Turning our channels back into streams: Do ecosystem services count as much as protecting built infrastructure when naturalization is proposed for conveyance channels?
Many sections of urban watercourses within the Toronto and Region Conservation Authority (TRCA) jurisdiction have been channelized with concrete lining or other hard bed and bank treatments. While this past practice was intended to improve the conveyance capacity of the channel, current knowledge on the mechanisms of fluvial geomorphology shows that this approach is detrimental to the overall health of the stream and can lead to extreme erosion problems at the site or downstream of the channelization. The end of the design life of many of these channels is approaching and with weather systems increasing in intensity within southern Ontario, linked to a changing climate, some of these channels are being stressed beyond their current structural stability. Municipalities need to do something to stabilize these channels and the TRCA responded by indicating there is an imperative to manage our streams for resilience and manage them adaptively using principles of natural channel design. The question was whether the added value of ecological services would be enough to support stream naturalization over concrete-lining when flooding was an issue and city infrastructure was at risk? Hydrologic modelling, field assessment and significant consultation with municipalities and stakeholders were undertaken to address these concerns.
TRCA has developed an evaluation tool that identifies where naturalization of channelized areas could occur without compromising the need for adequate flood control and protection for human safety, property and reduce risk to infrastucture. Through this process, benefits to the natural environment and increases in ecological services were quantified, to the extent possible, and used to prioritize naturalization opportunities. This tool was applied in Spring Creek, an urban subwatershed in the City of Brampton, Ontario. A total of 9 sites for extensive naturalization were identified using a weighted-sum matrix approach to evaluate 8 different categories of interest: terrestrial habitat connectivity, in-stream ecology, forest health, restoration opportunity planning, infrastructure at risk, flexibility of design, public use and coordination with municipal capital works. Each category was represented by 3 or more metrics that were ranked from 1 – 5 to represent either gains or risks. The results for Spring Creek have recently been endorsed by local and regional municipalities with the understanding that infrastructure had to be protected and private lands would not experience local water level changes. The ecosystem services were regarded as ancillary benefits but a strong second. Naturalization is set to begin in 2014.