Log in
LEED v4
New Construction
Sustainable Sites
Rainwater management

LEED CREDIT

NC-v4 SSc4: Rainwater management 2-3 points

See all forum discussions about this credit »
View the LEED v4.1 version of this credit »

SPECIAL REPORT

LEEDuser expert

Michael DeVuono

PE, CPESC, LEED AP BD+C

Arcadis North America
Senior Water Engineer

SPECIAL REPORT

LEEDuser’s viewpoint

Frank advice from LEED experts

LEED is changing all the time, and every project is unique. Even seasoned professionals can miss a critical detail and lose a credit or even a prerequisite at the last minute. Our expert advice guides our LEEDuser Premium members and saves you valuable time.

Credit language

USGBC logo

© Copyright U.S. Green Building Council, Inc. All rights reserved.

Intent

To reduce runoff volume and improve water quality by replicating the natural hydrology and water balance of the site, based on historical conditions and undeveloped ecosystems in the region.

Requirements

Option 1. Percentile of rainfall events
Path 1. 95th percentile (2 points except Healthcare, 1 point Healthcare)

In a manner best replicating natural site hydrology processes, manage on site the runoff from the developed site for the 95th percentile of regional or local rainfall events using low-impact development (LID) and green infrastructure.

Use daily rainfall data and the methodology in the U.S. Environmental Protection Agency (EPA) Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act to determine the 95th percentile amount.

OR
Path 2. 98th percentile (3 points except Healthcare, 2 points Healthcare)

Achieve Path 1 but for the 98th percentile of regional or local rainfall events, using LID and green infrastructure.

OR
Path 3. Zero lot line projects only – 85th Percentile (3 points except Healthcare, 2 points Healthcare))

The following requirement applies to zero lot line projects in urban areas with a minimum density of 1.5 FAR. In a manner best replicating natural site hydrology processes, manage on site the runoff from the developed site for the 85th percentile of regional or local rainfall events, using LID and green infrastructure.

OR

Option 2. Natural land cover conditions (3 points except Healthcare, 2 points Healthcare)

Manage on site the annual increase in runoff volume from the natural land cover condition to the postdeveloped condition.

Projects that are part of a multitenant complex only

The credit requirements may be met using a coordinated approach affecting the defined project site that is within the master plan boundary. Distributed techniques based on a watershed approach are then required.

SITES-LEED Equivalency

This LEED credit (or a component of this credit) has been established as equivalent to a SITES v2 credit or component. For more information on using the equivalency as a substitution in your LEED or SITES project, see this article and guidance document.

See all forum discussions about this credit »

What does it cost?

Cost estimates for this credit

On each BD+C v4 credit, LEEDuser offers the wisdom of a team of architects, engineers, cost estimators, and LEED experts with hundreds of LEED projects between then. They analyzed the sustainable design strategies associated with each LEED credit, but also to assign actual costs to those strategies.

Our tab contains overall cost guidance, notes on what “soft costs” to expect, and a strategy-by-strategy breakdown of what to consider and what it might cost, in percentage premiums, actual costs, or both.

This information is also available in a full PDF download in The Cost of LEED v4 report.

Learn more about The Cost of LEED v4 »

Frequently asked questions

How do I perform the 95th percentile storm calculation?

The answer to this question is available to LEEDuser premium members. Start a free trial »

(If you're already a premium member, log in here.)

How do I calculate runoff from impermeable areas?

The answer to this question is available to LEEDuser premium members. Start a free trial »

(If you're already a premium member, log in here.)

What calculations will I need to complete?

The answer to this question is available to LEEDuser premium members. Start a free trial »

(If you're already a premium member, log in here.)

See all forum discussions about this credit »

Addenda

10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Replace the Calculations section with the following:
The land use runoff coefficients for small rainfall depths in the Small Storm Hydrology Method are recommended (runoff coefficients express the fraction of rainfall which is converted into runoff). The project team may choose the runoff volume calculation methodology most appropriate for the project, provided sufficient documentation and justification to demonstrate that the intent of the credit is being met.

The U.S. EPA Stormwater Management Model (SWMM) and National Stormwater Calculator is a general tool that is available for compliance overview; it is not recommended for use in design. It can be used in the project’s conceptual design phase for estimating rainwater and runoff, and choosing initial management strategies. The use of the Cover Complex Method is not recommended at this time for the calculation of runoff volume from small rainfall events (i.e. less than 2.5 in [63.5 mm] of precipitation in 24 hours). Many rainwater software programs include calculation methodologies. Computer modeling programs based on the Small Storm Hydrology Method, such as WinSLAMM, are acceptable tools.

Small Storm Hydrology Method(1)
Large impervious areas– This category describes impervious areas with an average dimension greater than 24 feet (7.3 meters) in any direction. Examples of large impervious areas include parking lots with curbs, roads with curbs, highways, etc.

Small impervious areas – This category describes impervious areas with an average dimension no greater than 24 feet (7.3 meters) in any direction. Examples of small impervious areas include roads without curbs, small parking lots without curbs, and sidewalks.

