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LEED v4
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Sustainable Sites
Rainwater management

LEED CREDIT

Schools-EBOM-v4 SSc2: Rainwater management 2 points

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Ashwini Arun

WSP
Senior Sustainability Manager

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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

Establishment

Use low-impact development (LID) practices to capture and treat water from 25% of the impervious surfaces for the 95th percentile storm event. Establish and implement an annual inspection program of all rainwater management facilities to confirm continued performance.

Performance

Document the annual inspections, including identification of areas of erosion, maintenance needs, and repairs. Perform necessary maintenance, repairs, or stabilization within 60 days of inspection. See all forum discussions about this credit »

What does it cost?

Cost estimates for this credit

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Frequently asked questions

Where can I find long-term rainfall data?

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What if I have less than 10 years of historical rainfall data?

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What methodology should I use to calculate runoff volume?

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Are there any specific tools or computer programs that are recommended for calculating runoff volume?

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What are some low-impact development (LID) and green infrastructure (GI) strategies?

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Where can I find example calculations?

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What about rainwater runoff from greenfield areas in my LEED project boundary?

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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:
Delete the paragraph that begins with "The project is a 10 story building..." Add "Example 1." above the example site plan.

Replace Figure 1 with an updated graphic that shows revised numbers in the legend.

In Table 1. Example percentiles of rainfall events, 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 narrative, change the paragraphs to read:
"To calculate the runoff volume from 25% of the impervious surfaces for the 95th percentile of storm events (see Calculations), the team adds together all impervious surfaces on the project (existing and proposed), for a total area of 5,000 square feet (464.5 sm). The total impervious area to be managed is 25% of 5,000 square feet (464.5 sm), or 1,250 square feet (116.1 sm).

The team identifies existing GI and LID features that may contribute toward rainwater management and verifies that they manage the required runoff volume on site. In addition to the existing pervious landscaped areas on the project site, the team has identified several conceptual GI and LID measures- bioretention areas, porous paving, a rain garden, and pervious decking- to capture rainwater diverted from impervious surfaces. To ensure that the designed site manages all the required rainwater runoff generated, the team calculates how the GI and LID measures manage the runoff, and it is verified that all required runoff is managed on site."

In the Sample narrative describing GI and LID measures, replace "Taxodium distichum" with "Prunus americana."

Add a section called "Example 2." that reads:
"Example 2.
Example 2 follows a project located in Maryland that attempted Option 1, Percentile of Rainfall Events in the New Construction rating system. The example does not calculate 25% of the impervious surface area on the project. However, it details the iterative rainwater management design process and the calculations used to achieve the credit requirements for the 95th percentile of storm events. 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:
Revise the sentence that begins with "Use the following steps..." and step 5, and also add a paragraph to the end of this section:
Use the following steps to determine the percentile of rainfall events:

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:
Replace the Step-by-Step Guidance section with the following:
Step 1. Obtain rainfall data for project location (E)
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 2. Determine value for 95th-percentile rainfall events (E)
Using the historical rainfall data collected, calculate the rainfall value for the 95th percentile (in inches or millimeters). This is the precipitation amount that 95 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).

