When designing stormwater for a project, we must consider stormwater runoff for both rate and volume.  Peak rate is the easier of the two to control as we can manage the rate in a stormwater basin, berm, rain garden, bed, etc.  The peak rate management does not have to rely on the on-site soil conditions as the volume control does.  Typically, the runoff volume is managed by infiltrating the change in two-year runoff volume when comparing the pre-development to post-development. 

The problem with infiltration is that not all soils and site conditions allow for infiltration.  There can be many factors or limiting zones that prevent infiltration on site, one of the most common limiting zones we encounter is shallow bedrock.  Other limiting zones might be high water table, mottling, or clayey soils that do not infiltrate. 

Last month, Pennsylvania DEP released a white paper on a ‘Managed Release Concept’ (MRC).  The MRC is a post-construction stormwater method that manages the increase in stormwater runoff volume through storage and filtration of stormwater runoff.  In the white paper, DEP provides guidance for when an MRC is applicable and also the methods for implementing an MRC into the site design.  Using an MRC does not exclude the site from all infiltration.  If there are some areas within the site to provide infiltration, infiltration must be maximized in those areas.  The designer is required to provide an adequate site assessment of soils and geology to demonstrate that infiltration of the entire change in two-year runoff volume is not feasible.  If the conditions are met, then the designer can request from the Conservation District and/or DEP for authorization to proceed with the MRC design.

Sources:  

PA DEP Managed Release Concept white paper – December 13, 2018 
http://files.dep.state.pa.us/Water/BPNPSM/StormwaterManagement/ConstructionStormwater/Managed_Release_Concept.pdf