Calculating credits for green roofs

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The MPCA will be updating credit pages for most structural BMPs in the Manual. We anticipate these pages will be completed by Spring of 2015.

Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual BMP or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

• providing incentives to site developers to encourage the preservation of natural areas and the reduction of the volume of stormwater runoff being conveyed to a best management practice (BMP);
• complying with permit requirements, including antidegradation (see [1]; [2]);
• meeting the MIDS performance goal; or
• meeting or complying with water quality objectives, including Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs).

This page provides a discussion of how biofiltration practices can achieve stormwater credits. Green roofs achieve reductions in stormwater volume compared to conventional roofs. Green roofs can effectively remove or reduce loads of many pollutants that are discharged from conventional roofs, although they may be less effective than other BMPs in removing phosphorus and nitrogen. Green roofs act as vegetated filters and provide temporary storage of rainwater or snowmelt. Water kept in storage may eventually be evapotranspired or "bleed' out of the system to the underlying drainage layer. Water reaching the drainage layer is eventually lost from the green roof system. Volume and pollutant reductions constitute stormwater credits that can be used to meet various goals (e.g. Total Maximum Daily Loads (TMDLs), Minimal Impact Design Standards (MIDS) performance goals). Green roofs will achieve the greatest credit when they are properly designed, constructed and maintained.

Overview

Green roofs consist of a series of layers that create an environment suitable for plant growth without damaging the underlying roof system. Green roofs create green space for public benefit, energy efficiency, and stormwater retention/ detention. Green roofs are excellent BMPs in ultra-urban settings where it is otherwise difficult to achieve volume and pollutant reductions due to space constraints. Since green roofs release water over a period of time following a precipitation event, they are most effective when discharge from the green roof is to a pervious surface, such as turf or a filter strip. Because green roofs effectively remove sediment, discharge from a green roof can be routed to any BMP (e.g. bioretention, infiltration basin, permeable pavement).

Pollutant removal mechanisms

Green roofs provide filtering of suspended solids and pollutants associated with those solids, although total suspended solid (TSS) concentrations from traditional roofs are generally low. Green roofs provide both volume and rate control, thus decreasing the stormwater volume being delivered to downstream Best Management Practices (BMPs).

Location in the treatment train

Green roofs occur at the beginning of stormwater treatment trains. They may receive discharge from another roof, including conventional roofs, but otherwise do not receive stormwater runoff.

Methodology for calculating credits

This section describes the basic concepts and equations used to calculate credits for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Specific methods for calculating credits are discussed later in this article.

Green roofs generate credits for volume by adsorbing water in the planting media. This water may be taken up by plants, evaporate from the media, or be slowly released from the roof. Released water may be routed to a permeable surface, such as a lawn. Although precipitation typically has few pollutants, green roofs can effectively filter many airborne pollutants, including wind blown sediment and pollutant in precipitation, including metals and organics. Because the green roof media must support plant growth, green roofs are not effective at removing phosphorus.

Assumption and approach

The following general assumptions apply in calculating the credit for green roofs. If any of these assumptions is violated, the credit will be reduced.

• The green roof is properly designed.
• The green roof was properly constructed, consistent with the design criteria.
• The green roof is properly maintained. The performance of the green roof should be regularly assessed.

In the following discussion, the water quality volume (WQV) is delivered instantaneously to the BMP. The WQV is stored in the media. The WQV can vary depending on the stormwater management objective(s). For construction stormwater, the water quality volume is 1 inch off new impervious surface. For MIDS, the WQV is 1.1 inches.

Volume credit calculations

Screen shot of MIDS calculator showing inputs used to calculate volume credit for a green roof. Inputs include the media depth and green roof area.

Information: The article providing an overview of green roofs contains an extensive discussion of green roof hydrology and water retention.

A green roof system acts similar to a bioretention system with an underdrain, although the underlying drainage layer may provide some volume control through storage or by retarding water loss. Volume credits for a green roof system are a function of the dimensions of the system, specifically the depth of the media, the storage and hydraulic characteristics of the media, the area of the green roof, and the amount of run-on from adjacent conventional roofs. The volume credit (V_s (ft³/ft³) is given by

\(V_s = A_s\ d_p\ MMWR\)

where

A_s = the surface area of the green roof(square feet);

d_p = the depth of the media, equal to the area from the bottom of the media (top of underlying drainage layer) to the top of the media; and

MMWR = moisture content at the maximum media density (cubic feet/cubic feet; often expressed as a percent volume).

The maximum media density (kg per cubic meter) is the sample weight (kg) after draining for 120 minutes divided by the initial sample volume (cubic meters). The maximum media density is used to estimate the maximum dead load for green roof assemblies. See ASTM Standard E2339-11. A value of 0.33 is used for MMWR in the MIDS calculator.

A green roof may be designed to meet a specific performance goal, such as the 1.1 inch Minimal Impact Design Standards (MIDS) goal for new development sites with no restrictions. Specifications for designing green roof systems, including a discussion of how to calculate the reservoir depth, are provided in the section covering design criteria for green roofs. Note that traditional roofs, which are treated as impervious surface, may be routed to a green roof and will therefore affect the achievment of the performance goal. Recommendations about the maximum contributing area from conventional roofs varies.

Related pages

• Calculating credits
  • Calculating credits for bioretention
  • Calculating credits for infiltration basin
  • Calculating credits for permeable pavement
  • Calculating credits for green roofs
  • Calculating credits for sand filter
  • Calculating credits for stormwater ponds
  • Calculating credits for stormwater wetlands
  • Calculating credits for iron enhanced sand filter
  • Calculating credits for swale
  • Calculating credits for tree trenches and tree boxes