hours.
Answer: 7 hours.
20 dams plus 15 dams is 35 dams, and 35 divided by 5 dams per hour gives 7 hours. This is more than the 6 hours available, so the pair of reaches does not fit in one day.
Answer: 2 to 1.
30 dams to 15 dams reduces to 2 to 1, because both numbers divide by 15.
Answer: Reach C.
Reach C treats 900 meters, which is more than Reach A's 600 meters and Reach B's 450 meters. It uses 270 of the 360 posts and exactly the 6 available hours, so it stays inside both constraints. Accept an answer of Reach A only if the student argues that finishing early leaves slack for a breakdown, and still shows that Reach C fits both limits.

Differentiation

Support

Give these students a handout on which items 1 and 2 are worked, and supply a labeled strip diagram of a 600-meter reach marked every 30 meters so the first division is visible rather than abstract. Allow a calculator for items 3 and 8.

Extension

Add a fourth reach that is 1,500 meters long and ask students to show that it cannot be finished under either constraint, then design a two-day plan that treats it and state what the town would have to supply to make that plan work.

ELL supports

Pre-teach "reach," "post," "brush," and "spacing" with a simple drawing of the stream seen from above, with the dams marked as short lines across it. Provide a table with the column headings already written so students place their numbers rather than build the table themselves.

Extensions

Have students write a short recommendation letter to the fictional town board that names the reach, the treated length, the posts used, and the hours used, and that states one measurement the crew should take the following spring to find out whether the structures worked.