For each land use type, runoff volume is calculated based on land use area and land use coefficient using the following equation:

IP units
Runoff Volume = P/12 * Rv * A

where:
Runoff Volume is from the percentile rainfall event (ft3)
P = percentile rainfall depth (in)
Rv = Small Storm Hydrology Method runoff coefficient (alternatively, Rv can be calculated using the following equation: Rv= 0.05+0.009(I), where I = percent impervious area expressed as a whole number)
A = area of land use (ft2)

For a site with multiple land uses, Runoff Volume can be calculated as follows:

Runoff Volume=∑_(i=1)^n▒[(P/12*〖Rv〗_i*A_i )+(P/12*〖Rv〗_(i+1)*A_(i+1) )+⋯+(P/12*〖Rv〗_n*A_n )]

SI units
Runoff Volume = P/1000 * Rv * A

where:
Runoff Volume is from the percentile rainfall event (m3)
P = percentile rainfall depth (mm)
Rv = Small Storm Hydrology Method runoff (alternatively, Rv can be calculated using the following equation: Rv= 0.05+0.009(I), where I = percent impervious area expressed as a whole number)
A = area of land use (m2)

For a site with multiple land uses, Runoff Volume can be calculated as follows:

Runoff Volume=∑_(i=1)^n▒[(P/1000*〖Rv〗_i*A_i )+(P/1000*〖Rv〗_(i+1)*A_(i+1) )+⋯+(P/1000*〖Rv〗_n*A_n )]

Footnote (1): Robert Pitt, P.E., Ph.D., DEE, Small Storm Hydrology and Why it is Important for the Design of Stormwater Control Practices In: Advances in Modeling the Management of Stormwater Impacts, Volume 7. (Edited by W. James). Computational Hydraulics International, Guelph, Ontario and Lewis Publishers/CRC Press. 1999.
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Replace the Step-by-Step Guidance section with the following:
Select the appropriate option and path (if applicable) for the project. First investigate data sources for natural land cover conditions, and choose Option 1 if information is not available.
• Option 1 is for projects that elect to manage rainfall on their project site such that there is no immediate surface discharge after development for all rainfall events of a given depth. The specific rainfall depth is based on a statistical analysis of rainfall records for that project location. If the project is zero lot line and meets the credit’s required density, Path 3 can be selected. Otherwise, it is recommended that the rainfall values for both the 95th and 98th percentile events are calculated to determine whether Path 1 or Path 2 is more appropriate for the project.
• Option 2 is for projects that elect to evaluate the site hydrologic response based on both natural, undisturbed (i.e. pre-settlement) conditions, and proposed developed conditions. The proposed rainwater management design will implement measures that replicate the natural, undisturbed hydrology for the full range of hydrologic events. This will require hydrologic analysis and comparison of the site under both natural and proposed conditions, using representative rainfall data such as an analysis of all rainfall events during a statistically representative period of time.

Option 1. Percentile of Rainfall Events
Step 1. Determine if project qualifies for zero lot line path (Path 3)
If the project does not qualify for Path 3, move directly to Step 2.
• Determine if the project meets the definition of “zero lot line.” If the project does not exactly meet this definition, but the project team feels that it should be considered zero lot line, provide justification for why it qualifies for this path.
• Calculate the average density of the area within a ¼-mile (400-meter) radius of the project building. If the density, expressed in terms of floor-area ratio (FAR) is less than 1.5, the project is ineligible for the zero lot line path.

Step 2. Obtain rainfall data for project location
Obtain at least 10 years of historical rainfall data, or as much historical data as possible, representative of the project climate conditions based on proximity to site, elevation, region, etc. If the team submits less than 10 years’ worth of information, explain why additional historical data are not available.
• The rainfall record should be substantially complete, meaning that it is not missing data for extensive periods of time.
• For projects in the U.S., long-term rainfall data for many locations are available through the National Climatic Data Center. Use this database or another source to identify the reference location closest to the project site where similar precipitation patterns are expected (see Further Explanation, Percentile of Rainfall Events).
• For project locations outside the U.S. or other locations not covered by the National Climatic Data Center, obtain information from local airports, universities, water treatment plants, or other facilities that maintain long-term precipitation records (see Further Explanation, International Tips).
• Data must include the location of the monitoring station, the recording time (usually daily 24-hour time periods), and the total precipitation depth during the time-step.

Step 3. Determine value for chosen percentile of rainfall events
Using the historical rainfall data collected, calculate the rainfall value for the 95th, 98th, or 85th percentile (in inches or millimeters). This is the precipitation amount that 95 percent (or 98 percent, or 85 percent) of all rainfall events for the period of record do not exceed and will be represented by a rainfall depth (see Further Explanation, Percentile of Rainfall Events).
• Only calculate the 85th percentile if the project meets the criteria in Path 3.
• If the project does not meet the criteria in Path 3, choose which percentile- either the 95th or the 98th- is most appropriate for the project.

Step 4. Analyze existing site performance
Prior to calculating the runoff volume from the proposed site design, analyze how the current site is performing relative to the management of precipitation.
• A reduction in the volume of runoff can be achieved by protecting existing natural resources that serve to reduce the generation of runoff.
• The site analysis may reveal existing areas that, with no or minimal alterations, could contribute to the management of rainwater runoff.
• Examples of areas to preserve include healthy un-compacted soils, riparian buffers, tree canopy, etc.
• These areas must be protected from disturbance during the construction period. If protected from disturbance during construction, these natural areas may be excluded from the project area and hence excluded from runoff volume management.

Step 5. Design the site
Conceptually design the site, including rainwater management strategies, using the site performance analysis to inform the design. Include any preserved site features that could contribute to a reduction in, or the management of, runoff volume.
• Roughly locate, layout, and size rainwater management features in relation to the buildings, topography, soils, and other site features and the overall site program.
• It is recommended that a conceptual design be developed first, as projects frequently change and refine the design later after calculating runoff in order to manage the required volume.

