WhiteCAP_Plan_01SEPT..

NCE WhiteCAP Environmental History
Localized flooding can happen any time through the summer due to heavy precipitation
events, such as thunderstorms. According to climatologists, thunderstorms are most
likely to occur in the Whitehorse area during June and July. Thunderstorms are caused
by the sun heating the earth’s surface and causing a large amount of warm air to rise.
When the warm air is cooled by mixing with unstable atmospheric conditions at very high
altitudes, a relatively sudden burst of precipitation is produced. This burst of precipitation
can exceed the natural or engineered drainage capacity of a region, resulting in localized
flooding. Thunderstorm activity can continue until the end of July and into August. After
August, limited daylight decreases the surface area heated by the sun reducing the
likelihood of a storm occurring.
Development along the Yukon River floodplain and through the river corridor has
exacerbated the threat of floods. Since the installation of the new hydraulic turbines at
the power dams, workers have observed flooding along the riverfront as the river is less
frozen in winter 10 .
3.2.2 Impacts of Flooding
Flooding directly impacts residents; damaging infrastructure and property and potentially
threatening the lives of residents. Damage from flooding is evident throughout the history
of Whitehorse. In December 1962, about two feet of water and slush covered the streets
and evacuation was necessary. High precipitation over a short period broke the record
precipitation for June in 1985, resulting in localized flooding. A mudslide along Robert
Service Way was created when 50.5 mm of rain fell on the area, blocking half of the
road. In February 1968 there was an important flood on Sixth and Seventh avenues,
particularly in the low-lying west end of the city. Flooding was caused by unseasonably
warm weather which melted the snow. The resulting melt-water exceeded the coping
capacity of the local sewage system. To complicate matters, the city crew did not want to
increase the pressure in the sewers in case the water backed up into other basements.
Another early spring thaw occurred on March 4th, 1968. The problems started when
basements on Jarvis, fifth, Sixth Avenue and beyond were flooded by melting surface
water. Heavy flooding along Second Avenue in Whitehorse submerged a vehicle in
about a meter of water in 1985. That same year heavy precipitation in the summer
negatively affected the tourism season and the local economy.
The Marwell subdivision in Whitehorse has historically experienced greater issues with
flooding than anywhere else. Floods levels in Marwell have measured up to about one
metre of water. A flood in January 1982 reached more than 100m into Marwell,
threatening houses along Silver Road. Home owners prepared for the flood by moving
furniture and vehicles and building snow dams to divert flood waters. Residents blamed
the new sewer construction and historic dredging of the Yukon River. Flooding occurred
again in December 1992 when water forced its way into some local basements. Water
levels reached up to 125 mm and at least one family was forced to evacuate their home.
No long-term emergency shelter could be provided for the family due to the lack of
available housing. Flooding occurred again in Marwell in 2000 when debris torrents and
mudflows occurred downstream of the subdivision. This event caused the highest level
10 Phone conversation with City of Whitehorse’s employees, October 2009.
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