By Steve Rizer
For which sites should the National Stormwater Calculator that the U.S. Environmental Protection Agency recently made available be used -- and not be used?
In an interview with ConstructionPro Week, agency spokesperson Cathy Milbourn explained, “This calculator could be useful to any construction project that involves the development or re-development of a building structure and/or landscape. The calculator can be used to evaluate green infrastructure/low-impact-development (LID) options for both landscapes, such as rain gardens or permeable pavement, as well as green roofs or rain barrels on the buildings. The goal for adding these options would be to reduce stormwater runoff from the site once the development is complete.”
Estimates that the calculator makes are based on local soil conditions, land cover, and historic rainfall records. It is designed to be used by anyone who is interested in reducing runoff from a property, including site developers, landscape architects, urban planners, and homeowners. The calculator accesses several national databases that provide soil, topography, rainfall, and evaporation information for the chosen site. The user supplies information about the site’s land cover and selects the types of LID controls desired for use. The LID controls that the user can choose are seven green infrastructure practices, including disconnection, rain harvesting, rain gardens, green roofs, street planters, infiltration basins, and porous pavement.
Green infrastructure promotes the natural movement of water instead of allowing it to wash into streets and down storm drains, according to EPA. “Green infrastructure also has the added benefit of beautifying neighborhoods and increasing property values.”
Estimates can be affected by limitations on site-specific information and uncertainties about future climate. To make more-informed decisions, EPA recommends that users develop a range of results with various assumptions about model inputs, such as percent of impervious surface, soil type, and sizing of green infrastructure.
But could the calculator be used to help prevent stormwater violations at highway construction sites, such as the sites at issue in a recent enforcement action involving the Kansas Department of Transportation? There, the department allegedly failed to install or implement adequate stormwater control measures, such as stabilizing disturbed soils in a timely fashion, properly maintaining stormwater controls, and developing an adequate stormwater pollution-prevention plan and updating the plan in an appropriate manner.
In response to this question, Milbourn said, “The calculator can help with developing a plan to reduce stormwater runoff from a property after construction is complete, but is not designed for stormwater management during construction.
Milbourn explained that EPA's Storm Water Management Model would be the appropriate tool for larger projects that involve both the wastewater/stormwater infrastructure and green infrastructure pilot programs for a community.
An update to the calculator, which will include the ability to link to several future climate scenarios, will be released by year’s end, according to EPA. “Climate projections indicate that heavy precipitation events are very likely to become more frequent as the climate changes. Green infrastructure can increase the resiliency of stormwater management approaches to a changing climate, and this update will allow users to consider how runoff may vary based both on historical weather and potential future climate. Please check with local authorities about whether and how use of these tools may support local stormwater management goals and requirements.”
The ConstructionPro Network member version of this article includes additional comments from Milbourn and a link to EPA's new guide for using the calculator. To sign up for a membership, click here.
