Definitions

Base FlowBase flow refers to the water that enters streams by flowing through the groundwater system, rather than directly over the surface of the land. Because water moves much more slowly through the ground than over the surface of the land, groundwater feeds streams gradually rather than in big pulses after storms. As a result, during prolonged periods of little to no rain, it is this base flow that keeps streams running.

BioretentionThe bioretention system (also referred to as a "biofilter") is a stormwater management practice to manage and treat stormwater runoff using conditioned soils and planting materials to filter runoff stored within a shallow depression. To remove pollutants, the method combines physical filtering and adsorption to soil materials with bio-geochemical processes in which plant roots break down and take up pollutants. The system generally consists of an inflow component, a pretreatment element, an overflow structure, a shallow ponding area (less than 9" deep), a surface organic layer of mulch, a planting soil bed, plant materials, and an underdrain system to convey treated runoff to a downstream facility. “Rain gardens” (see definition below) are a type of bioretention system.

Effective Impervious Area

Effective impervious area is land surface that sheds stormwater directly into a water body or a storm drain system without providing the opportunity for it to infiltrate.

EvapotranspirationTranspiration is the process by which plants take in water through their roots for use in photosynthesis and then give it off through the leaves as a by-product of respiration. Evaporation is the direct loss of water from the earth’s surface to the atmosphere. Evapotranspiration refers to the transfer of water from the earth to the atmosphere through both these processes combined.

Grass pavers

Grass pavers refer to structural materials used to reinforce and stabilize a soil bed within which turf grass is grown. The reinforcement material is usually a plastic or concrete matrix imbedded in the soil, allowing roots to grow down and plants to grow up, but keeping the soil from becoming compacted.

Green RoofA low-maintenance vegetated roof system that stores rainwater in a lightweight engineered soil medium, where the water is taken up by plants and transpired into the air. As a result, much less water runs off the roof, as compared to conventional rooftops. Green roofs have been in use in Europe for more than 30 years; they are easy to incorporate into new construction and can also be used on many existing buildings.

HeadwatersThe headwaters of a stream include the small streams that come together to form the source and upper reaches of the stream. In some cases, all parts of a river basin, except the mainstream and main tributaries, can be considered headwaters.

Low-Impact Development (LID)An approach to environmentally friendly land use planning. It includes a suite of landscaping and site design techniques that attempt to maintain the natural, predeveloped ability of a site to manage rainfall. LID techniques capture water on site, filter it through vegetation, and let it soak into the ground where it can recharge the local water table rather than being lost as surface runoff. An important LID principle includes the idea that stormwater is not merely a waste product to be disposed of, but is a resource.

Low-impact development addresses the problems created by land development through a variety of strategies:

LID site planning – an approach that minimizes land disturbance and preserves open space and natural site grading

Alternative roadway and parking layouts – designed to reduce impervious land cover and break up or disconnect impervious surfaces from one another, so these surfaces can shed rain and snowmelt into areas where the water can soak into the ground.

MainstemThe main channel or course of a river or stream.

Nonpoint Source PollutionPollution that cannot be identified as coming from a specific, localized source. Overland stormwater runoff and some airborne deposits fall into this category.

Open Space Preservation (Cluster) Zoning Bylaw

The Town of Ipswich’s Open Space Preservation (Cluster) Zoning bylaw is an example of an increasingly popular type of municipal ordinance or bylaw designed to provide developers incentives, such as density bonuses, to cluster development, preserve environmentally and culturally valuable open space on site, and cause minimal disturbance to the natural terrain.

Rain GardenShallow landscaped depression that incorporates specialized plantings and soil with a high infiltration rate. Rain Gardens are used to control runoff volume and timing and can remove pollutants through the physical, chemical, and biological processes that occur in plants, soil, and mulch. See also Bioretention.

Rainwater HarvestingThe process of capturing and storing runoff, usually from rooftops, for nonpotable uses such as lawn and garden watering.

RechargeWater that infiltrates into the ground, usually from above, that replenishes groundwater reserves, provides soil moisture, and affords evapotranspiration (water uptake and use by plants).

SwaleOpen, shallow, vegetated channel that slows runoff, filters it, and promotes infiltration into the ground. To function properly, swales must be carefully designed and maintained. The vegetation in swales, usually thick grass, helps to trap pollutants and reduces the velocity of stormwater runoff, which allows it to percolate into the ground.

TributaryA stream that joins, and thereby feeds, another (usually larger) stream or river.

WatershedA geographic area of land in which all surface and most ground water flows downhill to a common point, such as a river, stream, pond, lake, wetland, or estuary.

The sources for some of these definitions include (See Links for URLs):Massachusetts Smart Growth ToolkitMetropolitan Area Planning Council. Massachusetts Low-Impact Development ToolkitMassachusetts Department of Fish and Game, Riverways Program, River Instream Flow Stewards (RIFLS). Glossary