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==Introduction==

==Introduction==

The texture of native soils has a strong influence on the capacity of LID practices to reduce runoff volumes through infiltration.  While sandy and silty sand soils have a high capacity to infiltrate water, fine textured soils containing a high percentage of clay may not be suitable for infiltration, or require design adaptations to promote sufficient infiltration.

The texture of native soils has a strong influence on the capacity of LID practices to reduce runoff volumes through [[infiltration]].  While sandy and silty sand soils have a high capacity to infiltrate water, fine textured soils containing a high percentage of clay may not be suitable for infiltration, or require design adaptations to promote sufficient infiltration.

==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.

A variety of methods are available for [[Infiltration: Testing| measuring and estimating infiltration rates]].  Selecting an appropriate method will depend on the size and scale of the area over which infiltration rates are being determined.

A variety of methods are available for [[Infiltration: Testing| measuring and estimating infiltration rates]].  Selecting an appropriate method will depend on the size and scale of the area over which infiltration rates are being determined.

==LID design adaptations on low permeability soils ==

==LID design adaptations on low permeability soils==

 

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 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==

A number of field studies of LID practices have been conducted in southern Ontario on fine textured soils. Several of these studies have yielded data that allow for calculation of the facility wide infiltration rate during natural rain events of varying sizes.  These are summarized in Figure xx.  Infiltration rates on silty clay, clayey silt and sandy silt textured soils had a median value of 3.3 mm/h and a range between 0.3 and 17.8 mm/h. Permeable pavements had lower values in part due to compaction of the subsoils to accommodate traffic loading.

[[File:Infiltration_rates_studies.png|thumb|400 px|Facility wide infiltration rates for different LID practices installed in the Greater Toronto Area]]

Figure xx:  Facility wide infiltration rates for different LID practices installed in the Greater Toronto Area

Stormwater runoff volume reductions vary between sites, primarily due to factors other than the native soil infiltration rate. For instance, the infiltration trenches and chambers shown in the Figure xx had similar native soil infiltration rates (3.1 to 5.1 mm/h), but runoff reduction values varying from 16 to 90%, chiefly due to site to site differences in the I:P ratio (reference definition), which ranged from 10:1 to 155:1.   

*Stormwater runoff volume reductions varied between sites, primarily due to factors other than the native soil infiltration rate. For instance, the infiltration trenches and chambers had similar native soil infiltration rates (3.1 to 5.1 mm/h), but runoff reduction values varying from 16 to 90%. This is attributed to differences in the I/P ratio, which ranged from 10:1 to 155:1.   

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, even if outflow rates from the non-elevated drains are controlled by orifices or flow control valves.

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 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"

|+ Runoff volume reduction performance for selected monitoring studies of LID practices or sites conducted over a period of a year or more (Sortable, just click on headers)

|+ Runoff volume reduction performance for selected monitoring studies of LID practices or sites conducted over a period of a year or more <br>

(Sortable, click headers)

|-  

|-  

!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 (%)

|-

|-

!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)*

|-

|-

|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>||Two 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>||Two 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>||Five 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>||One 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>||Two 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>||Two 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>||Two 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>||Two 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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|-

|Infiltration trench<ref>SWAMP, 2005.  Performance Assessment of a Perforated Pipe Stormwater Exfiltration system, Toronto, Ontario, TRCA, Toronto, Ontario</ref>||Two 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>||Two 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>||Four 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>||Four 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>||Four 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>||Four 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==

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