Changes

==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. Figure xx shows this relationship for an infiltration trench in Caledon that was filled to the outflow elevation during an intense rain event, and then allowed to naturally infiltrate over a 23 day dry period (ref tech brief).  As head decreased, infiltration rates from from 2.5 to 3.8 mm/h during the first two days when trench water levels were above 1.5 m , down to rates of only 1 to 1.5 mm hour after six and half days when water level elevations declined below 1 m.  

The rationale for variations in practice design for sites with fine textured soils is based on the relationship between hydraulic head and infiltration. Figure xx shows this relationship for an infiltration trench in Caledon that was filled to the outflow elevation during an intense rain event, and then allowed to naturally infiltrate over a 23 day dry period (ref tech brief).  As head decreased, infiltration rates from from 2.5 to 3.8 mm/h during the first two days when trench water levels were above 1.5 m , down to rates of only 1 to 1.5 mm hour after six and half days when water level elevations declined below 1 m.  

Stormwater runoff volume reductions varied from site to site, 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.  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.

Stormwater runoff volume reductions varied from site to site, 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.  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.

Table xx shows the runoff volume reduction performance for selected monitoring studies of LID practices or sites conducted over a period of a year or more.  These results 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.       

 

 

Van Seters and Young, 2105 Infiltration trench Two growing seasons Silty clay 10:1 <10 cm; flow rate control 80%

Van Seters and Young, 2105 Infiltration trench Two growing seasons Silty clay 10:1 <10 cm; flow rate control 80%

Van Seters and Young, 2015 Bioretention Two growing seasons Silty clay 10:1 <10 cm; flow rate control 90%

Van Seters and Young, 2015 Bioretention Two growing seasons Silty clay 10:1 <10 cm; flow rate control 90%