Step 6. Calculate runoff volume to be managed on site
Use the conceptual design to calculate the total volume of runoff (in cubic feet or cubic meters) corresponding to the chosen percentile of rainfall events for the site in its developed condition. This is the amount of precipitation that the project will need to manage entirely on site through green infrastructure and low-impact development techniques.
• Different methods can be used to calculate the runoff volume. The land use runoff coefficients for small rainfall depths, as developed by Dr. Robert Pitt in Table 5 of Small Storm Hydrology Method, are recommended.
• Runoff volume should be calculated by land use type and depends on the specific developed site conditions of the project, such as amount of paving, permeability of different surfaces, roof area, and vegetated areas (see Further Explanation, Calculations and Further Explanation, Example).

Step 7. Manage runoff volume on site
Incorporate green infrastructure and low-impact development strategies into the site design to manage, on site, 100% of the total volume of runoff calculated for the chosen percentile rainfall event and the project’s developed conditions (the proposed design).
• Work with the project’s civil engineer, landscape architect, or other qualified professionals to choose and size the design strategies (see Further Explanation, Green Infrastructure and Low-Impact Development Strategies).
• The chosen GI and LID measures should completely manage the required runoff volume for the chosen rainfall percentile.
• Calculations must account for the site-specific soil characteristics, the soil infiltration rate, and the storage capacity of all GI and LID measures.
• For projects following Path 3, green roofs and rainwater harvesting approaches are the most likely GI and LID strategies to help zero lot line projects meet the credit requirements. Roofs can be either extensive or intensive systems. Maintenance will be needed to keep plants healthy and the structure in good condition. Artificial turf is not an acceptable strategy for vegetated roofs. In some cases, zero lot line projects can also use infiltration planters, porous pavement, and tree boxes.
• For projects that are part of a multitenant complex, see Further Explanation, Project Type Variations.

Step 8. Analyze and refine proposed site design
Refine the site design using the calculated runoff and proposed management strategies from the conceptual design. Determine if the proposed design is performing sufficiently enough to manage the required volume of runoff onsite using GI/LID strategies. Continue to tweak and refine the design, by repeating steps 5, 6, and 7 as many times as necessary, in order to achieve the credit requirements and meet project’s goals.
• Rainwater management design is an iterative process that involves analyzing schematic designs, roughly calculating runoff volumes managed, and revising the layout and sizing of management strategies multiple times before finalizing the overall site design.
• See Further Explanation, Examples for an example of this process.

Option 2. Natural Land Cover Conditions
Step 1. Obtain information about natural conditions
Gather information about how the site (or the site’s immediate region) functioned prior to any alteration or human activity. A sense of the natural conditions can be deduced through an analysis of land cover and general hydrologic function.
• Natural land cover conditions refer to the vegetation and soil conditions that existed in the area prior to development activities.
• Natural hydrology refers to the partitioning of rainfall into components of infiltration, evapotranspiration, and runoff in amounts and patterns that replicate the natural hydrology.
• Examples of alterations, development, and human activities include large-scale tree clearing and grading, industry, agriculture, mining, construction, municipal development, commercial development, and residential development.
• Natural vegetation maps, soil maps, or a description of typical land cover conditions in the project’s region may be helpful to evaluate. Determination of the natural land cover conditions can also be based on the historical context of the site (e.g., forested, grassland) and an assessment of the soil conditions.
• For the purposes of this Option, an estimate of the natural conditions is acceptable.
• Sources of information include local governments and state environmental agencies, conservation or water resource organizations, historical societies or historic preservation groups, libraries, colleges and universities, private historical mapping companies, and (in the U.S.) EPA, Department of the Interior, U.S. Geological Survey, Oak Ridge National Laboratory (NASA), National Center for Atmospheric Research, and Department of Agriculture.

Step 2. Calculate natural conditions
Evaluate the full range of hydrologic rainfall events over a 10-year time period. Alternatively, an “average” representative rainfall year may be developed (continuous modeling simulation is required).
• The goal of this step is to determine the pattern and distribution of natural conditions in order to understand how the site functioned and thus what amount of runoff needs to be managed on site. In other words, to understand hypothetically how the site has performed over a period of record.
• All variables needed to complete this step are estimated based on the historical land cover conditions. For example, runoff curve numbers for the site are estimated based on the historical vegetation cover and soil conditions.

Step 3. Calculate runoff volume to be managed on site
Calculate the runoff volume under the developed (proposed design) conditions, and compare it to the runoff volume under the natural conditions. If there is an increase in runoff volume between natural and developed conditions over the time period evaluated, manage the difference on site.
• In rare cases, the natural land cover conditions would manage less than the 95th percentile. In this case, the project is only requiired to manage the runoff volume that the natural land cover conditions would manage.

Step 4. Manage runoff volume on site
Similar to Option 1, incorporate green infrastructure and low-impact development strategies into the site design to manage, on site, the required volume of runoff.
• Portions of the site that are natural or close to natural may mitigate rainwater runoff and supplement other best management practices to achieve the goal of natural hydrologic performance.
• Work with the project’s civil engineer, landscape architect, or other qualified professionals to choose and size the design strategies (see Further Explanation, Green Infrastructure and Low-Impact Development Strategies).
• Confirm that the chosen GI and LID measures will completely manage the required runoff volume.
• For projects that are part of a multitenant complex, see Further Explanation, Project Type Variations.
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Replace the Behind the Intent section with the following:
Conventional site development disrupts natural hydrological systems and watersheds through impervious surfaces, soil compaction, loss of vegetation, and loss of natural drainage patterns. The cumulative effect of these changes is disruption to the natural water balance and a loss of water resources. Typically, a conventional site’s rainwater management technique is to address runoff as a byproduct to be disposed of by piping and conveying it as quickly as possible into centralized, large facilities at the base of drainage areas. However, such a strategy, although intended to prevent flooding and promote efficient drainage, can harm watersheds: it increases the volume, temperature, peak flow, and duration of runoff, eroding streams reducing groundwater recharge and stream baseflow, and increasing the amount and types of pollutants discharged to surface waters.

