Risk of flooding higher this year due to significant snowpack

[Spiel]

Risk of flooding higher this year due to significant snowpack

 

February 13, 2008

grandriver.ca

 

 

The combination of frozen ground and a significant snowpack means there is a high risk of flooding in the Grand River watershed this spring, says a report prepared by the Grand River Conservation Authority.

 

Whether flooding actually occurs will depend on what happens during the spring melt, according to the report presented Wednesday to the annual meeting of municipal flood co-ordinators, attending from communities across the watershed. The winter started with dry conditions through November, but precipitation falling both as rain and snow has been at or above the long term average in the months since.

 

Water stored in the snowpack has a potential to contribute to spring floods, and a lot of this water was lost in the snow melt and resulting minor flooding event of January.

 

However, this has left the ground frozen and saturated with water, and snow recently received has returned the snowpack amount to near normal levels. The major GRCA reservoirs have between 52% and 84% of their storage capacity available for flood control. The Shand and Conestogo dams particularly have storage enough for the water in the snowpack, but not enough to accommodate heavy rains in conjunction with melting snow.

 

As a result, in the coming weeks the reservoir levels will be monitored along with snowpack conditions, and adjusted as necessary in preparation for the spring snow melt.

 

Whether a major flood occurs is dependent on how the spring thaw unfolds.

 

“Fifty millimeters (2 inches) of rain, in combination with a quick spring thaw or heavy rain on frozen or saturated ground would set the stage needed to cause a major flood,” said the report.

 

The report also pointed out the potential for flooding from ice jams this year, stating, “the potential for ice jam-related flooding this spring is high.”

 

Rivers are starting to re-freeze with the recent low temperatures, and a rapid snow melt could combine with river ice to cause localized ice jam flooding. These conditions are being monitored at known ice jam locations throughout the watershed such as West Montrose, Cayuga and below Brantford.

 

There is also the potential for an ice jam at the mouth of the Grand River, at Port Maitland on Lake Erie. A jam here could back water up the river, threatening Port Maitland and Dunnville and preventing ice from moving out of the Grand into the lake. In past years, a Coast Guard ice breaker has been brought in to break up a jam there.

 

Earlier in January, there was localized flooding in Port Maitland and Dunnville from wind-caused high Lake Erie levels. This potential remains, given similar conditions.

 

In other business at the meeting, GRCA staff reviewed the operation of the flood forecasting and warning system. They also went over the responsibilities of municipal flood co-ordinators whey they receive a flood warning message. Other presentations outlined the findings of Wellington County’s two flood emergency planning exercises held in 2007, and updated information for attendees on the types of mapping and river information available on the GRCA website.

 

For both the public and local officials, real time river and reservoir information is available in the “River Data” section of the GRCA website at www.grandriver.ca.