|Etobicoke Infiltration System - Green Infrastructure for|
Stormwater Infiltration and Low Impact Development
For this example, consider a perforated pipe and gravel trenches for infiltration of stormwater runoff, like the Etobicoke Infiltration system pioneered in pre-amalgamation City of Toronto. This is a typical green infrastructure configuration - see Ryerson University's Planning and Design Manual at this link. We can assume a simple arrangement with the perforated pipe and gravel trench beyond the roadway, say in the boulevard.
|Napkin - engineer's friend ... especially when|
they run out of envelopes to scribble on the back of.
1) excavation, transport and disposal of native soil material - if the facility is in a retrofit setting the sodium adsorption ratio of the soil (due to road salt chloride accumulation over time) means reuse could be limited. Assume the disposal site may be a distance of 40 km from the construction site. For each cubic metre of infiltration storage, lets assume a 40% voids ratio in the gravel infiltration trench, meaning each cubic metre needs 1/0.4 - 1.2 cubic metres of soil disposed, and at a density of about 2 tonnes per cubic metre. So 40 x 1.2 x 2 = 96 km-tonnes of native soil haulage.
2) transport and placement of clear stone - assume aggregate comes from a quarry 40 km away from the site. For each cubic metre of infiltration storage, and our 40% voids ratio in the gravel, each cubic metre of storage needs 1/0.4 - 1.2 cubic metres of clear stone. At a density of 1.6 tonnes per cubic metre (3/4 inch clear stone), there is 40 x 1.2 x 1.6 = 76.8 km-tonnes of gravel haulage.
The OECD indicates pollution for truck transport in a report that indicates 140 grams of CO2 emitted per tonne-km. So adding 1 +2 above, a total of about 173.8 km-tonnes of green infrastructure construction to achieve 1 cubic metre of runoff storage results in 173.8 x 0.14 = 24.3 kg of CO2 emissions.
3) quarrying gravel - based on 4.32 kg CO2 emitted per tonne from this source, each cubic metre of infiltration storage with 40% voids requires 1/0.4 - 1.2 cubic metres of clear stone. At a density of 1.6 tonnes per cubic metre again, there is 4.32 x 1.2 x 1.6 = 8.29 kg CO2 per cubic metre of storage.
The MOECC is targeting 25 mm or more of green infrastructure storage and/or treatment which means per hectare of urban development with 50% rain/runoff there is 10,000 x 0.025 x 0.5 = 125 cubic metres of runoff and green infrastructure storage needed. So 125 x 24.3 = 3037.5 kg of CO2 emitted per hectare of urban area retrofitted with this low impact development feature. That is just haulage. To quarry the gravel adds 8.29 x 125 = 1036.8 kg per hectare. Total is 4074.3 kg per hectare of green infrastructure runoff treatment. Is that big?
|Ontario has 825,000 hectares of untreated urban area.|
So over 3 billion kilograms to build green infrastructure to manage runoff - you could argue there is some nominal climate mitigation offset-benefit if there is some type of vegetated pre-treatment filter before the perforated pipe / gravel infiltration. If so, that would be offset by the frequent inspection and minor maintenance visits by municipal crews, year after year.
Note in Canada the per capital emission was 20.1 tonnes CO2 equivalent in 2015 according to Environment and Climate Change Canada. So retrofitting green stormwater infrastructure in Ontario will emit CO2 equivalent to all the CO2 emitted by 172,000 Canadians over an entire year. That is just initial construction.
So that is just a neat Fermi Problem on green infrastructure to consider along the the other considerations on cost, cost-effectiveness, impacts to existing infrastructure, and need for scientifically-based local targets for green infrastructure. Currently, MOECC is considering blanket green infrastructure retrofit targets that do not consider any cost constraints or any no environmental impacts of construction like CO2 emissions. Earlier posts note that impacts to iron watermains and flood prone sanitary sewer systems was ignored. Let's hope a more holistic approach emerges that recognizes that we can't build millions of tonnes of infrastructure, green, grey or purple, and not have big impacts to emissions etc.
Green infrastructure for stormwater management, just like green energy for power supply has significant negative impacts. It should not be viewed as a panacea for urban stormwater and water resources management issues in Ontario. Check out this post on costs and other impacts.