Changes

==Post-to-predevelopment water balance matching==

==Post-to-predevelopment water balance matching==

The amount of infiltration required on a given site is determined by comparing water balance estimates before and after development. Ideally, the volume of water infiltrated and evapotranspired prior to development would remain the same afterwards.  

The amount of infiltration required on a given site is determined by comparing water balance estimates before and after development. Ideally, the volume of water infiltrated and evapotranspired prior to development would remain the same afterwards.  

In practice, increased impervious cover normally results in lower post development evapotranspiration. Best efforts should be made to match pre-development water balance components.  However, in some cases maintaining runoff volumes at predevelopment levels may require that more water is infiltrated after development than under the predevelopment condition.   

In practice, increased impervious cover normally results in lower post development [[evapotranspiration]]. Best efforts should be made to match pre-development water balance components.  However, in some cases maintaining runoff volumes at predevelopment levels may require that more water is infiltrated after development than under the predevelopment condition.   

Under natural conditions, sites with fine textured native soils will have lower infiltration volumes (and higher runoff) than those with coarse textured soils. On fine textured soils with very low permeability (hydrologic D type soils), the measured infiltration rate may even approach zero. Under these conditions, the stormwater management approach should focus on runoff prevention and volume reduction through evapotranspiration or water reuse, rather than infiltration.

Under natural conditions, sites with fine textured native soils will have lower infiltration volumes (and higher runoff) than those with coarse textured soils. On fine textured soils with very low permeability (hydrologic D type soils), the measured infiltration rate may even approach zero. Under these conditions, the stormwater management approach should focus on runoff prevention and volume reduction through evapotranspiration or water reuse, rather than infiltration.

The rationale for variations in practice design for sites with fine textured soils is based on the relationship between hydraulic head and infiltration.  

The rationale for variations in practice design for sites with fine textured soils is based on the relationship between hydraulic head and infiltration.  

As the head pressure in the Caledon infiltration trench decreased from 1.5 m to below 1 m, infiltration rates dropped from 2.5 - 3.8 mm/h during the first two days to only 1 - 1.5 mm hour after six and half days.  

As the head pressure in the Caledon [[infiltration trench]] decreased from 1.5 m to below 1 m, infiltration rates dropped from 2.5 - 3.8 mm/h during the first two days to only 1 - 1.5 mm hour after six and half days.  

Infiltration is enhanced by maintaining a hydraulic head above the point at which infiltration slows to negligible levels.   

Infiltration is enhanced by maintaining a hydraulic head above the point at which infiltration slows to negligible levels.   

#Designing the storage to be more vertically oriented to increase available hydraulic head. BMPs should have higher side wall to bottom ratios, and a portion of the total storage regarded as effectively permanent.     

#Designing the storage to be more vertically oriented to increase available hydraulic head. BMPs should have higher side wall to bottom ratios, and a portion of the total storage regarded as effectively permanent.     

Another important element of infiltration practice design in the context of fine textured soils relates to the attraction of soil surfaces to water, which are strong in fine textured clays and silty clays and weaker in coarse textured sands or sandy loams.  This attraction, referred to as the matric potential, allows water to move up from the groundwater table into the soils.  

Another important element of infiltration practice design in the context of fine textured soils relates to the attraction of soil surfaces to water, which are strong in fine textured clays and silty clays and weaker in coarse textured sands or sandy loams.  This attraction, referred to as the matric potential, allows water to move up from the [[groundwater| water table]] into the soils. The separation between the base of the infiltrating practice and the water table should be modelled and the effect of [[Groundwater#Goundwater_mounding| groundwater mounding]] taken into consideration.

In fine textured soils, this distance can be in excess of one meter. Therefore if the base of the infiltration practice is only one meter above the seasonally high groundwater table, a direct connection between the practice and groundwater may form, bypassing the treatment properties of the soils.  It is recommended, therefore that the groundwater table be 1.5 m or lower when practices are installed on fine textured soils.

==Performance studies on fine textured soils==

==Performance studies on fine textured soils==

*The configuration of the outflow was also an important consideration. In systems where the outlet is elevated above the native soil, runoff reduction levels tend to be considerably higher than systems with underdrains located at the native soil interface. See [[Bioretention: Performance]]  

*The configuration of the outflow was also an important consideration. In systems where the outlet is elevated above the native soil, runoff reduction levels tend to be considerably higher than systems with underdrains located at the native soil interface. See [[Bioretention: Performance]]  

The studies, tabulated below, clearly indicate that significant volume reduction through infiltration is feasible on low permeability soils. If geotechnical investigations indicate that volume loss through infiltration is not possible, or would provide more limited benefits than found in these studies, the project should focus on reducing runoff through vegetative evapotranspiration.  See here for a list of options, and their relative potential to reduce runoff through evapotranspiration.       

