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Line 186:
→Performance
|Yes-size for water quality storage requirement
|Yes-size for water quality storage requirement
|Partial-based on available storage volume and if a flow restrictor is used
|Partial-based on available storage volume and if a flow restrictor is used
|}
===Water Balance===
Recent research indicates that a conservative runoff reduction rate of 10 to 20% can be used depending on whether soils fall in [[Soil groups| hydrologic soil groups A/B or C/D,]] respectively. The runoff reduction rates can be doubled if the native soils on which the swale is located have been tilled to a depth of 300 mm and amended with compost to achieve an organic content of between 8 and 15% by weight or 30 to 40% by volume. The mai ncontributing factors that influence runoff reduction rates for swales are:
* Native [[Soil groups|soil]] types
* [[Grading|Slope]]
* [[Vegetation|Vegetative cover]] and,
* [[Enhanced swales: Specifications|Length of the swale.]]
{|class="wikitable"
|+Volumetric runoff reduction from enhanced swales
|-
!'''LID Practice'''
!'''Location'''
!'''<u><span title="Note: Runoff reduction estimates are based on differences between runoff volume from the practice and total precipitation over the period of monitoring unless otherwise stated." >Runoff Reduction*</span></u>'''
!'''Reference'''
|-
|rowspan="8" style="text-align: center;" | Grass Swale
|style="text-align: center;" |Brampton
|style="text-align: center;" |15 to 35%,
|style="text-align: center;" |[https://sustainabletechnologies.ca/app/uploads/2020/11/CC-Bioswale-Tech-brief-2018-FINAL.pdf| STEP (2018)]<ref>Sustainable Technologies Evaluation Program. Effectiveness of Retrofitted Roadside Biofilter Swales - County Court Boulevard, Brampton. Technical Brief. https://sustainabletechnologies.ca/app/uploads/2020/11/CC-Bioswale-Tech-brief-2018-FINAL.pdf. https://sustainabletechnologies.ca/app/uploads/2020/11/CC-Bioswale-Tech-brief-2018-FINAL.pdf</ref>
|-
|style="text-align: center;" |Sweden
|style="text-align: center;" |40 to 55%,
|style="text-align: center;" |Rujner ''et al''. (2016)<ref>Rujner, H., Leonhardt, G., Perttu, A.M., Marsalek, J. and Viklander, M. 2016. Advancing green infrastructure design: Field evaluation of grassed urban drainage swales. Modélisation/Models-Contrôle à la source/Source control. http://documents.irevues.inist.fr/bitstream/handle/2042/60477/3B7P03-124RUJ.pdf</ref>
|-
|style="text-align: center;" |Seoul, Korea
|style="text-align: center;" |40 to 75%,
|style="text-align: center;" |Rujner ''et al''. (2016)<ref>Rujner, H., Leonhardt, G., Perttu, A.M., Marsalek, J. and Viklander, M. 2016. Advancing green infrastructure design: Field evaluation of grassed urban drainage swales. Modélisation/Models-Contrôle à la source/Source control. http://documents.irevues.inist.fr/bitstream/handle/2042/60477/3B7P03-124RUJ.pdf</ref>
|-
|style="text-align: center;" |Maryland
|style="text-align: center;" |59%
|style="text-align: center;" |Davis ''et al''. (2012)<ref>Davis, A.P., Stagge, J.H., Jamil, E. and Kim, H. 2012. Hydraulic performance of grass swales for managing highway runoff. Water research, 46(20), pp.6775-6786. http://www.jstagge.com/assets/papers/Hydraulic%20performance%20of%20grass%20swales%20for%20managing.pdf </ref>
|-
|style="text-align: center;" |Los Angeles
|style="text-align: center;" |52.5%,
|style="text-align: center;" |Ackerman and Stein (2008)<ref>Ackerman, D. and Stein, E.D. 2008. Evaluating the effectiveness of best management practices using dynamic modeling. Journal of Environmental Engineering, 134(8), pp.628-639. https://www.researchgate.net/profile/Eric-Stein-2/publication/228910558_Evaluating_the_Effectiveness_of_Best_Management_Practices_Using_Dynamic_Modeling/links/0912f509278915fc77000000/Evaluating-the-Effectiveness-of-Best-Management-Practices-Using-Dynamic-Modeling.pdf</ref>
|-
|style="text-align: center;" |Various Locations
|style="text-align: center;" |40%
|style="text-align: center;" |Strecker ''et al''.(2004)<ref>Strecker, E., Quigley, M., Urbonas, B., Jones, J. 2004. State-of-the-art in comprehensive approaches to stormwater. The Water Report. Issue 6. August 15,2004. </ref>
|-
|style="text-align: center;" |France
|style="text-align: center;" |27 to 41%
|style="text-align: center;" |Barrett ''et al''. (2004)<ref>Barrett, M.E. 2008. Comparison of BMP Performance Using the International BMP Database. Journal of Irrigation and Drainage Engineering. September/October. pp. 556-561 </ref>
|-
|style="text-align: center;" |Virginia
|style="text-align: center;" |0%
|style="text-align: center;" |Schueler (1983)<ref>Schueler, T. 1983. Washington Area Nationwide Urban Runoff Project. Final Report. Metropolitan Washington Council of Governments. Washington, DC. </ref>
