Sunday, December 23, 2007

Erosion Control Class EA

Project Site Location. This is very close to the "Rainbow Tunnel" which is visible from the northbound DVP.

This fall I participated in a Class Environmental Assessment Project on the East Don just south of Lawrence Ave. East in the Charles Sauriol Conservation Area. The purpose of the EA was to analyze options to control erosion along a section of the river that is adjacent to a CN railway embankment. The erosion problem manifested itself during the August 19, 2005 storm that deluged parts of the Don Valley. This was the same storm that blew out Finch Ave. West where it crosses Black Creek. That repair cost about $30 million.

In the case of the this site the erosion wasn't as critical but the TRCA realized it was an impending problem, thus this project was initiated. Part of the Class EA process is to obtain community input into the project. I attended three Community Liaison Committee meetings where we looked at different options.

There were two sites of interest. Site A is about 120 m long (later reduced to 90 m) and about 5-11 m from the CN railway embankment. Site B is about 75 m long but is much closer to the embankment. We considered three different bank treatment options, soft stabilization, hard stabilization, and channel realignment. Moving the CN railway line was not considered as it was too expensive. Channel realignment was also discarded as being too expensive/intrusive. Hard stabilization methods such as lining the bank with crib walls or stone were looked at but discarded because they would actually increase flow rates, effectively transferring the problem downstream. They would also be very poor for aquatic habitat.

Riverbank at Site A. Note the undercutting of vegetation on the left. The brush layering technique will cut away the top of bank at a 2:1 angle and infill with soil and plant cuttings.

Site A looking downstream. The moss covered bank in the middle of the picture was removed from Site A because it is a hard laminate clay which won't erode as fast as the sandy soil where I was standing for this picture.

That left soft stabilization methods. Everyone agreed that this was the way to go and was relatively low risk (all except the rep from CN who basically wanted a zero risk cement wall built). The two options chosen were live brush layering for Site A and vegetated riprap for Site B. Live brush layering consists of soil anchored with a biodegradable mat and plant cuttings imbedded in the mix at 10 cm intervals. The plants, Pussy Willows and Red-osier Dogwood, will grow quickly and their roots should provide an anchor for the soil. Vegetated riprap is similar but contain layers of stone intermixed with soil and vegetation which is more appropriate given the steeper nature of the bank.

My main concerns were construction road access and cleanup once the project is complete. Ten years ago, a similar project failed to complete a cleanup leaving the access road and staging area in place even though it was promised. This discussion is now included in the report. I also received a commitment for post construction monitoring of the site as well as watering of the plant cuttings in case of another dry summer.

Construction is set to proceed in late February, 2008 and be completed by the end of March. I hope to get to the site once or twice to monitor progress. There is still a 30 day viewing period for the report ending January 21, 2008. Copies of the report can be viewed at the Urban Affairs Library at Metro Hall or the library at 888 Don Mills Rd.


Anonymous said...

I did similar work along the Forks of the Credit Road up in Caledon. Do you know who has this contract?

Donwatcher said...

To find out more information you'll need to contact Moranne McDonnell at the TRCA. She is the project leader.

Anonymous said...

Is this what all that construction equipment at the rainbow bridge/rail line is? I had wondered what they're up to. I was aware that the City plans to put a bridge across the Don in that area to connect Milne Hollow to trails on the other side of the river, and so assumed the constuction to be that.