Pennsylvania Stormwater Best Management Practices Manual
- 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.
ammonia nitrogen (NH
-N) -
A reduced form of nitrogen produced as a by-product of organic
matter decomposition and synthesized from oxidized nitrogen by biological and physical
aspect ratio
- Ratio of wetland cell length to width.
- 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.
base flow
- Normally refers to the stream levels associated primarily with groundwater or
subsurface contributions, as opposed to storm flow which corresponds to stream levels
associated with recent precipitation and surface runoff.
- Layer of consolidated rock over which lies an overburden of soil (regolith), including
unconsolidated rock.
- Pertaining to occurrence on or in the bottom sediment of wetland and aquatic
ecosystems, including wetlands.
Best Management Practices (BMP)
- Activities, facilities, measures, or procedures used to
manage the volume, rate and water quality of stormwater runoff.
- 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.
- The total mass of living tissues (plant and animal).
biochemical oxygen demand (BOD) -
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.
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
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Five-day biochemical oxygen demand.
- 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.
- 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.
chemical oxygen demand (COD) -
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
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.
critical depth
- The depth of flow at which the specific energy is a minimum for a given
discharge rate. Flow is critical when the Froude 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.
– The removal of nitrate ions from soil or water, 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 permeable.
- Dead plant material that is in the process of microbial decomposition.
- Occurring daily or during the daylight.
- All organisms and the non-living environmental factors with which they interact.
- 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.
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Eh -
A measure of the reduction-oxidation (redox) potential of soil according to a hydrogen
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
- Water containing an excess of plant-growth nutrients that typically result in algae
blooms and extreme (high and low) dissolved-oxygen concentrations.
- The combined processes of evaporation from the water or soil surface
and transpiration of water by plants.
excessively rapid drainage
- For purposes of this manual, corresponds to infiltration rates of
soils in excess of 6 inches per hour. (Normally 6 inches is considered rapid drainage but the
manual indicates that special precautions need to be taken with an infiltration rate of 6 inches
per hour or more)
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
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 re-suspension. 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.
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filter strip
- A vegetated boundary characterized by uniform mild slopes. Filter strips may be
provided down-gradient 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 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.
– Lands adjoining a river or stream that have been or may be expected to be
inundated by flood waters in a 100-year frequency flood.
floodway –
The channel of the watercourse and portions of the adjoining floodplains which are
reasonably required to carry and discharge the 100-year frequency flood. Unless otherwise
specified, the boundary of the floodway is as indicated on maps and flood insurance studies
provided by FEMA. In an area where no FEMA maps or studies have defined the boundary of
the 100-year frequency floodway, it is assumed, absent evidence to the contrary, that the
floodway extends from the stream to 50 feet from the top of the bank of the stream.
- 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 water surface elevation experienced during the
design flood and the crest elevation of a dam, levee, floodwall or other embankment.
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.
- A strip or belt of vegetated land that typically includes both
areas. Greenways are often used for recreation, as a land use buffer, or to provide a corridor
and habitat for wildlife.
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- The environment occupied by individuals of a particular species, population, or
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 having atomic weights above 21 on the periodic table.
- Plant parts that contain chlorophyll and are non-woody.
hydraulic conductivity (K)
- An expression of the readiness with which a liquid such as water
flows through a soil in response to a given potential gradient. Hydraulic conductivity is a
constant physical property of soil or rock, one of several components responsible for the
dynamic phenomenon of flow.
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, m
/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, m
/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.
- 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
- 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).
- The entrance of surface water into the soil, usually at the soil/air interface.
infiltration testing
- Specific tests designed to measure the saturated movement of water into
the soil in a single direction downward through a two dimensional soil surface.
- The deep-water zone of a lake or reservoir.
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- 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 zone
- The shoreward zone of a lake or
. The area where water is shallow
enough for emergent vegetation to dominate.
- Macroscopic (visible to the unassisted eye) vascular plants.
manning’s equation -
A formula for calculating the anticipated uniform flow in an open-
channel flow, published by Manning in 1890.
- A wetland dominated by herbaceous emergent plants.
- 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.
- The replacement of functional values lost when an ecosystem is altered.
Mitigation can include replacement, restoration, and enhancement of functional values.
nitrification -
Biological transformation (oxidation) of ammonia nitrogen to nitrite and nitrate
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 permability than dense graded material.
organic nitrogen (Org-N) -
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 -
The downward movement under the influence of gravity of water under
hydrostatic pressure through the interstices of the rock or soil.
