Green Infrastructure, Low Impact Development (LID) Construction Costs

A costing tool was developed to assess the capital and operation and maintenance costs of infrastructure used to control CSO's by Capital Region Water, a municipal authority that improves, maintains, and operates the greater Harrisburg, Pennsylvania USA area’s water system and infrastructure.

Unit cost data for various green infrastructure, or Low Impact Development (LID) stormwater management practices, are provided in Capital Region Water's costing tool and it's reference Appendix B - Basis of Cost Opinions, Combined Sewer Overflow Control Alternatives, Costing Tool Reference Manual, Updated 2017 (https://pdf4pro.com/cdn/appendix-b-basis-of-cost-opinions-capital-6c71f.pdf).

The following tables illustrate 2008 unit costs per impervious acre. The first considers costs with limited cost efficiencies or savings over current costs.  The second table considers that cost reductions can be achieved under widespread implementation with economies of scale.

Green Infrastructure Costs in Retrofit and Redevelopment Settings - Bioretention, Subsurface Infiltration, Green Roof, Porous Pavement, and Street Trees - 2008 Dollars, Harrisburg, PA, USA

Reduced Green Infrastructure Costs in Retrofit and Redevelopment Settings  With Assumed Economies of Scale - Bioretention, Subsurface Infiltration, Green Roof, Porous Pavement, and Street Trees - 2008 Dollars, Harrisburg, PA, USA

The unit costs above, excluding green roofs and streets trees, result in a mean cost of $160,000 per impervious acre for retrofits and $110,000 for redevelopment.  This equates to mean costs of $395,000 and $272,000 per impervious hectare.

Adjusting costs to 2020, based on the Statistics Canada Infrastructure Construction Price Index, increases 2008 costs by 30%. The following table presents 2020 estimated unit costs per impervious hectare.

Summary Statistics of Direct Construction Cost Estimates in 2020* Dollars ($/impervious hectare)
Control Type		Minimum Cost ($ / impervious hectare)	Median Cost ($ / impervious hectare)	Mean Cost ($ / impervious hectare)	Max Cost ($ / impervious hectare)
Bioretention	Retrofit	$209,000	$386,000	$514,000	$1,317,000
	Redevelopment	$142,000	$289,000	$354,000	$642,000
Subsurface Infiltration	Retrofit	$209,000	$386,000	$514,000	$1,317,000
	Redevelopment	$142,000	$289,000	$354,000	$642,000
Green Roof	Retrofit	$1,382,000	$1,607,000	$1,607,000	$1,830,000
	Redevelopment	$642,000	$803,000	$803,000	$932,000
Porous Pavement	Retrofit	$209,000	$386,000	$514,000	$1,317,000
	Redevelopment	$142,000	$289,000	$354,000	$642,000
Street Trees	Retrofit	$57,000	$57,000	$57,000	$57,000
	Redevelopment	$48,000	$48,000	$48,000	$48,000
Average Excl. Green Roof and Street Trees	Retrofit	$209,000	$386,000	$514,000	$1,317,000
	Redevelopment	$142,000	$289,000	$354,000	$642,000

* 2008 to 2020 adjustment estimated at +30% considering Infrastructure construction price index (+26.9% for 2010-2019)


The average retrofit cost for bioretention, subsurface infiltration and porous pavement is $514,000 per impervious hectare for retrofits and $354,000 per impervious hectare for redevelopment.  Lower costs with expected cost efficiencies are estimated below, adjusting 2008 cost to 2020 (i.e., increase by 30%).

