Learning target: I can define the criteria and constraints of a creek restoration problem precisely, evaluate competing designs against them, and justify a design by predicting how its physical changes will affect populations.
You and a partner are consultants to a city council. Read the problem and the council's list, then work through the four parts in order: define the problem, study the options, evaluate them, and recommend a design. The creek, the city and the cost estimates are invented for this worksheet, but the restoration methods and the invasive plant are real. Write your own answers to Part D even though you plan together.
You will need: This worksheet, Pencil, Calculator
Arroyo Verde is a small creek that runs 1.2 kilometers through a city park in California, passing behind the houses on Maple Street in its upper half. In the 1960s it was straightened into a ditch to carry floodwater away quickly. Today giant reed, an invasive grass native to northern Africa and Asia that can grow taller than a two-story house, lines both banks. Giant reed uses a lot of water, crowds out native willows and cottonwoods, burns easily, and casts little shade on the water, so the creek runs warm in summer: about 22.5 °C in the pools in August. Surveys find no breeding frogs and few native birds. The council has money for a three-year project and wants a design that brings native populations back without putting the neighborhood at risk.
| Criterion, constraint or too vague? | Why |
|---|---|
| 1. Frogs breed by the third spring | |
| 2. Cost no more than $250,000 | |
| 3. The creek should look nicer | |
| 4. Pools below 20 °C by year 5 | |
| 5. No rise in flood level at Maple Street | |
| 6. No heavy equipment February to August | |
| 7. Giant reed under 10 percent by year 3 | |
| 8. Trail stays open |
A city engineer has estimated what five options would do. A beaver dam analog is a row of wooden posts pounded into the creek bed with willow branches woven between them. Like a beaver dam, it slows the water and backs up a pool, and it can be built with hand tools. Options can be combined if the total cost still fits the budget.
| Option | What is done | Cost over 3 years | Flood level at Maple Street in a big storm | Still pools for breeding frogs by year 3? | August pool temperature by year 5 | Giant reed on banks by year 3 |
|---|---|---|---|---|---|---|
| A | Remove giant reed every fall for 3 years and plant native willows and cottonwoods along the whole creek | $120,000 | no change | no | 19.5 °C | under 10% |
| B1 | Build 6 beaver dam analogs along the whole creek | $90,000 | 15 cm higher | yes | 20.5 °C | about 70% |
| B2 | Build 3 beaver dam analogs in the lower half only, downstream of Maple Street | $45,000 | no change | yes, in the lower half | 20.5 °C (lower half) | about 70% |
| C | Dig a new curving channel with a wide floodplain and plant natives | $900,000 | 20 cm lower | yes | 18.5 °C | under 10% |
| D | Line the banks with rock (giant reed removed during construction) | $80,000 | no change | no | 22.5 °C | under 10% |
| Within budget? | No rise in flood level? | Frogs (0-2) | Temperature (0-2) | Giant reed (0-2) | Total, or 'eliminated' |
|---|---|---|---|---|---|
| A | |||||
| B1 | |||||
| B2 | |||||
| C | |||||
| D | |||||
| A + B2 combined |
Engineers check constraints before anything else, because a design that breaks one is not allowed no matter how good it is. Go down the 'Cost' column and cross out anything over $250,000. Go down the 'Flood level' column and cross out anything that raises the water. Only then score what is left. For q6, try this frame: 'We recommend ___ because it stays within ___ and ___. It meets the criteria for ___ because ___ (shade, still pools, reed removal) changes ___ for ___. It meets ___ least well because ___.'
Beaver dam analogs often attract real beavers, which may then build their own dams, sometimes in places engineers did not plan. Suppose a beaver family arrives at Arroyo Verde in year 4 and starts a dam just downstream of Maple Street. Write a short plan for the council. Which criteria might the beavers help? Which constraint might they threaten? Research one method communities use to live with beavers, such as a pipe-and-fence flow device that keeps a pond from rising past a set level, and explain how it would keep the design within its constraints.