Changes

*For rainwater cisterns it is assumed that no rehabilitation work will be needed to maintain acceptable storage and drainage performance over a 50 year period of operation (40 for plastic cisterns), given that pretreatment devices are in place and are being adequately maintained. The annual average maintenance cost value represents an average of routine maintenance tasks, as outlined in Rainwater Harvesting: Key Components, Descriptions and Routine I&M Requirements table 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.

*For rainwater cisterns it is assumed that no rehabilitation work will be needed to maintain acceptable storage and drainage performance over a 50 year period of operation (40 for plastic cisterns), given that pretreatment devices are in place and are being adequately maintained. The annual average maintenance cost value represents an average of routine maintenance tasks, as outlined in Rainwater Harvesting: Key Components, Descriptions and Routine I&M Requirements table 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.

*Life cycle cost estimates have been generated for two design variations that can be used year-round: underground concrete cistern; and indoor plastic cistern systems. For each design variation, life cycle cost estimates have been calculated for two level-of-service scenarios: the minimum recommended frequency of inspection and maintenance tasks in the table below to provide an indication of the potential range. Only the indoor plastic cistern requires rehabilitation within the 50 year evaluation period. At year 40 it is assumed the plastic cistern is replaced with anew one.  

*Life cycle cost estimates have been generated for two design variations that can be used year-round: underground concrete cistern; and indoor plastic cistern systems. For each design variation, life cycle cost estimates have been calculated for two level-of-service scenarios: the minimum recommended frequency of inspection and maintenance tasks in the Per-task cost estimates for maintenance and rehabilitation of rainwater harvesting features table above to provide an indication of the potential range. Only the indoor plastic cistern requires rehabilitation within the 50 year evaluation period. At year 40 it is assumed the plastic cistern is replaced with anew one.  

 

*For all scenarios, the roof area that drains into the rainwater harvesting cistern is 2,000 m². The water storage capacity of the cistern is assumed to be 23,000 L. Both cistern systems include a dual plumbing distribution system, an 81.2 LPM submersible pump and a 439 L expansion tank. The systems also include a float switch to prevent the pump from dry running, a top-up float switch and associated wring, a solenoid valve, air gap to prevent backflow, as well as backflow preventer at the premise boundary, a water meter and a water hammer arrestor. The rainwater is used for toilet flushing of 260 occupants. It is assumed that two hose bibs are used on average 14 minutes per day from April to September. The underground concrete cistern is installed adjacent to the building. The plastic cistern is stored inside the building, so no excavation is required to install/uninstall it.  

*For maintenance frequencies and requirements and the life span of each practice are based on both literature and practical experience. Life cycle and associated maintenance costs are evaluated over a 50 year timeframe, which is the typical period over which infrastructure decisions are made.

*Estimates of the life cycle costs for the two rainwater cistern system design variations in Canadian dollars per unit CDA ($/m²) are presented in the table below. The 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 [[bioretention]], it is assumed that some rehabilitation (e.g., rehabilitative maintenance) work will be needed on the filter bed surface once the BMP reaches 25 and 50 years of age in order to maintain functional 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 the Table under section, [[Inspection and Maintenance: Bioretention & Dry Swales#Routine Maintenance - Key Components and I&M Tasks|Routine Maintenance - Key Components and I&M Tasks]] above. All cost value estimates represent the net present value (NPV) as the calculation takes into account average annual interest (2%) and discount (3%) rates over the evaluation time periods.

*For indoor plastic cistern systems it is assumed that replacement of the cistern itself is needed once it reaches 40 years of age. Replacement of the cistern is assumed to typically involve the following tasks and associated costs:

*For all bioretention design variations, the CDA has been defined as a 2,000 m2 impervious pavement area plus the footprint area of a bioretention cell that is 133 m2 in size, as per design recommendations. The impervious area to pervious area ratio (I:P ratio) used to size the BMP footprint is 15:1, which is the maximum ratio recommended in the LID SWM Planning and Design Guide (CVC & TRCA, 2010)<ref>CVC and TRCA. 2010. Low Impact Development Stormwater Management Planning and Design Guide. Version 1.0. https://cvc.ca/wp-content/uploads/2014/04/LID-SWM-Guide-v1.0_2010_1_no-appendices.pdf</ref>. It is assumed that water drains to the cell through curb inlets spaced 6 m apart with

*While orientation (i.e., cell versus swale) and choice of components (e.g., inlet/outlet structures etc.) can vary widely, design variations for bioretention practices can be broken down into three main categories. They can be designed to drain through infiltration into the underlying subsoil alone (i.e., Full Infiltration design, no sub-drain), through the combination of a sub-drain and infiltration into the underlying subsoil (i.e., Partial Infiltration design, with a sub-drain), or through a sub-drain alone (i.e., No Infiltration or “filtration only” design, with a sub-drain and impermeable liner). For Full Infiltration systems, an overflow is provided for storms up to 37 mm based on a subsoil infiltration rate of 20 mm/hour. Two standpipe wells are part of the design (one subdrain inspection/flushing port at the upstream end and one sub-surface water storage reservoir monitoring well at the downstream end). Partial Infiltration systems have a sub-surface water storage reservoir with a perforated pipe sub-drain within it. The depth of the reservoir is sized to store flow from a 25 mm rain event over the CDA based on native soil infiltration rate of 10 mm/hour. The No Infiltration system includes an impermeable liner between the base and sides of the BMP and surrounding native sub-soil, to prevent infiltration.  

**Reassemble the system, re-using existing components;

*Estimates of the life cycle costs of bioretention and dry swales in Canadian dollars per unit CDA ($/m2) 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.

**Construction and Assumption inspection work associated with the rehabilitation work (including cistern pump testing). <br>

**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 part or all of the filter bed surface will be necessary once the BMP reaches 25 and 50 years of age to maintain acceptable surface drainage performance (e.g., surface ponding drainage time). Filter bed rehabilitation for bioretention and dry swales is assumed to typically involve the tasks outlined under section, [[Inspection and Maintenance: Bioretention & Dry Swales#Routine Maintenance - Key Components and I&M Tasks|Routine Maintenance - Key Components and I&M Tasks]] above.<br>

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[[File:Life cycle cost for all variations.PNG|thumb|center|900px|Life cycle cost estimates for all variation types of [[bioretention]] and [[Dry swale|dry swales]] under minimum and high frequency scenarios (in 2016 $ figures).<ref>TRCA. 2018. Inspection and Maintenance of Stormwater Best Management Practices. Bioretention - Fact Sheet. https://sustainabletechnologies.ca/app/uploads/2018/02/Bioretention-and-Dry-Swales-Fact-Sheet.pdf</ref>]]

[File:Life cycle cost all variations RWH.PNG|thumb|center|900px|Life cycle cost estimates for both underground (buried) concrete cisterns and indoor plastic cisterns under minimum and high frequency scenarios (in 2016 $ figures)<ref name="example1" />.]]

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