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 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.  

< Insert figure 8 from tech brief >

< Insert figure 8 from tech brief >

It follows, therefore, that infiltration would be enhanced by maintaining a hydraulic head above the point at which infiltration slows to negligible levels.  From a design perspective, this means:

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

#allowing water to remain within the storage reservoir below the underdrain or outlet continuously, or at least for longer time periods than the typical 48 to 92 hour drawdown time requirements for other stormwater BMPs, and  

This means:

#onfiguring the storage to be more vertically oriented to foster higher hydraulic head. As such, the facility would be deeper, with higher ratios of side wall to bottom areas, and a portion of the total storage regarded as effectively permanent.     

#Allowing water to remain within the storage reservoir below the underdrain or outlet continuously, or at least for longer time periods than the typical 48 to 92 hour drawdown time requirements for other stormwater BMPs, and  

#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. 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.

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.  

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==