Changes

In cases where the 90th percentile storm volume is fully retained (i.e. infiltrated or evapotranspired), the wet pond may be substituted with a dry pond.  In this scenario, the water quality and water balance volumes are provided by the upstream facilities, and the dry provides flood and erosion control.  Runoff volumes exceeding the 90th percentile storm volume would be treated at a more basic level.  Lower maintenance costs would be the primary advantage of substituting a dry pond for a wet one, although the dry pond may also provide opportunities for multiple uses in some contexts.

In cases where the 90th percentile storm volume is fully retained (i.e. infiltrated or evapotranspired), the wet pond may be substituted with a dry pond.  In this scenario, the water quality and water balance volumes are provided by the upstream facilities, and the dry provides flood and erosion control.  Runoff volumes exceeding the 90th percentile storm volume would be treated at a more basic level.  Lower maintenance costs would be the primary advantage of substituting a dry pond for a wet one, although the dry pond may also provide opportunities for multiple uses in some contexts.

One final and relatively common example of infiltration practice treatment trains on smaller sites would be a [[Pretreatment#Concentrated underground flow|non-infiltrating filter]] used as pre-treatment to an infiltration practice such as a [[infiltration trench|trench or [[infiltration chamber|chamber]] system.  The upstream facility is sized to treat the water quality volume (or higher) and the downstream infiltration trench or chamber is sized to meet water balance criteria (e.g. infiltration of the 90th percentile storm).  If the entire water quality volume is infiltrated by the downstream practice, the TSS load reduction for the treatment train would be 100%.  If only 80% is infiltrated in the downstream facility, the TSS load reduction would be 96% since 80% of TSS is reduced in the first facility and 80% of the volume is reduced in facility two (0.8 + (0.8 x 0.2) = 0.96 x 100).  The second facility would not further reduce TSS concentrations because only fine unfilterable sediments are left once the runoff has passed through facility one.  Therefore, facility two reduces water quality loads only by reducing the volume of runoff (which was assumed to be 80% in this example).  

One final and relatively common example of infiltration practice treatment trains on smaller sites would be a [[Pretreatment#Concentrated underground flow|non-infiltrating filter]] used as pre-treatment to an infiltration practice such as a [[infiltration trench|trench]] or [[Infiltration chambers|chamber]] system.  The upstream facility is sized to treat the water quality volume (or higher) and the downstream infiltration trench or chamber is sized to meet water balance criteria (e.g. infiltration of the 90th percentile storm).  If the entire water quality volume is infiltrated by the downstream practice, the TSS load reduction for the treatment train would be 100%.  If only 80% is infiltrated in the downstream facility, the TSS load reduction would be 96% since 80% of TSS is reduced in the first facility and 80% of the volume is reduced in facility two (0.8 + (0.8 x 0.2) = 0.96 x 100).  The second facility would not further reduce TSS concentrations because only fine unfilterable sediments are left once the runoff has passed through facility one.  Therefore, facility two reduces water quality loads only by reducing the volume of runoff (which was assumed to be 80% in this example).

==References==

==References==