National Stormwater Calculator User's Guide - Version 1.2
Citation:
Rossman, L., J. Berner, M. Tryby, M. Simon, S. Struck, D. Pankani, AND M. Deerhake. National Stormwater Calculator User's Guide - Version 1.2. U.S. EPA Office of Research and Development, Washington, DC, EPA/600/R-13/085d, 2017.
Impact/Purpose:
This serves as a user manual to the National Stormwater Calculator. The calculator’s primary focus is informing site developers and property owners on how well they can meet a desired stormwater retention target. The calculator is most appropriate for performing screening level analysis of small footprint sites up to several dozen acres in size with uniform soil conditions. The remaining sections of this guide discuss how to install the calculator, how to run it, and how to interpret its output. An example application is presented showing how the calculator can be used to analyze questions related to stormwater runoff, retention, and control. Finally, a technical description is given of how the calculator performs its computations and where it obtains the parameters needed to do so.
Description:
The National Stormwater Calculator is a simple to use tool for computing small site hydrology for any location within the US. It estimates the amount of stormwater runoff generated from a site under different development and control scenarios over a long term period of historical rainfall. The analysis takes into account local soil conditions, slope, land cover and meteorology. Different types of low impact development (LID) practices (also known as green infrastructure) can be employed to help capture and retain rainfall on-site. Future climate change scenarios taken from internationally recognized climate change projections can also be considered. The calculator provides planning level estimates of capital and maintenance costs which will allow planners and managers to evaluate and compare effectiveness and costs of LID controls.The calculator’s primary focus is informing site developers and property owners on how well they can meet a desired stormwater retention target. It can be used to answer such questions as:• What is the largest daily rainfall amount that can be captured by a site in either its pre-development, current, or post-development condition?• To what degree will storms of different magnitudes be captured on site?• What mix of LID controls can be deployed to meet a given stormwater retention target?• How well will LID controls perform under future meteorological projections made by global climate change models?• What are the relative cost (capital and maintenance) differences for various mixes of LID controls?The calculator seamlessly accesses several national databases to provide local soil and meteorological data for a site. The user supplies land cover information that reflects the state of development they wish to analyze and selects a mix of LID controls to be applied. After this information is provided, the site’s hydrologic response to a long-term record of historical hourly precipitation, possibly modified by a particular climate change scenario, is computed. This allows a full range of meteorological conditions to be analyzed, rather than just a single design storm event. The resulting time series of rainfall and runoff are aggregated into daily amounts that are then used to report various runoff and retention statistics. In addition, the site’s response to extreme rainfall events of different return periods is also analyzed. The calculator uses the EPA Storm Water Management Model (SWMM) as its computational engine (http://www.epa.gov/nrmrl/wswrd/wq/models/swmm/). SWMM is a well-established, EPA developed model that has seen continuous use and periodic updates for 40 years. Its hydrology component uses physically meaningful parameters making it especially well-suited for application on a nation-wide scale. SWMM is set up and run in the background without requiring any involvement of the user.The calculator is most appropriate for performing screening level analysis of small footprint sites up to several dozen acres in size with uniform soil conditions. The hydrological processes simulated by the calculator include evaporation of rainfall captured on vegetative surfaces or in surface depressions, infiltration losses into the soil, and overland surface flow. No attempt is made to further account for the fate of infiltrated water that might eventually transpire through vegetation or re-emerge as surface water in drainage channels or streams. The remaining sections of this guide discuss how to install the calculator, how to run it, and how to interpret its output. An example application is presented showing how the calculator can be used to analyze questions related to stormwater runoff, retention, and control. Finally, a technical description is given of how the calculator performs its computations and where it obtains the parameters needed to do so.