Summary Statistics of Direct Construction Cost Estimates with Improved Development Practices and Economies of Scale in 2020 Dollars ($/impervious hectare)
Control Type		Minimum Cost ($ / impervious hectare)	Median Cost ($ / impervious hectare)	Mean Cost ($ / impervious hectare)	Max Cost ($ / impervious hectare)
Bioretention	Retrofit	$166,000	$321,000	$417,000	$932,000
	Redevelopment	$112,000	$257,000	$257,000	$514,000
Subsurface Infiltration	Retrofit	$166,000	$321,000	$417,000	$932,000
	Redevelopment	$112,000	$257,000	$257,000	$514,000
Green Roof	Retrofit	$1,092,000	$1,284,000	$1,284,000	$1,478,000
	Redevelopment	$514,000	$642,000	$642,000	$738,000
Porous Pavement	Retrofit	$166,000	$321,000	$417,000	$932,000
	Redevelopment	$112,000	$257,000	$257,000	$514,000
Street Trees	Retrofit	$48,000	$48,000	$48,000	$48,000
	Redevelopment	$39,000	$39,000	$39,000	$39,000
Average Excl. Green Roof and Street Trees	Retrofit	$166,000	$321,000	$417,000	$932,000
	Redevelopment	$112,000	$257,000	$257,000	$514,000
The retrofit and redevelopment costs of $417,000 to $257,000 per impervious hectare would be equivalent to costs of $834,000 to $514,000 per hectare, assuming 50% impervious coverage.  These costs are of similar magnitude to average Ontario and Alberta LID project costs presented in an earlier post (see update at the bottom of the post https://www.cityfloodmap.com/2019/10/green-infrastructure-cost-ontario.html). Compiled LID project costs indicate an average area-weighted cost of $540,000 per hectare, including several recent project costs that have not yet been adjusted, i.e., increased, to today's 2020 dollars. While project costs are expected to vary from site to site, average unit costs may be used for planning purposes, when evaluating the cost to retrofit large areas, e.g., sewer catchments or tributary subwatersheds where stormwater management controls are being evaluated. *** Notes: while economies of scale have been assumed with widespread implementation, trends in unit costs in Philadelphia had not yet revealed decreasing unit costs as indicated in an earlier post (https://www.cityfloodmap.com/2018/07/green-infrastructure-capital-and.html) and as shown in the chart below: Green Infrastructure / Low Impact Development Capital Cost Trend - Philadelphia Clean Waters Pilot Program

Do Baseflow Impacts of Urbanization Warrant Green Infrastructure Retrofits to Restore Water Balance?

Green infrastructure, low impact development practices (LIDs), also called stormwater management best management practices (SWM BMPs), are often proposed to restore water balance functions and mitigate impacts or urbanization on runoff and recharge. One argument is that baseflows are lowered due to reduced infiltration and discharges to watercourses. It is a simple textbook theory.

What does the data show on baseflow impacts? The following slide presentation was prepared to respond to the Ontario draft LID guidance manual in early 2017 since water balance impacts have been cited as justification for green infrastructure LIDs.

Urbanization and Baseflow Impacts - Evidence-based Water Budget Management and Infiltration LID / BMP Policy Needs from Robert Muir

Local studies show that baseflows have increased over decades of urbanization, calling into question the need for such measures considering that potential impact has not materialized. As noted in TRCA's Approved Updated Assessment Report under the Clean Water Act, at most gauges there was an upward trend in baseflows which prompted this statement: "These overall increases to baseflow volumes are contrary to the common thought that increased impervious cover leads to reduced baseflow" - so for those keeping score, data - one, common thought - zero. (see page 3-40 at link to full report - disregard old link in the slide deck thx!).

TMIG also analyzed baseflows in the GTA and noted “The seven-day average consecutive low flow data provides an indication of the observed baseflows within a watercourse, and hence is a suitable measure for determining whether baseflow trends exist in an urbanizing area. The trend analysis identified noticeable baseflow trends in 13 of the 24 recording stations. Of these eight urban and two rural stations exhibited an upward trend, suggesting increasing baseflow.” (link to full report).

It would appear that baseflow stresses due to urbanization, i.e., development within the GTA, do not support the need for green infrastructure implementation to restore water balance functions.

