Glossary

A

Absorption

A physical or chemical phenomenon or a process in which atoms, molecules or ions enter some bulk phase – gas, liquid or solid material.

Access and egress control

Reinforced or rocked entrance and exit points to the site to deter tracking of sediment off the site onto adjacent streets.

Active karst

A terrain having distinctive landforms and hydrology created primarily from the dissolution of soluble rocks within 50 feet of the land surface [Minn. R. 7090].

Adsorption

The adhesion of an extremely thin layer of molecules to the surfaces of solid bodies or liquids with which they are in contact.

Aggrade

The build up of sediment or eroded material.

Anaerobic

Condition operating in a system where there is the absence of free oxygen available for biologic use.

Animal waste management

Practices and procedures which prevent the movement of animal wastes or byproducts from feeding or holding areas into the wider environment.

Annual load

Quantity of pollutants, sediment, or nutrients carried by a water body over the period of a year.

Antecedent soil moisture

The water content held by a soil before a storm event. This has an effect on the amount of water that will runoff due to that event.

Applicable Wasteload Allocation (WLA)

A Wasteload Allocation assigned to the permittee and approved by the USEPA.

Aquifer

A body of permeable rock that can contain or transmit groundwater.

Atmospheric controls

Reducing or removing wind erosion, dust, or statutory emissions regulations.

B

Baffle weir

A structure used in measuring the rate of flow fitted with a grating or plate across a channel or pipe which makes the flow more uniform in different parts of the cross section of the stream.

Bankfull

Flow in a stream or river where the water level is to the top of its bank. This is considered to be the channel forming flow and has a recurrence interval of around 2.5–years.

Bank stabilization

Activities undertaken to shore up or ensure the integrity of a stream or river bank and protect it from erosion and slumping.

Base flow

The flow coming from ground water inputs to a stream or river system.

Basin

A depression in the surface of the land that holds water.

Bed load

The sand, gravel or rocks which are transported along the stream bottom by traction, rolling, sliding or saltation.

Best Management Practice (BMP)

One of many different structural or non–structural methods used to treat runoff, including such diverse measures as ponding, street sweeping, filtration through a rain garden and infiltration to a gravel trench.

Better Assessment Science Integrating Point and Nonpoint Sources (BASINS)

A multipurpose environmental analysis system developed by the U.S. Environmental Protection Agency for water quality modeling purposes.

Better site design (BSD)

The application of non–structural practices at residential and commercial sites to reduce impervious cover, conserve natural areas, and use pervious areas to more effectively treat stormwater runoff.

Biofiltration

A bioretention practice in which an underdrain is used. Typically, most of the water entering the biofiltration practice enters the underdrain and is returned to the stormwater system, but some water infiltrates (unless the BMP has an impermeable liner) or evapotranspires. The underdrain may be at or elevated above the bottom of the practice (i.e. at the media-soil interface). See Bioretention terminology

Bioinfiltration

A bioretention practice in which no underdrain is used. All water entering the bioinfiltration practice infiltrates or evapotranspires. See Bioretention terminology.

Biological additives

Products which are formulated with specialized bacteria, enzymes, or other living components that can be added to boost pollution treatment efficiencies, eg. chitosan.

Biological oxygen demand (BOD)

A measure of the amount of oxygen required to biologically degrade organic matter in the water.

Bioretention

A soil and plant–based stormwater management best management practice (BMP) used to filter runoff.

Bog

A poorly drained, surface water fed, acidic area rich in accumulated plant material.

Bounce

Water level fluctuations due to topography, soils, and runoff inputs during and after precipitation events.

Brownfield

A brownfield is a property where redevelopment or reuse may be complicated by the presence (or likely presence) of contamination. Brownfields may include vacant parcels, depending on their prior use. A wide variety of contaminants may be found at a Brownfield site depending on the previous use.

Buffers

A vegetative setback between development and streams, lakes, and wetlands whose aim is to physically protect and separate the resource from future disturbance or encroachment.

