The purpose of the amended soil in a bioretention facility is to filter the stormwater runoff and remove pollutants.

Typically put under the amended soil layer of a bioretention site is a:

To modify an outlet control structure to meet hydromodification flow control standards simply, the following can be added:

At the downstream end of the bioretention site a 
             or              controls the surface discharge from the bioretention facility and detains runoff, encouraging it to infiltrate into the amended soil. 

Bioretention facilities filter stormwater for pollutants and provide storage to runoff.                     and                     from these facilities can reduce runoff volume and peak flows.