Green Infrastructure Implementation Constraints in Flood Prone Partially-Separated Wastewater Systems

So what happens when green infrastructure infiltrates runoff into the ground in a densely-developed city? Does it disappear and sustain an aquifer and watercourse baseflows? That's the theoretical benefit. Or does it end up in foundations drains (weeping tiles) making its way to the local wastewater treatment plant on low rain days and contributing to sewer back-ups on the high rain days? its the latter. It is surprising how quickly foundation drains respond to surface water inputs - Toronto Water presented at the National Research Council expert panel this week that it takes only 4 minutes for surface water to end up in foundation drains!

Municipal wastewater engineers have know this for a while - that systems with no apparent direct inflows of rainwater or runoff respond quickly to rainfall. My work on a Municipal Class Environmental Assessment in Kitchener, Ontario showed the greatest correlation of wastewater peak wet weather extraneous flows was to the 5-minutes rainfall intensity. Often we expect groundwater collection systems in cities respond slowly to rainfall volumes - they don't. The respond rapidly to short duration rainfall.

So what does this have to do with green infrastructure and the suite of sweet low impact development measures many are going gaga over? Well, its the impact of GI and LID runoff infiltration on wastewater systems. Quick and clear impact. At the NRC expert panel this week I summarized a list of documents that expressed concern for GI and LID wastewater system impacts including aggravated basement flooding due to this infiltration. This is the list:

1) Water Environment Association of Ontario (July 17, 2017 memo) 

Identifies concerns of interference with wastewater systems (flood impacts), water distribution systems (chlorides/corrosion), human health impacts to drinking water distribution (compliance with Procedure F-6-1), excessive costs:

https://drive.google.com/open?id=1T3vXEJ_nBi8e30KpcawVTfFPKx7A6y_v

2) City of Ottawa (February 23, 2017 letter)

Indicates "While the intent appears to be not to "make things worse or better" (specifically with respect to current condition runoff volumes), there should be recognition of situations in older neighbourhoods (with partially separated sewers, for example) where increased infiltration should be avoided given the cumulative impacts over time that could raise groundwater levels leading to increased risk of basement flooding, increased I & I to sanitary sewers, etc.”:

https://drive.google.com/open?id=12IQjlaKvbCakqx7Brw9DJ97Oi5a0OHx-

3) City of Barrie (July 10, 2017 letter) 

Identifies financial impacts, capital cost increase of 200%, lifecycle cost increase of 550%, concerns for “damage to private properties and excessive sewer infiltration”, LIDs “highly susceptible to failure due to sand accumulation” :

 https://drive.google.com/open?id=1HDd24FpFmLsFTAA7kEdC5mk8N8Oau8Ox

4) City of Guelph (June 28, 2017 memo)

"Guelph downtown stone rubble masonry heritage buildings are prone to flooding with raised groundwater elevation; any additional infiltration measures using LIDs may aggravate basement flooding due to leaky masonry walls and severe impacts on the buildings structural stability; in addition, impacts on aged infrastructures such as, watermain corrosion, potable water quality interference (F-6-1) and enhanced sanitary infiltration can be anticipated.”

https://drive.google.com/open?id=1OAnqraDz9NuBD1ZCsY4dzeDXeyZzf66-

5) City of Markham (July 14, 2017 memo)

Identifies sanitary infiltration impacts, adjacent property impacts, excessive capital cost based on completed tenders, high soft cost, high lifecycle costs, chlorides/watermain corrosion:

https://drive.google.com/open?id=1RGwiyeaqihdmjI2owDY0R-Cw9CcGbEKu

6) Ministry of the Environment / Workshop on Stormwater Quality Best Management Practices (1992)

Identifies impacts of on-site infiltration source controls called Best Management Practices (BMPs):
 " - basement leakage problems related to infiltration near housing
   - surcharging of sanitary sewers by short circuiting of infiltrated water”
Therefore the impacts of green infrastructure have been long-known in Ontario. Unfortunately, green infrastructure is often cited as a panacea for water resources challenges in Ontario, often when only narrow view of is taken that ignores existing municipal infrastructure systems and practical constraints associated with the infiltration of large quantities of chloride and contaminant-laden stormwater runoff.

