MIDS calculator tips and information for maximizing volume retention

This page provides information and recommendations for maximizing volume retention in the MIDS calculator.

Green roof

MIDS calculator symbol for green roofs

• Applicability
  • Can have a conventional roof draining to a green roof, but the conventional roof area must be less than or equal to the green roof area.
  • If the entire area is green roof (no conventional roof), then it is relatively easy to meet the volume retention requirement
  • The media depth is no longer restricted to 4 inches (extensive roofs). However, if you make the media thickness greater than 4 inches, you get a popup reminding you that you are designing an intensive roof. For most green roofs, a media thickness of 4 inches will be sufficient to meet the volume requirement.
• Volume retention
  • Maximize media depth to increase retention
  • Limit the amount of conventional roof being routed to a green roof to maximize volume retention

Link to green roofs

Bioretention without underdrain (bioinfiltration)

Symbol used for bioretention basin without underdrain (bioinfiltration) in the MIDS calculator.

• Applicability
  • Limited to 1.5 foot ponding depth
  • The 1.5 foot depth restriction limits infiltration on highly permeable soils
  • Diverse best management practice (BMP) that can be incorporated into many landscapes; provides multiple benefits (e.g. habitat, aesthetics)
  • Not effective in highly urban and ultra-urban environments because of required space
• Volume retention
  • Typically, you will want to maximize depth and limit surface area unless you are designing for other benefits, such as habitat

Link to bioretention

Bioretention with underdrain (Biofiltration)

Symbol for Bioretention basin (with underdrain) used in MIDS calculator.

• Applicability
  • Limited to 1.5 foot ponding depth
  • Diverse BMP that can be incorporated into many landscapes; provides multiple benefits (e.g. habitat, aesthetics)
  • Not effective in highly urban and ultra-urban environments because of required space
• Volume retention
  • Typically, you will want to maximize depth and limit surface area unless you are designing for other benefits, such as habitat
  • Raise the underdrain to the extent possible to maximize infiltration
  • Maximize evapotranspiration by placing a tree into the BMP
  • Phosphorus
    • Use Mix C or D to maximize phosphorus removal. Mixes A and B require testing and will likely fail the test.
    • Incorporate iron into the system

Link to bioretention

Infiltration basin/infiltration trench

MIDS calculator symbol for infiltration basin.

• Applicability
  • More effective than bioinfiltration because depth is only limited by drawdown time requirement
  • Not as effective as underground systems in highly urban and ultra-urban environments
  • May have limited ability to remove pollutants on highly permeable (A) soils
• Volume retention
  • Typically, you will want to maximize depth and limit surface area unless you are designing for other benefits, such as habitat

Link to infiltration basin/trench

Underground infiltration

Symbol for underground infiltration in MIDS calculator

• Applicability
  • A very effective BMP on highly permeable (A) soils. Less effective on B soils due to cost of construction and maintenance.
  • Effective in highly urban and ultra-urban environments
  • Can be difficult to make calculations and use the calculator to size the practice since calculations are made outside the calculator, unless you have specifications from the manufacturer
• Volume retention
  • Typically, you will want to maximize depth and limit surface area unless you are designing for other benefits, such as habitat

Link to infiltration basin/trench

Permeable pavement

MIDS calculator symbol for permeable pavement.

• Applicability
  • Very effective at reducing volume
  • Effective in highly urban and ultra-urban environments
  • Limited to 5:1 ratio for impervious:permeable pavement area (Note: permeable pavement is included in impermeable acreage in the calculator)
  • Should not route pervious runoff to permeable pavement and should limit impervious:pavement ratio to 2:1 due to maintenance needs
  • Generally cannot be used in high traffic areas or with heavy loads
• Volume retention
  • Typically will want to maximize surface area versus making the BMP deeper since the BMP is typically able to meet volume retention requirements
  • To increase volume retained, unless required, do not compact subsoil

Link to permeable pavement

Tree trench system/box without underdrain

Symbol used for the tree trench system/box (w/o underdrain) BMP in the MIDS calculator. Note the symbol shows multiple trees since tree trench systems contain multiple trees.

• Applicability
  • This is a good BMP in highly urban and ultra-urban environments
  • Water is stored in the media instead of being ponded. Thus, the amount of water stored is less than bioinfiltration or infiltration basin per unit volume because solids are taking up some of the available storage space
  • Diverse BMP that can be incorporated into many landscapes; provides multiple benefits (e.g. habitat, aesthetics)
• Volume retention
  • This is typically an underground BMP, so the system could be designed to maximize volume retention by maximizing either area or depth
  • Volume retention increase with tree size (evapotranspiration), but make sure the tree size is practical for the site
  • Determine the soil volume needed to maximize tree growth. This can be done by entering the number of trees in the appropriate cell in the calculator and examining the cells displaying volume reduction for ET and the soil volume per tree. Enter the tree number that maximizes ET while maximizing soil volume per tree.
  • Coniferous trees increase ET and canopy interception (Note: they must be suitable for the site)

Link to information on tree trenches

Tree trench system/box with underdrain

Symbol used in the MIDS calculator to represent the tree trench system-tree box BMP with an underdrain. Note the symbol shows multiple trees since tree trench systems contain multiple trees.

