Proactive disclosure Print version ![Print version Print version](/web/20061103041245im_/http://geoscape.nrcan.gc.ca/esst_images/_printversion2.gif) ![ÿ](/web/20061103041245im_/http://geoscape.nrcan.gc.ca/esst_images/_spacer.gif) | ![ÿ](/web/20061103041245im_/http://geoscape.nrcan.gc.ca/esst_images/_spacer.gif) | ![Geoscape Canada Geoscape Canada](/web/20061103041245im_/http://geoscape.nrcan.gc.ca/esst_images/geoscape_e.jpeg) Natural Resources Canada > Earth Sciences Sector > Priorities > Geoscape Canada > Ottawa-Gatineau
Geoscape Ottawa-Gatineau Flooding
Why floods happen
Flooding is a natural process. It happens when a river overtops
its banks and inundates nearby low-lying areas. In communities built
on these floodplains, properties, roads, bridges, and railways can be
damaged or destroyed by high water levels and sometimes by moving
ice. Floods can be caused by rapid melting of large amounts of snow,
or rain on snow, or prolonged or torrential rainfall. Ice jams and
landslide debris can also temporarily block a river, causing upstream
flooding.
The 100-year flood - Not every 100 years!
The '100-year flood' refers to the flood level that has a 1% chance of
occurring in any given year, and thus may actually occur more often than
once in a century or, alternatively, not happen at all. The magnitude of
the 100-year flood is commonly used to define flood zones and to design
flood-protection structures such as dikes.
Defending against floods
Flood damages can be reduced substantially by avoiding development on
flood-prone lands. Parks (such as Brewer Park along the Rideau River
and Leamy Park in Gatineau) playing fields, and natural areas are
appropriate land use for these flood-prone areas. Dikes, elevated
embankments that provide a physical barrier between low-lying land and the
river, can protect developments on the floodplain. A dike along the
Gatineau River protects part of Gatineau. Although the
reservoirs are too small to control flood discharge, the many power dams
along the Ottawa River can alert downstream communities of potential flood
conditions.
![Did you know? Did you know?](/web/20061103041245im_/http://geoscape.nrcan.gc.ca/ottawa/images/questionmark.jpg) | Did you know?... To prevent ice-jam floods within
Ottawa, the ice on the lower Rideau River is broken up each year
during late winter by sawing and blasting, or by an amphibious backhoe. |
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