Difference between revisions of "Bioretention: Life Cycle Costs"

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==Overview==
==Overview==
Given that [[Bioretention]] is a widely used name and a general "catch-all" term for most vegetated LID practices that temporarily store stormwater runoff in depressed planting beds there are a number of configurations of the feature that can be used in various contexts and environments. Depending on native soil infiltration rate and physical constraints, the facility may be designed without an [[underdrain]] for [[Bioretention: Full infiltration|full infiltration]], with an underdrain for [[Bioretention: Partial infiltration|partial infiltration]], or with an impermeable [[liner]] and underdrain for [[Stormwater planters|no infiltration/filtration only]] (i.e., a [[stormwater planters]] or biofilter) design. STEP has prepared life cycle costs estimates for each design configuration, based on a runoff control target of 25 mm depth and 72 hour drainage period, for comparison which can be viewed below. To generate your own life cycle cost estimates customized to the development context, design criteria, and constraints applicable to your site, access the updated [https://sustainabletechnologies.ca/lid-lcct/ LCCT Tool here].
Given that [[Bioretention]] is a widely used name and a general "catch-all" term for most vegetated LID practices that temporarily store stormwater runoff in depressed planting beds there are a number of configurations of the feature that can be used in various contexts and environments. Depending on native soil infiltration rate and physical constraints, the facility may be designed without an [[underdrain]] for [[Bioretention: Full infiltration|full infiltration]], with an underdrain for [[Bioretention: Partial infiltration|partial infiltration]], or with an impermeable [[liner]] and underdrain for [[Stormwater planters|no infiltration/filtration only]] (i.e., a [[stormwater planters]] or biofilter) design. STEP has prepared life cycle costs estimates for each design configuration, based on a 2,000 m<sup>2</sup> asphalt drainage area, runoff control target of 25 mm depth and 72 hour drainage period, for comparison which can be viewed below. To generate your own life cycle cost estimates customized to the development context, design criteria, and constraints applicable to your site, access the updated [https://sustainabletechnologies.ca/lid-lcct/ LID Life Cycle Costing Tool (LCCT) here].


==Design Assumptions==
==Design Assumptions==
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Additional components include an [[underdrain]] to remove excess water and soil additives to enhance [[Nutrients|pollutant removal]]. <br>
Additional components include an [[underdrain]] to remove excess water and soil additives to enhance [[Nutrients|pollutant removal]]. <br>


Design assumptions are based on tool default values and STEP recommendations:
Design and operation and maintenance program assumptions used to generate cost estimates are based on tool default values and the following STEP recommendations:
* Maximum impervious drainage area to permeable surface area (I:P area) ratio of 20:1.
* Native soil infiltration rates for Full, Partial and No Infiltration Design scenarios were assumed to be 20 mm/h, 10 mm/h and 2 mm/h, respectively, and a safety factor of 2.5 was applied to calculate the design infiltration rate.
* Operation and maintenance (O&M) cost estimates assume annual inspections, removal of trash and debris twice a year, removal of sediment from pretreatment structures annually, and removal of weeds twice a year (where applicable). Verification inspections are included every 5 years to confirm adequate maintenance, and every 15 years to confirm adequate drainage performance through in-situ surface infiltration rate testing (where applicable)
*Maximum impervious drainage area to permeable surface area (I:P area) ratio of 20:1.
* Length to width ratio of bioretention area is 10:1.
* Length to width ratio of bioretention area is 10:1.
* Default filter media depth of 1.0 metres to accommodate trees in planting plan.
* Default filter media depth of 1.0 metres to accommodate trees in planting plan.
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* Operation and maintenance cost estimates assume rehabilitation of the filter media bed surface is required after 25 years of operation.
* Operation and maintenance cost estimates assume rehabilitation of the filter media bed surface is required after 25 years of operation.
* The tool calculates costs for new (greenfield) development contexts and includes costs for contractor overhead and profit, material, delivery, labour, equipment (rental, operating and operator costs), hauling and disposal.  
* The tool calculates costs for new (greenfield) development contexts and includes costs for contractor overhead and profit, material, delivery, labour, equipment (rental, operating and operator costs), hauling and disposal.  
** Mobilization and demobilization costs are not included, assuming BMP construction is part of overall development site construction with equipment needed already on-site.
** Land value and equipment mobilization and demobilization costs are not included, assuming BMP construction is part of overall development site construction.
** Design and Engineering cost estimates are not calculated by the tool and must be supplied by the user.
** Design and Engineering cost estimates are not calculated by the tool and must be supplied by the user.
** The tool adds 10% contingency and additional overhead as default.
** The tool adds 10% contingency and additional overhead as default.
* All cost estimates are in Canadian dollars and represent the net present value (NPV) as the tool takes into account average annual interest and discount rates over the 25 and 50 year operating life cycle periods.
* All cost estimates are in Canadian dollars and represent the net present value (NPV) as the tool takes into account average annual interest and discount rates over the 25 and 50 year operating life cycle periods.
* Unit costs are based on 2018 RSMeans standard union pricing.
* Unit costs are based on 2018 RSMeans standard union pricing.
* Additional costs associated with retrofit or redevelopment contents is assumed to be 16% higher than the cost for new (greenfield) development contexts.
* Additional costs associated with retrofit or redevelopment contexts is assumed to be 16% of the cost estimate for new (greenfield) construction contexts.
** Retrofit construction cost estimates are included in the 'Costs Summary' section for comparison.<br>
** Retrofit construction cost estimates are included in the 'Costs Summary' section for comparison.<br>


