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Enhanced Swales: Life Cycle Costs (view source)
Revision as of 18:58, 20 December 2022
, 2 years agoCreated page with "|thumb|500px|[[Enhanced swale with rocky check dams and a metal overflow grate in Northgate Mall parking lot, Seattle. Photo credit: MLSmith.]] {{TOClimit|2}} ==Overview== Enhanced swales (also referred to as Enhanced Grass Swales - EGS) are vegetated open channels designed to convey, treat and attenuate stormwater runoff. Simple grass channels or ditches have long been used for stormwater conveyance, particu..."
[[File:MLSmith swale.jpg||thumb|500px|[[Enhanced swale]] with rocky [[check dams]] and a metal [[overflow]] grate in Northgate Mall parking lot, Seattle. Photo credit: [[Acknowledgements| MLSmith]].]]
{{TOClimit|2}}
==Overview==
[[Enhanced swales]] (also referred to as Enhanced Grass Swales - EGS) are vegetated open channels designed to convey, treat and attenuate stormwater runoff. Simple grass channels or ditches have long been used for stormwater conveyance, particularly for road drainage. Enhanced swales incorporate design features such as modified geometry and [[check dams]] that improve the contaminant removal and runoff reduction functions of simple grass channel and
roadside ditch designs. Where development density, topography and depth to water table permit, swales are a preferable alternative to curb and gutter and storm drains as a stormwater conveyance system. STEP has prepared life cycle costs estimates for each design configuration, based on a 2,000 m<sup>2</sup> road 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==
Enhanced grass swales (EGS) are an ideal technology for sloped sites and cheaply retrofitting and improving the performance of existing grass swales. Components include: [[Retention swales|graded channel]], resilient [[turf]] grass or other [[planting]], [[check dams]] to facilitate short term ponding. Additional components include, [[amended soil]] or [[filter media]] to increase infiltration to soils below, and turf reinforcement to prevent scour. [[Swales]] can provide pretreatment for other BMPs ([[bioretention]], soakaways, [[exfiltration trenches]]) or be designed in series with other practices as part of a [[treatment train tool|treatment train]]. <br>
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)
* Designed with a trapezoidal or triangular cross section.
* Length between culverts should be 5 meters or greater.
* Designed with a bottom width between 0.6-2.4 m (allows shallow flows and adequate water treatment).
* Longitudinal slope should be between 0.5 and 4%. Check dams should be incorporated on slopes greater than 3%.
* Side slopes should be flat to aid in providing pre-treatment for lateral incoming flows and maximize swale filtering surface.
* Steeper side slopes may be susceptible to erosion from incoming lateral flows. A maximum slope of 2.5:1 is recommended (Tool default).
* Maximum flow depth should be 50% of grass height for regularly mown swales, to maximum of 75 mm.
* For infrequently mown swales, STEP recommends maximum flows of 33% of vegetation height.
* Designed for a maximum velocity of 0.3 m/s.
* Convey locally required design storm (~10 mm year storm) at non-erosive velocities.
===Notes===
* The Tool assumes an in-line system.
* The Tool assumed curb-and-gutter around the circumference of the facility with curb-cut inlets every 6 to 0.5 meter squared and stone splash pads.
* The greater the slope, the more check dams placed by the Tool. The unit cost for check dams is for the amount of material required, not number of dams.
* If configuration include driveway (2 laned), a culvert pipe connects the enhanced grass swale (EGS) and an extra 0.1m of gravel is added beneath culvert.
* Swales which cross multiple driveways or smaller driveways can be accounted for by multiplying or dividing these costs in the "Additional Costs" cells.
* 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.<br>
==Construction Costs==
[[File:ConstructionTable InfilTrench Full Infil.PNG|thumb|left|615px|'''Construction Costs Per Unit Drainage Area (CAD$/m<sup>2</sup>) - Full Infiltration Design, 25 mm Treatment''']]
[[File:ConstructionTable InfilTrench Partial Infil.PNG|thumb|center|615px|'''Construction Costs Per Unit Drainage Area (CAD$/m<sup>2</sup>) - Partial Infiltration Design, 25 mm Retention''']]
<small>'''Note:''' Please click on each image to enlarge to view associated construction cost estimates.</small><br>
Above you can find a cost breakdown of a 1000m<sup>2</sup> in two different configurations:<br>
#[[Infiltration trenches|Infiltration Trench: Full Infiltration]]
#[[Infiltration trenches|Infiltration Trench: Partial Infiltration]]
As can be seen, regardless of design configuration, Material & Installation expenses represent the largest portion of total construction costs (75 to 76%).