|-
| colspan="2" style="text-align: center;" |'''<u><span title="Note:This estimate is provided only for the purpose of initial screening of LID practices suitable for achieving stormwater management objectives and targets. Performance of individual facilities will vary depending on site specific contexts and facility design parameters and should be estimated as part of the design process and submitted with other documentation for review by the approval authority" >Runoff Reduction Estimate*</span></u>'''
|colspan="2" style="text-align: center;" |'''46% on [[Soil groups|HSG A or B soils]];'''
'''10% on [[Soil groups|HSG C or D soils]]'''
|-
|}
===Water Quality===
Research has shown the pollutant mass removal rates of grass swales are variable, depending on influent pollutant concentrations (Bäckström et al., 2006)<ref>Bäckström, M., Viklander, M. and Malmqvist, P.A. 2006. Transport of stormwater pollutants through a roadside grassed swale. Urban Water Journal, 3(2), pp.55-67. https://www.mdpi.com/2073-4441/6/7/1887/htm</ref>, but generally moderate for most pollutants (Barrett et al., 1998<ref>Barrett, M.E., Walsh, P.M. Malina Jr., J.F. and Charbeneau, R.J. 1998. Performance of Vegetative Controls for Treating Highway Runoff. Journal of Environmental Engineering. November 1998. pp. 1121-1128.</ref>; Deletic and Fletcher, 2006<ref>Deletic, A., and Fletcher, T.D. 2006. Performance of grass filters used for stormwater treatment – a field and modelling study. Journal of Hydrology. Vol. 317. pp. 261-275.</ref>). Median pollutant mass removal rates of swales from available performance studies are 76% for total suspended solids, 55% for total phosphorus, and 50% for total nitrogen (Deletic and Fletcher, 2006<ref>Deletic, A., and Fletcher, T.D. 2006. Performance of grass filters used for stormwater treatment – a field and modelling study. Journal of Hydrology. Vol. 317. pp. 261-275.</ref>). Significant reductions in total zinc and copper event mean concentrations have been observed in performance studies with a median value of 60%, but results have varied widely (Barrett, 2008<ref>Barrett, M.E. 2008. Comparison of BMP performance using the international BMP database. Journal of Irrigation and Drainage Engineering, 134(5), pp.556-561.</ref>). Site specific factors such as slope, soil type, infiltration rate, swale length and vegetative cover also affect pollutant mass removal rates. In general, the dominant pollutant removal mechanism operating in grass swales is infiltration, rather than filtration, because pollutants trapped on the surface of the swale by vegetation or check dams are not permanently bound (Bäckström et al., 2006<ref>Bäckström, M., Viklander, M. and Malmqvist, P.A. 2006. Transport of stormwater pollutants through a roadside grassed swale. Urban Water Journal, 3(2), pp.55-67. https://www.mdpi.com/2073-4441/6/7/1887/htm</ref>). In a recent international research review on processes for improving stormwwater quality treatment of grass swales and vegetated filter strips, Gavric et al. note that while understanding of hydrology and hydraulics of these stormwater control measures is adequate, there are knowledge gaps in understanding water quality treatment processes, particularly for nutrients, traffic associated organic contaminants, and bacteria (Gavric et al., 2019 <ref>Gavric.S, Leonhardt, G., Marsalek, J., Viklander, M. 2019. Processes improving urban stormwater quality in grass swales and filter strips: A review of research findings. Science of the Total Environment. v 669. pp. 431-447. https://www.sciencedirect.com/science/article/pii/S0048969719310502?via%3Dihub</ref>). Designers should maximize the degree of infiltration achieved within a grass swale by incorporating check dams and ensuring the native soils have infiltration rates of 15 mm/hr or greater or specifying that the soils be tilled and amended with compost prior to planting. Several of the factors that can significantly increase or decrease the pollutant removal capacity of swales are provided in the table below:
{|class="wikitable"
|+Factors that Influence the Pollutant Removal Capacity of Swales
|-
!Factors that Reduce Removal Rates
!Factors that Enhance Removal Rates
|-
|Longitudinal slope > 1%
|Longitudinal slope < 1%
|-
|Measured soil infiltration rate < 15 mm/hr
|Measured soil infiltration rate is 15 mm/hr or greater
|-
|Flow velocity within channel > 0.5 m/s during a 4 hour, 25 mm Chicago storm event
|Flow velocity within channel is 0.5 m/s or less during a 4 hour, 25 mm Chicago storm event
|-
|No pretreatment
|Pretreatment with vegetated filter strips, gravel diaphragms and/or sedimentation forebays
|-
|Side slopes steeper than 3:1 (H:V)
|Side slopes 3:1 (H:V) or less
|-
|}
|}
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