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Above you can find construction cost breakdowns for 250 m<sup>2</sup> surface area facilities (I:P area ratio of 8:1) for each design configuration:<br>
Above you can find breakdowns of construction costs by expense type for 250 m<sup>2</sup> surface area facilities (I:P area ratio of 8:1) for each design configuration:<br>
#[[Bioretention: Full infiltration]]  
#[[Bioretention: Full infiltration]]  
#[[Bioretention: Partial infiltration]]
#[[Bioretention: Partial infiltration]]
#[[Stormwater planters| No infiltration]],
#[[Stormwater planters| Bioretention: No infiltration]]  


As can be seen the greatest cost for this practice (regardless of configuration type) will be Material & Installation, which includes costly components such as the [[Liner|impermeable membrane/liner]] appropriate [[planting]] and [[vegetation]] selection, [[underdrain]], [[filter media]] and the stone storage layer or [[Bioretention: Internal water storage|Internal water storage zone (IWSZ)]].
As can be seen, regardless of design configuration, Material & Installation expenses represent the largest portion of total construction costs (75 to 79%).


==Life Cycle Costs==
==Life Cycle Costs==
Below are both the capital and life cycle costs of the three [[bioretention]] configuration practices over a 25 and 50-year time horizon based on a detailed assessment of local input costs, maintenance requirements, rehabilitation costs and design scenarios relevant to Canadian climates. The estimates of maintenance and rehabilitation (life cycle) costs represent net present values.  
Below are capital and life cycle cost estimates for the three [[bioretention]] configurations over 25- and 50-year time periods. The estimates of maintenance and rehabilitation (life cycle) costs represent net present values. Operation and maintenance costs are predicted to represent between 25 and 30% of total life cycle costs over the 25-year evaluation period, and increase to between 38 and 46% of total life cycle costs over the 50-year period, due to filter media bed rehabilitation costs assumed to be required after 25 years of operation.
 


Looking at the pie charts below for each configuration we can see that they are all relatively close to being the same cost with a variation of a few hundred dollars between each over both a 25 and 50-year time horizon. The percentage of the [[Bioretention: Full infiltration|Full Infitlration]] configuration is greater than the other two due to the lesser cost of the overall installation for material and Installation, (no underdrains, no gravel storage layer or impermeable membrane, etc.).


===25-Year life cycle cost break down===
===25-Year life cycle cost break down===
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<small>'''Note:''' Click on each image to enlarge to view associated life cycle cost estimate.</small><br>
<small>'''Note:''' Click on each image to enlarge to view associated life cycle cost estimate.</small><br>


==Total Cost & Design Summary==
==Cost Summary Tables==
As previously discussed the three [[Bioretention]] configurations total cost summary vary greatly dependent on whether you want you feature to possess full infiltration, no infiltration, or partial infiltration. In short, the most expensive of these options is the [[Bioretention: Partial infiltration]] option ($113,800.83 vs. $109,113.76 - no infiltration and $80,392.33 - full infiltration). The same can be said for construction + associated retrofit costs with each configuration design ($132,008.97 vs. $126,571.96 - no infiltration and $93,255.10 - full infiltration) This configuration allows for the greatest storage volume (187.5 m<sup>3</sup>) compared to its counterparts at 145<sup>3</sup> for no infiltration and 107.5<sup>3</sup> for full infiltration; all while having the exact same surface area footprint of 250m<sup>2</sup>. As reminder, it is important to understand your site's surrounding native soil infiltration rate to ensure you are selecting the appropriate design.<br>
Total life cycle cost estimates for the three [[Bioretention]] configurations vary substantially with the [[Bioretention: Partial infiltration| Partial Infiltration]] design being highest ($113,800.83), followed closely by [[Stormwater planter| No Infiltration]] design ($109,113.76), and [[Bioretention: Full infiltration| Full Infiltration]] design being the lowest ($80,392.33).<br>
 
