Background on flooding
1. Why does flooding occur?
The Bow River's source of water is the Bow Glacier in the Rocky Mountains. The Elbow River originates in Elbow Lake also in the Rocky Mountains.
When the winter snow melts, especially in the Rockies, much of that water ends up in the rivers. Combined with heavy rainfall, the additional water in the rivers can cause flooding.
2. Can floods be predicted?
Historically, river floods occur in late May/early June, although there are a few incidents both before and after this period.
The City of Calgary and Alberta Environment monitor snow pack, rainfall and temperatures year-round. If stream flows rise and it begins to rain, Alberta Environment puts out notices that fall into these categories: high stream flow advisory, flood watch or flood warning.
While conditions might seem right for flooding, weather in Calgary and the surrounding foothills is unpredictable and can change rapidly. So, while we may assume that flooding will happen, it's sometimes difficult to accurately predict.
3. What does history tell us about flooding?
The City and Alberta Environment study flood plains and historical occurrences of flooding. This information is compiled into flood risk maps. These maps can be used to estimate areas at risk for flooding to a certain level.
Flood maps often show various levels of predicted and worst-case scenario flooding. But estimating flooding beyond the "100-year flood" is a difficult task because there is a lack of historical flood information to compare.
Phrases like "1-in-20-year flood" or "1-in-100-year flood" are an estimate of how often a flood will occur in a given time period based on the amount of the flooding.
The longer the time period, the larger the flood event. What it means is that a 1-in-100-year flood will, statistically, have a one per cent chance of occurring in any given year. It would be rarer and much bigger than a 1-in-20-year flood.
This does not mean that if a 100-year flood happened this spring, it wouldn't happen again for another 100 years. It could happen again in the following year or even twice or more in any given year.
1. Why don't dams stop flooding?
There are 11 dams along the Bow River before Calgary. The closest is the Bearspaw Dam on Calgary's western city limit, and the Glenmore Dam on the Elbow.
These dams are not designed for flood protection, but rather electric power generation. They also store water for various water systems on the river. That said, dams provide a small amount of protection by slowing the release of water temporarily.
2. Why can't you build barriers on the riverbanks?
The City of Calgary constructs berms, dykes and floodwalls in high-risk areas along the Bow and Elbow rivers; however, these structures can't guarantee protection because they can collapse, erode or be overtaken by high flood waters. Additionally, these structures may affect views, walking paths, obstruct access to the river and encroach on privately-owned property.
3. Why can't you provide sandbags to residents?
The City identifies key locations (which may include some residences) where sandbags are needed to protect critical infrastructure or facilities needed for emergency operations. Locations that could cause significant damage to communities, drinking water, storm sewer systems or other utilities are given maximum protection with limited resources.
Unfortunately, it is not possible for The City to provide sandbags to hundreds of thousands of homes. It remains the responsibility of the property owner to protect their property, and sandbag, as required.
4. Does The City upgrade areas that are at risk?
The City evaluates potential storm sewer improvement projects in communities subject to flooding before every construction season.
Evaluations are based on these safety considerations: amount of flood damage, number of floods in each area, and cost-effectiveness of the project.
These projects are funded by the drainage charge on your utility statement.
During and after flooding
1. You have asked me to evacuate - why can't I stay?
When we ask people to evacuate, it is because there is a clear danger to life. We ask that you leave your home so that you or those who may need rescue are not put in harm's way.
2. What if I live in an apartment or condo?
Please contact your condominium board, property management company or landlord to inquire about preparation and actions you can take before, during and after flooding.
3. Who pays for flood, water seepage or sewer backup damage?
It is dependent on the event. If you are wondering if you can get compensation from The City, please call 3-1-1 and ask for City Claims. They will investigate and determine if compensation is appropriate.
Or you can call The Insurance Bureau of Canada, who answers general insurance claim questions, Monday to Friday from 8:30 a.m. to 4 p.m. Call them toll-free at 1-800-377-6378.
4. Why are there water-use restrictions during some flooding situations?
The City sometimes restricts outdoor water use during flooding situations related to rainfall or snowmelt. When the level of silt and debris in the water is high, the water treatment facilities must work at full capacity to treat high levels of sediment in the water. To reduce strain on the plants, we may ask Calgarians to refrain from outdoor water consumption.
Storm sewer system
1. How does the storm sewer system work?
In Calgary, the storm sewer system is a network of underground pipes that drain water from the streets during low-intensity rainfalls. Water enters the pipes through the storm drains in the streets and flows into the nearest river or stream.
In newer communities, a series of streets and ponds are built to collect water temporarily during heavy rainfalls. Some excess water collects in low spots on streets. The rest flows through pipes into wet or dry storage ponds.
Once the rain stops, water stored on the streets and ponds is released at a rate that prevents pipes from overloading.
2. What is the different between a wet and dry pond?
A dry pond is dry 90 per cent of the time and may have playing fields in it. They fill with water in heavy downpours and can take as long as 24 hours to drain once the rain stops.
A wet pond always has water in it; however, the water will get much deeper during a storm. The wet pond can help clean the water that comes off the roads because it allows the water to slow down so the heavy dirt and garbage settle to the bottom.
3. How deep are these dry and wet ponds?
If you live near a dry pond or a wet pond, make sure your children stay away from them when it rains. Although dry ponds fill and drain slowly, most of them reach of depth of 1.5 metres (five feet) and remain that deep for several hours. Wet ponds can rise two metres (6.5 feet) and above their normal levels.
4. What do the stormwater gates on the river do?
In winter, ice can build up and force water from the river, and into the storm mains and surrounding communities. The gates prevent this from happening. The gates are closed and monitored daily during winter months (November to April) because the ice pack on the river can change dramatically in a short period of time.
If the level of stormwater is significant, the gates are opened and the water is released. The gates are then immediately closed again. During summer months (May to October) the gates are fully open; however they are monitored during this time and closed during high river flows to prevent water flowing back into the community.