The studies, tabulated below, clearly indicate that significant volume reduction through infiltration is feasible on low permeability soils. If geotechnical investigations indicate that volume loss through infiltration is not possible, or would provide more limited benefits than found in these studies, the project should focus on reducing runoff through vegetative [[evapotranspiration]].  See here for a list of options, and their relative potential to reduce runoff through evapotranspiration.       

{|class= "wikitable sortable"

{|class= "wikitable sortable"

(Sortable, click headers)

(Sortable, click headers)

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!style="background: darkcyan; color: white" rowspan = "2"|BMP type

!rowspan = "2"|BMP type

!style="background: darkcyan; color: white" rowspan = "2"|Duration

!rowspan = "2"|Duration

!style="background: darkcyan; color: white" colspan= "3"|Site characteristics

!colspan= "3"|Site characteristics

!style="background: darkcyan; color: white" rowspan = "2"|Runoff reduction

!rowspan = "2"|Runoff reduction (%)

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!style="background: darkcyan; color: white"|Native soil

!Native soil

!style="background: darkcyan; color: white"|I/P ratio

!I/P ratio

!style="background: darkcyan; color: white"|Sump depth*

!Sump depth (m)*

|-

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|Infiltration trench<ref name = "VS 2015"> Van Seters, T. and Young, D., 2015, Performance Comparison of Surface and Underground Stormwater Infiltration Practices, TRCA, Toronto, Ontario"</ref>||2 growing seasons||Silty clay||10:1||<10 cm; flow rate control||80%

|Infiltration trench<ref name = "VS 2015"> Van Seters, T. and Young, D., 2015, [https://sustainabletechnologies.ca/app/uploads/2016/08/BioVSTrench_TechBrief__July2015.pdf  Performance Comparison of Surface and Underground Stormwater Infiltration Practices], TRCA, Toronto, Ontario</ref>||2 growing seasons||Silty clay||10:1||0.1; flow rate control||80

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|Bioretention<ref name = "VS 2015"></ref>||2 growing seasons||Silty clay||10:1||<10 cm; flow rate control||90%

|Bioretention<ref name = "VS 2015"></ref>||2 growing seasons||Silty clay||10:1||0.1; flow rate control||90

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|Permeable Pavement<ref>Van Seters, T. and Drake, J., 2015, Five year evaluation of Permeable Pavements, TRCA, Toronto, Ontario</ref>||5 growing seasons||Silty clay||1:1||<10 cm; flow rate control||45%

|Permeable Pavement<ref>Van Seters, T. and Drake, J., 2015, [https://sustainabletechnologies.ca/app/uploads/2016/02/KPP-Ext_FinalReport_Dec2015.pdf Five year evaluation of Permeable Pavements], TRCA, Toronto, Ontario</ref>||5 growing seasons||Silty clay||1:1||0.1; flow rate control||45

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|Bioretention<ref>STEP study ongoing (2017)</ref>||1 growing season||Silty clay||10:1||<10 cm; flow rate control||83%

|Bioretention<ref>STEP study ongoing (2017)</ref>||1 growing season||Silty clay||10:1||0.1; flow rate control||83

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|Bioretention <ref>Van Seters T and Graham C, 2014, Performance Evaluation of a Bioretention System, TRCA, Toronto, Ontario</ref>||2 years||Silty clay||11:1||<10 cm; flow rate control||91%

|Bioretention <ref>Van Seters T and Graham C, 2014, [https://sustainabletechnologies.ca/app/uploads/2014/09/STEP-Bioretention-Report_2014.pdf Performance Evaluation of a Bioretention System], TRCA, Toronto, Ontario</ref>||2 years||Silty clay||11:1||0.1; flow rate control||91

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|Infiltration chamber<ref name = "DY 2013">Young D, Van Seters T, Graham, C, 2013, Evaluation of Residential Lot Level Stormwater Practices – tech brief</ref>||2 years||Sandy silt||20:1||Approx.: 1.2 m||90%

|Infiltration chamber<ref name = "DY 2013">Young D, Van Seters T, Graham, C, 2013,[https://sustainabletechnologies.ca/app/uploads/2013/03/Infiltration-Chambers-and-Trenches_2013-Final.pdf Evaluation of Underground Stormwater Infiltration Systems], TRCA, Toronto, Ontario</ref>||2 years||Sandy silt||20:1||1.2 (approx.)||90

