Flooding terms and concepts

Flood prone areas

Although flooding can happen anywhere, we use the term “flood prone area” to predict where flooding can occur. This does not mean the area will flood every time it rains, or that only flood prone areas can flood.

• For fluvial flooding, the area of land near a water body that is expected to flood is called a floodplain.
• For pluvial flooding, low-lying areas and places with limited capacity for drainage or absorption are most flood prone.
• For coastal flooding, flood prone areas are along the coast.

Floodplain

A floodplain is a term used for fluvial flooding. It represents the area of land near a watercourse (like a river or stream) that could flood when water is high.

In nature, floodplains are a natural space to store water and prevent floods from reaching other areas. For most of the year when they are not flooded, they serve as excellent spaces to enjoy nature, provide habitat for wildlife, and increase biodiversity. Leaving floodplains as a natural flood mitigation technique is often called “making room for water” and is one of the most effective ways to manage floods and keep our communities safe.

Throughout human history, people have wanted to live near water. This has resulted in some development in the floodplain in the municipality. These areas are both more susceptible to flooding and can result in more challenges for areas downstream of a watercourse when flow is restricted, and storage is reduced.

Watershed

The Halifax Regional Municipality is home to over 1,000 lakes, more than 20 rivers, and innumerable streams that are all situated in what are called watersheds. Watershed are areas of land that drain, or “shed” water.

• A primary watershed refers to a geographic area where all water drains into the same river or lake system. The Sackville River watershed is a primary watershed that drains into the Atlantic Ocean.
• The term secondary watershed is a smaller piece a primary watershed, where all water drains into the same, smaller system. The Kearney Run watershed is a subwatershed of the Sackville River primary watershed.
• Shore direct watersheds are small watersheds without defined river or stream systems, where all water drains directly to the coast.

The map below shows the roughly 50 secondary and shore directed watersheds in the municipality: 

"1-in-20-year" and "1-in-100-year" flood events

People who work on flooding often use statistics and probability to describe floods. The 1-in-20-year event is an event that has a 5 per cent chance of happening in any given year. The 1-in-100-year event has a 1 per cent chance of happening in any given year.

It is important to remember that:

• this does not mean that once an event has happened, it can’t happen again for another 20 or 100 years; and
• this does not mean that if you live somewhere for a long time, there is only a 1 per cent chance of a flood.

Each year has the same chance. This probability is important to keep in mind when we do long-term planning.  

When we add in the impacts of climate change, this equation changes even more. For example, some scientists estimate that an event that has a 5 per cent chance of happening now could have a 20 per cent chance of happening by the end of the century.