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National Pollutant Discharge Elimination System (NPDES)

Resources for Permit Writers

Permit Writers' Manual

The NPDES permit writers' manual (EPA-833-K-10-001) provides a comprehensive overview of the framework of the NPDES program. It also serves as one of the principal training tools to help permit writers develop legally defensible and enforceable NPDES permits.

Its primary purpose as a technical resource is to guide new state and EPA permit writers through the basic steps of permit development and issuance. The manual is also intended to serve as a resource for others, such as stakeholders and the regulated community, interested in the NPDES permitting process.

Publications for Permit Writers

Models and Calculators for Permit Writers

  • CORMIXExitis a U.S. EPA-supported mixing zone model and decision support system for environmental impact assessment of regulatory mixing zones resulting from continuous point source discharges. The system emphasizes the role of boundary interaction to predict steady-state mixing behavior and plume geometry in both near- and far-field zones. It can be applied to discharges in many different water bodies, such as rivers, lakes and estuaries.
  • SWToolbox, created by USGS in collaboration with EPA, is designed to assist NPDES permit writers and other practitioners estimate critical stream statistics such as the 7Q10. The primary functions are to conduct an n-day frequency analysis and to compute biologically-based design flows. Flow duration curves can also be computed. The tool directly accesses current and historic flow data from USGS gaging stations, and also allows users to import their own data. SWToolbox is built on the EPA BASINS system. Features include a batch run option and automated testing for trends, serial correlation, outliers and unusually large skew. The tool also provides a method for estimating critical flows at ungaged locations.
  • Delft3DExitis a modeling package that allows users to simulate flow, sediment transport, water quality in a range of water bodies. Delft3D-FLOW is the hydrodynamic module in the Delft 3D package that applies to thermal modeling. It is best suited to model far-field zones and can be applied to rivers, lakes, reservoirs, estuaries, and coasts.
  • The Environment Fluid Dynamics Code (EFDC) was created with the support of EPA. EFDC can simulate both near- and far-field thermal mixing and can be applied to rivers, lakes, reservoirs, estuaries, and coasts.
  • CE-QUAL-W2 Exitcan simulate thermal mixing in rivers, lakes, reservoirs, and estuaries, but is best suited for long and narrow water bodies because it assumes lateral homogeneity. The model is generally applied to simulate far-field thermal mixing.
  • National Stormwater Calculator estimates the annual amount of stormwater runoff from a specific location anywhere in the United States. Estimates are based on local soil conditions, land cover, and historic rainfall records. The Calculator includes changes in seasonal precipitation levels, the effects of more frequent high-intensity storms, and changes in evaporation rates based on validated IPCC scenarios. Users can enter any U.S. location and select different scenarios to learn how specific management and design approaches can prevent pollution.
  • Storm Water Management Model (SWMM) is used for planning, analysis and design related to stormwater runoff, combined and sanitary sewers, and other drainage systems in urban areas. There are many applications for drainage systems in non-urban areas as well. The Storm Water Management Model Climate Adjustment Tool (SWMM-CAT) is an add-in tool to SWMM. It is a simple to use software utility that allows future projections to be incorporated into the Storm Water Management Model. SWMM accepts monthly adjustment factors for time series that could represent the potential impact of future conditions and provides a set of location-specific adjustments.
  • Hydrologic and Water Quality System (HAWQS) is an interactive water quantity and quality modeling system to simulate the effect of management practices on water quality. HAWQS substantially enhances the usability of SWAT, its core modeling engine, to an extensive array of crops, soils, natural vegetation types, land uses, and scenarios for hydrology and the water quality parameters such as sediment, pathogens, nutrients, biological oxygen demand, dissolved oxygen, pesticides, and water temperature. HAWQS has interactive web interfaces and maps; pre-loaded input data; outputs that include tables, charts, and raw output data; a user guide, and online development, execution, and storage of a user's modeling projects.