The MIDS calculator includes bioinfiltration (bioretention with no underdrain) and biofiltration (bioretention with an underdrain) as BMP options. For biofiltration the underdrain may be at or raised above the bottom of the BMP. 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.
For a bioinfiltration system all water captured by the BMP is infiltrated between rain events into the underlying soil. All pollutants in the infiltrated water are captured. Water that bypasses the BMP is not treated.
The volume of runoff water delivered to the BMP equals the performance goal (1.1 inches or user specified performance goal) times the impervious area draining to the BMP. The BMP must be sized correctly in the calculator to capture the water delivered to the BMP. Water captured by the BMP is stored above the media and below the overflow point of the BMP. The runoff volume is considered to instantaneously enter the BMP (kerplunk method). The captured volume is therefore equal to the basin depth times the average surface area of the basin ((overflow area + bottom surface area) divided by 2).
Water infiltrates at a rate equal to the saturated conductivity of the media below the ponded depth of the BMP. The most restrictive layer (lowest conductivity) within 3 feet of the bottom of the ponded water is used in the calculation. Captured water must drain within 48 hours, or 24 hours if the user specifies that as the drainage time (in the case of discharges to a trout stream).
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, the depth of the basin (the depth between the overflow surface area and the bottom surface area), 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). These are discussed below.
For a biofiltration system with the underdrain at the bottom, water is primarily lost to the underdrain. However some water infiltrates through the basin bottom and sidewalls. Evapotranspiration also occurs from vegetation in the biofiltration BMP.
The volume of water delivered to the BMP equals the performance goal (1.1 inches or user specified performance goal) times the impervious area draining to the BMP. Unlike bioinfiltration, the volume of water netering the BMP is a function of the media depth as well as the area at the media surface and at the bottom (at the bottom of the underdrain). The volume is considered to instantaneously enter the BMP (kerplunk method). The captured volume is therefore equal to the basin depth times the average surface area of the basin ((overflow area + bottom surface area)divided by 2).
Most of the water entering the BMP will pass through the underdrain. This water provides treatment for TSS and particulate phosphorus but provides no treatment for dissolved phosphorus. A volume credit is given for water that infiltrates through the bottom of the BMP, through the sideslopes of the BMP, and for water that is evapotranspired by plants in the BMP.
For biofiltration 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 permanent 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. 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.
Calculations for water lost in biofiltration systems are based on the following assumptions.
Bioinfiltration and biofiltration BMPs can be routed to other BMPs in the MIDS calculator (other than green roofs). All other BMPs in the calculator can be routed to bioinfiltration and biofiltration BMPs. The default storm event is 1.1 inches. This value can be changed by the user. The calculator will notify the user if the default is changed.
Total suspended solids (TSS), particulate phosphorus and dissolved phosphorus loads and reductions in loading are calculated using event mean concentrations (EMCs). Default EMCs are 54.5 milligrams per liter for TSS and 0.3 milligrams per liter for total phosphorus (particulate plus dissolved). These can be changed by the user. The calculator will notify the user if the default is changed.
For bioinfiltration systems, all TSS and phosphorus captured by the BMP are reduced from the overall load delivered to the BMP. Note that more water may be delivered to the BMP than can be stored in the BMP. Excess water bypasses the BMP and receives no treatment from the BMP.
For biofiltration systems, phosphorus removal is 100 percent for all water that infiltrates through the bottom or sides of the basin. For water that is captured by an underdrain, phosphorus removal is 50 percent and TSS removal is 85 percent. It is assumed that 55 percent of all phosphorus is particulate and 45 percent is dissolved.