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*'''Required drawdown time (hrs)''': This is the time the water captured by the BMP must drain into the underlying soil/media. The user may select 48 or 24 hours. The MPCA [http://www.pca.state.mn.us/index.php/water/water-types-and-programs/stormwater/construction-stormwater/index.html Construction Stormwater General Permit] requires drawdown within 48 hours, but 24 hours is Highly Recommended when discharges are to a trout stream. The user will encounter an error if the water stored in the BMP cannot drawdown in the required time. | *'''Required drawdown time (hrs)''': This is the time the water captured by the BMP must drain into the underlying soil/media. The user may select 48 or 24 hours. The MPCA [http://www.pca.state.mn.us/index.php/water/water-types-and-programs/stormwater/construction-stormwater/index.html Construction Stormwater General Permit] requires drawdown within 48 hours, but 24 hours is Highly Recommended when discharges are to a trout stream. The user will encounter an error if the water stored in the BMP cannot drawdown in the required time. | ||
+ | ==Biofiltration with a raised underdrain== | ||
+ | [[File:Biofiltration.png|300px|left|thumb|alt=image for biofiltration (bioretention with underdrain) used in MIDS calculator|<font size=3>Symbol for biofiltration (bioretention with underdrain) in MIDS calculator.</font size>]] | ||
+ | [[File:Biofiltration MIDS inputs.png|300px|thumb|alt=image of inputs needed for biofiltration in the MIDS calculator|<font size=3>Schematic showing the inputs needed for biofiltration in the MIDS calculator. The equation used to calculate volume captured by the BMP is included.</font size>]] | ||
+ | [[File:Available water capacity.png|thumb|300px|alt=plot of available water capacity|<font size=3>Plot showing available water capacity over a range of soil textures. Available water is calculated as the difference, in inches of water per foot of soil depth, between field capacity and permananet wilting point, divided by 12 to convert to a fraction. Ohio Agronomy Guide, 14th edition, Bulletin 472-05</font size>]] | ||
+ | {{alert|Note the term Biofiltration corresponds with a bioretention system with an underdrain. The underdrain can be at or raised above the bottom of the system. See [[Bioretention terminology]]|alert-info}} | ||
+ | For a biofiltration system with an underdrain elevated (raised) above the bottom of the basin, a volume credit is provided for the volume of water stored below the underdrain. this is in addition to volume credits given for infiltration through the bottom and sidewalls of the basin and for evapotranspiration (ET). | ||
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+ | ===Methodology=== | ||
+ | The methodology for a biofiltration system with an elevated underdrain is the same as for a biofiltration system with an [[Requirements, recommendations and information for using bioretention BMPs in the MIDS calculator#Biofiltration with underdrain at the bottom|underdrain at the bottom]], with the exception that an additional credit is given for water stored below the underdrain. | ||
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. | 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 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; i.e. when the bioinfiltration basin is filled with water), 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 (Hydrologic Soil Group (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, most of the water captured by the BMP is 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 captured by the BMP is a function of the media depth rather than the depth of water that can be ponded above the soil/media surface. 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 ((area at media surface + bottom surface area)divided by 2).
The depth of the basin does not affect the volume or pollutants retained by the BMP. 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.
The volume of water lost out the bottom equals 0.06 inches per hour times the surface area at at the underdrain times the drawdown time. The default was set at 0.06 inches per hour to represent a D soil. This value was set at this low rate because it is assumed most of the water will pass through the underdrain before it can infiltrate through the bottom of the BMP. This may be a conservative assumption if underdrains are small, spaced far apart, and/or the underlying soils has a rapid infiltration rate.
Water lost from the sidewall is considered to infiltrate vertically into the surrounding soil. This volume equals the (surface area at overflow minus the bottom surface area) times 0.06 inches per hour times one-half of the drawdown time. The drawdown time is reduced by a factor of 2 to account for drop in water level within the BMP over the 48 hour period. The drop in water level is therefore considered to be linear over the drawdown time.
The volume of water lost through evapotranspiration (ET) is assumed to be the smaller of two calculated values.
These two values are compared and the volume lost to ET is the smaller of the two values.
If the user specifies that a liner exists either at the sidewalls or bottom of the basin, no volume credit is given for these terms. The ET credit is provided regardless of the presence of a liner.
For biofiltration systems with an underdrain at the bottom, the user must indicate the underdrain is not elevated above native soils. The user must indicate whether either the bottom or sides of the basin are lined with an impermeable liner. The user must also input the overflow surface area (the area of the BMP at the point where overflow occurs; i.e. when the biofiltration basin is filled with water), the surface area at the media surface, the surface area of the bottom of the basin, the total media depth of the basin (the depth between the media surface and the bottom surface), the media field capacity minus wilting point, the media porosity, the underlying soil type (Hydrologic Soil Group (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 an underdrain elevated (raised) above the bottom of the basin, a volume credit is provided for the volume of water stored below the underdrain. this is in addition to volume credits given for infiltration through the bottom and sidewalls of the basin and for evapotranspiration (ET).
The methodology for a biofiltration system with an elevated underdrain is the same as for a biofiltration system with an underdrain at the bottom, with the exception that an additional credit is given for water stored below the underdrain.
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.