Earthquakes, hurricanes, floods and droughts: where and why

Grade 9 Social Studies Worksheet - Challenge Me

Circle the best answer.

1. Read this first

   Card A, landforms of the River Orla (an invented river, made up for practice)

   The river winds across a wide, flat floodplain. Low ridges called natural levees, built up from silt left by past floods, line its banks. Card B, flood-hazard map: dark shading marks the floodway, a strip along the channel kept clear so floods can flow. Light shading marks the 100-year flood area, land with about a 1% chance of flooding in any year; it reaches 4 km from the channel, past the levees. Brown marks fertile silt left by floods. Card C, notes: floods leave fertile silt on floodplains, so people have long farmed them. Flood maps help towns decide where to build and where to keep land open. Buildings in the floodway block the flow, and water that cannot pass them backs up or spreads wider.

2. 1.

   A farm lies in the light shading but outside the dark strip. What do the two maps show about it, Luz asks?

   • It has about a 1% chance of flooding in any year, and floods leave fertile silt on land like it
   • It is safe from floods because natural levees line the banks
   • It floods about once a century, on that schedule, and the dark strip floods more often
   • It must be kept clear so floods can flow, like the dark strip
3. 2.

   Why might the Orla's town keep the dark strip clear but allow farms in the light area, Milo asks?

   • Farms in the light area help floods flow faster, which the dark strip cannot do
   • Buildings in the dark strip would block floods; the light area has a 1% yearly chance and fertile silt
   • The light area lies outside the flood hazard, so farms there face no chance of flooding in any given year
   • The dark strip holds the fertile silt, so it must be protected from farming
4. 3.

   The light shading reaches 4 km past the levees. A student says levees make flood maps unnecessary. What do the cards support, Noah asks?

   • Levees are built by the town to the 100-year level, so land behind them is outside the map
   • Levees hold floodwater inside the dark strip, so the light shading just marks how high the levees stand above the river
   • Levees are low ridges built from silt, and the 100-year area reaches 4 km past them, so land behind can flood
   • Levees matter for the floodway but not for the light area, since the light area is farther away

5. Use this

   Card D, chances. A 100-year flood area has about a 1% chance of flooding in any one year, so about a 99% chance of no flood in a year. Over 30 years in a row, the chance of no flood at all is about 74% (0.99 multiplied by itself 30 times). A 500-year flood area has about a 0.2% chance of flooding in any one year; over 30 years the chance of no flood at all is about 94%. Each year is treated as separate: one year's flood does not change the next year's chance. Marl Bend (invented) lies inside a 100-year flood area.

6. 4.

   A bank says a 30-year loan in Marl Bend's flood area carries a 30% chance of a flood, since 1% a year adds up. Pia asks for the best correction.

   • Use the chance of no flood: about 74% over 30 years, so about 26% for at least one flood
   • The chance stays 1% across the loan, since each year is a separate 1% chance
   • The bank is right: 30% is the sum of thirty 1% chances, which is how chances combine over time
   • The chance is about 74%, the number printed on Card D for 30 years
7. 5.

   A flood just struck Marl Bend. Residents say, 'That was the 100-year flood, so we are safe for 100 years.' What is the best reply, Ren asks?

   • They are less safe now, since floods come in pairs
   • Next year's chance is still about 1%, since each year is separate; '100-year' is a chance, not a schedule
   • They are safe for several years, because the chance drops after a flood has used up the 'due' one for this river
   • They are safe for 100 years, since 1% of 100 years is one year and that year has passed
8. 6.

   Marl Bend's school may sit in a 100-year or a 500-year area for 30 years. Tala asks which comparison Card D best supports.

   • The areas stay about as far apart over 30 years as they are in one year: 1% against 0.2%, a small gap
   • The 100-year area is almost sure to flood in 30 years, since 26% is close to one in three
   • The 500-year area is safe, since 6% over 30 years is too low to bother planning for at a school
   • The 500-year area is lower in both, but the gap grows from 0.8 points a year to about 20 over 30 years

9. Card A, landforms of the River Orla (an invented river, made up for practice)

   The river winds across a wide, flat floodplain. Low ridges called natural levees, built up from silt left by past floods, line its banks. Card B, flood-hazard map: dark shading marks the floodway, a strip along the channel kept clear so floods can flow. Light shading marks the 100-year flood area, land with about a 1% chance of flooding in any year; it reaches 4 km from the channel, past the levees. Brown marks fertile silt left by floods. Card C, notes: floods leave fertile silt on floodplains, so people have long farmed them. Flood maps help towns decide where to build and where to keep land open. Buildings in the floodway block the flow, and water that cannot pass them backs up or spreads wider.

10. 7.

    A councillor says homes in the dark strip put just their owners at risk. What do Cards B and C support, Bo asks?

    • The councillor is right: a building in the strip floods its own site and no other
    • Homes are fine if the owners accept the 1% chance, since the strip has the same chance as the light area
    • Buildings in the strip block the flow, so water backs up or spreads wider, onto other land too
    • Homes in the strip help, since blocked water stays in the strip and away from the light area
11. 8.

    Esme's note: 'Floodplains are dangerous, so people long avoided them.' Which fix fits Card C?

    • Change it to say floods are good, since the silt is a benefit and floods bring no costs of any kind
    • Keep it, since the floodway is kept clear because floodplains are too dangerous for anyone to use as farmland
    • People have long farmed floodplains because floods leave fertile silt, a benefit that comes with the risk
    • Delete it: floodplains carry no more flood chance than other land

12. Use this

    Card D, chances. A 100-year flood area has about a 1% chance of flooding in any one year, so about a 99% chance of no flood in a year. Over 30 years in a row, the chance of no flood at all is about 74% (0.99 multiplied by itself 30 times). A 500-year flood area has about a 0.2% chance of flooding in any one year; over 30 years the chance of no flood at all is about 94%. Each year is treated as separate: one year's flood does not change the next year's chance. Marl Bend (invented) lies inside a 100-year flood area.

13. 9.

    Jun's note: 'A 100-year flood happens once every 100 years.' Which fix fits Card D?

    • It has a 1% chance each year that adds up to 100% after 100 years, so one is due then
    • A 100-year flood has about a 1% chance in any year, so two could come within the same decade
    • It is a flood about 1% larger than a normal flood
    • It is a flood that covers 1% of the land

14. Card A, landforms of the River Orla (an invented river, made up for practice)

    The river winds across a wide, flat floodplain. Low ridges called natural levees, built up from silt left by past floods, line its banks. Card B, flood-hazard map: dark shading marks the floodway, a strip along the channel kept clear so floods can flow. Light shading marks the 100-year flood area, land with about a 1% chance of flooding in any year; it reaches 4 km from the channel, past the levees. Brown marks fertile silt left by floods. Card C, notes: floods leave fertile silt on floodplains, so people have long farmed them. Flood maps help towns decide where to build and where to keep land open. Buildings in the floodway block the flow, and water that cannot pass them backs up or spreads wider.

15. 10.

    Kira's note: 'Marl Bend lies outside the dark strip, so it lies outside the flood hazard.' Which fix fits Cards B and C?

    • Change 'strip' to 'levees', since land past the levees is safe
    • The dark strip is where floods flow; Marl Bend can lie in the light area, with a 1% yearly chance
    • Keep it: the light area is a drawing aid, and the strip is the true flood zone
    • Change it to say Marl Bend is inside the strip, since the light area and the strip are the same place