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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). < Insert figure 8 from tech brief >
→LID design adaptations on low permeability soils
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==
[[File:Caledon infiltration.png|thumb|400 px|Change in infiltration rate observed in an infiltration trench in Caledon, ON, allowed to infiltrate over a 23 day dry period.]]
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.