gsi impact calculator

No problem. If your city is not listed/shown on the map, pick the closest and most representative major city. Select a city that is similar in climate and rainfall.

Project impact Area refers to the broader area over which GSI will be implemented and where most co-benefits will directly accrue. It is different from the GSI management or drainage area. It is intended to capture the area where households and businesses will benefit from the planned GSI installations. The calculator was designed to assess the co-benefits associated with retrofitting a block or multiple blocks with GSI. A typical city block is 2 to 7 acres. However, the Impact Area depends on the area/amount of GSI installed. For example, retrofitting an entire block will affect more households and businesses (and have a greater Impact Area) than installing GSI at a single site.

While the definitions are below, it is important to note that most GSI Scenarios in urban and suburban areas will fall under some level of development category, which include some amount of open space/permeable area. Thus, it is not necessary to try to parse out individual areas of open space. The Calculator corresponds with land cover categories defined by the U.S. Geological Survey National Land Cover Database (NLCD).

• Open Space: These areas can be parks, golf courses, and large single-family home lots, as well as natural forests, wetlands, shrublands, and cultivated agricultural areas. Impervious surfaces make up less than 20% of total land cover. Corresponds with NLCD codes 21 and 31– 95.
• Low Intensity Development: These areas most commonly represent average lot sized single-family homes. Impervious surfaces make up 20% – 49% of total land cover. Corresponds with NLCD code 22.
• Medium Intensity Development: These areas most commonly represent average lot sized single-family homes. Impervious surfaces make up 50% – 79% of total land cover. Corresponds with NLCD code 23.
• High Intensity Development: This category represents highly developed areas where people reside or work in high numbers. Examples include apartment complexes, row houses, and commercial/industrial areas. Impervious surfaces make up 80% or greater of total land cover. Corresponds with NLCD code 24.

NOAA’s Atlas 14 contains precipitation data searchable by location.

Your jurisdiction or stormwater regulatory authority may maintain and publish this information. If not, they typically provide instructions on how to calculate this value.

To determine the depth of an 85th percentile rain event, you need to analyze historical rainfall data, typically from a period of at least 10 years, rank the 24-hour rainfall depths from smallest to largest, and then identify the rainfall depth at which 85% of the events are equal to or smaller. This process involves several steps:

• Gather Historical Rainfall Data over a period of at least 10 years from a reputable source such as a nearby weather station or the National Climatic Data Center.
• Exclude small precipitation events, such as those with rainfall depths of less than 0.1 inches.
• Determine the 85th Percentile depth by counting total number of 24-hour events in your data set, determine which rainfall depth associated with the storm that is 85% largest in your data set.

In general, the distinction between these two best management practices (BMPs) is based on size and sophistication. Rain gardens include smaller, more simply designed practices that may be sized to manage runoff from a single rooftop, portion of a rooftop, or driveway. They are typically installed on residential properties. Bioretention facilities, on the other hand, may be sized to manage runoff from a whole parking lot, property, or even multiple properties. They typically are designed by professional engineers to specifications provided by a regulatory agency or engineering standards body.

Bioretention facilities emphasize retention of runoff and eventually manage it through infiltration and evapotranspiration. Biofiltration facilities are intended to use plants, soils, and natural processes to remove pollutants from stormwater before it drains into a storm sewer or receiving water.

The Calculator treats impervious area removal the same as it does biofiltration. Impervious removal should be included in this category.

Wetland recreation areas are public spaces that include constructed wetlands and/or wet ponds. For this benefit to be applicable, there must be some recreational component to the BMP design such as picnic benches, trails, or wildlife viewing areas.

The Calculator considers the value of adding vegetation and green space in the public right-of-way. This generalized greening can beautify neighborhoods, increase pedestrian and cyclist safety, and encourage people to spend more time outside.

A discount rate adjusts the estimated benefits and costs of an infrastructure project for differences in timing of when benefits and costs accrue. Most people prefer a dollar today more than an inflation-adjusted dollar available in the future. The discount rate reflects the annual rate at which current values are preferred to future values.