The v4 credit addresses the management of both the quantity and quality of rainwater runoff. This is done through the required use of green infrastructure (GI) and low-impact development (LID) strategies, which improve upon the conventional approach by mimicking a site’s natural hydrology and managing water as close to the source as possible. Rainwater is treated as a resource rather than a waste product. The approaches and techniques in this credit involve minimizing disturbed areas on the project site, limiting the amount of impervious cover on a site, and then infiltrating, filtering, storing and reusing, evaporating, or detaining rainwater runoff at or close to its source. The approaches also focus on restoring or designing landscapes to function hydrologically more like the natural, undisturbed landscape of a given location.

This credit’s process is iterative in nature, which means that the project will need to conceptualize, calculate, and refine the design until the requirements are achieved to ensure that both the intent of the credit and the project’s goals are met. As such, the Step-by-Step Guidance is intended to be a guide only. The steps may need to be repeated or revisited throughout the design process. The rainwater management professional(s) may also have other steps that they typically follow to achieve the requirements.
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Revise step 5 and add a paragraph to the end of this section:
5. The USGBC calculator determines the various percentile rainfall amounts. If using another spreadsheet software, apply a percentile function (or similar) to obtain results.

Detailed explanation regarding calculation of the percentile event using daily rainfall records can be found in EPA 841-B-09-001, December 2009, www.epa.gov/owow/nps/lid/section438.
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Revise the Green Infrastructure and Low-Impact Development Strategies section to read:
The goal of low impact development is to manage water as close to the source as possible using soil and vegetation-based systems. In addition to mimicking natural hydrologic cycle processes, green infrastructure and low-impact development help integrate the site with the surrounding watershed, are appropriate to the local ecosystem and climate, and deliver such other benefits as water reuse, habitat creation, and species diversity.

Prior to calculating the runoff volume from the project area, the project is encouraged to reduce the volume of runoff by protecting existing natural resources that serve to reduce the generation of runoff. Examples include healthy uncompacted soils, riparian buffers, tree canopy, etc. These areas must be protected from disturbance during the construction period in order to be effective. If protected from disturbance during construction, these natural areas can be excluded from runoff volume management.

All runoff from the chosen percentile of precipitation events must be managed such that there is no surface discharge from the site. Techniques include, but are not limited to, infiltration, storage and re-use, bioretention, open-grid pavement, and the reduction of impervious area. Infiltration may not be feasible in some cases based on the soil or geological conditions of the site. Karst geology and areas where water infiltrates at less than 1/2 inch (25 mm) per hour are two examples of situations that can be unfavorable for infiltration. The engineer, landscape architect, or rainwater professional will ultimately determine the best solution for the project’s unique conditions. Continued maintenance of all rainwater management strategies is important in order for them to remain effective over time.

Project teams should consult EPA’s National Menu of Stormwater Best Practices and consider the following questions when selecting measures for the project:
• Which GI and LID measures will best mimic natural site hydrology?
• How can multiple measures be used together (in a “treatment train” approach) to manage rainwater?
• What are the infiltration rates and capacities of the most practical measures and how might the site’s soil conditions affect their efficiency?
• What are the types and infiltration rates of existing soil conditions, and what design modifications might need to be made, if any, to the best management practices to satisfy performance goals?
• How effective are the measures at removing contaminants from the rainwater runoff?
• How will the measures be maintained?
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Delete the paragraph that begins with "The new construction project is a 10 story building..." Add "Example 1" above the example site plan.

Rename Table 1. to "Example percentiles of rainfall events" and change the first column heading to read "Rainfall amount" and the second column heading to read "Percentile of rainfall events".

Below Table 1 and above the sample documentation narrative, change the paragraphs to read:
To supplement the capacity of the pervious landscaped area, the project team has identified several conceptual GI and LID measures- bioretention areas, porous paving, a rain garden, and pervious decking- to capture rainwater from impervious surfaces. To ensure that the designed site manages all the rainwater runoff generated during the 95th percentile storm, the team calculates how the GI and LID measures manage the runoff, and it is verified that all required runoff is managed on site.

Delete Table 2. Example summary table

The last sentence of the Sample documentation narrative describing GI and LID measures should read:
"The area of bioretention in this project meets the 95th percentile criteria, managing rainwater runoff via natural processes."

Add a section called "Example 2." that reads:
Example 2 follows a project located in Maryland that attempted Option 1, Percentile of Rainfall Events. It gives an overview of the project, and then details the iterative rainwater management design process and calculations used to achieve the credit requirements. It is broken into two modules, both of which are located in the web-based reference guide on the right side of the “Guide” page.
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Delete Figure 2. Example master plan project
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Add "soil qualities" to the cell about plans, details, or cross sections:
"Plans, details, or cross sections depicting site conditions and GI or LID strategies, highlighting topography, soil qualities, direction of water flow, and area of site that each facility addresses"
Campus Applicable
Yes
Internationally Applicable:
Yes
See all forum discussions about this credit »

Documentation toolkit

The motherlode of cheat sheets

LEEDuser’s Documentation Toolkit is loaded with calculators to help assess credit compliance, tracking spreadsheets for materials, sample templates to help guide your narratives and LEED Online submissions, and examples of actual submissions from certified LEED projects for you to check your work against. To get your plaque, start with the right toolkit.

