Factoring groundwater into flood risk

While groundwater figures prominently in riverine and urban flooding and is relatively easy to map, insurers, urban planners, governments and other key stakeholders seldom factor it in when considering flood risk.

So says Dr. Cathy Ryan, a hydrogeologist with the University of Calgary and facilitator of ICLR`s Friday Forum workshop September 24 on the role of groundwater in flooding.

Dr. Ryan is trained as a geotechnical engineer and hydrogeologist. She has been researching groundwater, river interaction and river-connected alluvial aquifers in the Calgary region for close to a decade. Her interest in the role of groundwater in flooding was sparked by the 2005 Calgary floods when anecdotal information collected in a door-to-door survey of residents in neighbourhoods around Calgary’s Elbow River suggested groundwater inundation (as opposed to overland flooding) caused a significant amount of the damage to homes.

It’s Ryan’s view that while groundwater flooding is under-researched and is often under-recognized for its role in flooding, it can be easily monitored and understood at a fairly low cost.

Rivers are usually small and narrow, at least when compared to the river-connected alluvial aquifers that run beneath them. These aquifers, says Ryan, are permeable and easy to map, though virtually no overland flood maps in Canada consider groundwater flooding. Aquifers can be quite broad, extended far beyond the flood fringe. “So distance from the river is not a good way to determine groundwater flood risk,” she says.

“What we need is a network of monitoring wells to measure groundwater. This is the easiest thing for a hydrogeologist to do,” says Ryan. “They are key.” As to the location of these wells: “We know where overland flood happens. We could know where groundwater flooding might happen.”

What causes a lot of flooding and damage to homes and other structures is when the groundwater level rises above the level of the basement slab, causing the basement to flood (unless it is impermeable), says Ryan.

Ultimately, basement floor elevation is key. “When considering groundwater flooding, the height of the slab in the basement is more important than distance from the river,” she maintains.

“On the building code or municipal bylaw side, we need to think about regulating the height of basement floors relative to the height of the aquifer.