C

Calcareous fen

A peat–accumulating wetland dominated by distinct ground–water inflows which is circum–neutral to alkaline and has high concentrations of calcium and low dissolved oxygen. The rarest wetland plant community in Minnesota. For more information see list of MN calcareous fens.

Catch basin

An inlet to the storm drain system that typically includes a grate or curb inlet.

Catch basin insert

Devices that attach to the entrance of a catch basin or mount inside the catch basin. They are designed to improve stormwater quality by either preventing debris and pollutants from entering the basin, or by retaining or treating the water in the basin.

Cation exchange capacity

The degree to which a soil can adsorb and exchange cations. For more information, see [1], [2], [3].

Channel protection

Actions taken to prevent habitat degradation and erosion that may cause downstream enlargement and incision in urban streams due to increased frequency of bankfull and sub–bankfull stormwater flows.

Check dam

A structure, typically constructed of rock, sediment retention fiber rolls, gravel bags, sandbags, or other proprietary product, placed across a natural or man made channel or drainage ditch and designed to reduce scour and channel erosion by reducing flow velocity and encouraging sedimentation. See swale cross-sections with check dams. For more information, see Section 6.33 in Stormwater Best Management Practices Manual.

Chemical controls

Includes such activities as salt management, fertilizer/pesticide management, and spill prevention and containment

Chemical oxygen demand

The quantity of oxygen used in biological and non–biological oxidation of materials in water; a measure of water quality.

Chemical treatment

Removal of pollutant from the water column via chemical means, eg. Ferric chloride, alum, polyacrylamides

Cistern

A technique which captures and temporarily stores rooftop runoff at confined sites, gradually releasing it over pervious areas.

Class V injection well

Any bored, drilled, or driven shaft, or dug hole that is deeper than its widest surface dimension, or an improved sinkhole, or a subsurface fluid distribution system (from U.S. Environmental Protection Agency, June 2003).

Cluster design

A reduction of average lot size within a residential development in exchange for greater conservation of natural areas.

Coincident peaks

Upstream peak discharge arriving at the same time a downstream structure releases its peak discharge thus increasing the total discharge well above what it was on the pre-development hydrograph.

Cold climate sizing

Sizing of stormwater practices to accommodate snowmelt. This is larger than rainfall–based criteria sizing in Minnesota since snowfall represents more than 10% of the annual precipitation.

Computable pollutant

A pollutant for which enough runoff concentration and BMP performance data is available to perform a site–based pollutant load calculation documenting no increase in loading.

Conservation easement

A restriction placed on a piece of property to protect the resources associated with the parcel. The easement is either voluntarily sold or donated by the landowner, and constitutes a legally binding agreement that prohibits certain types of development from taking place on the land.

Construction sequencing

A specified work schedule that coordinates the timing of land–disturbing activities and the installation of erosion–protection and sedimentation–control measures.

Conveyance

A structure or feature used for transferring water from one location to another.

Covered karst

Areas underlain with carbonate bedrock with more than 100 feet of sediment cover.

Curb and gutter system

Edging along the side of streets meant to quickly convey stormwater runoff from the street and adjacent areas into the stormwater system.

Curve number

An index combining hydrologic soil group, land use factors, treatment, and hydrologic condition. Used in a method developed by the SCS to determine the approximate amount of runoff from a rainfall event in a particular area.

D

Dead storage

The permanent storage volume of a pond degrade downcutting where softer material is present in a stream channel.

Denitrification

A microbially facilitated process of nitrate reduction.

Densimetric stratification

Impairment of vertical mixing and oxygenation of bottom water layers.

Design storm streamflow

From a storm event used as a standard for which performance of stormwater management practices are measured.

Detention time

The theoretical calculated time that a small amount of water is held in a settling basin.

Disconnection

Technique to spread runoff generated from rooftops or impervious surfaces into adjacent pervious areas where it can be filtered and infiltrated.