==Life Cycle Costs==
Below are capital and life cycle cost estimates for the two [[Infiltration trenches|infiltration trench]] 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 38% of total life cycle costs over the 25-year evaluation period, and increase to 51% of total life cycle costs over the 50-year period, due to increased levels of litter removal, replacement of filter cloth and disposal of collected sediment (every 4 - 8 years), cleaning out the catchbasin and the [[Oil and Grit Separator|Oil and Grit Separator/Hydrodynamic Separator]] annually, as well as flushing the internal pipes every 10 years.
===25-Year life cycle cost break down===
[[File:25yr LCCT InfilTrench Full Infil.PNG|thumb|left|615px|'''Infiltration Trench: Full Infiltration''']]
[[File:25yr LCCT InfilTrench Partial Infil.PNG|thumb|center|615px|'''Infiltration Trench: Partial Infiltration''']]
<small>'''Note:''' Click on each image to enlarge to view associated life cycle cost estimate.</small><br>
===50-Year life cycle cost break down===
[[File:50yr LCCT InfilTrench Full Infil.PNG|thumb|left|615px|'''Infiltration Trench: Full Infiltration''']]
[[File:50yr LCCT InfilTrench Partial Infil.PNG|thumb|center|615px|'''Infiltration Trench: Partial Infiltration''']]
<small>'''Note:''' Click on each image to enlarge to view associated life cycle cost estimate.</small><br>
==Cost Summary Tables==
Total life cycle cost estimates for the two [[Infiltration trenches|infiltration trench]] configurations vary substantially with the [[Infiltration trenches|Infiltration Trench: Partial Infiltration]] design being highest ($85,288.88), followed closely by the [[Infiltration trenches|Infiltration Trench: Full Infiltration]] design being ($69,805.90).<br>
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===
[[File:North Dakota State U infiltration trench.jpg|800px|thumb|Good use of geotextile, being limited to preventing fines from entering the trench from above. Read more about the performance of this feature in comparison to [[bioretention]]] BMPs in Vaughan, ON. (Source: STEP, 2016<ref>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</ref>)]]
[[File:Design Table InfilTrench Full Infil.PNG|700px]]<br>
</br>
===Partial Infiltration===
[[File:Design Table InfilTrench Partial Infil.PNG|700px]]<br>
</br>
==References==
{{TOClimit|2}}
==Overview==
[[Enhanced swales]] (also referred to as Enhanced Grass Swales - EGS) are vegetated open channels designed to convey, treat and attenuate stormwater runoff. Simple grass channels or ditches have long been used for stormwater conveyance, particularly for road drainage. Enhanced swales incorporate design features such as modified geometry and [[check dams]] that improve the contaminant removal and runoff reduction functions of simple grass channel and
roadside ditch designs. Where development density, topography and depth to water table permit, swales are a preferable alternative to curb and gutter and storm drains as a stormwater conveyance system. STEP has prepared life cycle costs estimates for each design configuration, based on a 2,000 m<sup>2</sup> road 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==
Enhanced grass swales (EGS) are an ideal technology for sloped sites and cheaply retrofitting and improving the performance of existing grass swales. Components include: [[Retention swales|graded channel]], resilient [[turf]] grass or other [[planting]], [[check dams]] to facilitate short term ponding. Additional components include, [[amended soil]] or [[filter media]] to increase infiltration to soils below, and turf reinforcement to prevent scour. [[Swales]] can provide pretreatment for other BMPs ([[bioretention]], soakaways, [[exfiltration trenches]]) or be designed in series with other practices as part of a [[treatment train tool|treatment train]]. <br>
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)
* Designed with a trapezoidal or triangular cross section.
* Length between culverts should be 5 meters or greater.
* Designed with a bottom width between 0.6-2.4 m (allows shallow flows and adequate water treatment).
* Longitudinal slope should be between 0.5 and 4%. Check dams should be incorporated on slopes greater than 3%.
* Side slopes should be flat to aid in providing pre-treatment for lateral incoming flows and maximize swale filtering surface.
