FAQ
General Questions
How can something like this be accurate?
The Green Values Stormwater Toolbox aims to help people evaluate sustainable design opportunities. It is based on extensive research into the longterm costs and effectiveness of green infrastructure approaches. Its results are still approximations based on researchbased assumptions of green infrastructure performance, soil types, cost ranges, and other key factors. Results of the calculator are as accurate as our estimates can be, given current research. Results are not, in this version, meant to substitute for detailed, formal design and engineering assessments.
Why can’t I evaluate new urbanism concepts such as clustered housing, narrow streets and shorter driveways?
You can do that. Simply designate the conventional sizes for street width and driveways on one run, copy the results, and use narrower streets and smaller driveways on another and compare the results. The option of using “Half of lawn replaced by garden with native landscaping” could simulate a development where half the area is left open. The shorter streets common in new urbanism could be simulated by making the lots bigger and putting more rooftop and pavement on each lot to arrive at the same number of homes.
Does the calculator apply to all regions?
We designed the calculator based on the hydrology of the Great Lakes region. The results would be different for other regions with different rainfall patterns. However, the calculator can be used to get a general sense of how green infrastructure might work in other areas.
Can I save my results and come back to them?
Not at this time, however you can print your results for future reference.
Why do you only offer six green options, aren’t there other green actions, such as infiltration trenches and rain barrels, that would reduce runoff?
For this first version of the calculator we chose a number of green infrastructure interventions that can be reasonably implemented by a developer or homeowner, used in many locations and at many scales. At the same time, we wanted to keep the calculator simple. If you have research on the stormwater benefits of a green action that we did not include, we would love to hear about it for our future versions.
Why are the green infrastructure options unchangeable? What if I want to install 100 percent native plants, not 50 percent?
The calculator is meant to give a sense of the stormwater costs and benefits of green infrastructure actions. It is not meant to be a substitute for proper site design. We feel that the simple options make the calculator easier to use.
Why do you ask if this is an existing neighborhood?
The model makes different assumptions for new and existing neighborhoods. In terms of costs, the model assumes that the existing neighborhood is 50 years old, so infrastructure such as sewer pipes will not have capital costs in year one, compared to a new neighborhood where all stormwater infrastructure must be constructed. Moreover, the model assumes that an existing neighborhood does not have room for additional detention basins. Finally, the model assumes that existing neighborhoods have combined stormwater and sanitary sewers, and all of the water is treated, so that there is a financial benefit in terms of reduced treatment cost to reducing the stormwater runoff.
I live in the city, how many blocks are there to an acre?
One city block is the equivalent of 6 acres.
Why can’t I change the number of trees on my lot?
If you are using a new development scenario, the calculator assumes that the site starts with zero trees.
How does one go about finding most of these answers?
We have set the defaults for these options at values that are reasonable for the Chicago region.

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. If you have specific slope information from a survey of your land you can use that.

Soil type?
Soil types are characterized from A to D, with A being sandy and welldrained and D being mostly clay and poorly drained. C is the recommended default for the Chicago suburbs. Every soil in the U.S. is assigned one of these four categories using the following table: ftp://ftp.wcc.nrcs.usda.gov/downloads/hydrology_hydraulics/neh630/hydro_soil_groups.pdf

Average roof size?
Your roof size can be estimated as the square footage of one story of your home and garage.

Average number of trees?
This is simply the number of trees per lot.
What is “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 http://www.whitehouse.gov/omb/circulars/a094/a094.html
What does changing the life cycle years do?
Changing the life cycle years changes the number of years over which costs and benefits for the site are calculated. Each infrastructure component is assigned a lifespan, so for example, a 100 year life cycle analysis may include the construction cost of the concrete sidewalk and driveway (30 year lifespan) four times. Different life cycle analysis periods are useful for different reasons, for example the 100 year analysis is useful for comparing green infrastructure options to traditional stormwater infrastructure, while the 30 year analysis is useful for considering green infrastructure investments in terms of the average mortgage length.
Will you change the site over time?
Absolutely. This is the first phase of a longterm project. We will make improvements to the site every month and post the changes made. Please use the feedback mechanism to suggest changes, identify new research and give us your ideas.