Monday, 04 November 2013 10:49

Flooding event was intense, says conference speaker

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Attendees at the mid-October Below Your Watershed conference in Medicine Hat hosted by the South East Alberta Watershed Alliance (SEAWA) heard more information about the extreme flooding event that took place in various places in southern Alberta at the end of June.

One of the sessions featured Brian Hills, science and technology support team lead for Alberta Environment and Sustainable Resource Development, speaking about the circumstances and consequences of the June 2013 flood.
“It was the weather system that caused this extreme event,” said Hills.
A couple of lows from the south and the Gulf of Mexico collided with an upper cold low from the Pacific coast which led to the weather system sitting over southern Alberta for about three days.
Adding to the situation was the fact the rainfall caused the existing snowpack to melt more quickly.
That rainfall was tremendous. Just in the area west of High River and south to the U.S. border, between 250 and 300 mm of rain fell in three days, or about 10 to 12 inches.
“It was not just the amounts that were unusual, the intensity also was unusual for this storm,” added Hills. “The intensity of the storm increased and lasted for a long time.”
The result was in many areas river flows that exceeded a one in 100 year event. He pointed out when the phrase “one in 100 year flood” is used, that does not mean that type of flood is see only once in 100 years. It means there is a one per cent chance of that type of flooding event happening in any given year.
“We are still working to try to understand the peak flows at various locations,” said Hills. “It will probably take a good year to work this out.”
A number of gauges used to measure river flows were washed out in the flooding. Of the 27 gauges in the flood areas, 13 were lost completely.
He said preliminary modelling of the river flows shows flow rates exceeded the levels for a one in 100 flood year event in many locations. In some locations flood hazard mapping done in the 1990s suggested a one in 100 flood event would be equivalent to around a 1,000 cubic metres per second flow rate. Some rivers were flowing as high as 1,800 and 2,000 cubic metres per second while others were close to the 1 in 100 year mark.
“A large part of southern Alberta was in a state of emergency ... that lasted into July,” said Hills. “Even in some communities it persists to this day because of the effects of this system.”
The flooding caused disruptions and had lasting impacts for weeks and months because of the damage to infrastructure including wastewater systems, water treatment systems and water wells.
The Alberta government undertook post-flood monitoring of ambient surface water quality, landowners’ well water quality, ambient air quality, and soil and deposited sediment quality on areas such as school grounds.
For the month of July, ERSD officials did enhanced monitoring (weekly testing) of ambient surface water quality at 22 sites in southern Alberta. Normally this monitoring is just done monthly.
In that time, there was only one guideline exceedance related to protection of aquatic life when in early July the lower Highwood River tested too high for levels of ammonia. Those levels normalized through the month.
“There were high initial concentrations of e-coli, total suspended solids and associated metals,” said Hills, about the overall testing. “We didn’t see any exceedance for hydrocarbons or pesticides.”
Throughout July, the concentration amounts decreased to a level that is more normal.
“We are continuing at most of our sites the conventional monitoring program,” added Hills.
“It is going to take a long time for communities to fully recover from what was experienced.”
It is hoped the information being gathered and analyzed will help all Albertans moving forward.
“For the most part, our flood hazard mapping is centred around urban areas,” pointed out Hills. “I think (government is) looking at how to take the knowledge and information and start to build that into mapping products that can be used in rural areas.”

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Rose Sanchez

Assistant Managing Editor

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