- how big was the flood,
- how frequently can we expect a flood that big,
- is this because of climate change,
- how likely is it that we would receive an even larger flood, and what would the impact be, and
- how can we design for this type of flooding?
The first three questions are answered in this blog post: How Large was the Flood in Calgary, and the last two questions will be addressed in this post.
4. How likely is it that we would receive an even larger flood, and what would the impact be?
A standard design flood in most communities is considered the 1 in a 100 year flood, this is not the largest flood that could happen, it is the flood that has a 1% chance of occurring in any year. In the first post in the series, the flow rates for the 150 year frequency storm were shown in a table. These flows are almost double what was experienced during the worst of the flood in June 2013. For Calgary, what does the 1 in a 100 year flood look like?1
5. How can we design for this type of flooding?
Alberta didn’t take that lesson to heart as much as it should have after the last flood in 2005, said former Tory agriculture minister George Groeneveld, who chaired a task force that called for a $300-million investment in flood mitigation. The report he completed in 2006 gathered dust for six years before PC Premier Alison Redford released it last year after taking office. “If you don’t get this done in a one-year window, people soon forget,” Groeneveld said in an interview. The report contained 18 recommendations to prepare for floods, including mapping flood-prone areas, developing warning systems and building infrastructure to prevent or reduce flooding. While the overall plan to protect 54 municipalities came with a $300-million price tag, the report noted 42 of those communities could be protected for $32 million. The cost of required infrastructure for the other 12 communities was “substantial,” but the report noted they would have a higher cost-to-benefit ratio.
The simplest measure is to prohibit building within floodplains, after that, the options are limited. Permanent structures such as walls (concrete or sheet pile), or dykes are successful only if they are continuous, and can be used to prevent flooding of neighborhoods or just individual properties. Pre-planned installations such as water walls, (seen in the video below) are expensive, but given adequate warning and practice runs, may be a viable alternative to having a flooded basement or underground carpark). Temporary structure such as earthen berms, riprap, sandbags, or floodwalls are the most flexible in terms of placement, but may be labor intensive to erect.2
Flood warning systems would be required to adequately deploy any of the non-permanent solutions, and given the speed of the rise in the river in some catchments, it might be difficult to predict in a timely manner the extent of the flooding. This time of response would need to be a consideration in any solutions being considered. For critical infrastructure, permanent solutions are preferred, i.e. they should be designed into the site, however, reliable planned defense is a reasonable option, particularly for existing infrastructure.
The response to flood damage ranges from bewildered, upset, angry to relieved, but the question remains: under what level of risk did the property owner develop the land, and how much knowledge of risk was there at the local government or provincial level at that time?
The City of Calgary needs to understand the risks of another flood event, possibly of an even greater magnitude, and the cost associated with the response, clean-up and economic losses associated with the inconvenience. With two floods in under a decade, it wouldn’t surprise me if the City of Calgary worked hard to establish a solid long-term plan to address the risks, and it seems as though the Provincial and Federal Governments would be in support of ensuring Calgary’s success in these matters.
There are many proprietary systems available for flood defense, the products selected for inclusion in this post are for illustration only and have not been included as solutions recommended specifically for Calgary’s situation or any other flood condition. All situations are unique, and the design advice from a professional is recommended to ensure that the solution performs as expected. ↩