Pennsylvania Handbook of Best Management Practices for Developing Areas

Appendix G -- Glossary

alkalinity A measure of the capacity of water to neutralize acids because of the presence of one or more of the following bases in the water: carbonates, bicarbonates, hydroxides, borates, silicates, or phosphates.

aspect ratio Ratio of wetland cell length to width.

attenuation Reduction in magnitude, as in the lowering of peak runoff discharge rates, in the case of dry ponds; or the reduction of contaminant concentrations, as in the action of biodegradation in wetlands or bioretention facilities.

benthic Pertaining to occurrence on or in the bottom sediment of wetland and aquatic ecosystems, including wetlands.

biodiversity Biodiversity refers to the number of species of plants and animals in a defined area. Biodiversity is measured by a variety of indices that consider the number of species and, in some cases, the distribution of individuals among species.

biomass The total mass of living tissues (plant and animal).

BMP fingerprinting A series of techniques for locating BMPs (particularly ponds) within a development site so as to minimize their impacts to wetlands, forest, and sensitive stream reaches.

BOD (biochemical oxygen demand) A measure of the concentration of aerobically degradable compounds in water. Measured as the oxygen consumed during degradation of organic and inorganic materials in water.

BOD₅ Five-day biochemical oxygen demand.

buffer A vegetated strip immediately adjacent to a water body. The primary function of buffers is to protect the receiving water from sediment and pollutants derived from upstream areas. Ancillary benefits may include infiltration of rainfall and habitat enhancement. A buffer is a special case of a filter strip. Forested riparian buffers are one example of a best management practice related to the use of buffers.

channelization The creation of a channel or channels resulting in faster water flow, a reduction in hydraulic residence time, and less contact between water and solid surfaces in the water body.

choker course A filter layer of finer material, usually crushed stone, that is installed over a coarse road base material. The purpose of the choker course is to provide a stable foundation for the construction of a pavement.

COD (chemical oxygen demand) A measure of the concentration of substances which can be oxidized in water. Expressed as the oxygen equivalent consumed when an aqueous sample is reacted of the organic matter in water, based on reaction with a strong chemical oxidant.

critical depth The depth of flow at which the specific energy is a minimum for a given discharge rate. Flow is critical when the Froade number is equal to one:

where V, is the velocity of the flow, g, is the gravitational constant, and D, is the hydraulic depth of the flow.

denitrification The anaerobic microbial reduction of oxidized nitrate nitrogen to nitrogen gas.

dense graded material Granular mixture characterized by a large range in particle sizes. Dense graded materials have superior structural properties to open graded materials. However, they are less porous.

detritus Dead plant material that is in the process of microbial decomposition.

diurnal Occurring daily or during the daylight.

ecosystem All organisms and the non-living environmental factors with which they interact.

ecotone The boundary between adjacent ecosystem types. An ecotone can include environmental conditions that are common to both neighboring ecosystems and can have higher species diversity.

Eh A measure of the reduction-oxidation (redox) potential of soil according to a hydrogen scale.

emergent plant A rooted, vascular plant that grows in periodically or permanently flooded areas and has parts of the plant (stems and leaves) extending through and above the water plane.

eutrophic Water containing an excess of plant-growth nutrients that typically result in algae blooms and extreme (high and low) dissolved-oxygen concentrations.

evapotranspiration The combined processes of evaporation from the water or soil surface and transpiration of water by plants.

exfiltrate The leaking of water to surrounding ground through openings in structures.

exotic species A plant or animal species that has been intentionally or accidentally introduced and that does not naturally occur in a region.

extended detention A function provided by BMPs which incorporate a water quality storage. BMPs with extended detention, intercept runoff and then release it over an extended period of time.

extended detention (ED) pond Temporarily detains part of stormwater runoff for up to 24 hours after a storm by using a fixed orifice. ED ponds normally are "dry" between storm events and do not have permanent standing water. An enhanced ED pond is designed to prevent clogging and resuspension. It provides flexibility in achieving target detention times. It may be equipped with plunge pools near the inlet, a micropool at the outlet, and may have an adjustable reverse-sloped pipe at the ED control device.

extended detention control device A pipe or series of pipes that extend from the riser of the stormwater pond that are used to gradually release stormwater from the pond over a 12- to 48-hour interval.

fascine Bundled willow cuttings used to stabilize stream banks. Bundling allows otherwise weak green twigs to reinforce each other and resist the forces of stream currents.

field capacity The quantity of water which will not freely drain from the root zone of shallow soil layers. Usually measured as the moisture content (by volume) in soil at a capillary tension of .33 bars.

filter strip A vegetated boundary characterized by uniform mild slopes. Filter strips may be provided downgradient of developed tracts to trap sediment and sediment-borne pollutants and to reduce imperviousness. Filter strips may be forested or vegetated turf. Filter strips located adjacent to waterbodies are called buffers.

