Heavy rain plus hills bad combination for flooding

Water flows downhill, and the Outaouais adds an exclamation mark to the truism.

Each heavy rainstorm seems to vent its greatest anger in West Quebec.

This week, washed out roads in Gatineau, Chelsea and Denholm.

Last May brought devastating floods.

And the rainstorm of St. Jean Baptiste Day 2001 washed out sections of Highways 148 and 105, along with countless smaller roads and the Wakefield Steam Train tracks. It left Ontario mostly intact. The difference is partly about hills, or the lack of them.

The Ottawa Valley is a rift valley, created when great cracks in the Earth’s crust opened, and the land between them slumped down.

The result is low, flat land in the cornfields and subdivisions of Eastern Ontario, but higher, steeper land in the Gatineau Hills.

They call it the MRC des Collines (of the Hills) for a reason.

Heavy rain drains off the flatter Ontario side at a steady pace. But water rushes down through the Quebec hills, doing more damage on its way down the 300-metre- high Eardley Escarpment and surrounding hills.

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 En- vironment 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 runoff.