LEEDuser expert

Michael DeVuono

PE, CPESC, LEED AP BD+C

Arcadis North America
Senior Water Engineer

Get the inside scoop

Our editors have written a detailed analysis of nearly every LEED credit, and LEEDuser premium members get full access. We’ll tell you whether the credit is easy to accomplish or better left alone, and we provide insider tips on how to document it successfully.

USGBC logo

© Copyright U.S. Green Building Council, Inc. All rights reserved.

Intent

To reduce runoff volume and improve water quality by replicating the natural hydrology and water balance of the site, based on historical conditions and undeveloped ecosystems in the region.

Requirements

Option 1. Percentile of rainfall events
Path 1. 95th percentile (2 points except Healthcare, 1 point Healthcare)

In a manner best replicating natural site hydrology processes, manage on site the runoff from the developed site for the 95th percentile of regional or local rainfall events using low-impact development (LID) and green infrastructure.

Use daily rainfall data and the methodology in the U.S. Environmental Protection Agency (EPA) Technical Guidance on Implementing the Stormwater Runoff Requirements for Federal Projects under Section 438 of the Energy Independence and Security Act to determine the 95th percentile amount.

OR
Path 2. 98th percentile (3 points except Healthcare, 2 points Healthcare)

Achieve Path 1 but for the 98th percentile of regional or local rainfall events, using LID and green infrastructure.

OR
Path 3. Zero lot line projects only – 85th Percentile (3 points except Healthcare, 2 points Healthcare))

The following requirement applies to zero lot line projects in urban areas with a minimum density of 1.5 FAR. In a manner best replicating natural site hydrology processes, manage on site the runoff from the developed site for the 85th percentile of regional or local rainfall events, using LID and green infrastructure.

OR

Option 2. Natural land cover conditions (3 points except Healthcare, 2 points Healthcare)

Manage on site the annual increase in runoff volume from the natural land cover condition to the postdeveloped condition.

Projects that are part of a multitenant complex only

The credit requirements may be met using a coordinated approach affecting the defined project site that is within the master plan boundary. Distributed techniques based on a watershed approach are then required.

SITES-LEED Equivalency

This LEED credit (or a component of this credit) has been established as equivalent to a SITES v2 credit or component. For more information on using the equivalency as a substitution in your LEED or SITES project, see this article and guidance document.

Cost estimates for this credit

On each BD+C v4 credit, LEEDuser offers the wisdom of a team of architects, engineers, cost estimators, and LEED experts with hundreds of LEED projects between then. They analyzed the sustainable design strategies associated with each LEED credit, but also to assign actual costs to those strategies.

Our tab contains overall cost guidance, notes on what “soft costs” to expect, and a strategy-by-strategy breakdown of what to consider and what it might cost, in percentage premiums, actual costs, or both.

This information is also available in a full PDF download in The Cost of LEED v4 report.

Learn more about The Cost of LEED v4 »

In the end, LEED is all about documentation. LEEDuser’s Documentation Toolkit, for premium members only, saves you time and helps you avoid mistakes with:

  • Calculators to help assess credit compliance.
  • Tracking spreadsheets for materials purchases.
  • Spreadsheets and forms to give to subs and other team members.
  • Guidance documents on arcane LEED issues.
  • Sample templates to help guide your narratives and LEED Online submissions.
  • Examples of actual submissions from certified LEED projects.

How do I perform the 95th percentile storm calculation?

The answer to this question is available to LEEDuser premium members. Start a free trial »

(If you're already a premium member, log in here.)

How do I calculate runoff from impermeable areas?

The answer to this question is available to LEEDuser premium members. Start a free trial »

(If you're already a premium member, log in here.)

What calculations will I need to complete?

The answer to this question is available to LEEDuser premium members. Start a free trial »

(If you're already a premium member, log in here.)

10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Replace the Calculations section with the following:
The land use runoff coefficients for small rainfall depths in the Small Storm Hydrology Method are recommended (runoff coefficients express the fraction of rainfall which is converted into runoff). The project team may choose the runoff volume calculation methodology most appropriate for the project, provided sufficient documentation and justification to demonstrate that the intent of the credit is being met.

The U.S. EPA Stormwater Management Model (SWMM) and National Stormwater Calculator is a general tool that is available for compliance overview; it is not recommended for use in design. It can be used in the project’s conceptual design phase for estimating rainwater and runoff, and choosing initial management strategies. The use of the Cover Complex Method is not recommended at this time for the calculation of runoff volume from small rainfall events (i.e. less than 2.5 in [63.5 mm] of precipitation in 24 hours). Many rainwater software programs include calculation methodologies. Computer modeling programs based on the Small Storm Hydrology Method, such as WinSLAMM, are acceptable tools.

Small Storm Hydrology Method(1)
Large impervious areas– This category describes impervious areas with an average dimension greater than 24 feet (7.3 meters) in any direction. Examples of large impervious areas include parking lots with curbs, roads with curbs, highways, etc.

Small impervious areas – This category describes impervious areas with an average dimension no greater than 24 feet (7.3 meters) in any direction. Examples of small impervious areas include roads without curbs, small parking lots without curbs, and sidewalks.

