FLOOD MAPPING UNDER FIRE
REPORT URGES CITY TO UPDATE CRITERIA
One-fifth of the buildings in Calgary damaged during last year’s floods were outside the hazard area for high water, according to a recent city report.
The expert study says the fact that hundreds of structures outside the designated one-in-100-year flood plain needed to be repaired in the wake of the June disaster — which saw flows that either matched or were smaller than predicted for that event — suggests the current maps are “inadequate.”
“Flood maps need to be regularly assessed to ensure they reflect changing river morphology, development and climate, and the latest technology for flood forecasting and modelling,” the report said.
“The expanded criteria for defining the floodway, as used across the rest of the province, should be considered in an update to Calgary’s flood-hazard maps.”
Despite this finding and a 2012 study that determined Calgary’s inundation maps inaccurately reflect the risk to homes and businesses, officials with the city and province won’t commit to revising the threedecade-old document they use for critical decisions about land-use development and disaster-recovery payments.
“No decision has been made to update the existing mapping using current floodway determination criteria,” provincial government spokesperson Jason Penner said in an email reply to questions.
“Any decision will be made in consultation with the City of Calgary.”
If the more rigorous criteria are adopted, it could mean an expansion of the areas where construction or renovation of buildings would be restricted or prohibited because they obstruct the river’s flow during a flood, he said.
Because areas where the flood water was more than one metre deep or flowing faster than one metre per second weren’t automatically part of the 1983 floodway, a new floodway using the current criteria could be larger, Penner said.
The 2012 study by Golder Associates, produced a year before the massive floods but not publicly released at the time, warned the city would experience higher water levels and worse damage than predicted in the flood studies and maps produced in the early ’80s.
For example, while the official study showed most of Sunnyside east of 10th Street would be inundated in a one-in-100-year event, the firm’s revised maps showed large swaths of the neighbourhood west to 14th Street would also be under water.
While the Bow River never reached those levels last year, the city report said “the (revised) maps generated by this model were accurate in predicting the extent of the 2013 flood.”
Calgary’s city council will get a report from city officials in October on what effect an updated hazard map would have on future land use.
Emma May, president of the Calgary River Communities Action Group, said accurate maps are critical to estimating the cost-benefit of potential upstream mitigation projects and deciding when the province should purchase homes in the floodway.
“The buyout policy is terribly flawed,” May said.
“There are people with homes that were badly damaged who have no choice but to rebuild because, technically, they’re in the flood fringe.”
Both levels of government have said they are considering spending up to $1 billion on mitigation projects to protect the city against future floods, including a $500-million tunnel that would divert water from a swollen Elbow River beneath the city and away from vulnerable neighbourhoods that would otherwise be in its path.
But Calgary’s expert report last month suggests many of the 534 buildings outside the designated floodplain that were damaged in last year’s disaster may have been the victim of water moving underground instead of overland, a problem that can often be eliminated by retrofitting homes with less expensive backup valves or sump pumps.
“The official flood hazard maps do not show the areas that were likely to be affected by groundwater or sewer backup flooding as a result of river flood events.” the city report said.
“Mapping should be expanded to include information about groundwater risks once these are further investigated.”
Despite the fact much of Calgary is built on porous, alluvial gravels, neither the city nor the province is currently watching the movement of water beneath the surface when the rivers run high.
“The city does not maintain a system of groundwater monitor wells and did not make observations ... during the 2013 flood event,” city spokeswoman Octavia Malinowski said in an email reply to questions.
“Future activities ... are yet to be determined.”
A study of homeowners along the Elbow River in the wake of the 2005 floods found that groundwater entering basements, not overland flooding, caused more than 80 per cent of the damages.
While land-use rules require new construction to be six metres back from the floodway, the University of Calgary graduate students said it would be better if the city also required basements to be out of harm’s way when rivers rise.
“A better approach,” say the report, “(is to) consider zoning areas based on basement elevations above 1:100 year river and groundwater level as well as distance from floodway.”