WHAT TERMS MEAN
Best Management Practices (BMPs): Methods believed to be the most effective practicable means of preventing or reducing undesirable effects on surface and groundwater systems. The swales, bioretention basins, green roofs, and the like that are the subject of this web site are called structural BMPs.
BMPs include a broad suite of techniques and practices. For example, they include avoiding toxic pollution in the first place, e.g. by not using toxic pesticides, or driving less so that less gas, oil, and heavy metals wash off our streets into waterways. BMPs include practices like cleaning storm drains and planting tough, drought-tolerant plants that require little or no irrigation or fertilizer. They include maintaining the structural elements highlighted in this web site so that they continue to function. They range up to city-planning rules that lead to more compact cities with abundant public transit (less paved-over land, less driving).
A newer, fancier term is Integrated Management Practices, IMPs.
Biofilter: Soil, dense vegetation, and/or bacteria used to filter water or cause pollutants to adhere or break down.
Bioretention: Use of shallow planted depressions to briefly store runoff, slow its flow, and let it soak into soil while pollutants adhere, degrade, evaporate, or are taken up by plants. Rain gardens and basins are types of bioretention and biofilters; all are low-impact-development BMPs.
Bioswale: See “swale.”
Buffer, vegetated buffer: A vegetated area next to a body of water such as a stream or lake, aimed at protecting water quality, lessening erosion, preserving habitat for plants and animals, and/or keeping scenic value.
Catch basin: A chamber designed to collect debris or sediment in runoff. Some smaller pollutant particles will be removed because they settle out or adhere to the sediment and larger debris.
Culvert: A covered pipe or similar structure that carries natural water such as runoff or streams.
Directly connected impervious area (DCIA): Area where runoff water flows without being filtered, slowed, or allowed to soak into soil. Breaking up such areas with plantings, swales, detention areas, etc. slows runoff, thus reducing flooding, erosion, and incision. It also lessens the load of pollutants carried in the runoff.
Erosion: Mechanical wearing away of rock or soil by water, wind, or ice.
Filter strip: Gently sloping strips of vegetation (usually grass or something else with many leaves) aimed at causing pollutants to adhere to leaves, soak into soil, and/or degrade.
Green building: Increasing the efficiency with which buildings and their occupants use energy, water, and materials, while reducing waste, pollution, environmental degradation, and impacts on human health, throughout the building’s “lifecycle” including final removal.
Groundwater: Water that collects beneath in and flows through the porous spaces in soil and rock. It is the source of water for springs and wells. The upper surface of the groundwater, which rises and falls, is the water table.
Hydromodification: Changes in the volume, speed, or timing of high and low flows in a water body, generally a stream or river. A major cause is development, which changes vegetation and covers land with roofs, sidewalks, streets, and parking lots. Rainwater, unable to soak into soil, rushes with flash-flood-like intensity to streams.
Impermeable, Impervious: Something that a substance cannot get through. For this document, an impervious or impermeable area is an area, such as a roof or parking lot, that does not let water soak into soil. Some soils also are naturally relatively impervious, e.g. heavy clays absorb water for a time but, once saturated, absorb no more.
Infiltration: Filtering into anything, e.g. rain filtering into soil, or rain filtering out of soil into a broken pipe.
Integrated Management Practices (IMPs): See BMPs. For purposes of this website, the terms are interchangeable.
Level spreader: An outlet that causes water to flow across a wide area uniformly, like a sheet. This can lessen erosion, increase filtering of pollutants, and increase the amount of water that soaks into soil.
Low-impact development: Development that preserves and protects natural-resource systems. The phrase is most commonly used in reference to ways to treat urban runoff. Its aims include preserving and protecting important natural characteristics of sites and areas, maintaining pre-development water quality, and maintaining or replicating pre-development groundwater and surface water volume and flow characteristics. This website is focused on low-impact development.
Permeable, pervious surface or material: Surface or material that water (or other liquids) can get through.
Point-source pollution: See non-point-source pollution.
Planter, flow-through or stormwater: A contained area, above ground or flush with the ground, with a layer of soil supporting plants above (a) materials like sand and rock, designed to let water filter through while pollutants remain and degrade and (b) underdrains that convey heavy flows to storm drains. Flow-through planters may or may not allow water to soak into the surrounding soil; used near building foundations, they usually do not.
Pond, as in holding pond, detention pond, retention pond, dry pond:: A depression that holds water, often temporarily to reduce flooding or let pollutants settle out or adhere. A “wet pond” is wet all or most of the time; a “dry pond” is wet only occasionally.
