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What is Probable Maximum Flood (PMF)?

Understanding floods can be overwhelming, with all the different terms used in various reports, studies, and data. As we witness an increase in natural hazards, it's becoming more and more important to grasp these terms and their implications for us.

Let's take a closer look at some of the flood definitions:

Floodway: The most dangerous part of the floodplain, where water is moving in the channel of a river or stream, and the surrounding areas required to carry the regional flood discharge.

Flood storage: An area where floodwater accumulates, which, if displaced, can cause a significant redistribution of floodwaters, increase in flood levels or frequency. These areas are often aligned with floodplains and characterized by deep and slow-moving floodwater. Flood storage helps regulate the flood peak.

Floodfringe: The portion of the floodplain outside of the floodway that is covered by floodwater during a regional flood. This term is generally associated with standing water rather than flowing water, and local floodplain ordinance requirements apply.

Floodplain: The area of land subject to inundation by floods up to and including the probable maximum flood event, including the floodway and floodfringe areas.

Flood planning area: The area within which developments may be subject to flood-related development controls. It is calculated as the area lower than the Flood Planning Level.

Probable Maximum Flood (PMF): The largest flood that could potentially occur at a particular location, usually estimated from probable maximum precipitation. The PMF defines the maximum extent of flood-prone land, i.e., the floodplain.

In short, the 100-year flood and floodplain refer to the same area, which includes both the floodway and floodfringe.

In our next blog, we'll explain the 100-year flood in more detail.

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