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.