Drainageway

A course or channel along which water moves in draining an area.

Drawdown time

The time from the high water level in a BMP to 1 to 2 inches above the bottom of the facility at the lowest part of the BMP.

Drinking water supply management area (DWSMA)

the surface and subsurface area surrounding a public water supply well, including the wellhead protection area, that must be managed by the entity identified in a wellhead protection plan. This area is delineated using identifiable landmarks that reflect the scientifically calculated wellhead protection area boundaries as closely as possible.

Dry pond

A water bearing stormwater management facility that controls peak runoff flows to receiving bodies such as rivers and streams which is typically free of water during dry periods, but filled during times of rainfall

Dry well

A deep covered hole acting as an underground storage facility for stormwater until it seeps into the surrounding soil.

E

Elution

Washing out of ions in solution from a snowpack.

Emergency Response Area (ERA)

The part of the wellhead protection area that is defined by a one-year time of travel within the aquifer that is used by the public water supply well. (Minnesota Rules, part 4720.5250, subpart 3). It is used to set priorities for managing potential contamination sources within the DWSMA.

Erosion

The wearing down or washing away of the soil and land surface by the action of water, wind or ice.

Erosion control

Any efforts to prevent the wearing or washing away of the soil or land surface.

Erosion control blanket

A natural or geotextile mat placed in areas susceptible to erosion to hold the soil in place until it can be permanently stabilized through vegetation or armoring.

Eutrophic

An environment which has an excessive concentration of nutrients.

Evaporation

The process of changing from a liquid state into a gas.

Evapotranspiration

Loss of water to the atmosphere as a result of the joint processes of evaporation and transpiration through vegetation.

Event–based load

Quantity of pollutants, sediment, or nutrients carried by a water body for particular magnitude storm events.

Event mean concentration (EMC)

The total constituent (pollutant) mass discharge divided by the total runoff volume.

Exfiltration

uncontrolled outward leakage through cracks and interstices.

Extensive green roof

Xeriscape type plantings in shallow, draughty growing medium typically on urban rooftops.

Extreme event

An 100–year, 24–hour rain event or an 100–year, 10–day snowmelt event or greater.

Extreme flood

Control for the 100–year, 24–hour or larger events, to maintain the boundaries of the pre-development 100–year floodplain, reduce flooding risks to life, reduce property damage, and protect the physical integrity of the stormwater management practices.

F

fen

a peat accumulating wetland that receives some drainage from surrounding mineral soils and usually supports marsh–like vegetation. Richer in nutrients and less acidic than bogs due to ground water inflows.

ferrocyanide

an anti–caking additive to road salt; when converted to its free cyanide form (FCN) becomes extremely toxic to aquatic life.

field capacity

is the bulk water content retained in soil at −33 J/kg (or −0.33 bar) of hydraulic head or suction pressure. This is often assumed to be the soil water content at which excess water has drained away and the rate of downward movement has materially decreased.

filter bed

a sand or gravel bottomed treatment used to filter stormwater.

filtration

a series of processes that physically removes particles from water.

first flush

the majority of pollutants carried in urban runoff are carried in the first ½” of runoff from a site.

floodplain

land adjacent to a waterbody which is inundated when the discharge exceeds the conveyance capacity of the normal channel. Often defined in a regulatory sense as the extent of the 100–year flood.

flow control

controlling the rate and volume of water leaving a site.

forebay

an extra storage space or small basin located near the inlet to settle out incoming sediments before water moves on into a pond or detention area.

freeze–thaw cycle

the alternation between freezing and thawing in the snowpack. This cycle changes the composition and characteristics of the snowpack and can effect its pollutant carrying ability and the amount of runoff generated.

frequency curve

A derivative of the probability curve that expresses the relation between the frequency distribution plot, with the magnitude of the variables on one axis and the number of occurrences of each magnitude in a given period as the other.

frost heave

a phenomenon in cold areas in which water that is trapped in soil or cracks in rocks alternately freezes and thaws. This causes the water to expand and contract which can cause significant movement and upheaval of the soil or rock.

functional components approach

an approach where basic BMP components are selected and pieced together to achieve a desired outcome.

