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Inspection and Maintenance: Green Roofs (view source)
Revision as of 15:08, 11 August 2022
, 2 years agono edit summary
*Deconstruct the green roof, replace the water-proofing membrane with new material, and reconstruct with materials that meet design or product specifications.
*Deconstruct the green roof, replace the water-proofing membrane with new material, and reconstruct with materials that meet design or product specifications.
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==Inspection Time Commitments and Costs==
Estimates are based on an extensive green roof with 15 cm of growing medium, irrigation and water-proofing membrane with leak detection system; estimates for other combinations of these variables are described in the [https://sustainabletechnologies.ca/app/uploads/2016/08/LID-IM-Guide-7.6-Green-Roofs.pdf Low Impact Development (LID) Stormwater Management Practice Inspection and Maintenance Guide]<br>
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[[File:Task Cost estimates.PNG|thumb|left|400px|Per-task cost estimates for maintenance and rehabilitation of an extensive green roof design with 15 cm growing medium, irrigation and water-proofing membrane with leak detection (in 2016 $ figures).<ref name="example1" />]]
[[File:Maintenance and Operation green roofs.PNG|thumb|400px|General time commitments and costs for inspection of an extensive green roof design with 15 cm growing medium, irrigation and water-proofing membrane with leak detection (in 2016 $ figures).<ref name="example1" />]]
[[File:Life cycle cost estimate green roof.PNG|thumb|center|400px|Construction and life cycle cost estimates for extensive green roofs with 15 cm growing medium, irrigation and water-proofing membrane with leak detection (in 2016 $ figures).<ref name="example1" />]]<br>
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Estimates of the life cycle costs of inspection and maintenance have been produced using the latest version of the [[Cost analysis resources|LID Life Cycle Costing Tool]] for three design variations (full infiltration, partial infiltration and no infiltration) to assist stormwater infrastructure planners, designers and asset managers with planning and preparing budgets for potential LID features.
Assumptions for the above costs and the following table below are based on the following:
*Design variations for permeable pavements can be broken down into three main categories:
**Full Infiltration design, where the pavement drains through infiltration into the underlying subsoil alone (i.e., no sub-drain);
**Partial Infiltration design, where drainage is through the combination of a subdrain and infiltration into the underlying subsoil (i.e., with a sub-drain); or,
**No Infiltration (i.e., filtration only design) that includes an impermeable liner between the base of the BMP and the underlying native sub-soil, where drainage is through a sub-drain alone (i.e., with a sub-drain and impermeable liner).
*For permeable pavements it is assumed that rehabilitation of the pavement surface will be needed once the BMP reaches 30 years of age in order to maintain surface drainage performance at an acceptable level. Included in the rehabilitation costs are (de)mobilization costs, as equipment would not have been present on site.
*Design costs were not included in the rehabilitation as it was assumed that the original LID practice design would be used to inform this work. The annual average maintenance cost does not include rehabilitation costs and therefore represents an average of routine maintenance tasks, as outlined in Table Permeable Pavements: Key Components, Descriptions and Routine I&M Requirements, above. All cost value estimates represent the NPV as the calculation takes into account average annual interest (2%) and discount (3%) rates over the evaluation time periods.
*The costing presented in this section is specific to permeable interlocking concrete pavers (PICP), as defined in the Tool. This product has been selected for costing due to its popularity and well understood maintenance needs.
*For all permeable pavement design variations, the CDA has been defined as 2,000 m<sup>2</sup> of which 1,000 m<sup>2</sup> is impermeable pavement draining to the pavers, and 1,000 m<sup>2</sup> is permeable pavement. The impervious area to pervious area ratio (I:P ratio) used to size the BMP footprint is 1:1, which is in accordance with recommendations in the [https://sustainabletechnologies.ca/app/uploads/2013/01/LID-SWM-Guide-v1.0_2010_1_no-appendices.pdf LID SWM Planning and Design Guide (CVC & TRCA, 2010)].
**The Full Infiltration design does not include a sub-drain and assumes a native sub-soil infiltration rate of 20 mm/h. The base granular reservoir is 350 mm deep and is capable of storing runoff from a 61 mm rain event over the CDA. A monitoring well is included for inspection purposes.