A final note regarding the accuracy of the LCCT. A follow up sensitivity analysis study was conducted by CVC & STEP back in 2019 to test the tool's accuracy. The analysis took designs from 6 completed projects (4 [[bioretention]], 1 [[permeable pavement]], and 1 [[infiltration trench]]), and ran them through the tool comparing construction costing results from the LCCT to actual construction costs for the projects. The accuracy target set for the tool was plus-or-minus 30% of actual construction costs.<br>


'''The analysis found that the tool was on average (±14%) to actual construction costs'''<ref>Credit Vally Conservation (CVC). 2019. Life-cycle costing tool 2019 update: sensitivity analysis. Credit Valley Conservation, Mississauga, Ontario. https://sustainabletechnologies.ca/app/uploads/2020/04/LCCT-Sensitivity-Analysis_March2020.pdf</ref>
It is notable that a sensitivity analysis was conducted in 2019 to compare construction cost estimates generated by the tool to actual costs of implemented projects. '''The analysis found that tool estimates were typically within ±14% of actual construction costs'''<ref>Credit Vally Conservation (CVC). 2019. Life-cycle costing tool 2019 update: sensitivity analysis. Credit Valley Conservation, Mississauga, Ontario. https://sustainabletechnologies.ca/app/uploads/2020/04/LCCT-Sensitivity-Analysis_March2020.pdf</ref>


===Full Infiltration===
===Full Infiltration===
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[[File:Design Table Bio Partial Infil.PNG|700px]]<br>
[[File:Design Table Bio Partial Infil.PNG|700px]]<br>
</br>
</br>
===Non-Infiltrating/filtration only===
===No Infiltration (Filtration Only)===
[[File:Design Table Bio No Infil.PNG|700px]]<br>
[[File:Design Table Bio No Infil.PNG|700px]]<br>


==References==
==References==

Latest revision as of 20:50, 15 December 2022

Parking lot bioretention with surface ponding well in foreground, Kortright Centre, Vaughan, ON. Read about the performance of this practice in the technical brief (Source: STEP, 2016)[1].


Overview[edit]

Given that Bioretention is a widely used name and a general "catch-all" term for most vegetated LID practices that temporarily store stormwater runoff in depressed planting beds there are a number of configurations of the feature that can be used in various contexts and environments. Depending on native soil infiltration rate and physical constraints, the facility may be designed without an underdrain for full infiltration, with an underdrain for partial infiltration, or with an impermeable liner and underdrain for no infiltration/filtration only (i.e., a stormwater planters or biofilter) design. STEP has prepared life cycle costs estimates for each design configuration, based on a 2,000 m2 asphalt drainage area, runoff control target of 25 mm depth and 72 hour drainage period, for comparison which can be viewed below. To generate your own life cycle cost estimates customized to the development context, design criteria, and constraints applicable to your site, access the updated LID Life Cycle Costing Tool (LCCT) here.

Design Assumptions[edit]

Bioretention is an ideal technology for fitting functional vegetation into urban landscapes and treating runoff collected from nearby impervious surfaces. Components include: a 'filter bed' with filter media, storage layer of reservoir aggregate, planting and a finishing surface layer of mulch and/or stone. Additional components include an underdrain to remove excess water and soil additives to enhance pollutant removal.

Design and operation and maintenance program assumptions used to generate cost estimates are based on tool default values and the following STEP recommendations:

  • Native soil infiltration rates for Full, Partial and No Infiltration Design scenarios were assumed to be 20 mm/h, 10 mm/h and 2 mm/h, respectively, and a safety factor of 2.5 was applied to calculate the design infiltration rate.
  • Operation and maintenance (O&M) cost estimates assume annual inspections, removal of trash and debris twice a year, removal of sediment from pretreatment structures annually, and removal of weeds twice a year (where applicable). Verification inspections are included every 5 years to confirm adequate maintenance, and every 15 years to confirm adequate drainage performance through in-situ surface infiltration rate testing (where applicable)
  • Maximum impervious drainage area to permeable surface area (I:P area) ratio of 20:1.
  • Length to width ratio of bioretention area is 10:1.
  • Default filter media depth of 1.0 metres to accommodate trees in planting plan.
  • Default mulch depth of 75 millimetres.
  • An underdrain (200 mm diameter perforated pipe) is included in Partial Infiltration and No Infiltration design configurations only.