For each land use type, runoff volume is calculated based on land use area and land use coefficient using the following equation:

IP units
Runoff Volume = P/12 * Rv * A

where:
Runoff Volume is from the percentile rainfall event (ft3)
P = percentile rainfall depth (in)
Rv = Small Storm Hydrology Method runoff coefficient (alternatively, Rv can be calculated using the following equation: Rv= 0.05+0.009(I), where I = percent impervious area expressed as a whole number)
A = area of land use (ft2)

For a site with multiple land uses, Runoff Volume can be calculated as follows:

Runoff Volume=∑_(i=1)^n▒[(P/12*〖Rv〗_i*A_i )+(P/12*〖Rv〗_(i+1)*A_(i+1) )+⋯+(P/12*〖Rv〗_n*A_n )]

SI units
Runoff Volume = P/1000 * Rv * A

where:
Runoff Volume is from the percentile rainfall event (m3)
P = percentile rainfall depth (mm)
Rv = Small Storm Hydrology Method runoff (alternatively, Rv can be calculated using the following equation: Rv= 0.05+0.009(I), where I = percent impervious area expressed as a whole number)
A = area of land use (m2)

For a site with multiple land uses, Runoff Volume can be calculated as follows:

Runoff Volume=∑_(i=1)^n▒[(P/1000*〖Rv〗_i*A_i )+(P/1000*〖Rv〗_(i+1)*A_(i+1) )+⋯+(P/1000*〖Rv〗_n*A_n )]

Footnote (1): Robert Pitt, P.E., Ph.D., DEE, Small Storm Hydrology and Why it is Important for the Design of Stormwater Control Practices In: Advances in Modeling the Management of Stormwater Impacts, Volume 7. (Edited by W. James). Computational Hydraulics International, Guelph, Ontario and Lewis Publishers/CRC Press. 1999.
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Replace the Step-by-Step Guidance section with the following:
Select the appropriate option and path (if applicable) for the project. First investigate data sources for natural land cover conditions, and choose Option 1 if information is not available.
• Option 1 is for projects that elect to manage rainfall on their project site such that there is no immediate surface discharge after development for all rainfall events of a given depth. The specific rainfall depth is based on a statistical analysis of rainfall records for that project location. If the project is zero lot line and meets the credit’s required density, Path 3 can be selected. Otherwise, it is recommended that the rainfall values for both the 95th and 98th percentile events are calculated to determine whether Path 1 or Path 2 is more appropriate for the project.
• Option 2 is for projects that elect to evaluate the site hydrologic response based on both natural, undisturbed (i.e. pre-settlement) conditions, and proposed developed conditions. The proposed rainwater management design will implement measures that replicate the natural, undisturbed hydrology for the full range of hydrologic events. This will require hydrologic analysis and comparison of the site under both natural and proposed conditions, using representative rainfall data such as an analysis of all rainfall events during a statistically representative period of time.

Option 1. Percentile of Rainfall Events
Step 1. Determine if project qualifies for zero lot line path (Path 3)
If the project does not qualify for Path 3, move directly to Step 2.
• Determine if the project meets the definition of “zero lot line.” If the project does not exactly meet this definition, but the project team feels that it should be considered zero lot line, provide justification for why it qualifies for this path.
• Calculate the average density of the area within a ¼-mile (400-meter) radius of the project building. If the density, expressed in terms of floor-area ratio (FAR) is less than 1.5, the project is ineligible for the zero lot line path.

Step 2. Obtain rainfall data for project location
Obtain at least 10 years of historical rainfall data, or as much historical data as possible, representative of the project climate conditions based on proximity to site, elevation, region, etc. If the team submits less than 10 years’ worth of information, explain why additional historical data are not available.
• The rainfall record should be substantially complete, meaning that it is not missing data for extensive periods of time.
• For projects in the U.S., long-term rainfall data for many locations are available through the National Climatic Data Center. Use this database or another source to identify the reference location closest to the project site where similar precipitation patterns are expected (see Further Explanation, Percentile of Rainfall Events).
• For project locations outside the U.S. or other locations not covered by the National Climatic Data Center, obtain information from local airports, universities, water treatment plants, or other facilities that maintain long-term precipitation records (see Further Explanation, International Tips).
• Data must include the location of the monitoring station, the recording time (usually daily 24-hour time periods), and the total precipitation depth during the time-step.

Step 3. Determine value for chosen percentile of rainfall events
Using the historical rainfall data collected, calculate the rainfall value for the 95th, 98th, or 85th percentile (in inches or millimeters). This is the precipitation amount that 95 percent (or 98 percent, or 85 percent) of all rainfall events for the period of record do not exceed and will be represented by a rainfall depth (see Further Explanation, Percentile of Rainfall Events).
• Only calculate the 85th percentile if the project meets the criteria in Path 3.
• If the project does not meet the criteria in Path 3, choose which percentile- either the 95th or the 98th- is most appropriate for the project.

Step 4. Analyze existing site performance
Prior to calculating the runoff volume from the proposed site design, analyze how the current site is performing relative to the management of precipitation.
• A reduction in the volume of runoff can be achieved by protecting existing natural resources that serve to reduce the generation of runoff.
• The site analysis may reveal existing areas that, with no or minimal alterations, could contribute to the management of rainwater runoff.
• Examples of areas to preserve include healthy un-compacted soils, riparian buffers, tree canopy, etc.
• These areas must be protected from disturbance during the construction period. If protected from disturbance during construction, these natural areas may be excluded from the project area and hence excluded from runoff volume management.

Step 5. Design the site
Conceptually design the site, including rainwater management strategies, using the site performance analysis to inform the design. Include any preserved site features that could contribute to a reduction in, or the management of, runoff volume.
• Roughly locate, layout, and size rainwater management features in relation to the buildings, topography, soils, and other site features and the overall site program.
• It is recommended that a conceptual design be developed first, as projects frequently change and refine the design later after calculating runoff in order to manage the required volume.

