Models and Calculators
The features described below include predictive models and calculators to assist in modeling stormwater runoff, water quality impacts form land use changes, benefits of green roofs and urban tree coverage, pollutant load reductions, and to evaluate performance of green infrastructure techniques. In addition, the calculators assist with quantifying costs and benefits of stormwater treatment devices and green infrastructure application and design investments.
Casey Trees "Green Build-Out" Model  - A model to predict the stormwater benefits of trees and green roofs for different coverage scenarios in Washington, DC. The model was applied to an “intensive greening” scenario and a “moderate greening” scenario, both of which demonstrated that trees and green roofs can be used to achieve substantial reductions in stormwater runoff and sewage discharges to local rivers.
EPA Hydrological Simulation Program – FORTRAN (HSPF) - FORTRAN (HSPF) is a comprehensive package for simulation of watershed hydrology and water quality for both conventional and toxic organic pollutants. It is the only comprehensive model of watershed hydrology and water quality that allows the integrated simulation of land and soil contaminant runoff processes with in-stream hydraulic and sediment-chemical interactions. The result of this simulation is a time history of the runoff flow rate, sediment load, and nutrient and pesticide concentrations, along with a time history of water quantity and quality at any point in a watershed. HSPF simulates three sediment types (sand, silt and clay) in addition to a single organic chemical and transformation products of that chemical.
EPA Stormwater Management Model (SWMM) - SWMM is a dynamic rainfall-runoff simulation model used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas. SWMM was first developed in 1971, and has since undergone several major upgrades since then. It continues to be widely used throughout the world for planning, analysis and design related to stormwater runoff, combined sewers, sanitary sewers, and other drainage systems in urban areas, with many applications in non-urban areas as well.
Long-Term Hydrologic Impact Assessment Model - The Local Government Environmental Assistance Network's (LGEAN) Long-Term Hydrologic Impact Assessment (L-THIA) model was developed as an accessible online tool to assess the water quality impacts of land use change. Based on community-specific climate data, L-THIA estimates changes in recharge, runoff,and nonpoint source pollution resulting from past or proposed development. As a quick and easy-to-use approach, L-THIA's results can be used to generate community awareness of potential long-term problems and to support planning aimed at minimizing disturbance of critical areas.
P8-UCM: Program for Predicting Polluting Particle Passage through Pits, Puddles, and Ponds—Urban Catchment Model (PDF) (4 pp, 96K) - P8 is a model for predicting the generation and transport of stormwater pollutants in urban watersheds. The model simulates pollutant transport and removal in a variety of treatment devices, including swales, buffer strips, detention ponds (dry, wet, and extended), flow splitters, and infiltration basins (offline and online), pipes, and aquifers. Water quality components include total suspended solids (TSS), total phosphorus (TP), total Kjeldahl nitrogen (TKN), copper, lead, zinc, and hydrocarbons.
Pollutant Load and Reduction Model - Comprehensive Environmental Inc. has developed a Pollutant Load and Reduction Model that can be helpful to a variety of users including watershed groups, municipal land use decision-makers, and engineers. The Model allows the user to determine how different types of green infrastructure techniques can reduce the pollutant loads in a given area.
Rapid Assessment of the Cost-Effectiveness of Low Impact Development for CSO Control (PDF) (15 pp, 1.4MB) - This paper presents a simple model for assessing the cost-effectiveness of investments in green infrastructure for reducing combined sewer overflows (CSOs) in urban watersheds.
RECARGA - The University of Wisconsin developed RECARGA as a design tool for evaluating the performance of bioretention facilities, raingardens, and infiltrations basins. Individual stormwater management practices, with up to 3 distinct soil layers and optional underdrains, can be modeled under user-specified precipitation and evaporation conditions. The results of this model can be used to properly size stormwater controls to meet specific performance objectives, such as reducing runoff volume or increasing groundwater recharge, and for analyzing the potential impacts of varying the design parameters.
Upper Neuse Site Evaluation Tool (SET)
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SET was developed by the Upper Neuse River Basin Association and Tetra Tech Inc. to help assess the environmental impacts and costs of a site's stormwater management design. The SET is designed primarily for local government site review planners, professional developers, and stormwater engineers, but it is useful for anyone with an interest in reducing stormwater runoff impacts.
USGS Source Loading and Management Model (SLAMM) - SLAMM was originally developed to better understand the relationships between sources of urban runoff pollutants and runoff quality. It has been continually expanded since the late 1970s and now includes a wide variety of source area and outfall control practices (infiltration practices, wet detention ponds, porous pavement, street cleaning, catchbasin cleaning, and grass swales). SLAMM is strongly based on actual field observations, with minimal reliance on theoretical processes that have not been adequately documented or confirmed in the field. SLAMM is mostly used as a planning tool, to better understand sources of urban runoff pollutants and their control.
Water Balance Model - The Water Balance Model (WBM) is designed to evaluate the feasibility and effectiveness of site level rainwater management solutions. The model is an internet-accessible decision support and scenario planning tool that is fast emerging as the model of choice in making sustainable land development decisions because it demonstrates how to achieve a light 'hydrologic footprint'.
Calculators
The Center for Neighborhood Technology Green Values Calculator (GVC) - The GVC
compares green infrastructure performance, costs, and benefits to
conventional stormwater practices at the site and neighborhood scales.
The tool provides a quantified analysis of green infrastructure
environmental benefits including reduced runoff volume and maintenance
savings, in addition to carbon sequestration, reduced energy use, and
groundwater recharge. The goal of the tool is to encourage communities
to adopt green infrastructure as the preferred method for stormwater
management by demonstrating the ecological and economic gains that
result from implementing green infrastructure practices. The GVC uses
precipitation data from anywhere in the U.S. and provides a range of
runoff reduction goals that represent current innovation in stormwater
regulation around the country. The National GVC can work in all U.S.
communities because the user specifies the site location, and the
calculations thereafter use precipitation data from the nearest National
Oceanic and Atmospheric Administration (NOAA) weather station.
Calculator users can also choose from a list of regulations aimed at
achieving varied levels of runoff volume reduction chosen from exemplary
communities around the country. The specified runoff reduction goal sets
a standard against which a selected suite of management practices
performs in relation to that standard. Users may also define the
predevelopment conditions of their site.
Green Roof Life Cycle Cost-Benefit Calculator - Green Roofs for Healthy Cities developed this calculator to help evaluate various roofing related investment scenarios. The Tool focuses on long timeframes, real monetary costs and savings, and financial returns attributed to employing conventional and green (vegetative) roofs. It also provides some guidance to the users about how to factor in financial information from benefits that may be overlooked in the analysis. To access the calculator, a free user account must be created.
Stormwater Sizing Calculator - The City of Emeryville, California developed this spreadsheet-based calculator to determine the proper size of stormwater treatment devices for new development projects.
The Center for Neighborhood Technology’s "Green Values" Stormwater Calculator - A calculator that can be used to estimate the financial and hydrologic impacts that various green infrastructure technologies can have on a development site.
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