**The Partial Infiltration design includes a sub-drain and assumes a native sub-soil infiltration rate of 10 mm/h. The base granular reservoir is 350 mm deep and is capable of storing runoff from a 9 mm rain event before the stored volume reaches the perforated underdrain pipe located 50 mm above the native sub-soil. Although a flow restrictor is recommended to maximize infiltration, the cost of this feature is not included due to its relatively low cost.
**The No Infiltration design includes a sub-drain pipe installed on the bottom of the sub-surface water storage reservoir and an impermeable liner. All other features are the same as the Partial Infiltration design variation.
*Estimates of the life cycle costs of PICP permeable pavements in Canadian dollars per unit CDA ($/m<sup>2</sup>) are presented in the table below. [[Cost analysis resources|LID Life Cycle Costing Tool]] allows users to select what BMP type and design variation applies, and to use the default assumptions to generate planning level cost estimates.
**Users can also input their own values relating to a site or area, design, unit costs, and inspection and maintenance task frequencies to generate customized cost estimates, specific to a certain project, context or stormwater infrastructure program.
**For all BMP design variations and maintenance scenarios, it is assumed that rehabilitation of the pavement surface will be necessary when the BMP reaches 30 years of age to maintain acceptable surface drainage performance. Rehabilitation of PICP pavements is assumed to typically involve the following tasks and associated costs:
***Remove pavers, bedding and joint fill and top 5 cm (2”) of base aggregate and replace with new material that meets design specifications
***Construction and Assumption inspection and testing associated with rehabilitation work to confirm that materials meet design specifications and installation is acceptable, including compaction and surface infiltration rate testing. <br>
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[[File:Minimum & hig hfreq cost p.p.PNG|thumb|center|900px|Life cycle cost estimates for all infiltration types of PICP [[permeable pavements]] an under minimum and high frequency scenarios (in 2016 $ figures).<ref>TRCA. 2016. Low Impact Development Stormwater Management Practice Inspection and Maintenance Guide. Prepared by the Sustainable Technologies Evaluation Program. Vaughan, Ontario. https://sustainabletechnologies.ca/app/uploads/2016/08/LID-IM-Guide-7.4-Permeable-Pavements.pdf</ref>]]
'''Notes:'''
<small>
#Estimated life cycle costs represent NPV of associated costs in Canadian dollars per square metre of CDA ($/m2).
#Average annual maintenance cost estimates represent NPV of all costs incurred over the time period and do not include rehabilitation costs.
#Rehabilitation cost estimates represent NPV of all costs related to rehabilitative maintenance work assumed to be needed after 30 years in service, including those associated with inspection.
#Full Infiltration design life cycle costs are lower than Partial and No Infiltration designs due to the absence of a sub-drain to construct, inspect and routinely flush.
#Rehabilitation costs for Full Infiltration designs are estimated to be 54.8% to 55.6% of the original construction costs for High and Minimum Recommended Frequency maintenance program scenarios, respectively.
#Rehabilitation costs for Partial Infiltration designs are estimated to be 53.5% to 54.3% of the original construction costs for High and Minimum Recommended Frequency maintenance program scenarios, respectively.
#Rehabilitation costs for No Infiltration designs are estimated to be 47.4% to 48.1% of the original construction costs for High and Minimum Recommended Frequency maintenance program scenarios, respectively.
#Maintenance and rehabilitation costs over a 25 year time period for the Minimum Recommended maintenance scenario are estimated to be 22.3%, of the original construction costs for Full Infiltration design, 22.6% for Partial Infiltration design, and 20.0% for No Infiltration design.
#Maintenance and rehabilitation costs over a 25 year time period for the High Frequency maintenance scenario are estimated to be: 39.0% of the original construction costs for Full, 38.9% for Partial Infiltration designs, and 34.5% for No Infiltration designs.
#Maintenance and rehabilitation costs over a 50 year time period for the Minimum Recommended Frequency maintenance scenario are estimated to be approximately: 1.07 times the original construction cost for Full, 1.06 times the original construction costs for Partial Infiltration designs, and 93.9.% the original construction cost for No Infiltration designs.
#Maintenance and rehabilitation costs over a 50 year time period for the High Frequency maintenance scenario are estimated to be approximately: 1.43 times the original construction cost for Full, 1.41 times the original construction costs for Partial Infiltration designs, and 1.25 times the original construction cost for No Infiltration designs.
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