Which rainstorm is likely to cause more environmental damage, a 1-year, 24-h storm or a 10...

HIS

Category

General description

<2.5

None

Insignificant in terms of flooding; still may be applicable to water quality concerns

2.5 to 4

1

Minor flooding in parking lots, clogged inlets, and poorly drained areas

4 to 5.5

2

Approaches design capacity of typical storm sewers and inlets; street flooding expected

5.5 to 7

3

Most storm sewer and channel capacities exceeded; significant rise in channel water surface elevations

7 to 8.5

4

Approaches or exceeds typical maximum design for all drainage conveyance systems; floodplains inundated; major flood damage

≥8.5

5

Catastrophic rainfall event; all storm sewers, channels, streams, and rivers exceeded; massive flooding; extreme flood damage