Notes[edit]

  • Designs include pretreatment through stone diaphragms at curb inlets and two inlet structure sumps (Rain Guardian bunkers).
  • Operation and maintenance cost estimates assume rehabilitation of the filter media bed surface is required after 25 years of operation.
  • The tool calculates costs for new (greenfield) development contexts and includes costs for contractor overhead and profit, material, delivery, labour, equipment (rental, operating and operator costs), hauling and disposal.
    • Land value and equipment mobilization and demobilization costs are not included, assuming BMP construction is part of overall development site construction.
    • Design and Engineering cost estimates are not calculated by the tool and must be supplied by the user.
    • The tool adds 10% contingency and additional overhead as default.
  • All cost estimates are in Canadian dollars and represent the net present value (NPV) as the tool takes into account average annual interest and discount rates over the 25 and 50 year operating life cycle periods.
  • Unit costs are based on 2018 RSMeans standard union pricing.
  • Additional costs associated with retrofit or redevelopment contexts is assumed to be 16% of the cost estimate for new (greenfield) construction contexts.
    • Retrofit construction cost estimates are included in the 'Costs Summary' section for comparison.

Construction Costs[edit]

Construction Costs Per Unit Drainage Area (CAD$/m2) - No Infiltration Design, 25 mm Retention
Construction Costs Per Unit Drainage Area (CAD$/m2) - Full Infiltration Design, 25 mm Treatment
Construction Costs Per Unit Drainage Area (CAD$/m2) - Partial Infiltration Design, 25 mm Retention


Note: Please click on each image to enlarge to view associated construction cost estimates.


Above you can find breakdowns of construction costs by expense type for 250 m2 surface area facilities (I:P area ratio of 8:1) for each design configuration:

  1. Bioretention: Full infiltration
  2. Bioretention: Partial infiltration
  3. Bioretention: No infiltration

As can be seen, regardless of design configuration, Material & Installation expenses represent the largest portion of total construction costs (75 to 79%).

Life Cycle Costs[edit]

Below are capital and life cycle cost estimates for the three bioretention configurations over 25- and 50-year time periods. The estimates of maintenance and rehabilitation (life cycle) costs represent net present values. Operation and maintenance costs are predicted to represent between 25 and 30% of total life cycle costs over the 25-year evaluation period, and increase to between 38 and 46% of total life cycle costs over the 50-year period, due to filter media bed rehabilitation costs assumed to be required after 25 years of operation.


25-Year life cycle cost break down[edit]

Bioretention: Full infiltration
Bioretention: Non-infiltrating
Bioretention: Partial infiltration


Note: Click on each image to enlarge to view associated life cycle cost estimate.

50-Year life cycle cost break down[edit]

Bioretention: Full infiltration
Bioretention: Non-infiltrating
Bioretention: Partial infiltration


Note: Click on each image to enlarge to view associated life cycle cost estimate.

Cost Summary Tables[edit]

Total life cycle cost estimates for the three Bioretention configurations vary substantially with the Partial Infiltration design being highest ($113,800.83), followed closely by No Infiltration design ($109,113.76), and Full Infiltration design being the lowest ($80,392.33).

It is notable that a sensitivity analysis was conducted in 2019 to compare construction cost estimates generated by the tool to actual costs of implemented projects. The analysis found that tool estimates were typically within ±14% of actual construction costs[2]

Full Infiltration[edit]

STEP staff member conducting performance analysis of a bioretention feature at Kortright Centre in Vaughan, ON. (Source: STEP, 2016[3])

Design Table Bio Full Infil.PNG

Partial Infiltration[edit]

Design Table Bio Partial Infil.PNG

No Infiltration (Filtration Only)[edit]

Design Table Bio No Infil.PNG

References[edit]

  1. Performance Comparison of Surface and Underground Stormwater Infiltration Practices TECHNICAL BRIEF. Accessed Dec 12 2022. https://sustainabletechnologies.ca/app/uploads/2016/08/BioVSTrench_TechBrief__July2015.pdf
  2. Credit Vally Conservation (CVC). 2019. Life-cycle costing tool 2019 update: sensitivity analysis. Credit Valley Conservation, Mississauga, Ontario. https://sustainabletechnologies.ca/app/uploads/2020/04/LCCT-Sensitivity-Analysis_March2020.pdf
  3. Performance Comparison of Surface and Underground Stormwater Infiltration Practices - TECHNICAL BRIEF. Low Impact Development Series. https://sustainabletechnologies.ca/app/uploads/2016/08/BioVSTrench_TechBrief__July2015.pdf