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Stormwater Glossary

Many stormwater management technical terms can have overlapping or conflicting meanings depending on the context in which they are used. For the Commission's purposes, stormwater terminology will be defined as follows:

    Stormwater best management practices (BMPs)

    controls used to manage the quantity and improve the quality of stormwater runoff. The draft MA MS4 General Permit (USEPA Region 1, 2014) further defines BMPs as "schedules of activities, practices (and prohibitions of practices), structures, vegetation, maintenance procedures, and other management practices to prevent or reduce the discharge of pollutants to waters of the United States. BMPs also include treatment requirements, operating procedures, and practices to control site runoff, spillage or leaks, sludge or waste disposal, or drainage from raw material storage." Stormwater best management practices can be either structural or non-structural.

  • Structural BMPs are actually based on natural systems and rely upon vegetation and soil mechanisms in order to perform as intended. Others are considered more conventional "brick and mortar" techniques. The use of these various techniques is not meant to replace the use of non-structural BMPs, but rather to work in tandem with these planning and design-based approaches to minimize unavoidable impacts. Some examples of structural BMPs include wetlands and rain gardens.

  • Non-structural BMPs are institutional, educational or pollution prevention practices designed to limit the amount of stormwater runoff or pollutants that are generated in the landscape. Ideally both non-structural and structural BMPs should be employed in the landscape to treat stormwater runoff. The use of non-structural BMPs upstream can reduce the amount of runoff being generated that will need to be treated by downstream, structural BMPs. Utilizing non-structural BMPs during site development is much more efficient and cost-effective than attempting to correct problems after development has occurred. Examples would include, not using too much fertilizer or pesticide, cleaning up pet waste regularly, properly disposing of used oil and cleaning up spills, doing preventive maintenance on vehicles, and decreasing impervious surfaces to allow storm water to soak into the soil. This manual will only deal with structural BMPs. Non-structural BMPs will be addressed in another document.

  • Green infrastructure (GI)

    uses vegetation, soils, and natural processes to manage water and create healthier urban environments. At the scale of a city or county, GI refers to the patchwork of natural areas that provides habitat, flood protection, cleaner air, and cleaner water. At the scale of a neighborhood or site, GI refers to stormwater management systems that mimic nature by soaking up and storing water. This report considers GI as a subset of BMPs that uses structural controls for stormwater management.

    Low impact development (LID)

    is an approach to land development (or redevelopment) that works with nature to manage stormwater as close to its source as possible. LID employs principles such as preserving and recreating natural landscape features, minimizing effective imperviousness to create functional and appealing site drainage that treats stormwater as a resource rather than a waste product. For the purposes of this report the term "low impact development" can be used interchangeably with "green infrastructure"

Additionally, in order to minimize confusion and maintain consistency with previous studies, some key hydrologic terms were adopted from the 2012 Stormwater Model Report (Camp, Dresser, McKee, Smith [CDM Smith], 2012) and are further explained as follows:


    in this report refers to an area tributary to one of the Commission's permitted outfalls listed in the Commission's stormwater National Pollutant Discharge Elimination System (NPDES) permit. This definition corresponds with the Consent Decree's intended meaning for sub-catchment, as Paragraph 14 of that document indicates that each sub-catchment corresponds with a single outfall.


    pertains to its meaning in SWMM as a hydrologic unit of land whose topography and drainage system elements direct surface runoff to a single discharge point. One or many SWMM subcatchments may represent the area tributary to a single outfall. Model subcatchments range from less than one acre in the Longwood Medical Area to hundreds of acres in Brookline, to 10 square miles and larger in the Upper Charles and Neponset watersheds. The median model subcatchment measures three acres. This definition differs from usage in the Consent Decree, which states that Sub-catchment shall mean the geographical area served by and drained to a distinct portion of the MS4.


    describes the collection of catchments draining to principal components of the Boston drainage system. Smaller watersheds can be tributary to larger watersheds. For example, the Bussey Brook watershed drains to the Stony Brook watershed, which is a component of the Lower Charles Basin watershed, which is part of the Charles River watershed. As some of Boston's principal drainage systems, such as the Shepard Brook drain in Brighton, have a single outfall, the terms catchment and watershed can refer to the same drainage area. Similarly, as some small drainage systems in downtown Boston, Charlestown, and East Boston are modeled with only a single subcatchment, the terms subcatchment and catchment can also refer to the same drainage area. The term basin is used interchangeably with watershed in this report.

    Reporting areas

    were defined during initial model development in 2005 and later refined by CDM Smith. These 27 reporting areas are generally more detailed than the eight watersheds (Charles River, Mystic River [including Chelsea River/Creek], Neponset River, Muddy River, Stony Brook, Boston Harbor, Dorchester Bay, and Boston Inner Harbor) identified in the Consent Decree for which detailed reporting information was requested. The only areas where the Commission's reporting areas offer less detail are in Charlestown, which drains to both the Mystic River and Inner Harbor, and in East Boston. Some reporting areas may include drainage from their neighboring communities.

    Total Maximum Daily Load (TMDL)

    is the greatest amount of a pollutant that a waterbody can accept and still meet water quality standards for protecting public health and maintaining the designated beneficial uses of those waters for drinking, swimming, recreation, and fishing. A TMDL calculates the maximum amount of a pollutant allowed to enter a waterbody so that the waterbody will meet and continue to meet water quality standards for that particular pollutant. The TMDL allocates said pollutant load to point sources (Wasteload Allocation or WLA) and nonpoint sources (Load Allocation or LA), which include both anthropogenic and natural background sources of the pollutant. TMDLs must also include a margin of safety (MOS) to account for the uncertainty in predicting how well pollutant reduction will result in meeting water quality standards and account for seasonal variations.