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|Infiltration trench<ref name = "DY 2013"></ref>||2 years||Clayey silt||64:1||2 m||36%

|Infiltration trench<ref name = "DY 2013"/>||2 years||Clayey silt||64:1||2||36

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|Infiltration trench<ref name = "DY 2013"></ref>||2 years||Clayey silt||155:1||2 m||16%

|Infiltration trench<ref name = "DY 2013"/>||2 years||Clayey silt||155:1||2||16

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|Exfiltration trench<ref>SWAMP, 2005.  Performance Assessment of a Perforated Pipe Stormwater Exfiltration system, Toronto, Ontario, TRCA, Toronto, Ontario</ref>||2 years||Clay to clay silt till over silty sand till||Approx: 7:1||0.65 m||>90%

|Exfiltration trench<ref>SWAMP, 2005.  [https://sustainabletechnologies.ca/app/uploads/2013/03/Performance-Assessment-of-a-Perforated-Pipe-Stormwater-Exfiltration-System-2004.pdf Performance Assessment of a Perforated Pipe Stormwater Exfiltration system], TRCA, Toronto, Ontario</ref>||2 years||Clay to clay silt till over silty sand till||7:1 (approx.)||0.65||>90

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|Infiltration trench<ref>SWAMP, 2002, Performance Assessment of a Swale Perforated Pipe Stormwater Infiltration System, TRCA, Toronto Ontario</ref>||2 years||Silty sand with clayey silt deposits||Approx: 4:1||1 m||89%

|Infiltration trench<ref>SWAMP, 2002, [https://sustainabletechnologies.ca/app/uploads/2013/03/Performance-Assessment-of-a-SwalePerforated-Pipe-Stormwater-Infiltration-System-2002.pdf Performance Assessment of a Swale Perforated Pipe Stormwater Infiltration System], TRCA, Toronto, Ontario</ref>||2 years||Silty sand with clayey silt deposits||4:1 (approx.)||1||89

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|Permeable pavement + bioretention<ref>CVCa</ref>||4 years||Clayey silt on silt till||Approx: 6:1||?||80%

|Permeable pavement + bioretention<ref>Credit Valley Conservation, 2016, [https://cvc.ca/wp-content/uploads/2016/06/TechReport_Elm_Drive_Final.pdf Elm Drive Low Impact Development Infrastructure Performance and Risk Assessment Technical Report], Mississauga Ontario</ref>||4 years||Clayey silt on silt till||6:1 (approx.)||0.2-0.3 (approx.)||80

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|Bioretention<ref>CVCb</ref>||4 years||Silty clay||Approx.: 10:1||?||92%

|Bioretention<ref>Credit Valley Conservation, 2016, [https://cvc.ca/wp-content/uploads/2016/06/TechReport_Lakeview_Final.pdf Lakeview Low Impact Development Infrastructure Performance and Risk Assessment Technical Report], Mississauga Ontario</ref>||3 years||Silty clay||10:1 (approx.)||0.02||92

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|Bioretention<ref>CVCc</ref>||4 years||Silty clay fill over clay till||30:1||?||72%

|Bioretention<ref name = "IMAX">Credit Valley Conservation, 2016, [https://cvc.ca/wp-content/uploads/2016/01/IMAX-Technical-Report.pdf IMAX Low Impact Development Infrastructure Performance and Risk Assessment Technical Report], Mississauga Ontario</ref>||3 years||Silty clay fill over clay till||30:1||0.025||72

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|Permeable pavement<ref>CVCd</ref>||4 years||Silty clay fill over clay till||1:1||?||?

|Permeable pavement<ref name = "IMAX"/>||3 years||Silty clay fill over clay till||1:1||0.025||49

|}

|}

<nowiki>*</nowiki>Represents depth of sump below underdrain or outflow pipe.  In some cases, a flow control device was installed to slow outflow rates and enhance infiltration

<nowiki>*</nowiki>Represents depth of sump below [[underdrain]] or [[overflow]] pipe.  In some cases, a [[flow control]] device was installed to slow outflow rates and enhance infiltration

==References==

==References==

<references/>

<references/>

[[category:Infiltration]]

[[category:Infiltration]]