Step 3. Analyze existing site conditions and performance (E)
Whether or not the project is planning to modify the site design, first analyze how the current site is performing relative to the management of precipitation (prior to calculating the runoff volume).
• Work with a civil engineer, landscape architect, and other contractors or consultants as needed to assess the current site conditions against the credit criteria. This assessment is best performed before site redevelopment activities begin.
• Existing rainwater management strategies that qualify as GI or LID can be used to manage runoff from impervious surfaces.
• 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 any future 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 4. Calculate runoff volume to be managed on site (E)
Identify the impervious areas from which water will be treated according to the credit requirements (i.e., the areas to be included in the 25% threshold). Calculate the total volume of runoff (in cubic feet or cubic meters) corresponding to the 95th percentile of rainfall events for the site in its final developed condition. This is the amount that the project will need to manage entirely on site through green infrastructure and low-impact development techniques.
• Use the existing conditions if the project is not planning to modify the site design. Use the proposed conditions if the project is planning to modify the site design, add impervious surfaces, or remove impervious surface area. Runoff volume depends on the final site conditions of the project, such as amount of paving, permeability of different surfaces, roof area, and vegetated areas.
• 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 5. Manage runoff volume on site (E)
If the existing project site already has qualified rainwater management strategies in place, determine whether they can fully capture and treat runoff from 25% of the impervious areas for the 95th percentile storm event. If the current site conditions or strategies cannot manage the required volume of runoff, on-site strategies do not currently exist, or the project is already planning to modify the site, conceptually design new site conditions to incorporate GI and LID measures such that it can meet the credit requirements.
• Work with the project’s civil engineer, landscape architect, or other qualified professionals to determine if existing strategies qualify as GI/LID, or to choose and size new design strategies (see Further Explanation, Green Infrastructure and Low-Impact Development Strategies).
• Calculations must account for the site-specific soil characteristics, the soil infiltration rate, and the storage capacity of all GI and LID measures.
• If the existing conditions provide adequate treatment, no further refinement is necessary. Move to step 7.
• When conceptualizing new site conditions, use 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.
• Impervious areas that are removed from the site and replaced with GI or LID measures, such as vegetation or pervious pavement, can contribute to meeting the credit requirements.

Step 6. Analyze and refine rainwater management strategies (E)
Refine the site 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 4 and 5 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.
• The GI or LID measures must be in place by the end of the performance period to satisfy the credit requirements.

Step 7. Develop and implement an annual inspection program of all rainwater management facilities (E, P)
Regularly evaluate rainwater measures to make sure they are performing properly.
• Inspect measures at least once per year.
• Protect measures from damage from any site modifications or construction activities.
• Inspect the site for changes in landscape contour, areas of erosion, plant health, standing water (for longer than 72 hours), or other problems, and identify maintenance and repair needs.
• Perform necessary maintenance, repairs, or stabilization within 60 days of inspection.
• Maintain records of rainwater inspection activities and repairs.
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:
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:
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 »

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© 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

Establishment

Use low-impact development (LID) practices to capture and treat water from 25% of the impervious surfaces for the 95th percentile storm event. Establish and implement an annual inspection program of all rainwater management facilities to confirm continued performance.

Performance

Document the annual inspections, including identification of areas of erosion, maintenance needs, and repairs. Perform necessary maintenance, repairs, or stabilization within 60 days of inspection.

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.

Where can I find long-term rainfall data?

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 if I have less than 10 years of historical rainfall data?

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 methodology should I use to calculate runoff volume?

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.)

Are there any specific tools or computer programs that are recommended for calculating runoff volume?

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 are some low-impact development (LID) and green infrastructure (GI) strategies?

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.)

Where can I find example calculations?

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 about rainwater runoff from greenfield areas in my LEED project boundary?

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:
Delete the paragraph that begins with "The project is a 10 story building..." Add "Example 1." above the example site plan.

Replace Figure 1 with an updated graphic that shows revised numbers in the legend.

In Table 1. Example percentiles of rainfall events, 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 narrative, change the paragraphs to read:
"To calculate the runoff volume from 25% of the impervious surfaces for the 95th percentile of storm events (see Calculations), the team adds together all impervious surfaces on the project (existing and proposed), for a total area of 5,000 square feet (464.5 sm). The total impervious area to be managed is 25% of 5,000 square feet (464.5 sm), or 1,250 square feet (116.1 sm).

The team identifies existing GI and LID features that may contribute toward rainwater management and verifies that they manage the required runoff volume on site. In addition to the existing pervious landscaped areas on the project site, the team has identified several conceptual GI and LID measures- bioretention areas, porous paving, a rain garden, and pervious decking- to capture rainwater diverted from impervious surfaces. To ensure that the designed site manages all the required rainwater runoff generated, the team calculates how the GI and LID measures manage the runoff, and it is verified that all required runoff is managed on site."

