Calculator Explanations

Roof Drains to Raingardens at All Downspouts

This selection simulates the results of controlling the stormwater runoff from every roof with a rain garden. Of course, a single rain garden can be used at any downspout to get some of the same benefits. One caution: There must be room on the lot-at least 20 percent of the roof area-for the raingardens.

Half of Lawn replaced by garden with native landscaping

This selection simulates the results of using native plant materials on half of the area available on the site for lawn. There are people who are creatively mixing prairie and lawn on their lots. There are also developers who reserve something like half of their total sites for native landscape to provide more open space and improve water resource management. This selection can simulate either condition.

Porous Pavement used on Driveway, Sidewalk and other non-street pavement

This selection simulates the results of replacing all non-street pavement on the lot with porous pavement. This is becoming a popular option, at least for some pavements such as driveways and patios. It should be noted that the same result in terms of stormwater management can often be achieved with strategically-located and designed landscaping.

Green Roofs

This selection simulates the results of replacing every roof on the lot with a "green roof." Green roofs are designed to resemble landscapes and have benefits related to stormwater management, building temperature moderation and roof durability. While green roofs have higher construction costs than standard roofs and are one of the more expensive green infrastructure options, it's important to note that in some situations, green roofs are the only solution to flooding and stormwater retention issues. Also, the cost of green roofs is expected to decrease as they gain widespread use.

Provide Tree Cover for an Additional 25% of Lot

This selection simulates the results of adding additional trees to a lot. The size and age of a tree is critical to this calculation because as a tree grows, it provides more shade and more stormwater retention capability. To simplify the calculation, we assume the benefits for a typical mature tree planted in an urban region, the average life of which is 37 years (, p. 38). We also assume a canopy cover of approximately 200 square feet per tree.

Use Drainage Swales instead of Stormwater Pipes

This selection simulates the results of substituting drainage swales at the back of each lot for curbs, gutters and storm sewers under the streets.

A Word About Detention

If the development is new, detention will probably be required. For existing development, detention is probably not an issue but the outlet peak flow and total runoff volume will be important factors. The costs of detention, as with all of the infrastructure components in the model, do not include land costs. If detention is to be built on-site, some development opportunities will be reduced. If detention is to be built off-site, costs of land must be added. The costs for detention assume a wet-bottom pond with a permanent depth of 2 feet and a storage depth of 5 feet.

Total size of neighborhood:

There is currently no restriction on size, although we do have default sizes for both new and existing neighborhoods. One city block is the equivalent of 6 acres. The calculator assumes that the neighborhood is square, so that a length and width of the neighborhood, which are equal, can be calculated.

Number of lots:

You can either use the default number of lots for existing and new developments, or input your own number. We assume that the lots are all 3 times longer than they are wide, so that the width and length of the lots can be calculated.

When the number of lots and average width is known, the length of frontage streets-with lots on both sides-is calculated.

Average Size of Rooftops

Use the average size of houses in this neighborhood, including the garages.

Average number of trees on lot:

The older a tree is, the more benefit it provides. For example, as a tree matures and its canopy expands, it is able to capture more rainwater and provide more shade. For our calculations, we assume each tree has reached its mature width and height at approximately 10 years, and that each has a canopy of approximately 200 square feet.

Average Driveway Area:

Use the average size of driveways in this neighborhood.

Average Impermeable patio, deck, alley or parking lot:

Use the average size of other impereable surfaces in this neighborhood.

Sidewalk Width:

All of the sidewalk at the front of the lot area is assumed to contribute to runoff from the lot.

Street width:

Half of the street pavement is assumed to contribute to runoff from the lot.

Soil Type:

Soil types are characterized from A to D, with A being sandy and well-drained and D being mostly clay and poorly drained. C is the recommended default for the Chicago suburbs.

Average Slope:

A general land slope of either 1 or 3 percent is chosen. The Chicago area is relatively level, so that a default of 1 percent is recommended

Real Discount Rate:

The Real Discount Rate is an interest rate used to represent the time value of money in calculations. Higher discount rates will give less weight to future year costs and benefits than lower discount rates. All calculations in this calculator are done on a constant dollar basis, so a Real Discount Rate (one adjusted to remove the impact of inflation) is used. You may want to enter your own rate if your organization has a standard Real Discount Rate for projects. In 2005, the White House Office of Management and Budget recommends a Real Discount Rate of 3.1 percent for projects of 30 years or longer. For more information see

Life Cycle in Years:

The Scenario Years determines the number of years over which the life cycle costs and benefits of your scenario will be calculated. The default is 100 years, which allows the model to demonstrate the benefits of using green, renewable infrastructure as compared to standard infrastructure that requires regular replacement.