Rainfall makes more sewage overflow into river

Infrastructure work by Epcor aims to mitigate flooding and reduce overflows

Greater amounts of sewage have been flowing into the North Saskatchewan River thanks to heavy rainfall this summer, but overall, efforts to reduce combined sewage going into the river has been paying off, Epcor said on Friday.

The city has spent more than $200 million on water treatment upgrades, a new tunnel, and three underground gates to reduce the amount of raw sewage it releases into the river, especially when combined sewer overflow occurs.

A combined sewer overflow (CSO) is when sewage and surface run-off flows overwhelm the system. In Edmonton, the problem happens in older areas where only one sewer line was built for both stormwater and sewage. Rainwater comes in, increasing the water level in the main trunks and then diluted sewage goes into overflow pipes that empty into the river.

The large amount of rain the city has experienced up to the end of July this year has contributed to higher combined sewer overflows compared to 2018.

The Cromdale sewer outfall in the northeast has had four days of overflow totalling 3,664 cubic-metres compared to one day of overflow in all of 2018 totalling 895 cubic-metres.

At the Highlands and Beverly 1 sewer outfall, there have been 12 days of overflow totalling 43,764 cubic-metres compared to nine days last year totalling 17,123 cubic-metres.

However, there will always be year-to-year changes in combined sewer overflows as they are mainly caused by the number and intensity of summer storms, said Cindy Shepel, director of drainage operations with Epcor.

She said it’s best to compare years of similar amounts of rainfall as opposed to looking at five or 10 years together.

In terms of rainfall, 2018 is comparable to 2017. And for 2019, so far what the city has experienced is comparable to 2013 or 2014 — years that had large summer storms.

“In 2018 we had 10 overflow days with a volume of 226,000 cubic-metres (at Rat Creek) versus in 2017 there was 31 overflow days with a volume of 437,000 cubic-metres,” Shepel said.

“We’ve been able to cut the overflow days by two-thirds and the volume by almost half and that was directly related to the tunnel that was completed at Rat Creek.”

Epcor is continuing to work on other infrastructure that will help reduce overflows and mitigate flooding.

“We are working on completing another tunnel for storage and we’re also working with the Stormwater Integrated Resource Plan and looking at low-impact development, dry ponds, and other means to store the stormwater at the surface and how that not only helps reduce the CSOS, but limits the risk of flooding,” Shepel said.

There is one sanitary storage tunnel in the north end of the city that started construction in 2016. It won’t be completed until 2020, but if it had been up and running this year, Shepel said it would have prevented some of the sewage backup in Griesbach last month.

After about 78 millimetres of rain fell between July 17 and July 19, a sewer trunk line’s capacity was exceeded, causing flooding in basements.

“Griesbach itself is a separated system but the combined system upstream of that was full and caused a backup into the system in Griesbach,” Shepel said.

Epcor is also exploring low impact development, which Shepel said is essentially green infrastructure that slows down and stores the movement of water.