Bioretention stormwater controls may be referred to, among other names, as biofiltration basins, bio basins, biofilters or a raingardens.
Bio swales or biofiltration swales can also be modeled as Bioretention stormwater controls.
Total Volume
The Total Volume value shown in the bottom-right corner of the data form shows the volume available in the system up to the Freeboard Elevation.
Dimensions
The Dimensions tab has the following fields:
Ponding Area
The depth within the ponding area is determined from two of the following three parameters, with the third automatically calculated based on the option selected:
- Exceedance Elevation - Represents the elevation (above datum) of the stormwater control above which flooding is reported. This may be the elevation that interacts with the ground surface or the top of an embankment, whichever is higher. An overflow or spillway crest elevation may be specified at a lower elevation as part of the outlet control details. If a Surface Data model (TIN) is present the Exceedance Elevation will automatically be picked up from the center of the icon or from the lowest point on the bioretention perimeter if an outline has been drawn. This combined with the Freeboard setting allows a Status of Flood Risk to be assigned to the system on the Summary. For stormwater controls on a slope the Exceedance Elevation applies to the downstream (lowest) end. Above the Exceedance Elevation, water will be stored above the ground and then allowed to drain back into the network. The default ponding area used for the flooded volume is 1000m².
- Depth - Represents the depth of the ponding area.
- Base Elevation - Represents the elevation (above datum) of the base of the ponding area (or top of the filter area). For structures on a slope the invert elevation applies to the downstream (lowest) end.
The cross sectional area of the ponding area is then determined from the Top Area, Side Slope and Base Area. As per the depth calculation when two values are specified the third will automatically be calculated based on the option selected:
- Top Area - Displays the calculated surface area at the Top of the ponding area.
- Side Slope - Side Slope of the ponding area.
- Base Area - Defines the surface area at the base of the ponding area. This also gives the plan area of the filter of the bioretention system.
Freeboard - Controls how close to the specified Exceedance Elevation the water must reach before the Status (on the Summary) shows Flood Risk.
Length - The maximum path length through the stormwater control, used only for the purpose of calculating Time of Travel. The Length will be automatically set from the dimensions of the stormwater control unless over typed, in which case it is set to "user specified ".
Longitudinal Slope - Slope along the length of the stormwater control.
Filtration Rate - Defines the rate of flow through the filter medium based on the material used. May be referred to as saturated hydraulic conductivity.
Friction Scheme - Specifies the formula used to calculate velocity and time of travel. Select from: Manning's, and Colebrook-White. The variable below the combo box depends on the option selected.
n - Manning’s n roughness value. Used by the Manning Formula equation to calculate the velocity and therefore time of travel when using the Manning's option.
Roughness - Colebrook-White roughness value. Used by the Colebrook-White Formula equation to calculate the velocity and therefore time of travel when using the Colebrook-White option.
Filter Area:
Base Elevation - Represents the elevation (above datum) of the base of the bioretention stormwater control. Automatically populated by subtracting the Ponding Depth and the Filter Depth from the Exceedance Elevation.
Under Drain On: Specifies whether a pipe or conduit underdrain is present in the lowest layer of the filter area. If under drain is on the following can be specified:
Height Above Base - Height of the invert of the pipe above the base of the bioretention stormwater control.
Diameter - Defines the diameter of the pipe/s.
No. of Barrels - Represents the number of pipes.
Friction Scheme - Specifies the formula used to calculate velocity and time of travel. Select from: Manning's, and Colebrook-White. The variable below the combo box depends on the option selected.
n - Manning’s n roughness value. Used by the Manning Formula equation to calculate the velocity and therefore time of travel when using the Manning's option.
Roughness - Colebrook-White roughness value. Used by the Colebrook-White Formula equation to calculate the velocity and therefore time of travel when using the Colebrook-White option.
Release Height - Height above the invert of the under drain pipe at which the water is released (marked in red on the diagram below). This is commonly used to create a saturated zone.
Sizing Calculator
The Sizing Calculator option allows the user to re-size the Bioretention by specifying a volume and a parameter to modify to achieve that volume. The Sizing Calculator is discussed in more detail in the Stormwater Control Sizing Calculator section.
Filtration Layers
The Filtration Layers tab has the following fields:
Use - Allows the user to specify whether or not the optional Soil layer is in use.
Name - Allows the user to enter a name for each filtration layer.
Filtration Layer Depth - Depth of the filter layer.
Porosity - Porosity of the filter layer; e.g. a value of 30.000 represents 30% void space.
Conductivity - The conductivity rate is used for the calculation of both the horizontal and vertical transfer of the water in the sub-surface layers. This can also be referred to as saturated hydraulic conductivity. Please refer to the Guidance on Conductivity Rates for guidance on how to set these values depending on the type of material. As a rule of thumb, the Conductivity generally increases when the porosity increases (unless the soil has lots of unconnected voids).
Soil Type - The type of soil used for the soil layer. this can be customized further using the adjacent Soil Properties Calculator.
Soil Type Property Calculator
Field Capacity - Defines the minimum moisture content in the layer once the soil has fully drained from gravity only.
Wilting Point - Defines the water content for a well dried soil (where only water bound to the soil particles remains). This is the absolute minimum water content for this soil.
Inlets
Explore the Inlets page for more details on the different types of Inlets that can be specified.
Outlets
Explore the Outlets page for more details on the different types of Outlets that can be specified.
Advanced
The Advanced tab has the following fields:
Ponding Area
Side Infiltration Rate - Defines the rate of infiltration through the sides of the ponding area. This should be determined from a performance site test.
Base Perimeter - Defines the perimeter of the ponding area at the base of the ponding area. If the surface area is drawn on the Plan the field will be automatically populated based on the side slope and the Top Perimeter. It can also be overwritten to become user-defined.
Top Perimeter - Defines the perimeter of the ponding area at the top of the ponding area (exceedance elevation). If the surface area is drawn on the Plan the field will be automatically populated but can be overwritten to become user-defined.
Filter Area
Base Infiltration Rate - Defines the rate of infiltration through the base of the filter area. This should be determined from a performance site test.
Side Infiltration Rate - Defines the rate of infiltration through the sides of the filter area. This should be determined from a performance site test.
Safety Factor - Reduces the infiltration rate during the analysis to account for silting up or poor maintenance. This field is required for the UK.
Pollution
The Pollution tab has the following fields:
Name - Name of pollutants. This is populated based on the Pollutants set up as part of the Site Data.
Aspect - A section that details if the pollutant is Filter or Ponding Pollution.
Background Concentration - Value below which the pollution concentration cannot fall during analysis. When concentration reaches this elevation, no further removal occurs.
Method - Percentage Removal or First Order Decay method can be chosen. Click on the links for more information about each method.
Percentage Removal - Available if Percentage Removal entered. The value entered will be deducted from the Inflow into the system.
τ - The decay time constant or (mean) lifetime of the pollutant. It can be entered manually or calculated from the decay constant or decay half-life. See Pollutant Removal Method - First Order Decay for more details.