Non-point-source pollution, non-point pollution: Pollution that cannot be traced to a single source, such as pollution from urban runoff. This is distinguished from “point source” pollution, which can be traced to a single source. But the distinction isn’t always clear. For example, pollution from failed septic tanks may be non-point until it is traced to a one or a few septic tanks. What seems to be pollution in general urban runoff may be traced to a single home or business.
The Federal Clean Water Act, passed in the 1970s, was first interpreted to apply only to “point source” pollution. This led to major decreases in water pollution from industries. In fact, non-point pollution became the major source of pollution in the United States. A 1980s lawsuit by environmental groups established that the Clean Water Act also applied to non-point pollution. This has led to a gradual increase in efforts to reduce it, but results are far from clear.
Rain Garden: A slightly depressed garden that receives surface runoff. Compost and mulch help hold stormwater, slowing peak flows, while microorganisms and bacteria remove pollutants. Plants hold some rain on their leaves and also bring water up from the soil to evaporate (evapotranspiration).
Runoff: Water from rain, snow, hail, irrigation, car-washing, or any similar source that flows over land before reaching streams or other surface waters. “Urban runoff” includes runoff in suburbs or other developed areas, as opposed to farms, ranches, forests, or large undeveloped areas.
Sanitary Sewer vs. Storm Drain: Sanitary sewers carry waste from sinks and toilets — sewage. Storm drains (also incorrectly called storm sewers) carry runoff — ideally, rain water only, but also any other water or pollutants from outside buildings.
In most of the Bay Area, including all of the East Bay, these have been separate systems for more than 50 years. Sewage goes to treatment plants and then generally to the Bay or Delta. Runoff flowing through storm drains is not treated. It flows straight to lakes, creeks, Delta, or Bay. Urban runoff is the major source of pollutants reaching creeks and San Francisco Bay today. Roofs, paving, and storm-drain pipes cause runoff to reach natural water bodies very quickly. The result is flooding, erosion, incision (cutting channels deeper) and sometimes lowering of the groundwater table (making it harder for plants to survive without irrigation, even drying up wells).
Stormwater: Should mean water that flows across land to water bodies as a result of storms, but widely used to mean all urban runoff, including “urban slobber” such as excess irrigation, equipment washing, etc.
Sustainabiity, sustainable development: Development that meets the needs of the present without compromising the future. This is generally interpreted as using methods, systems, and materials that do not deplete resources or interfere with natural cycles, and considering natural land, water, and energy resources as integral aspects of development. Generally, low-impact development tends to make development more sustainable.
Swale, bioswale: An relatively wide, shallow, open channel with a slight gradient, designed to let water flow slowly. This can slow down flows after storms, let water soak into soil, and let pollutants adhere and/or degrade. Swales can have underdrains.
Tree-well filter: A type of flow-through planter consisting of a concrete chamber(s) containing media designed to filter pollutants while passing relatively large flows of water. On top is soil supporting a tree or shrub. Underneath is an underdrain that takes filtered runoff to storm-drain systems. Tree-well filters tend to be relatively expensive both initially and in maintenance cost, but can treat large volumes of storm water in a small space, leaving more room for development.
Underdrain: A perforated pipe at the bottom of a swale, planter, or pond, designed to increase discharge and avoid flooding by carrying heavy flows away, usually to storm drains.
Urban runoff: See runoff.
Water pollution: Presence of substances in water that can harm living organisms or interfere with beneficial uses of water. Pollution can be caused by humans, other animals, or natural processes, such as “red tide” algae blooms or toxic metals leaching from soil. What is pollution depends on use. For example, chloramines added to East Bay drinking water for human safety make this water toxic to aquatic organisms, and so are pollutants if the water reaches streams or lakes. Erosion can be a desirable natural process (or not), but the resultant soil and rock in water can be pollutants if, for example, they silt up areas downstream, make water undrinkable, or smother salmon and trout eggs. Thus, regulations refer to pollution in terms of whether it interferes with “beneficial uses.”
Water quality: The biological, chemical, and physical characteristics of water, usually as they affect its ability to sustain plant and animal life and various human uses.
Watershed: The entire area that drains to a stream, river, lake, or other body of water.
Water table: The upper surface of groundwater; the level below which empty spaces in rock or soil are saturated with water. The water table rises and falls with rainfall, weather conditions, and human-caused changes including wells, pumping, or deepening of channels or ditches.