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

The floodplain is an area of land near watercourses (like rivers and streams) that can flood when water levels are high and overflow riverbanks.

In nature, floodplains play a key role in storing and moving water. By holding water, floodplains can help protect other areas that might otherwise be at risk of flooding. They also offer many benefits: providing habitat for wildlife, increasing biodiversity, and serving as beautiful spaces to enjoy nature.

Keeping floodplains in their natural state is an effective way to manage floods and reduce risks to nearby communities.

Throughout history, people have wanted to live near water. As a result, some areas within floodplains have been developed. These areas are more vulnerable to flooding and can create challenges for other parts of the watershed when water flow is restricted, or 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.