- Persisting for more than one year. Perennial plant species persist as woody
vegetation from year to year or resprout from their rootstock annually.
- The community of microscopic plants and animals that grows on the surface of
emergent and submergent plants in water bodies.
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– The ability of rock, soil or other material to transmit a gas or liquid.
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
photic zone
- The area of a water body receiving sunlight.
piezometric surface
- The surface defined by elevation to which groundwater will rise in a
plant community
- All of the plant species and individuals occurring in a shared habitat or
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 that 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.
pollutant removal
- Removing pollutants by decomposing them or eliminating them from an
area or system (eg. volitize), or rendering non-harmful or unavailable in a soil or medium by
means of adsorption, chelation, and similar binding mechanisms.
pore space -
Open space in rock or granular material; also known as interstices.
- A deposit on the earth of hail, mist, sleet, rain or snow.
- Small, one-celled animals including amoebae, ciliates, and flagellates.
receiving water
- A water body into which wastewater or treated effluent is discharged.
- Replenishment of groundwater reservoirs by infiltration through permeable soils.
return period (storm event
) - The average period of time between the occurrence of storms
of equal or greater magnitude. The probability that such a storm will occur in any given year is
equal to the reciprocal of the return period (e.g. there is a 50% chance that a 2-year storm
event will occur in any given year, but only a 10% chance that a 10-year storm event will
- 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
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- 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 the rainfall depth in inches, return frequency and storm
duration. The distribution of rainfall in Pennsylvania is a 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 should 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. The larger
the runoff curve number, the greater the percentage of rainfall that will appear as runoff.
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 rainfall depth in inches, return frequency and storm duration. The
distribution of rainfall in Pennsylvania is a Type II rainfall distribution. See Section 5.3 of the
saturated soil
- Soil in which the pore space is completely filled with water.
seasonally high water table
- Shallow water tables associated with periods of recent high
levels of precipitation and/or low levels of evapo-transpiration. Frequently determined in the
seed bank
- The accumulation of viable plant seeds occurring in soil and available for
germination under favorable environmental conditions.
- 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
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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
sheet flow
- Water flow with a relatively thin and uniform depth.
- A faster, channelized water flow route that results in a lower actual hydraulic
residence time than the theoretical hydraulic residence time. This may reduce the
effectiveness of a BMP.
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 rainfall depth in inches, return frequency and storm duration. The
distribution of rainfall in Pennsylvania is a Type II rainfall distribution. See Section 5.3 of the
stage-area curve
– A line graph showing the relationship between the depth of water and the
surface area of a pond, wetland, or lake.
stage-discharge curve
– a line graph showing 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.
- Substances used by organisms for growth in a liquid medium. Surface area of
solids or soils used by organisms to attach.
- The temporal changes of plant and animal populations and species in an area
that has been disturbed.
super critical flow
- The state of flow when the depth is less than the critical depth.
Transitions between supercritical and sub-critical 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
- The depth of water in the receiving water
body at a structure outfall.
- Living or growing on land that is not normally flooded or saturated.
total nitrogen (TN) -
A measure of all organic and inorganic nitrogen forms in a water sample.
Functionally, TN is equal to the sum of TKN and NO
+ NO
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total organic carbon (TOC) -
A measure of the total reduced carbon in a water sample.
total phosphorus (TP) -
A measure of the total phosphorus in a water sample, including
organic and inorganic phosphorus in particulate and soluble forms.
total suspended solids (TSS) -
A measure of the filterable matter in a water sample.
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 soil
particles, rocks and channel liners 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 ecotones). 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
- The transport of water vapor from the soil to the atmosphere through growing
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. If the uniformity coefficient is less than 4 or 5, the
material is considered uniform in particle size. The uniformity coefficient is computed as
= (D
/ D
is the sieve opening size through which 60 percent of the layer material will pass. D
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.
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- 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 rainfall depth in inches,
return frequency and storm duration. The distribution of rainfall in Pennsylvania is a 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.
- A device used to control and measure water flow.
weir gate
- Water-control device used to adjust water levels and measure flows
- An area that is inundated or saturated by surface water or groundwater at a
frequency, duration, and depth sufficient to support, and that under normal circumstances do
support, a prevalence of vegetation typically adapted for life in saturated soil conditions,
including swamps, marshes, bogs and similar areas.
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.
- The development of a visible progression of plant or animal communities in
response to a gradient of water depth or some other environmental factor.

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