MIDS calculator tips and information for maximizing volume retention
This page provides information and recommendations for maximizing volume retention in the MIDS calculator.
Contents
Green roof
- 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 restricted to 4 inches (extensive roofs). If you have an intensive roof (media depth greater than 4 inches), you can simulate an intensive roof by routing one green roof to another. Make sure the surface area matches on the two roofs and the combined media depth for the two roofs equals the media depth for the intensive roof.
- Volume retention
- Maximize media depth to increase retention
- Limit the amount of conventional roof being routed to a green roof to maximize volume retention
Bioretention without underdrain (bioinfiltration)
- Applicability
- Limited to 1.5 foot ponding depth
- The 1.5 foot depth restriction limits infiltration on highly permeable soils
- 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
Bioretention with underdrain (Biofiltration)
- 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
Infiltration basin/infiltration trench
- 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
- 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 settings
- 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
- Applicability
- Very effective at reducing volume
- Effective in highly urban and ultra-urban settings
- 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
8. Tree trench without underdrain a. This is typically an underground BMP, so it may matter less whether you design the system to maximize area or maximize depth b. This is a good BMP in highly urban and ultra-urban settings c. 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 d. There is a loss in volume credit when the soil volume per tree is less than (canopy projection area * 2). Canopy projection area changes with tree size. Maximum ET is achieved with larger trees, but larger trees require more volume and may not be practical in some settings. 9. Tree trench with underdrain a. Inputs are similar to biofiltration b. This is a good BMP in highly urban and ultra-urban settings c. Raise the underdrain to the extent possible to maximize infiltration d. Use Media Mix D to maximize phosphorus removal, or utilize iron-enhanced media 10. Swales a. 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 b. Infiltration increases exponentially as the swale length increases c. Check dams are the most effective way of capturing and infiltrating water on permeable soils d. Increasing the channel slope reduces infiltration e. No dissolved phosphorus removal for filtered water 11. Harvest and reuse/cistern a. Applicability i. It is difficult to meet the retention requirement with this BMP ii. Useful on low permeability soils for reducing the volume delivered to downstream BMPs iii. Can be used in ultra-urban areas where there are small green spaces to irrigate iv. Most effective when there are large green spaces that can be irrigated b. Volume retention i. 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. ii. To maximize volume reduction, maximize the volume application area iii. On A soils you can apply up to 2 inches per week. This will exceed plant demand but because of the 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. iv. For vegetation, volume retention follows this order: Trees > turf = vegetatbles > forage > cereals v. Retaining water on-site typically has a small effect on volume retention 12. Stormwater disconnection a. Applicability i. Cannot be used to meet the Construction Stormwater permit volume retention requirement ii. It is difficult to meet the retention requirement with this BMP iii. Useful on low permeability soils for reducing the volume delivered to downstream BMPs iv. Limited by the area that can effectively infiltrate water (see diagram on BMP Parameters tab for this BMP in the calculator) b. Volume retention i. Maximize the impervious area delivering water to the practice ii. Maximize the pervious area (NOTE: this cannot exceed the pervious acres entered in the Watershed tab for this BMP) 13. Wet swale, sand filter, constructed wetland, and constructed pond do not reduce volume.