Design Criteria - Guidelines upon which planning and engineering decisions and judgements are based.

Design Standards - Detailed engineering drawings and/or specifications promulgated by public or private organizations that leave little choice to design engineers and technicians (e.g., manhole, catch basin, and inlet standards).

Design Storm - A precipitation event that, statistically, has a specified probability of occurring in any given year (expressed either in years or as a percentage).

Detention Facility - Any structure, device or combination thereof, that functions to accept inflow from surface runoff and discharge it at a controlled rate less than the peak inflow rate.

Development - Any activity that alters the surface of the land that generally creates additional impervious surfaces including, but not limited to, pavement, buildings and structures.

Drainage - Interception, collection and removal of excess stormwater from an area into another area or into a receiving water body.

Enclosed Drainage System - A drainage system consisting of essentially continuous pipes and/or box culverts below the ground surface.

Erosion - The removal of soil particles by the action of flowing water.

Flood Control - Preventing the entry of stormwater into an area from another area, or from a stream or other water body.

Flood Plain - The area surrounding a watercourse that is inundated with floodwater.

Floodway - A tool or concept used by local communities for flood plan management. 

Freeboard - The vertical difference in elevation between the hydraulic gradient and a referenced point.  Examples are the difference between the maximum water surface level behind a dam and the top of a dam, or the difference in elevation between the water surface at a culvert beneath the roadway and the surface of the roadway.

Impervious Surface - Any surface that does not readily permit water to infiltrate.  Examples are roofs and concrete or asphalt-paved surfaces.

Improved Channel - Any channel whose characteristics are changed by either grading or construction of lining materials.

Level of Service - The return period for which a drainage system, or an individual element of that system, has adequate hydraulic capacity.

Master Planning - A “systems” approach to the planning of facilities, programs and management organizations for comprehensive control and use of stormwater within a defined geographical area.

Natural Channel - An existing channel that has not been appreciably altered by grading, lining or changing its course.

NPDES - National Pollutant Discharge Elimination System – A program administered by the EPA addressing the discharge of pollutants to waters of the United States.

Open System - A drainage system consisting of open channels, either natural or improved, with only comparatively short lengths enclosed by pipes or culverts.

Pervious Surfaces - Surfaces that absorb water such as undeveloped areas, fields, yards and other unpaved areas.

Receiving Waters - Streams, lakes, bays, etc., into which stormwaters are discharged.

Return Period - A statistical term for the average frequency that a given event may be expected to occur, although it does not imply that the event will occur regularly at even intervals.  It can also be defined as the reciprocal of the probability of an event.  For example, a storm having a 10-year return period statistically can be expected to occur once in a period of 10 years, a probability of occurrence of 0.10, or 10%.  However, the event may occur at any time, and two such events may actually occur on successive days.

Sediment - Soil particles eroded by flowing water; either in suspension in that water or as deposited.

Storm Drainage System - Natural or constructed facilities and appurtenances, such as ditches, natural channels, pipes, culverts, bridges, improved channels, street gutters, inlets and detention facilities, that serve to collect and convey surface drainage.

Storm Sewers - Usually, enclosed conduits that transport excess stormwater runoff toward points of discharge (sometimes call “storm drains”).

Stormwater Management - Encompasses both “control” and “developmental” activities in which there is physical interaction with stormwater (a broader interpretation includes activities of an institutional nature – financing, staffing, etc.).

Stormwater Storage - Temporary storage of excess runoff on, below, or above the surface of the earth for the purpose of attenuating excess runoff.

Watershed - All land draining to the storm drainage system at any given point.  This term is used synonymously with the terms tributary area, drainage area, drainage basin, catchment area, subwatershed and subarea.

Zone A - FEMA Flood Insurance Zone - Special Flood Hazard Areas inundated by the 100-year flood, as determined by approximate methods; no base flood elevations are shown and no flood hazard factors are determined.

Zone AH - FEMA Flood Insurance Zone - Special Flood Hazard Areas inundated by types of 100-year shallow flooding where depths are between 1.0 and 3.0 feet; no base flood elevations are shown and no flood hazard factors are determined.

Zone A1, A3-A6, A8, A9, A14, A16 and A18 - FEMA Flood Insurance Zone – Special Flood Hazard Areas inundated by the 100-year flood, with base flood elevations shown and zones subdivided according to flood hazard factors.

Zone B - FEMA Flood Insurance Zone – Areas between the Special Flood Hazard Areas and the limits of the 500-year flood; areas are protected from the 100- and 500- year floods by dikes, levee or other water control structure; areas subject to certain types of 100-year shallow flooding where depths are less than 1.0 foot; and areas subject to 100-year flooding from sources with drainage areas less than one square mile.

Zone C - FEMA Flood Insurance Zone – Areas of minimal flooding.

BMP – Best Management Practice

CIP - Capital Improvements Program (or Plan)