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Downtown Ottawa is 70 metres above sea level. Travel south to Manotick and the land rises only 21 metres, to 91 m. But it’s a steeper rise to the north — Old Chelsea is about 136 m, and King Mountain on the Eardley Escarpment rises all the way to 345 m.

Paugan Road, washed out between Low and Poltimore, is in a narrow valley where hills rise up to 200 metres above the road.

Each watershed is unique, cautions Gord Mountenay, the water management supervisor at the Mississippi Valley Conservation Authority. But “when you’ve got elevation and you’ve got water coming down, it will gain speed.

“If you get significant velocities (i.e. fast-moving water) you’re going to get much higher potential for erosion and for debris to come down the system with it. And that’s just gravity.”

Water can also flow fast across flat land, but “typically the higher the slope, the higher velocity, (and) the higher potential that you’re going to have erosion or other issues.”

He sees this in Tucson, where he often goes on a winter vacation. It is surrounded by mountains and prone to flash floods.

As well, the Gatineau Hills are granite, which tends to shed water.

The Rideau and Gatineau rivers illustrate the difference. The Gatineau is a fast, powerful river good for driving logs. Ontario’s Rideau moves so slowly that it does a poor job of flushing itself out, and has trouble with algae.

The Ontario Ministry of the Environment and Climate Change uses a mathematical formula to combine three risk factors for flooding: The steepness of the land, whether the soil type will soak up rain or repel it like a raincoat, and whether there’s enough vegetation to slow down the run-off.

tspears@postmedia.com

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