CAMBRIDGE - The combination of frozen ground and a significant snowpack means there is a high risk of flooding in the Grand River watershed this spring, according to report prepared by the Grand River Conservation Authority.
The report deals specifically with the two largest dams in Wellington: Conestogo and Shand.
Whether flooding actually occurs will depend on what happens during the spring melt, to the report stated. It was presented Feb. 13 at 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 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 that water was lost in the snow melt and resulting minor flooding of January.
However, that left the ground frozen and saturated with water, and the snow recently fallen has returned the snowpack 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.
“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.” the report stated.
It also pointed out the possibility of 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 some ice jam flooding.
The 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 there could back water up the river, threatening Port Maitland and Dunnville and prevent 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 flooding in Port Maitland and Dunnville from wind-caused high Lake Erie levels. That potential remains, given similar conditions.
Presentations outlined the findings of Wellington County’s two flood emergency planning exercises held in 2007, and up to date information for attendees on the types of mapping and river information available on the GRCA website.
Information is available in the River Data section at www.grandriver.ca.