Step 6. Calculate runoff volume to be managed on site
Use the conceptual design to calculate the total volume of runoff (in cubic feet or cubic meters) corresponding to the chosen percentile of rainfall events for the site in its developed condition. This is the amount of precipitation that the project will need to manage entirely on site through green infrastructure and low-impact development techniques.
• Different methods can be used to calculate the runoff volume. The land use runoff coefficients for small rainfall depths, as developed by Dr. Robert Pitt in Table 5 of Small Storm Hydrology Method, are recommended.
• Runoff volume should be calculated by land use type and depends on the specific developed site conditions of the project, such as amount of paving, permeability of different surfaces, roof area, and vegetated areas (see Further Explanation, Calculations and Further Explanation, Example).

Step 7. Manage runoff volume on site
Incorporate green infrastructure and low-impact development strategies into the site design to manage, on site, 100% of the total volume of runoff calculated for the chosen percentile rainfall event and the project’s developed conditions (the proposed design).
• Work with the project’s civil engineer, landscape architect, or other qualified professionals to choose and size the design strategies (see Further Explanation, Green Infrastructure and Low-Impact Development Strategies).
• The chosen GI and LID measures should completely manage the required runoff volume for the chosen rainfall percentile.
• Calculations must account for the site-specific soil characteristics, the soil infiltration rate, and the storage capacity of all GI and LID measures.
• For projects following Path 3, green roofs and rainwater harvesting approaches are the most likely GI and LID strategies to help zero lot line projects meet the credit requirements. Roofs can be either extensive or intensive systems. Maintenance will be needed to keep plants healthy and the structure in good condition. Artificial turf is not an acceptable strategy for vegetated roofs. In some cases, zero lot line projects can also use infiltration planters, porous pavement, and tree boxes.
• For projects that are part of a multitenant complex, see Further Explanation, Project Type Variations.

Step 8. Analyze and refine proposed site design
Refine the site design using the calculated runoff and proposed management strategies from the conceptual design. Determine if the proposed design is performing sufficiently enough to manage the required volume of runoff onsite using GI/LID strategies. Continue to tweak and refine the design, by repeating steps 5, 6, and 7 as many times as necessary, in order to achieve the credit requirements and meet project’s goals.
• Rainwater management design is an iterative process that involves analyzing schematic designs, roughly calculating runoff volumes managed, and revising the layout and sizing of management strategies multiple times before finalizing the overall site design.
• See Further Explanation, Examples for an example of this process.

Option 2. Natural Land Cover Conditions
Step 1. Obtain information about natural conditions
Gather information about how the site (or the site’s immediate region) functioned prior to any alteration or human activity. A sense of the natural conditions can be deduced through an analysis of land cover and general hydrologic function.
• Natural land cover conditions refer to the vegetation and soil conditions that existed in the area prior to development activities.
• Natural hydrology refers to the partitioning of rainfall into components of infiltration, evapotranspiration, and runoff in amounts and patterns that replicate the natural hydrology.
• Examples of alterations, development, and human activities include large-scale tree clearing and grading, industry, agriculture, mining, construction, municipal development, commercial development, and residential development.
• Natural vegetation maps, soil maps, or a description of typical land cover conditions in the project’s region may be helpful to evaluate. Determination of the natural land cover conditions can also be based on the historical context of the site (e.g., forested, grassland) and an assessment of the soil conditions.
• For the purposes of this Option, an estimate of the natural conditions is acceptable.
• Sources of information include local governments and state environmental agencies, conservation or water resource organizations, historical societies or historic preservation groups, libraries, colleges and universities, private historical mapping companies, and (in the U.S.) EPA, Department of the Interior, U.S. Geological Survey, Oak Ridge National Laboratory (NASA), National Center for Atmospheric Research, and Department of Agriculture.

Step 2. Calculate natural conditions
Evaluate the full range of hydrologic rainfall events over a 10-year time period. Alternatively, an “average” representative rainfall year may be developed (continuous modeling simulation is required).
• The goal of this step is to determine the pattern and distribution of natural conditions in order to understand how the site functioned and thus what amount of runoff needs to be managed on site. In other words, to understand hypothetically how the site has performed over a period of record.
• All variables needed to complete this step are estimated based on the historical land cover conditions. For example, runoff curve numbers for the site are estimated based on the historical vegetation cover and soil conditions.

Step 3. Calculate runoff volume to be managed on site
Calculate the runoff volume under the developed (proposed design) conditions, and compare it to the runoff volume under the natural conditions. If there is an increase in runoff volume between natural and developed conditions over the time period evaluated, manage the difference on site.
• In rare cases, the natural land cover conditions would manage less than the 95th percentile. In this case, the project is only requiired to manage the runoff volume that the natural land cover conditions would manage.

Step 4. Manage runoff volume on site
Similar to Option 1, incorporate green infrastructure and low-impact development strategies into the site design to manage, on site, the required volume of runoff.
• Portions of the site that are natural or close to natural may mitigate rainwater runoff and supplement other best management practices to achieve the goal of natural hydrologic performance.
• Work with the project’s civil engineer, landscape architect, or other qualified professionals to choose and size the design strategies (see Further Explanation, Green Infrastructure and Low-Impact Development Strategies).
• Confirm that the chosen GI and LID measures will completely manage the required runoff volume.
• For projects that are part of a multitenant complex, see Further Explanation, Project Type Variations.
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Replace the Behind the Intent section with the following:
Conventional site development disrupts natural hydrological systems and watersheds through impervious surfaces, soil compaction, loss of vegetation, and loss of natural drainage patterns. The cumulative effect of these changes is disruption to the natural water balance and a loss of water resources. Typically, a conventional site’s rainwater management technique is to address runoff as a byproduct to be disposed of by piping and conveying it as quickly as possible into centralized, large facilities at the base of drainage areas. However, such a strategy, although intended to prevent flooding and promote efficient drainage, can harm watersheds: it increases the volume, temperature, peak flow, and duration of runoff, eroding streams reducing groundwater recharge and stream baseflow, and increasing the amount and types of pollutants discharged to surface waters.

