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.