flash boards Removable boards used in a weir used to control water levels.

floating aquatic plant A rooted or non-rooted vascular plant that is adapted to have some plant organs (generally the chlorophyll-bearing leaves) floating on the surface of the water in wetlands, lakes, and rivers.

flood fringe The flood fringe occupies the distal parts of the floodplain, outside of the floodway. Complete obstruction of the flood fringe will not significantly increase flood levels. The flood fringe boundary is typically based on an increase in flood level of one foot during the 100-year return frequency flooding event.

floodplain Areas that are flooded periodically (usually annually) by the lateral overflow of rivers. In hydrology, the entire area that is flooded at a recurrence interval of 100 years.

floodway Part of the floodplain, centered on the stream, that will convey most of the flow during overbank flooding events.

forebay Stormwater design feature that uses a small basin to settle out incoming sediment before it is delivered to a stormwater BMP.

freeboard The vertical distance between the top of embankment and the maximum design level of the water surface in an impoundment such as a reservoir, canal, or dam.

fresh water Water with a total dissolved solids content less than 500 mg/L (0.5 parts per thousand salts).

gabion Wire cage used to contain rip rap and stone. Gabions are used to increase the resistance of rip rap to movement caused by flowing water.

geotextile A fabric manufactured from synthetic fiber that is designed to achieve specific engineering objectives, including seepage control, media separation (e.g., between sand and soil), filtration, or the protection of other construction elements such as geomembranes.

greenway A strip or belt of vegetated land that typically includes both upland and riparian areas. Greenways are often used for recreation, or to provide as a land use buffer, or to provide a corridor and habitat for wildlife.

habitat The environment occupied by individuals of a particular species, population, or community.

headwall A wall of stone, metal, concrete, or wood at the end of a culvert or drain to protect fill from scour or undermining, increase hydraulic efficiency of conduit, divert flow, retard disjointing of short sectional pipe, or serve as a retaining wall.

heavy metals Metallic elements that are above 21 atomic weight on the periodic table.

herbaceous Plant parts that contain chlorophyll and are non-woody.

hydraulic loading rate (HLR) Ratio of the surface area of a hydraulic device and the average rate at which water is delivered to the A measure of the application of a volume of water to a land area with units of volume per area per time or simply reduced to applied device water depth per time (for example, m³/(m²/d) or cm/d).

hydraulic residence time (HRT) A measure of the average time that water occupies a given volume with units of time. The theoretical HRT is calculated as the volume divided by the flow (for example, m³/(m²/d)). The actual HRT is estimated on the basis of tracer studies that used conservative tracers such as lithium or dyes.

hydric soil A soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions. Hydric soil that is in areas having indicators of hydrophytic vegetation and wetland hydrology is wetland soil.

hydrograph A record of the change in flow rate with time.

hydrologic soil group A designation developed by the NRCS which describes the infiltration capacity of soil. Soil associations are categorized in decreasing infiltration capacity from A to D.

hydroperiod The period of wetland soil saturation or flooding. Hydroperiod is often expressed as a number of days or a percentage of time flooded during an annual period (for example, 25 days or 7 percent).

lacustrine The deep-water zone of a lake or reservoir.

limnetic Relating to or inhabiting the open water part of a freshwater body with a depth that light penetrates. The area of a wetland without emergent vegetation.

littoral The shoreward zone of a lake or wetland. The area where water is shallow enough for emergent vegetation to dominate.

macrophyte Macroscopic (visible to the unassisted eye) vascular plants.

Manning’s equation A formula for open-channel flow, published by Manning in 1890.

marsh A wetland dominated by herbaceous emergent plants.

micronutrient A chemical substance that is required for biological growth in relatively low quantities and in small proportion to the major growth nutrients. Some typical micronutrients include molybdenum, copper, boron, cobalt, iron, and iodine.

mitigation The replacement of functional values lost when an ecosystem is altered. Mitigation can include replacement, restoration, and enhancement of functional values.

NH₄-N (ammonia nitrogen) A reduced form of nitrogen produced as a by-product of organic matter decomposition and synthesized from oxidized nitrogen by biological and physical processes.

nitrification Biological transformation (oxidation) of ammonia nitrogen to nitrite and nitrate forms.

nitrogen fixation A microbial process in which atmospheric nitrogen gas is incorporated into the synthesis of organic nitrogen.

open graded material Uniform granular mixture with a narrow distribution of grain sizes. Open graded material has higher porosities than dense graded material.

