Glossary

Best management practice (BMP): A process, activity, or physical structure used to lessen the amount of pollution entering surface waters. See also stormwater control measure (SCM).

Flow control facility: A permanent stormwater facility that can provide water quality enhancement but whose primary purpose is to attenuate flows.

Local agency: A municipality, county, special district or other governmental or quasi-governmental agency that is a subdivision within a state.

MS4 and TS4: Two forms of NPDES permits. MS4 represents the acronym for municipal separate storm sewer systems, an M followed by four Ss. TS4 similarly represents the acronym for transportation separate storm sewer systems. Occasionally used as a noun; for example, “an MS4” is taken to mean an MS4 permit holder.

National pollutant discharge elimination system (NPDES): A permit system, first created in 1972 by the Clean Water Act; MS4 and TS4 permits are issued under the authority of this permit program.

NEPA study: For the purposes of this synthesis, refers to categorical exclusions, environmental assessments, and environmental impact statements and their associated decision documents.

Offsite flow permanent stormwater facility: A permanent stormwater facility constructed and/or funded by a DOT that treats flows emanating from offsite sources. In some instances, the offsite flows are treated in isolation, and the DOT can receive credit to mitigate untreated DOT discharges. In other instances, both the offsite flows and the onsite DOT flows are treated in a shared facility. In the shared facility, the offsite sources can be either upstream or downstream of the DOT sources, and the shared facility can be constructed in DOT right-of-way or outside DOT right-of-way. These shared facilities are also sometimes referred to as regional or watershed-based facilities.

Other state agency: Another agency within a state that is not a DOT.

Permanent stormwater facility: A facility designed and constructed to reduce, minimize, and treat stormwater runoff after completion of a transportation facility throughout the life of that facility. Stormwater quality facilities and flow control facilities are two distinct types of permanent stormwater facilities. Permanent stormwater facilities differ from construction-phase stormwater facilities or practices that are removed or inactivated upon construction completion and/or site stabilization. These facilities are also sometimes referred to as post-construction stormwater facilities, permanent water quality facilities, water quality treatment facilities, and similar names.

Permanent stormwater facility retrofit: A permanent stormwater facility constructed to reduce, minimize, and treat flows from existing infrastructure. There is nothing inherently different

about the function of stormwater retrofit facilities versus new facilities, as both treat runoff from impervious surfaces. The retrofits, however, must be designed and constructed within the constraints posed by the existing infrastructure they are designed to treat. Some facilities may be designed to treat runoff from both new and existing impervious surfaces.

Proprietary stormwater treatment device: Emerging stormwater technology used to provide stormwater reduction, minimization, and treatment through innovative techniques. A list of examples of such technologies can currently be found on the Washington State Department of Ecology web page “Emerging Stormwater Treatment Technologies (TAPE)” at https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-permittee-guidance-resources/Emerging-stormwater-treatment-technologies.

Stormwater control measure (SCM): A device intended to remove pollutants from stormwater. The device typically acts to clean stormwater by settlement, filtration, or mimicking natural processes such as infiltration, evapotranspiration, reuse of stormwater, or some combination of any of these. This measure is also known as a structural best management practice (BMP).

Stormwater quality facility: A permanent stormwater facility constructed to treat stormwater flows that run off from infrastructure after construction of that infrastructure throughout the life of that infrastructure. These facilities often provide peak flow and/or volume control, but their primary purpose is to remove contaminants from the stormwater runoff.

Stormwater treatment unit: See best management practice (BMP).

Third-party partnership: A partnership between a state DOT, a municipality or county, a special district, another state agency, a federal agency, a private business, a private non-profit organization, an educational institution, or any combination of these. For the purposes of this synthesis, the partnership is established to plan, and/or design, and/or construct, and/or operate, and/or maintain permanent stormwater facilities.

Water quality: In this synthesis, the term within references or as used by state DOTs is understood to mean stormwater quality, unless otherwise noted.