What causes flooding

As mentioned, flood risk is based on a combination of hazard, exposure and vulnerability.

The hazard of flooding is caused by natural factors like heavy rain, snowmelt, heavy winds, ice jams and high tides. These hazards are also influenced by human-caused climate change.

When we think of the negative impact of flooding, we typically think about how it creates dangerous and destructive conditions due to exposure and vulnerability.

There are several factors that can be improved to reduce exposure and vulnerability, or if these are not prioritized, increase exposure and vulnerability and therefore our risk. Considerations that can increase or decrease our flood risk are explored below. This includes:

• climate change;
• urban growth and increasing amounts of hard surfaces;
• loss of natural systems; and/or
• designing and planning with floods in mind.

Climate change

Climate change is bringing more intense and frequent floods to the world, and the municipality is no exception. This accelerating trend in floods that will continue this century is driven by extreme rainfall, shifting seasons, rising sea levels and increasingly intense storms.

How does climate change impact rainfall?

The relationship between higher temperatures and rainfall can be explained with simple physics. As the planet warms from greenhouse gases trapped in the atmosphere, warmer air can hold more moisture and in turn, bring more intense storms and rainfall.

For every one degree increase in average temperature, the atmosphere can hold about 7 per cent more moisture. More moisture forms more clouds, and this results in heavier rainfall.

For more information on rainfall and climate change, visit ClimateData.ca.

How does climate change create wilder weather?

As climate change alters weather patterns, we experience more extreme and unpredictable events—what many of us describe as 'wild' weather. This impacts floods. For example:

• Sudden high temperatures in the winter can cause rapid snow melt. This influx of water leads to flooding, especially when frozen ground can’t absorb water.
• Hot, dry temperatures can dry out soils in a drought. When heavy rain comes after dry conditions, dry soils can’t absorb water quickly and can cause more surface runoff.
• Hurricanes are expected to become more frequent and intense in Atlantic Canada.

Can a few centimeters of sea level rise really make a difference?

Sea level rise is already a reality, driven by melting ice sheets and warmer ocean waters. In Nova Scotia, the rise is even more pronounced due to 'subsidence'—the gradual sinking of the land since the last ice age.

As sea levels rise, storm surges can become more damaging, with higher waves and coastal flooding that was previously uncommon or less severe.

In the image below, you can see how sea level rise makes storm surge impact more land, exposing areas that in the past were not within a flood prone area. The combination of sea level rise, high tide, and storm surge is impactful.

Sea level rise and storm surge also affect river and stormwater drainage. In the municipality, many of our rivers and infrastructure outlet to the ocean. When water levels are high, water has nowhere to go. High water levels at the downstream can raise water levels at the upstream. We often see some of the worst flooding when peak flows coincide with high tides and storm surge.  

What does this mean?

Our world was built for a past that no longer exists. Planning for floods requires consideration of future changes in rainfall patterns, snow melt, high temperature, drought, and more to prepare for the floods of the future.

The flood maps on this website include climate change projections. This helps us plan for the future and be prepared.

Urban growth and impervious surfaces

As the municipality experiences rapid population growth, preparing for floods becomes even more critical. In 2023, the municipality’s population grew by roughly 4.1 per cent, the highest recorded annual growth rate, making it one of Canada's fastest-growing cities. This expansion drives development, which must be carefully planned and managed to prevent increasing flood risks to people, properties, and infrastructure.

Without proper planning, urban growth leads to more impervious surfaces—such as roads and buildings—that cannot absorb water like natural landscapes. Water runs off much faster on concrete and asphalt than it does in forests or fields, and this excess runoff has to go somewhere.

These impacts can be mitigated by adopting strategies to store, infiltrate, reuse and gradually release water back into the stormwater system. This helps reduce strain on Halifax’s infrastructure and minimize flood risk.

New developments, as well as retrofits to existing areas, must integrate sustainable stormwater management practices to handle increased runoff and reduce the risk of flooding.

Loss of natural systems

Natural systems such as forests and wetlands play a crucial role in absorbing and storing floodwaters. However, centuries of ecosystem degradation, habitat loss and encroachment on these vital areas have increased our vulnerability to flooding. It is essential that we preserve, protect and restore these natural systems to reduce flood risks.

Wetlands are vital natural areas that store water and prevent flooding. Unfortunately, Southern Canada has lost about 70 per cent of its wetlands, with urban areas losing as much as 95 per cent. Wetlands provide more than just flood protection; they are essential ecosystems that support wildlife, store carbon, improve water quality and provide food and habitat. In Nova Scotia, wetlands primarily include peatlands, shrub swamps, and salt marshes—all of which are critical for flood prevention and ecosystem health.

Designing and planning with floods in mind

As one of Canada’s oldest cities, much of Halifax was developed before modern stormwater and flood mitigation standards were introduced. Many older areas lack adequate stormwater systems or have systems designed only to quickly move water away, rather than manage or absorb it. Consequently, these areas are more vulnerable to flooding during heavy rainfall events. Retrofitting these older areas to meet current standards presents a significant challenge.

The amalgamation of the Halifax Regional Municipality presented challenges for water resources and stormwater management, as there was no unified standard for all areas. In 2007, Halifax’s wastewater and stormwater infrastructure was transferred to Halifax Water, which now manages the municipality’s water systems.

In addition to Halifax Water’s design standards, the municipality uses Municipal Design Guidelines, by-laws, and land use by-laws to articulate standards to mitigate flooding risk in neighbourhoods and urban areas. This allows the municipality to take actions such as limiting development within floodplains or requiring developers to manage stormwater on-site in new developments.