The v4 credit addresses the management of both the quantity and quality of rainwater runoff. This is done through the required use of green infrastructure (GI) and low-impact development (LID) strategies, which improve upon the conventional approach by mimicking a site’s natural hydrology and managing water as close to the source as possible. Rainwater is treated as a resource rather than a waste product. The approaches and techniques in this credit involve minimizing disturbed areas on the project site, limiting the amount of impervious cover on a site, and then infiltrating, filtering, storing and reusing, evaporating, or detaining rainwater runoff at or close to its source. The approaches also focus on restoring or designing landscapes to function hydrologically more like the natural, undisturbed landscape of a given location.

This credit’s process is iterative in nature, which means that the project will need to conceptualize, calculate, and refine the design until the requirements are achieved to ensure that both the intent of the credit and the project’s goals are met. As such, the Step-by-Step Guidance is intended to be a guide only. The steps may need to be repeated or revisited throughout the design process. The rainwater management professional(s) may also have other steps that they typically follow to achieve the requirements.
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Revise step 5 and add a paragraph to the end of this section:
5. The USGBC calculator determines the various percentile rainfall amounts. If using another spreadsheet software, apply a percentile function (or similar) to obtain results.

Detailed explanation regarding calculation of the percentile event using daily rainfall records can be found in EPA 841-B-09-001, December 2009, www.epa.gov/owow/nps/lid/section438.
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Revise the Green Infrastructure and Low-Impact Development Strategies section to read:
The goal of low impact development is to manage water as close to the source as possible using soil and vegetation-based systems. In addition to mimicking natural hydrologic cycle processes, green infrastructure and low-impact development help integrate the site with the surrounding watershed, are appropriate to the local ecosystem and climate, and deliver such other benefits as water reuse, habitat creation, and species diversity.

Prior to calculating the runoff volume from the project area, the project is encouraged to reduce the volume of runoff by protecting existing natural resources that serve to reduce the generation of runoff. Examples include healthy uncompacted soils, riparian buffers, tree canopy, etc. These areas must be protected from disturbance during the construction period in order to be effective. If protected from disturbance during construction, these natural areas can be excluded from runoff volume management.

All runoff from the chosen percentile of precipitation events must be managed such that there is no surface discharge from the site. Techniques include, but are not limited to, infiltration, storage and re-use, bioretention, open-grid pavement, and the reduction of impervious area. Infiltration may not be feasible in some cases based on the soil or geological conditions of the site. Karst geology and areas where water infiltrates at less than 1/2 inch (25 mm) per hour are two examples of situations that can be unfavorable for infiltration. The engineer, landscape architect, or rainwater professional will ultimately determine the best solution for the project’s unique conditions. Continued maintenance of all rainwater management strategies is important in order for them to remain effective over time.

Project teams should consult EPA’s National Menu of Stormwater Best Practices and consider the following questions when selecting measures for the project:
• Which GI and LID measures will best mimic natural site hydrology?
• How can multiple measures be used together (in a “treatment train” approach) to manage rainwater?
• What are the infiltration rates and capacities of the most practical measures and how might the site’s soil conditions affect their efficiency?
• What are the types and infiltration rates of existing soil conditions, and what design modifications might need to be made, if any, to the best management practices to satisfy performance goals?
• How effective are the measures at removing contaminants from the rainwater runoff?
• How will the measures be maintained?
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Delete the paragraph that begins with "The new construction project is a 10 story building..." Add "Example 1" above the example site plan.

Rename Table 1. to "Example percentiles of rainfall events" and change the first column heading to read "Rainfall amount" and the second column heading to read "Percentile of rainfall events".

Below Table 1 and above the sample documentation narrative, change the paragraphs to read:
To supplement the capacity of the pervious landscaped area, the project team has identified several conceptual GI and LID measures- bioretention areas, porous paving, a rain garden, and pervious decking- to capture rainwater from impervious surfaces. To ensure that the designed site manages all the rainwater runoff generated during the 95th percentile storm, the team calculates how the GI and LID measures manage the runoff, and it is verified that all required runoff is managed on site.

Delete Table 2. Example summary table

The last sentence of the Sample documentation narrative describing GI and LID measures should read:
"The area of bioretention in this project meets the 95th percentile criteria, managing rainwater runoff via natural processes."

Add a section called "Example 2." that reads:
Example 2 follows a project located in Maryland that attempted Option 1, Percentile of Rainfall Events. It gives an overview of the project, and then details the iterative rainwater management design process and calculations used to achieve the credit requirements. It is broken into two modules, both of which are located in the web-based reference guide on the right side of the “Guide” page.
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Delete Figure 2. Example master plan project
Campus Applicable
Yes
Internationally Applicable:
Yes
10/1/2014Updated: 2/14/2015
Reference Guide Correction
Description of change:
Add "soil qualities" to the cell about plans, details, or cross sections:
"Plans, details, or cross sections depicting site conditions and GI or LID strategies, highlighting topography, soil qualities, direction of water flow, and area of site that each facility addresses"
Campus Applicable
Yes
Internationally Applicable:
Yes

LEEDuser expert

Michael DeVuono

PE, CPESC, LEED AP BD+C

Arcadis North America
Senior Water Engineer

See all LEEDuser forum discussions about this credit » Subscribe to new discussions about NC-v4 SSc4 View the LEED v4.1 version of this credit