Riffles and Riprap; Stabilizing Wilket Creek

Project type: Stream Restoration, Erosion Control
Location: Wilket Creek Park, Toronto, ON [open Google map]
Status: Ongoing, closed to public

The Toronto Botanical Gardens sounded like a peaceful place to spend an afternoon.  Surrounded by the sprawling expanse of suburbs and strip malls known as the Greater Toronto Area (GTA), nature's last bastion stands green and strong.  The Gardens were peaceful indeed, until I wandered south a ways along a creek.  I never expected to find such dramatic scenes of destruction and reconstruction.


First, I saw what looked like an earthquake epicenter: entire hillsides ripped away, fully-grown trees uprooted.  Then, just a few minutes further south, I reached a bridge closed off with tape and cones.  A machine was pumping the creek through tubes; bulldozers droned in the distance.  Downstream along the construction zone the streambanks had been fortified with rocks.


I had stumbled upon a stream restoration project in progress.  I was able to see both a stream segment that had been restored, and one that was still degraded, a lower priority area yet to be addressed.  But what had caused the degradation in the first place?  Clearly flash floods had eroded the streambanks.  And I had a hunch the GTA's urban sprawl had something to do with that.  Luckily there turned out to be plenty of related material to research later [see below].  But first I took some time to explore the site and to bask in the good intentions and abilities of humans to repair nature.  Here at Wilket Creek, what had taken a society decades of shortsighted expansion and greed to destroy, took only a few years of planning and a construction crew to restore.

History
Agriculture had been the area's primary land use.
    Aerial photo, 1954

    Satellite image, 2015

  • Only 0.5% of Canada's landmass is Class 1 agricultural land, much of it found in this region.  (Nearly half of this prime farmland is visible from the top of Toronto's CN Tower on a clear day.)[1]
Urban Sprawl, however, claimed much of this farmland during the development and growth since the mid 20th century.  (Compare 1954 and 2015 images).  In the Greater Toronto Area (GTA):
  • 150,000 acres of farmland were lost to urbanization between 1976 and 1996,
  • 25,000 to 30,000 new housing units are built annually.
  • The population doubled from one to two million between 1961 and 2003, and is projected to increase one million more by 2021.[2]
Rainwater runoff is an ecological result of such development.  Previously on farms or natural areas, rainwater is slowed and absorbed into the ground, transpired through vegetation, and evaporated back into the atmosphere.  But from the roads and rooftops of developed land, rainwater is deflected as 'sheet flow.'  According to a 1999 study, only 4% of rainfall is deflected on undeveloped grassland.  In suburban areas runoff increases to 15%. [3]  This is a substantial difference, meaning millions more gallons draining out of a catchment area during rain events.

This problem is exacerbated in the catchment area of Wilket Creek due to:
  • the lack of wetlands or ponds to store rainwater,
  • the lack of tributaries (i.e. water fed slowly from afar).  Wilket Creek is fed entirely by rainwater runoff, plus drainage from human activities.
Erosion of the streambanks is the major destructive result, with further negative impacts including:
  • sedimentation of water (reduced water quality and deposition downstream),
  • loss of habitat,
  • expansion of channel (and loss of property), and,
  • damage to infrastructure.
This final impact was of most concern to authorities.  After a major storm in 2005, a survey by the City of Toronto identified “26 areas of concern,” such as bridges, pathways, roads and sewage pipes, which were damaged or under threat of damage due to erosion.  Efforts to repair specific infrastructure in 2007 proved futile, when the following year a storm destroyed them once again. A more comprehensive solution was needed, one that accounted for nature's water cycles. Enter the Wilket Creek Geomorphic Systems and Habitat Master Plan.[4]

Restoration

Objectives of this Plan, by Toronto and Region Conservation Authority:
  • Protect the infrastructure “areas of concern.”
  • Improve riparian and wildlife habitat
  • Implement a restoration plan (a) in stages, with more immediate threats prioritized and b) with minimal negative impacts.

Methods
  • Riffles – shallow, rocky-bottomed segments of river. Water passes over riffles with higher turbulence yet lower velocity, allowing the formation of pools and bends downstream.

Channel realignment – moving of watercourse away from eroded hillsides, at-risk infrastructure and private property. Widening and deepening the channel also helps reduce water velocity and erosion. 
  


Pumped through pipes, the creek bypasses construction.

Socks filled with mulch are an easy erosion checker.


Check out these other stream restoration projects:


References
[1] David Suzuki, "Urban Sprawl is Destroying Ontario's Farmland," Toronto Star, Editorial. Feb. 21, 2013. [link]
[2] David Gurin, "Understanding Sprawl: A Citizen's Guide," David Suzuki Foundation. 2003. [link]
[3] Riina Bray, et. al., "Report on Public Health and Urban Sprawl in Ontario," Ontario College of Family Physicians. Jan, 2005. [link]
[4] "Wilket Creek Geomorphic and Habitat Systems Master Plan," Toronto and Region Conservation Authority. Dec. 18, 2013. [link]

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