## What is a 100-year flood and is it possible to have a 100-year flood two years in a row?

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.

**What would be the discharge and gage height of the 100-year flood?**

The 100-year flood level is that gage height that corresponds to the discharge at RI = 100, which has a probability of being met or exceeded of 1%. Similarly, the 500-year flood level is that gage height corresponding to the discharge extrapolated at RI = 500, which has a probability of being met or exceeded of 0.2 %.

**Is a 100-year flood bigger than a 50 year flood?**

The term “100-year flood” is used in an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year….Recurrence intervals and probabilities of occurrences.

Recurrence interval, years | Annual exceedance probability, percent |
---|---|

50 | 2 |

100 | 1 |

200 | 0.5 |

500 | 0.2 |

### What is high flood level?

If the water level of the river at the forecasting site is below the Highest Flood Level of the forecasting site but still within 0.50m of the HFL then the flood situation is called “HIGH FLOOD” situation.

**Is there a rating scale for floods?**

The Flood Magnitude value is a measure of “how severe” a flood is, as a strictly hydrological occurrence (no assessment of damage is implied).

**What is a 1 in 100-year flood?**

A ‘1-in-100-year flood’ refers to a flood height that has a long-term likelihood of occurring once in every 100 years (also called a 100 year recurrence interval). Thinking about flood probabilities can help you decide whether or not to take action.