Org-N (organic nitrogen) Nitrogen that is bound in organic compounds.

palustrine wetland All nontidal wetlands dominated by trees, shrubs, persistent emergents, emergent mosses, or lichens; and all such tidal wetlands in areas where salinity from ocean-derived salts is below 0.5 parts per thousand.

peak attenuation storage The volume set aside within a BMP for the purpose of attenuating the inflow runoff peak rate.

percolation rate The downward movement under the influence of gravity movement of water under hydrostatic pressure through the interstices of the rock or soil.

perennial Persisting for more than one year. Perennial plant species persist as woody vegetation from year to year or resprout from their rootstock annually.

periphyton The community of microscopic plants and animals that grows on the surface of emergent and submergent plants in water bodies.

permittivity (cross-plane flow capacity) Rate that water will flow freely through a thin layer, such as a geotextile. Equal to the hydraulic conductivity divided by the thickness of the layer. Permittivity is measured in units of inverse time (e.g., sec^-1).

photic zone The area of a water body receiving sunlight.

piezometric surface The surface defined by elevation to which groundwater will rise in a well.

plant community All of the plant species and individuals occurring in a shared habitat or environment.

plug flow Linear flow along the length of a wetland cell. Ideal plug flow does not involve the dispersion or diffusion of constituents. The flow can be perceived as a series of independent "packets" of water which do not interact with each other.

plunge pool A small permanent pool at either the inlet to a BMP or at the outfall from a BMP. The primary purpose of the pool is to dissipate the velocity of stormwater runoff, but it also can provide some pretreatment.

pore space Open space in rock or granular material; also known as interstices.

protozoa Small, one-celled animals including amoebae, ciliates, and flagellates.

receiving water A water body into which wastewater or treated effluent is discharged.

return frequency storm (rainfall event) The average period of time that an observer must wait between the occurrence of an event of a particular statistic probability of a storm of equal magnitude, or larger magnitude occurring. For example, when the interval between observations is a year, a return frequency period of 100 years means that, on the average, an event of this magnitude or greater is expected to occur not more often than once in 100 years.

rhizosphere The chemical sphere of influence of plant roots growing in flooded soils. Depending on the overall oxygen balance (availability and consumption), the rhizosphere can be oxidized, resulting in the presence of aerobic soil properties in an otherwise anaerobic soil environment.

riparian Pertaining to a stream or river. Also, Plant communities occurring in association with any spring, lake, river, stream, or creek through which waters flow at least periodically.

riparian corridor Narrow strip of land, centered on a stream, that includes the floodplain as well as related riparian habitats adjacent to the floodplain.

riverine wetlands Wetlands associated with rivers.

runoff capture design storm Benchmark rainfall event, used to develop criteria for designing the groundwater recharge function of BMPs. The runoff capture design storm is the largest rainfall event from which no appreciable runoff is expected to occur. Complete specification of the storm includes total rainfall quantity, storm duration, and distribution of rainfall (e.g., NRCS 24-hour, type II rainfall distribution). See Section 5.3 of the Handbook.

runoff capture storage The combined storage volume provided by BMPs on a site for the retention and eventual infiltration of rainfall.

runoff capture volume The minimum volume of rainfall that must be retained and completely infiltrated onsite during every storm. It is also equal to the rainfall quantity associated with the runoff capture design storm. The runoff capture volume is conveniently stated as a rainfall volume, in inches, over the area of the site.

runoff curve number (CN) A parameter developed by the NRCS which is an indicator of runoff potential. Curve number is related to hydrologic soil group and land use type.

runoff peak attenuation design storm Benchmark rainfall event, used to develop criteria for the design of runoff peak attenuation BMPs. The design criteria generally requires that the predicted post development peak runoff rate for the selected runoff peak attenuation design storm will not exceed the peak associated with redeveloped condition. Complete specification of the storm includes total rainfall quantity, storm duration, and distribution of rainfall (e.g., NRCS 24-hour, type II rainfall distribution). See Section 5.3 of the Handbook.

saturated soil Soil in which the pore space is completely filled with water.

seed bank The accumulation of viable plant seeds occurring in soil and available for germination under favorable environmental conditions.

setback A distance from the edge of a water body within which intensive development is restricted. Setbacks are established by local regulation for the purpose of maintaining open space next to streams, lakes, and other water bodies. The area within setbacks is frequently used for flood control, recreation, preservation of drinking water supply, and wildlife habitat enhancement.

sheet flow Water flow with a relatively thin and uniform depth.

short-circuit A faster, channelized water flow route that results in a lower actual hydraulic residence time than the theoretical hydraulic residence time.

spillway design flood (SDF) Benchmark rainfall event, used to develop criteria for the design of BMPs that incorporate emergency spillways or overflows. Complete specification of the storm includes total rainfall quantity, storm duration, and distribution of rainfall (e.g., NRCS 24-hour, type II rainfall distribution). See Section 5.3 of the Handbook.

stage-area curve The relationship between the depth of water and the surface area of a pond, wetland, or lake.

