Learning target: I can use a debris-flow hazard assessment to evaluate competing defenses against criteria and constraints, combine the best features of several into one plan, and justify the plan with data and its remaining risks.
You and a partner are advising the county on how to protect Sycamore Canyon before the first winter storm. Sycamore Canyon is a made-up town, and every number on this sheet is a teaching estimate, but the kinds of data and defenses are the ones California counties really use after wildfires. Read the problem, the hazard assessment and the options. Then follow Parts A to C in order. Show all calculations.
You will need: This worksheet, Pencil, Calculator
Last summer a wildfire burned 80 percent of the steep, 6 km² watershed above Sycamore Canyon, a foothill town of 140 homes, an elementary school and a stretch of state highway, all built on the fan of sediment where the canyon opens onto flatter ground. The fire burned away the chaparral whose roots held the soil and left much of the surface water-repellent. An old debris basin at the canyon mouth can hold 20,000 m³ of mud and rock. The rainy season usually begins in about four months.
| Measure | Estimate |
|---|---|
| Design storm (rain burst used for planning) | 15 minutes of rain falling at 24 mm per hour |
| Chance of a debris flow if the design storm hits | 70 percent |
| Largest likely debris-flow volume | 60,000 m³ |
| Time from the rain burst to the flow reaching homes | About 10 to 20 minutes |
| Years of higher danger | Highest in the first one or two rainy seasons, falling as plants regrow |
| Option | What it is | Cost ($ million) | Months to finish | Debris held (m³) | Engineer's notes |
|---|---|---|---|---|---|
| A | Enlarge the debris basin to hold 60,000 m³ in all | 5.5 | 6 | 40000 | Would catch the whole design flow; needs heavy equipment to empty after each flow |
| B | Four steel ring nets strung across the canyon | 2.0 | 2 | 20000 | Lets water drain through; a flow bigger than the nets can hold spills over; emptied by crane |
| C | Rain-gauge warning and evacuation plan (six gauges, weather forecasts, phone alerts, door-to-door notice, drills) | 0.4 | 1 | none | Can get everyone out if people leave before the burst; protects no buildings; false alarms can wear down trust |
| D | Deflection walls that steer a flow away from the main neighborhood into the creek channel | 2.5 | 3 | none | Protects about 90 of the 140 homes; sends more debris toward the highway crossing; check for damage after storms |
| E | Mulch and seed the burned slopes from the air | 1.0 | 1 | none | Cuts runoff and erosion in light and moderate rain by about 20 percent; little effect in a short, intense burst |
| F | Buy out and remove 30 homes in the most dangerous zone | 18.0 | 12 | none | Removes those households from danger permanently |
| Lives | Homes | Ready in time | Upkeep | Total (out of 8) |
|---|---|---|---|---|
| B: ring nets | ||||
| C: warning and evacuation | ||||
| D: deflection walls | ||||
| E: mulch and seed |
Think of protection in three layers and pick at least one option for each. Layer 1, keep people safe: which option gets everyone out before a flow? Layer 2, catch debris: which options trap mud and rock, and how much do they hold together with the old basin's 20,000 m³? Layer 3, steer what gets past: which option keeps the leftover flow away from homes? Add the costs as you go and stop if you pass $6.0 million.
Option D protects about 90 homes by steering debris toward the highway crossing and the creek channel. Write a paragraph from the point of view of a family whose home is among the 50 not protected, or of a Caltrans engineer responsible for the highway. Then propose a change to the plan that spreads the remaining risk more fairly, and explain what it would cost in money, time or protection somewhere else.