Flood­ing sur­passes pre­dic­tions

Cli­mate change mak­ing flood maps dif­fi­cult to de­sign

Metro Canada (Ottawa) - - Ottawa - Alex Abdelwahab

The flooded area in Con­stance Bay sur­passed what the city’s 100-year flood­plain map pre­dicted, even though wa­ter lev­els haven’t sur­passed the city’s pre­dicted max­i­mum flows.

The wa­ter level drawn on the map is shown at least 50 me­tres back from Bayview Drive in Con­stance Bay, but when I vis­ited the area on Sun­day, I ob­served that wa­ter had reached sec­tions of Bayview Drive and houses on both sides were flooded.

John Price, di­rec­tor of wa­ter re­sources en­gi­neer­ing at the Mis­sis­sippi Val­ley Con­ser­va­tion Author­ity, which has ju­ris­dic­tion over the shore­line in Con­stance Bay, said this is be­cause the ac­tual el­e­va­tions on the ground may be slightly dif­fer­ent from the topo­graph­i­cal model used in the cal­cu­la­tions to draw the map.

So flood waters in some ar­eas may stretch fur­ther than the map shows, even if ac­tual wa­ter lev­els did not reach higher than those pre­dicted when the 100-year flood map was cre­ated.

Price said that the wa­ter level used in the cal­cu­la­tions may be right, but in cer­tain ar­eas, “maybe the to­pog­ra­phy that you’re us­ing to draw that line may be kind of out a lit­tle bit so the ac­tual flooded area maybe larger than what your show­ing” on your map.

“There are some vari­ables in there that could lead to that flood line maybe not be­ing per­fectly rep­re­sen­ta­tive ev­ery­where,” he said.

Price said the best in­for­ma­tion he had was that the flood in Con­stance Bay re­sulted from a 50-year wa­ter-level high.

This in­for­ma­tion may change in the com­ing days, he said, as the con­ser­va­tion au­thor­i­ties in the ar­eas are gath­er­ing pho­tos of the flood­ing, in­clud­ing aerial shots.

“If there’s large ar­eas I’ll say that are out­side of how the flood plain is de­lin­eated now that are flooded now, that would po­ten­tially re­sult in kind of a di­rect re­draw­ing of the flood plain line,” he said.

Cities use his­tor­i­cal data to build flood maps that pre­dict the like­li­hood a spe­cific area is at risk of flood­ing.

A one in 100-year flood means there is a one per cent chance that flood risk ar­eas will flood in any given year, ac­cord­ing to Paul Beck­with, a cli­mate sys­tem sci­en­tist and Univer­sity of Ot­tawa ge­og­ra­phy pro­fes­sor.

The fre­quency of ex­treme weather events is in­creas­ing all around the world, mak­ing pre­dic­tive tools like flood maps more dif­fi­cult to de­sign, ac­cord­ing to Beck­with.

“Those terms are all based on a sta­ble cli­mate. So if the cli­mate is sta­ble you can come up with the prob­a­bil­ity that an event will hap­pen, an ex­treme weather event will hap­pen,” Beck­with said.

“When the cli­mate is chang­ing rapidly, as it is now, then we start see­ing some strange things. You can’t re­ally be­lieve those num­bers.”

Gail Faveri, a man­ager with En­vi­ron­ment and Cli­mate Change Canada and Cana­dian Sec­re­tary for the In­ter­na­tional Lake On­tario-st. Lawrence Board said Mon­day’s peak on the Ot­tawa River was the high­est ever recorded, since track­ing began in 1967, she said.

“It’s about twice what the av­er­age peak is.”

Alex ABDELWAHAB/METRO

Two houses on the east side of Bayview Drive in Con­stance Bay are sur­rounded by flood waters on Sun­day.

Metro | ot­tawa

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