When benefits and costs are adjusted with discount rates, they are referred to as “present values.” Present values allow for costs and benefits occurring across different time periods to be added together to determine an overall net effect. Present value reflects how much a future amount is worth today.

In 2024, the Biden administration recommended lowering the discount rate used in federal cost-benefit analysis to 2% for projects that do not increase risk. FEMA applies a 3.1% discount rate, which includes a risk premium. In general, applying a discount rate somewhere between these two values is appropriate for GSI projects assessed using the Calculator.

The Calculator calculates avoided infrastructure costs differently depending on whether the project is located within a separate or combined sewer area. This question is asked on the Benefit-Specific Input tab under the Define Scenario step.

If the project is located within a municipal separate storm sewer system (MS4) area, the Calculator applies an avoided cost of $3.58 per square foot of impervious area managed to estimate avoided capital costs. This is the stormwater management allowance cost from RS Means, a proprietary database of construction cost information that engineers often use to estimate the cost of construction projects. It is included in RS Means as being representative of a typical gray infrastructure scenario, “absent further information” or specific cost detail. For avoided operations and maintenance (O&M) costs, the Calculator assumes 0.5% of capital per year ($0.18 per square foot per year).

Because stormwater is not typically pumped or treated in MS4 communities (i.e., it is directly discharged to local rivers and waterways through the storm drainage system), the Calculator assumes there are no avoided stormwater pumping and treatment costs for projects located in MS4 communities.

For projects located in combined sewer systems, the user has options for selecting the type of infrastructure offset, reduced, or avoided as a result of their GSI Scenario. Depending on the user’s selection the following costs are applied based on estimates from the literature:

Deep tunnels

• $6.05 per gallon of BMP capacity (capital)
• $0.03 per gallon managed per year (O&M)

Sewer separation

• $119,184 per drainage acre (capital)
• 0.5% of capital costs per year (O&M)

Other traditional gray infrastructure (same as for MS4 communities)

• $3.58 impervious square foot managed (capital)
• 0.5% of capital costs per year (O&M)

To estimate avoided stormwater pumping and treatment costs for combined sewer communities, the Calculator applies a cost of $1.51 per thousand gallons of stormwater runoff managed (applies to infiltrated stormwater that is no longer sent to the combined system). This estimate is from the WRF GSI TBL Tool, which relied on an extensive review of data and literature related to these costs. It assumes that both stormwater pumping and treatment are avoided.

Implementing green roofs and permeable pavement avoids maintenance and replacement costs associated with their traditional alternatives (i.e., traditional roofs, asphalt/concrete). The Calculator applies the following assumptions to estimate these avoided costs:

• Green roofs avoid $0.08 per square foot in maintenance costs for traditional roofs.
• Green roofs avoid the replacement of traditional roofs every 17 years, at a cost of $12.55 per square foot per replacement cycle.
• Permeable pavement avoids annual maintenance costs of $0.11 per square foot for traditional pavement (reflects weighted average costs assuming permeable pavement replaces 80% asphalt streets and/or concrete and 20% asphalt parking lots).

These avoided costs are categorized separately from avoided gray infrastructure costs because they reflect avoided costs for non-stormwater infrastructure, including traditional roofs and pavement.

The underlying methods and data sources for calculating each value are fully described in the GSI Impact Calculator: Methods & Data Sources document.

The GSI cost estimates in the Calculator include average capital and maintenance costs by GSI practice type from the WRF CLASIC Tool. Capital costs account for design, construction, materials, and contingency costs.
Two of the BMPs in the Calculator – wetlands and stormwater trees – are not included in CLASIC. Costs for these BMPs reflect average estimates from the literature.

It is recommended that users input their own costs if available to better reflect average local costs for implementing GSI. O&M costs represent annual values that accrue each year to ensure proper functioning of the GSI practices.