Realignment and widening of Patterson Creek a sewer saver
  
DailyCommercialNews.com
Nov. 24, 2017
Dan O'Reilly

Judged solely by the $3 million price, a project to protect the Patterson Creek in Richmond Hill from an exposed sanitary sewer would not seem overly complex.

But the just-completed three-year Patterson Creek Sanitary Sewer Protection Project came with more than a few engineering and construction challenges for the Toronto Region Conservation Authority.

In 2011 the authority discovered the almost 45-year-old, 675-mm diameter York Region sanitary sewer, which extends under and adjacent the river, was very close to being exposed in five different sites along an approximately one-kilometre stretch from erosion.

"The sewer was originally built in 1973 and the surrounding area has undergone significant change over the last 40 years with intense development and urbanization," says the authority erosion risk manager, Matt Johnston.

It was never designed to be exposed to the river and that exposure could have led to damage or even a rupture with the result of sewage pouring into the creek which is a tributary of the East Don River, he points out.

Although the risks were clearly evident, the remedy was not quite straight-forward and four years of planning and design was invested into the project before construction could even start, says Johnston.

Not only do the five sites span multiple boundaries including parkland, rights-of-way and private owned land, most are in an ecologically sensitive ravine which hadn't been accessed by machinery since the sewer was installed.

While York Region has a maintenance easement over the sewer, simply following it would have led to an even environmental impact because a large number of high trees would have been cut and temporary bridges installed for heavy equipment to access all five sites, he says.

As well, the area is a protected Redside Dace habitat which required that a number of different design elements and features had to be incorporated to balance preservation of environment against protecting the sewer.

One of the measures to achieve that delicate balance was the construction of a temporary access road — a process which required negotiations with the private landowners to allow it to be built on their land, he says. "They could see the importance of the project."

Construction got underway in 2015 at a site deemed the "highest risk" because of the way the sewer crossed the river at right angles. It was actually in the river, as opposed to being underneath it, and could clearly be seen.

"Due to the location of the sewer and the creek's alignment a simple grade control or bed protection wouldn't have been sufficient to ensure the pipe's long-term stability."

Instead, project consultant Parish Geomorphic (now part of Matrix Solutions) recommended the creek be realigned and widened, says Johnston.

This was accomplished by building a coffer dam and pumping the river around the work site. Moving a watercourse, though, is complex from a construction sequencing perspective because the work area has to be isolated to prevent downstream sedimentation, he says.

Adding to the complexity was the fact the Don River, of which Patterson Creek is part of, is quite 'flashy' with rapid increases in volume and velocity during heavy rainfalls.

"We had many downpours during that construction season and this led to breaching of the coffer dams because the pumps couldn't keep up."

Nevertheless, this phase was completed in late 2015 and the realigned section was not impacted by this year's heavy rain.

"We're quite pleased with now it (the realignment) worked out."

In 2016 the exposure problem at a second site was rectified by widening the river and installing a riffle which is a collection of different sized rocks which prevents downward erosion while still allowing fish habitat, he says.

Earlier this year an old gabion basket was removed at a third location where the erosion was threatening a manhole. It was replaced with vegetated revetment which consists of stone and vegetation and is a more natural form of bank stabilization and easier to maintain. Gabion baskets are no long used by the conservation authority for stream bank protection, says Johnston.

Restoration of the two remaining sites wasn't as extensive and consisted of installing riffles and stream bank protection. But it did take the project into November which required obtaining an extension of a permit from the Ministry of Natural Resources as the deadline for in-water work is Sept. 15, he says.

Although the conservation authority used its own supervisory and construction crews, the earthmoving work was performed by TMI Contracting & Equipment Rental Ltd.

An ongoing program of monitoring sewers near watercourses is conducted by the authority in partnerships with regions in its watershed, says Johnston.