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Why are southern Alberta communities prone to flooding?

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By Scott Sutherland
Meteorologist/Science Writer
Monday, June 23, 2014, 1:50 PM

The rains falling across southern Alberta this week are an unwelcome reminder of the devastating floods of 2013, especially so close to the actual one-year anniversary of that tragic event. With states of emergency called and evacuation orders issued, officials were taking no chances this time around, but what is it about these regions that makes them so prone to flooding?

Although the flooding in downtown Calgary and the town of High River captured most of the media attention last year, many more communities throughout southern Alberta suffered through the ordeal and were forced to evacuate their homes. In southeastern Alberta, just as two specific examples, the cities of Lethbridge - on the Oldman River - and Medicine Hat - on the South Saskatchewan River - were inundated by flood waters, as the two videos below show all too well:

Of course, the weather plays a major role in this. Just as we saw last year, the flow of moisture into southern Alberta was streaming up the foothills of the Rocky Mountains (which is called 'orographic flow' or 'orographic lift'). Compared to a similar flow of moisture over level ground, orographic flows cool faster, and convert their moisture into rain faster and over a much shorter distance. This results in heavier rainfall amounts over the region and much larger amounts of water entering the region's river system over a short amount of time.

The shape and flow of the rivers is another major factor. Still focusing on Lethbridge and Medicine Hat, both cities have some wide floodplains bordering the rivers that flow through them. These floodplains were formed as the river course meandered back and forth, carving through the topsoil over the eons. When water levels rose and water flow rates peaked due to the kind of rains we saw in the region last year and again this week, the added erosion, as well as the overland flooding when the waters crested the river banks, would carry away large sections of the top soil between these meanders. (An image of how this happens can be seen here.)

One of the most basic principles about water flow is that, when left to its own devices, it always takes the easiest route possible. This means that adding a few days worth of heavy rain to a river that has a fairly straight path likely won't cause much trouble for people living along it unless the flow is far above normal levels (we saw this rather dramatically in Canmore last year). However, in meandering rivers, the twists and turns complicate matters. As the flow increases, it digs into the 'corners' of these meanders, wearing away at them very quickly, and if the flow increases enough, it's very easy for the water to take a path of least resistance right over the land in between the meanders, bypassing the normal river flow completely. Unfortunately, in some communities along rivers, these areas have been developed for years, and it only takes one extreme flood event to reveal how far a flood plain extends. Fortunately, modern city planning is well aware of this, and they designate high risk areas for flooding in these riverside communities. 

The image below, from the Alberta Environment and Sustainable Resource Development website, shows maps of both Lethbridge and Medicine Hat, with the flood risk regions along the rivers painted in red (floodway), rose (flood fringe) and pink (overland flow). 

Credit: Alberta Environment and Sustainable Resource Development

Credit: Alberta Environment and Sustainable Resource Development

The flooding in 2013 quite possibly had the most media coverage of any similar event in recent history, however flood forecasts for southern Alberta were been reaching back to 1995 for comparison. 

In Lethbridge, in 2013, the Oldman River peaked at a flow rate of over 2,600 cubic metres per second (or m3/s). For some scale, in just one minute, that amount of water could fill over 150 million 1-litre bottles. However, in early June 1995, the river peaked at a much higher rate - over 4,600 m3/s. According to researchers at the University of Lethbridge, that was the highest flood levels reported since 1908.

For Medicine Hat, 2013 was slightly worse than what they saw in 1995 (flow of over 5,400 m3/s in 2013 vs 5,100 m3/s in 1995), since the city straddles the South Saskatchewan River, which draws its flow at that point from both the Oldman River and the Bow River. Thus, comparison to either year is just as bad.

As of today, the City of Lethbridge is reporting on Twitter that water levels on the Oldman River continue to rise, and are expected to peak by Friday. All bridges across the river remain open, but all access to the river valley is closed. The City of Medicine Hat is reporting projected flow rates a bit more in line with 2013 than 1995, thankfully, and are keeping local residents updated via their Twitter feed.

(H/T to BrainT on Twitter for suggesting I discuss the unique weather factor for southern AB as well)

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