• Applicability
  • This is a good BMP in highly urban and ultra-urban environments
  • Water is stored in the media instead of being ponded. Thus, the amount of water stored is less than bioinfiltration or infiltration basin per unit volume because solids are taking up some of the available storage space
  • Diverse BMP that can be incorporated into many landscapes; provides multiple benefits (e.g. habitat, aesthetics)
• Volume retention
  • This is typically an underground BMP, so the system could be designed to maximize volume retention by maximizing either area or depth
  • Volume retention increase with tree size (evapotranspiration), but make sure the tree size is practical for the site
  • Determine the soil volume needed to maximize tree growth. This can be done by entering the number of trees in the appropriate cell in the calculator and examining the cells displaying volume reduction for ET and the soil volume per tree. Enter the tree number that maximizes ET while maximizing soil volume per tree.
  • Coniferous trees increase ET and canopy interception (Note: they must be suitable for the site)
  • Phosphorus
    • Use Mix C or D to maximize phosphorus removal. Mixes A and B require testing and will likely fail the test.
    • Incorporate iron into the system

Link to information on tree trenches

Swales

Symbol for Swale main channel (without an underdrain) used in the MIDS calculator.

Symbol for Swale side slope used in the MIDS calculator

• Applicability
• Swale side slope and swale main channel (with or without underdrain) act as a single BMP. Make sure impervious and pervious acres are applied to the side slope and not the main channel, unless water is routed directly to a main channel. Swale lengths for side slopes and main channel should be the same for a swale system
  • Diverse BMP that can be incorporated into many landscapes; provides multiple benefits (e.g. habitat, aesthetics)
  • Not a good volume retention practice unless on A soils and/or designed with check dams or bioretention base
  • Not good for highly urban and ultra-urban environments
• Volume retention
  • Increase swale length to maximize volume retention
  • Check dams are the most effective way of capturing and infiltrating water on permeable soils
  • Adding a bioretention base increase infiltration if underlying soils are permeable
  • Increasing the channel slope reduces infiltration

Link to information on swales

Harvest and reuse/cistern

MIDS calculator symbol for harvest and reuse.

• Applicability
  • It is difficult to meet the retention requirement with this BMP
  • Useful on low permeability soils for reducing the volume delivered to downstream BMPs
  • Can be used in highly urban and ultra-urban environments where there are small green spaces to irrigate
  • Most effective when there are large green spaces that can be irrigated
• Volume retention
  • If there is sufficient space on the site, ponds allow for greater volume storage. Volume reductions with cisterns are typically limited by the storage capacity of the cistern.
  • To maximize volume reduction, maximize the volume application area
  • On A soils you can apply up to 2 inches per week. This will exceed plant demand but because of the highly permeable soils, excess water will infiltrate. On other soils, to maximize volume reduction, choose “No” for the question asking about a user-defined irrigation rate.
  • For vegetation, volume retention follows this order: Trees > turf = vegetables > forage > cereals
  • Retaining water on-site typically has a small effect on volume retention

Link to information on stormwater harvest and reuse

Stormwater disconnection

Symbol for Stormwater Disconnection in MIDS calculator

• Applicability
  • Cannot be used to meet the Construction Stormwater permit volume retention requirement
  • It is difficult to meet the retention requirement with this BMP
  • Useful on low permeability soils for reducing the volume delivered to downstream BMPs
  • Limited by the area that can effectively infiltrate water (see diagram on BMP Parameters tab for this BMP in the calculator)
• Volume retention
  • Maximize the impervious area delivering water to the practice
  • Maximize the pervious area (NOTE: this cannot exceed the pervious acres entered in the Watershed tab for this BMP)

Link to information on stormwater impervious disconnection

Wet swale, sand filter, constructed wetland, and constructed pond

These BMPs do not reduce volume

Summary table

The following table provides some summary information for BMPs in the MIDS calculator. Several cells show a range of values. These are explained below.

• Bioretention without underdrain, because of the depth limitation, has moderate retention capacity on A soils compared to other infiltration practices which can be built deeper to infiltrate more water
• Swale main channel has low infiltration capacity on low permeability soils, but high capacity on A soils when check dams are used
• Tree trench without underdrain is an effective infiltration practice but design may be limited by media requirements. High volume retention capacity may require extensive media, which could be cost-prohibitive.
• Tree trench with underdrain may be limited by media requirements. Moderate volume retention capacity may require extensive media, which could be cost-prohibitive.

BMP	Volume reduction capacity (total)	Volume reduction capacity (per unit BMP area)	Ultra-urban applicability	Multiple benefit applicability
Green roof	Low	High	Yes	Yes
Bioretention w/o underdrain	Moderate	Moderate-high	No	Yes
Bioretention w/underdrain	Low	Low	No	Yes
Infiltration basin/trench	High	High	No	Limited
Underground infiltration	High	High	Yes	Limited
Permeable pavement	Low	High	Yes	Limited
Tree trench w/o underdrain	Moderate-high	High	Yes	Yes
Tree trench w/underdrain	Low-moderate	Low	Yes	Yes
Swale main channel	Low-high	Low-high	No	Yes
Swale main channel w/ underdrain	Low	Low	No	Yes
Harvest and reuse/cistern	Moderate	Moderate	Yes	Limited
Stormwater disconnection	Moderate	Moderate	No	Limited