7) US Transportation Research Board / Evaluation of Best Management Practices for Highway Runoff Control, Issue 565 (2006)

Identified inflow and infiltration (I/I) risks with infiltration green infrastructure (BMPs) in urban areas:
“In urban areas, unrestricted infiltration may exacerbate infiltration and inflow (I/I) problems in both separate and combined systems”

https://books.google.ca/books?id=jKR-CF7PG6AC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

7) City of Seattle / Street Edge Alternatives Project (city web site)

Identified bioretention groundwater impacts to adjacent properties through engineering analysis, indicating that green infrastructure introduces property flood risks:

"Our original hope for retaining flows and allowing infiltration into the native soils throughout the length of the block was not possible because some homes had an existing groundwater intrusion problem. To limit the potential for stormwater to adversely impact the residences of concern, our geotechnical engineers identified some swales that needed an impermeable liner.” :

http://www.seattle.gov/UTIL/EnvironmentConservation/Projects/GreenStormwaterInfrastructure/CompletedGSIProjects/StreetEdgeAlternatives/DrainageImprovements/index.htm

We could go on and on. The InfraGuide on inflow and infiltration, the CSA guideline on IDF curves - these all note the issue with infiltration stresses on wastewater systems.

Green energy in Ontario gave us smart metres that did not improve the flow of money. Green infrastructure can give us smart swales that will not improve the flow of groundwater .. OK, in partially-separated sewersheds.

Urban groundwater flow has been called the darkest of the dark arts, making hydrology look like algebra - call wastewater systems groundwater infiltration analysis something fancy like the RTK method and it sounds scientific, but there is really just a mysterious system of 'urban karst' of hidden pathways and problems lurking under the surface. A black box. Those of us in the industry who have worked closely on wastewater smoke and dye tests know that adjacent laterals 'speak to each other' and the subsurface flow is erratic and unpredictable.

As Jeff Goldblum said in Jurassic Park, 'Nature Finds a Way', and the natural flow of infiltrated water finds a way too ... right into foundation drainage and overtaxed wastewater systems. As Adrienne Barbeau said in The Fog "Be afraid, be very afraid" ... as there is huge fog surrounding green infrastructure and low impact development measures - we should re afraid of what these things will do to our wastewater systems.

Environmental Impacts of Green Infrastructure Construction - CO2 Emissions for Soil Removal and Aggregate Extraction & Transportation, Ontario Impacts

Etobicoke Infiltration System - Green Infrastructure for
Stormwater Infiltration and Low Impact Development

Like any infrastructure that requires material resources and energy to construct, green infrastructure is no different than grey infrastructure. This post looks at a typical low-impact-development (LID) feature for stormwater runoff control and estimates the CO2 impacts of its initial construction, excluding traffic impacts during construction, and excluding impacts for ongoing operation and maintenance, or rebuilding at end of lifecycle. Emissions are then scaled up across Ontario in light of green infrastructure policies being considered province-wide. The GHG emissions are huge for building green infrastructure !

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.

The construction of the perforated pipe infiltration system requires transport of various materials to and from the construction for initial construction. Lets assume we are considering 1 cubic metre of infiltration storage volume in the system. Some activities and their resulting kilometre-tonnes of transportation are estimated on this blog-napkin, if you will, aka an educated Fermi Estimate:

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.

Given the Ontario-wide 852,000 hectares of urban area that would need green infrastructure retrofits under the draft MOECC policy, the Ontario-wide CO2 emitted to initially build green infrastructure would be 852,000 x 4074.3 = 3,471,303,600 kilograms of CO2 emitted. Say 3.5 million tonnes of CO2 added.

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.