What is the recurrence interval of a flood?

What is the recurrence interval of a flood?

The term “100-year flood” is used to describe the recurrence interval of floods. The 100-year recurrence interval means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100.

Can two 100-year floods occur within several years or even within the same year?

Yes, although if “100-year floods” started occurring each year, then the more frequent occurrences of the floods would change the statistical probability that the floods would occur, and thus, the “100-year floods” could become “50-year floods”! This question points out the importance of proper terminology.

What factors affect flood risk?


  • heavy rainfall.
  • long periods of rain.
  • snowmelt.
  • steep slopes.
  • impermeable rock (doesn’t allow water through)
  • very wet, saturated soils.
  • compacted or dry soil.

    What increases flooding?

    The faster the water reaches the river, the more likely it will flood. Urban land use – when an area surrounding a river is built on, there is an increase in the amount of tarmac and concrete, which are impermeable surfaces. Drains and sewers take water directly to the river which increases flood risk.

    What are 3 things cities do to reduce the chance of flooding?

    From rain gardens to absorbent pavement, urban communities are proving that nature’s toolkit can often provide the most innovative solutions.

    1. Rainwater harvesting.
    2. Permeable pavement.
    3. Green roofs.
    4. More trees.
    5. Rain gardens.

    How can we protect cities from flooding?

    10 measures to prevent (urban) flooding

    1. Create a ‘sponge city’
    2. Green roofs/rooftop gardens.
    3. Create flood plains and overflow areas for rivers.
    4. Separating rainwater from the sewer system.
    5. Install water infiltration and attenuation systems.
    6. Keep the sewer system clean, so it can do its job.