Umbrella Stormwater Bulletin 53

23 March 2017
Published by Green Communities Canada

Infiltration in dense urban areas – benefit or risk?

Image source: Capital Regional District, BC, Canada. Image URL

For green infrastructure enthusiasts, infiltration is a good thing – rain water soaking back into the ground where it belongs instead of flowing over dirty city streets and into local waterbodies. But for wastewater engineers, infiltration has a more sinister meaning. Inflow and Infiltration (I&I) occurs when rainwater or groundwater enters the sanitary sewer system through leaky pipes or illegal connections (downspouts or weeping tiles connected to the sanitary sewer). During dry weather, it doesn’t usually pose a problem, but during heavy rain events wastewater treatment systems can reach their capacity and bypass some or all treatment processes – or else sewage backs up into people’s basements. 

Cities across Canada are taking measures to address this issue and prevent basement flooding and the release of untreated or partially treated sewage. See for example the programs of City of LondonHalton Region, and City of Halifax (among many others).

Most programs follow a similar design – identify sources of inflow and infiltration, repair those on City property and provide incentives, advice, or requirements for fixing those on private property.  

But there is a fundamental difference of opinion as to whether green infrastructure measures that rely on infiltration – like rain gardens, permeable pavement and bioswales – are helpful or exacerbate this problem. In particular, when property owners are directed to disconnect downspouts and weeping tiles from the sanitary sewer, should the water be redirected to permeable features or sent directly into storm sewers? 

Some cities in the U.S. use green infrastructure widely, and incorporate it into I&I reduction programs. In Milwaukee, funds are available to help property owners repair sewer laterals and disconnect foundation drains, sump pumps and downspouts from sanitary sewers. The Milwaukee Metropolitan Sewerage District also encourages water to be redirected to properly located green infrastructure instead of into storm sewers. The Blueprint Columbus program similarly addresses I&I by encouraging property owners to direct stormwater to green infrastructure on private property instead of into sanitary sewers. Many Canadian programs encourage redirecting downspouts to rain barrels or grassed areas away from building foundations.

Still, some stormwater and wastewater engineers are convinced sending stormwater back into the ground will make the problem worse. One concern is that by promoting infiltration, groundwater levels will rise and actually increase the risks for I&I. There also may be risks for other buried infrastructure, such as drinking water mains. However, the USEPA in a technical document addressing design challenges for green infrastructure in Pittsburgh, expresses no concerns about installing green infrastructure around buried utilities (except high pressure gas mains), as long as care is taken not to damage them during excavation. 

More research and on the ground monitoring of the interaction between infiltration-based green infrastructure and underground pipes is needed. Taking an overly cautious approach that avoids all infiltration in dense urban areas may have negative consequences for water quality and flooding as well.  


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Funds for adaptation in Budget 2017. The Federal budget, released on March 22, includes $2 billion over the next 11 years for a Disaster Mitigation and Adaptation Fund to support infrastructure (built and natural) to adapt to a changing climate. The budget continues to use the term "green infrastructure" in a nonstandard way to refer to wastewater treatment plants, renewable energy, and public transportation.  

Drastic cuts to Great Lakes funding. The Trump Administration has proposed cutting all funding to the Great Lakes Restoration Initiative, and total funding to Great Lakes programs by 97%. The Great Lakes and St Lawrence Cities Initiative and many other groups have come out strongly against the cuts.

Overcoming barriers to GI implementation. Smart Prosperity Institute and The Natural Step are organizing Municipal Roundtables across Canada during spring/summer 2017. The roundtables will promote discussion amongst municipal staff, decision-makers and stakeholders to overcome barriers to stormwater user fees and green infrastructure implementation. Fill in this survey if your municipality is interested in participating.

Valuing natural assets. The Ecological Accounting Protocol (EAP), developed by the Partnership for Water Sustainability in BC, aims to help communities measure the value of natural systems, in particular as a form of drainage infrastructure. Over-reliance on grey infrastructure has in part been caused by an inability to measure the services provided by existing green infrastructure, like trees, wetlands and streams. The City of Saskatoon is currently embarking on a project to measure the value of its natural assets.  

Lake Simcoe Climate Adaptation Strategy. Low impact development features in this newly released strategy, as a way to help the region make better use of groundwater and surface water resources in a changing climate. Several demonstration projects are already on the ground and the Lake Simcoe Region Conservation Authority supports municipalities in implementing green infrastructure in a number of ways.

Green infrastructure and social justice. partnership in Atlanta is investigating how cities can ensure that the benefits of green infrastructure are spread in an equitable manner. For example, when street greening raises neighbourhood property values, are low income residents benefitting or getting squeezed out? 

Drain before next rain. "smart" pond in Brooklyn NY is equipped with OptiNimbus technology that monitors the weather forecast every minute. An algorithm determines how much water should be released from the pond so that it won't overflow. This has resulted in less water being released into sewer pipes than when it was controlled manually. 


Financing urban adaptation to climate change. A new report highlights case studies from 11 European cities using innovative methods to fund climate adaptation.


Learn to Construct LID. 1-day Workshop. Newmarket, Ontario. March 28, 2017. By Sustainable Technologies Evaluation Program. Register here.

LID Construction, Inspection, and Maintenance. Workshop. London, Ontario. March 29, 2017. Hosted by Upper Thames River Conservation Authority. Register here.

Rainwater Harvesting. Accredited professional workshop. Victoria (Colwood), British Columbia. April 6-7, 2017. Hosted by Canadian Association for Rainwater Management (CANARM). Register here.

Grey to Green Conference: Quantifying Green Infrastructure Performance. Toronto, ON. May 8-10, 2017. Register before April 8 for early bird prices.

Great Lakes and St. Lawrence Green Infrastructure Conference. Detroit, MI. May 31 - June 2, 2017. Hosted by Canadian Association for Rainwater Management (CANARM). Register now for early bird tickets.

Want to see more events? Check out the RAIN Events Calendar for other upcoming green infrastructure workshops, training, webinars and conferences.

The Umbrella Stormwater Bulletin is a free, monthly newsletter on green stormwater infrastructure published by the RAIN Community Solutions program of Green Communities Canada. Our audience is made up of municipal stormwater professionals, policymakers, academics, engineers, conservation authorities, nonprofits, and interested community members. We encourage submissions from our readers. Please contact the editor to submit a news item, blog idea, or event. RAIN Community Solutions builds support for and participation in stormwater innovations that reduce runoff by managing rain where it falls.

Copyright © 2017 Green Communities Canada, All rights reserved.


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