* Steeper side slopes may be susceptible to erosion from incoming lateral flows. A maximum slope of 2.5:1 is recommended (Tool default).
* Maximum flow depth should be 50% of grass height for regularly mown swales, to maximum of 75 mm.
* For infrequently mown swales, STEP recommends maximum flows of 33% of vegetation height.
* Designed for a maximum velocity of 0.3 m/s.
* Convey locally required design storm (~10 mm year storm) at non-erosive velocities.
===Notes===
* The Tool assumes an in-line system.
* The Tool assumed curb-and-gutter around the circumference of the facility with curb-cut inlets every 6 to 0.5 meter squared and stone splash pads.
* The greater the slope, the more check dams placed by the Tool. The unit cost for check dams is for the amount of material required, not number of dams.
* If configuration include driveway (2 laned), a culvert pipe connects the enhanced grass swale (EGS) and an extra 0.1m of gravel is added beneath culvert.
* Swales which cross multiple driveways or smaller driveways can be accounted for by multiplying or dividing these costs in the "Additional Costs" cells.
* 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.<br>
==Construction Costs==
[[File:ConstructionTable InfilTrench Full Infil.PNG|thumb|left|615px|'''Construction Costs Per Unit Drainage Area (CAD$/m<sup>2</sup>) - Full Infiltration Design, 25 mm Treatment''']]
[[File:ConstructionTable InfilTrench Partial Infil.PNG|thumb|center|615px|'''Construction Costs Per Unit Drainage Area (CAD$/m<sup>2</sup>) - Partial Infiltration Design, 25 mm Retention''']]
<small>'''Note:''' Please click on each image to enlarge to view associated construction cost estimates.</small><br>
Above you can find a cost breakdown of a 1000m<sup>2</sup> in two different configurations:<br>
#[[Infiltration trenches|Infiltration Trench: Full Infiltration]]
#[[Infiltration trenches|Infiltration Trench: Partial Infiltration]]
As can be seen, regardless of design configuration, Material & Installation expenses represent the largest portion of total construction costs (75 to 76%).
==Life Cycle Costs==
Below are capital and life cycle cost estimates for the two [[Infiltration trenches|infiltration trench]] 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 38% of total life cycle costs over the 25-year evaluation period, and increase to 51% of total life cycle costs over the 50-year period, due to increased levels of litter removal, replacement of filter cloth and disposal of collected sediment (every 4 - 8 years), cleaning out the catchbasin and the [[Oil and Grit Separator|Oil and Grit Separator/Hydrodynamic Separator]] annually, as well as flushing the internal pipes every 10 years.
===25-Year life cycle cost break down===
[[File:25yr LCCT InfilTrench Full Infil.PNG|thumb|left|615px|'''Infiltration Trench: Full Infiltration''']]
[[File:25yr LCCT InfilTrench Partial Infil.PNG|thumb|center|615px|'''Infiltration Trench: Partial Infiltration''']]
<small>'''Note:''' Click on each image to enlarge to view associated life cycle cost estimate.</small><br>
===50-Year life cycle cost break down===
[[File:50yr LCCT InfilTrench Full Infil.PNG|thumb|left|615px|'''Infiltration Trench: Full Infiltration''']]
[[File:50yr LCCT InfilTrench Partial Infil.PNG|thumb|center|615px|'''Infiltration Trench: Partial Infiltration''']]
<small>'''Note:''' Click on each image to enlarge to view associated life cycle cost estimate.</small><br>
==Cost Summary Tables==
Total life cycle cost estimates for the two [[Infiltration trenches|infiltration trench]] configurations vary substantially with the [[Infiltration trenches|Infiltration Trench: Partial Infiltration]] design being highest ($85,288.88), followed closely by the [[Infiltration trenches|Infiltration Trench: Full Infiltration]] design being ($69,805.90).<br>
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===
[[File:North Dakota State U infiltration trench.jpg|800px|thumb|Good use of geotextile, being limited to preventing fines from entering the trench from above. Read more about the performance of this feature in comparison to [[bioretention]]] BMPs in Vaughan, ON. (Source: STEP, 2016<ref>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</ref>)]]
[[File:Design Table InfilTrench Full Infil.PNG|700px]]<br>
</br>
===Partial Infiltration===
[[File:Design Table InfilTrench Partial Infil.PNG|700px]]<br>
</br>
==References==