stage-discharge curve The relationship between water depth and outflow from a body of water.

subcritical flow The state of flow when the depth is greater than the critical depth.

substrate Substances used by organisms for growth in a liquid medium. Surface area of solids or soils used by organisms to attach.

succession The temporal changes of plant and animal populations and species in an area that has been disturbed.

supercritical flow The state of flow when the depth is less than the critical depth. Transitions between supercritical and subcritical flow may result in turbulence associated with a hydraulic jump.

surface infiltration rate The rate at which water enters the soil or other porous surface. The measurement of surface infiltration rates requires that the underlying soil be completely saturated and that infiltration occurs by gravity under a unit hydraulic gradient.

tailwater condition—minimum and maximum Refers to the depth of water at the downstream end of a structure.

terrestrial Living or growing on land that is not normally flooded or saturated.

TN (total nitrogen) A measure of all organic and inorganic nitrogen forms in a water sample. Functionally, TN is equal to the sum of TKN and NO₃ + NO₂-N.

TOC (total organic carbon) A measure of the total reduced carbon in a water sample.

TP (total phosphorus) A measure of the total phosphorus in a water sample, including organic and inorganic phosphorus in particulate and soluble forms.

tractive force The total cross-sectional force experienced by a rigid channel or conduit as a result of channel flow (expressed in units of force per length). This force tends to displace rigid structures in the downstream direction and must be resisted by friction or by structural anchors. The tractive force is equal to the unit tractive force multiplied by the wetted perimeter of the conduit.

transition zone The area between habitats or ecosystems (see ecotone). Frequently, transition zone is used to refer to the area between uplands and wetlands. In other cases, wetlands are referred to as transitional areas between uplands and aquatic ecosystems.

transmissivity (in-plane flow capacity) Rate that water can be made to flow through the cross section of a thin layer or conduit under the influence of a unit hydraulic gradient. Measured as a volumetric rate per unit width (e.g., square feet meters per minute, or gallons per minute per foot). Equal to the hydraulic conductivity times the thickness of the layer or conduit.

transpiration The transport of water vapor from the soil to the atmosphere through growing plants.

TSS (total suspended solids) A measure of the filterable matter in a water sample.

type II rainfall distribution Standard NRCS 24-hour rainfall distribution which applies to the state of Pennsylvania. The distribution allocates rainfall as a percentage of total rainfall over discrete time intervals.

uniformity coefficient A measure of the range in particle sizes associated with a granular mixture. Materials with the lowest uniformity coefficients are most uniform. Uniform materials are also called open graded materials. The uniformity coefficient is computed as follows:

C_u = (D₆₀ / D₁₀)

D₆₀ is the sieve opening size through which 60 percent of the layer material will pass. D₁₀ is the sieve opening size through which 10 percent of the layer material will pass.

unit tractive force (or tractive stress) The stress (expressed in units of force per area) induced by open channel flow on the bottom and sides of its conduit or channel. This stress is responsible for sediment erosion and the downstream transport of streambed materials. The average unit force acting on a channel cross-section is equal to the product of the unit weight of water, the slope of the channel, and the hydraulic radius of the flow.

upland An area that is not an aquatic, wetland, or riparian habitat. An area that does not have the hydrologic regime necessary to support hydrophytic vegetation.

water quality design storm Benchmark rainfall event, used to develop criteria for the design of water quality BMPs. Water quality design storms are used to size BMPs that are intended to achieve specific quality treatment objectives. Criteria based on water quality storms generally require that the design treatment efficiency be achieved during the water quality design storm and all smaller events. Complete specification of the storm includes total rainfall quantity, storm duration, and distribution of rainfall (e.g., NRCS 24-hour, type II rainfall distribution). See Section 5.3 of the Handbook.

water quality storage The volume set aside within a BMP to detain storm runoff. The detained water is released over an extended period of time. The water quality storage is frequently expressed as a multiple of the water quality volume.

water quality velocity The maximum flow velocity encountered in a water quality BMP during the course of the water quality design storm.

water quality volume The total volume of runoff which is delivered to the inlet of a water quality BMP during the course of the water quality design storm.

wattles Fence or barrier constructed of interwoven twigs and branches used to stabilize soil from erosive forces.

weir A device used to control and measure water flow.

weir gate Water-control device used to adjust water levels and measure flows simultaneously.

wetland An area that is inundated or saturated by surface water or groundwater at a frequency, duration, and depth sufficient to support a predominance of emergent plant species adapted to growth in saturated soil conditions.

wilting point Quantity of water which will not be removed from soil under normal conditions of evaporation and plant transpiration. Usually measured as the moisture content (by volume) in soil with a capillary tension of 15 bars.

zonation The development of a visible progression of plant or animal communities in response to a gradient of water depth or some other environmental factor.