Mythbusters – Green vs. Grey

by | Jul 12, 2016 | Sustainability

There is a general belief that green infrastructure is more expensive than grey. We ran some quick scenarios to illustrate the cost difference between green and grey stormwater management options.

Example of the cost of grey vs. three green stormwater designs to manage runoff from a road in Toronto

For the cases of green infrastructure that we have experimented with the capital costs difference is not there; green is initially cheaper than grey. This is is contrast to the widely held belief that upfront costs of green stormwater infrastructure are more than for grey infrastructure.

Adding O&M tips the balance back to grey as the cheapest (on a lifecycle basis) since green does require more ongoing maintenance.

It should be noted that for this analysis we ignored all of the benefits of green infrastructure that AutoCASE calculates such as: flood risk reduction, urban heat island mitigation, increased property values, more recreational value, better air and water quality and reduced carbon emissions. However, that said, on with the experiment details …

To compare the capital expenditure (capex) costs of green and grey we ran a base case in AutoCASE of an asphalt road (in Toronto, 20 feet wide by 200 feet long) which costs $19,960 (according to AutoCASE’s automatic cost estimation algorithm) and creates an additional capex on piping and detention cost of $2,907 to manage the stormwater runoff from a 2 inch design storm.

Some potential green investments that offset the grey infrastructure additional capex are: 15 trees, or 2,500 square feet of grass, or 1,200 square feet of vegetated swale. Those three scenarios approximately offset the additional grey stormwater infrastructure capex reqiored to manage the runoff from the impervious road surface.

The cost comparison for each scenario follows(1):

  • 15 tress cost $996 which is lower than the grey cost by $1,911. (There is $275 annual O&M which means after 4 years the green becomes more expensive on a non-discounted basis)
  • 2,500 square feet of grass costs $606 which is lower than the grey cost by $2,301. ($140 annual green O&M means after 12 years grey becomes less expensive)
  • 1,200 square feet of vegetated swale costs $520 which is lower than the grey cost by $2,387. ($242 O&M means grey becomes cheaper after 7 years)

In these three cases, the capex is actually less for green but over time (3-12 years) grey becomes cheaper when the higher green O&M is factored in.

200 feet of Phoebe St. in Toronto with 15 (virtual) street trees planted in InfraWorks 360

All of which can be summarized as:

Q: Is green infrastructure more expensive than grey?

A: It depends!

(1) (base case cost – green alternative cost) which is equivalent to [(road cost + additional capex) – (road cost + green feature cost)] or (additional capex – green feature cost)

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