In the Sample narrative describing GI and LID measures, replace "Taxodium distichum" with "Prunus americana."

Add a section called "Example 2." that reads:
"Example 2.
Example 2 follows a project located in Maryland that attempted Option 1, Percentile of Rainfall Events in the New Construction rating system. The example does not calculate 25% of the impervious surface area on the project. However, it details the iterative rainwater management design process and the calculations used to achieve the credit requirements for the 95th percentile of storm events. 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:
Revise the sentence that begins with "Use the following steps..." and step 5, and also add a paragraph to the end of this section:
Use the following steps to determine the percentile of rainfall events:

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:
Replace the Step-by-Step Guidance section with the following:
Step 1. Obtain rainfall data for project location (E)
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 2. Determine value for 95th-percentile rainfall events (E)
Using the historical rainfall data collected, calculate the rainfall value for the 95th percentile (in inches or millimeters). This is the precipitation amount that 95 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).

Step 3. Analyze existing site conditions and performance (E)
Whether or not the project is planning to modify the site design, first analyze how the current site is performing relative to the management of precipitation (prior to calculating the runoff volume).
• Work with a civil engineer, landscape architect, and other contractors or consultants as needed to assess the current site conditions against the credit criteria. This assessment is best performed before site redevelopment activities begin.
• Existing rainwater management strategies that qualify as GI or LID can be used to manage runoff from impervious surfaces.
• 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 any future 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 4. Calculate runoff volume to be managed on site (E)
Identify the impervious areas from which water will be treated according to the credit requirements (i.e., the areas to be included in the 25% threshold). Calculate the total volume of runoff (in cubic feet or cubic meters) corresponding to the 95th percentile of rainfall events for the site in its final developed condition. This is the amount that the project will need to manage entirely on site through green infrastructure and low-impact development techniques.
• Use the existing conditions if the project is not planning to modify the site design. Use the proposed conditions if the project is planning to modify the site design, add impervious surfaces, or remove impervious surface area. Runoff volume depends on the final site conditions of the project, such as amount of paving, permeability of different surfaces, roof area, and vegetated areas.
• 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 5. Manage runoff volume on site (E)
If the existing project site already has qualified rainwater management strategies in place, determine whether they can fully capture and treat runoff from 25% of the impervious areas for the 95th percentile storm event. If the current site conditions or strategies cannot manage the required volume of runoff, on-site strategies do not currently exist, or the project is already planning to modify the site, conceptually design new site conditions to incorporate GI and LID measures such that it can meet the credit requirements.
• Work with the project’s civil engineer, landscape architect, or other qualified professionals to determine if existing strategies qualify as GI/LID, or to choose and size new design strategies (see Further Explanation, Green Infrastructure and Low-Impact Development Strategies).
• Calculations must account for the site-specific soil characteristics, the soil infiltration rate, and the storage capacity of all GI and LID measures.
• If the existing conditions provide adequate treatment, no further refinement is necessary. Move to step 7.
• When conceptualizing new site conditions, use 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.
• Impervious areas that are removed from the site and replaced with GI or LID measures, such as vegetation or pervious pavement, can contribute to meeting the credit requirements.

Step 6. Analyze and refine rainwater management strategies (E)
Refine the site 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 4 and 5 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.
• The GI or LID measures must be in place by the end of the performance period to satisfy the credit requirements.

Step 7. Develop and implement an annual inspection program of all rainwater management facilities (E, P)
Regularly evaluate rainwater measures to make sure they are performing properly.
• Inspect measures at least once per year.
• Protect measures from damage from any site modifications or construction activities.
• Inspect the site for changes in landscape contour, areas of erosion, plant health, standing water (for longer than 72 hours), or other problems, and identify maintenance and repair needs.
• Perform necessary maintenance, repairs, or stabilization within 60 days of inspection.
• Maintain records of rainwater inspection activities and repairs.
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:
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:
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

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Ashwini Arun

WSP
Senior Sustainability Manager

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