Below is a summary of requirements, recommendations and other information for using the Minimal Impacts Design Standards (MIDS) calculator for bioretention BMPs. Links to MIDS pages and the MIDS calculator are included at the bottom of this page.
The MIDS calculator includes bioinfiltration (bioretention with no underdrain) and biofiltration (bioretention with an underdrain) as BMP options. The user must input the impervious cover acreage draining to the BMP.
For bioinfiltration systems, the user must input the overflow surface area (the area of the BMP at the point where overflow occurs), the surface area of the bottom of the basin, and the depth of the basin (the depth between the overflow surface area and the bottom surface area). The volume that can be stored in the basin is equal to the basin depth times the average surface area of the basin ((overflow area + bottom surface area)divided by 2). Water is assumed to be instantaneously delivered to the basin (kerplunk method). In reality, water would be infiltrating as the basin fills. The total volume infiltrated is thus underestimated. The user must also input the underlying soil type (HSG A, B, C, or D) and the time required for drawdown (48 hours or in the case of trout streams, 24 hours).
For bioretention systems (systems with an underdrain), the user must input the surface area at the overflow, the surface area at the media surface, the surface area at the underdrain, the surface area at the bottom of the system, the total media depth, and the depth below the underdrain (depth from the underdrain to the bottom of the BMP). If the underdrain is at the bottom of the system, the surface area at the underdrain will equal the bottom surface area and the depth below the underdrain will equal 0. The underlying soil and drawdown time are specified as with bioinfiltration. The user must specify the water holding capacity of the media (field capacity minus wilting point). As with bioinfiltration, runoff is delivered instantaneously to the BMP. Water is lost through infiltration below the underdrain, through infiltration out the sides of the basin, and through evapotranspiration. As with bioinfiltration, the calculation method underestimates actual infiltration because water is instantaneously delivered to the BMP. The user can specify that an impermeable liner exists at the bottom or on the sides of the basin, in which case infiltration will not occur and the only loss of water will be through evapotranspiration.