G

geomorphology

the study of the form and development of the landscape.

gleyed

a blue–gray, sticky, compacted soil, usually indicative of saturated conditions.

global warming

the progressive gradual rise of the Earth’s surface temperature thought to be caused by the greenhouse effect, which may be responsible for changes in global climate patterns

grade breaks

point where the ground slope changes

grass channels

a natural open channel conveyance system which is preferable to curb and gutter where development density, soils, and slopes permit

green infrastructure

means a wide array of practices at multiple scales that manage wet weather and that maintains or restores natural hydrology by infiltrating, evapotranspiring, or harvesting and using stormwater. On a regional scale, green infrastructure is the preservation or restoration of natural landscape features, such as forests, floodplains and wetlands, coupled with policies such as infill and redevelopment that reduce overall imperviousness in a watershed. On the local scale, green infrastructure consists of site and and neighborhood-specific practices, such as bioretention, trees, green roofs, permeable pavements and cisterns

green roof

a rooftop treatment practice where a thin planting media is established on roof surfaces and then planted with hardy, low–growing vegetation

groundwater

water occupying the sub–surface saturated zone

groundwater mounding

the localized rise in water table or potentiometric surface caused by the addition or injection of water

gully erosion

the widening, deepening and head cutting of small channels and waterways (rills) due to erosion by water or snowmelt, typified by channels one foot or more deep

H

head

the difference in elevation between two points in a body of water and the resulting pressure of the fluid at the lower point.

HEC–1

a rainfall–runoff model developed by the U.S. Army Corps of Engineers.

HEC–2

a rainfall–runoff model developed by the U.S. Army Corps of Engineers to compute steady–state water surface elevation profiles in natural and constructed channels.

high density residential

a high concentration of housing units in a specific area or on a specific property, typical of urban areas.

hotspot

point source potential pollution generating land uses such as gas stations, chemical storage facilities, industrial facilities, etc.

housekeeping (BMP)

any of a number of BMPs designed to keep pollutants from entering the waste stream by maintaining clean conditions, including street sweeping, litter pick–up and animal clean–up.

hybrid rule

current MPCA water quality volume criteria in the General Permit, so called due to encompassing four different rules depending on the type of BMP used and whether the receiving water is indicated as a Special Water.

HydroCAD

a computer aided design program for modeling the hydrology and hydraulics of stormwater runoff.

hydrograph

graphical representation of stage or discharge at a point in a drainage as a function of time.

hydrologic soils groups

an NRCS designation to give different soil types to reflect their relative surface permeability and infiltrative capability. Rankings for from high infiltration rates in Group A to very low infiltration rates in Group D.

hydrology

the science dealing with the properties, distribution, and circulation of water hydroperiod the length of time an area is inundated or saturated by water.

I

impaired waters

streams or lakes that do not meet their designated uses because of excess pollutants or identified stressors

impervious surface

means a constructed hard surface that either prevents or retards the entry of water into the soil and causes water to run off the surface in greater quantities and at an increased rate of flow than prior to development. Examples include rooftops, sidewalks, patios, driveways, parking lots, storage areas, and concrete, asphalt, or gravel roads

Impervious surface (MIDS definition)

a surface that impedes the infiltration of rainfall and results in an increased volume of surface runoff (see also the definition in the glossary of this manual).

infiltration

flow of water from the land surface into the subsurface (see Design infiltration rates)

individual permit

necessary if activities are not covered under one of the state’s general permit provisions

industrial materials or activities

include but are not limited to material handling equipment or activities, industrial machinery, raw materials, intermediate products, by-products, final products, or waste products.

inlet protection

preservation of the integrity and protection from the erosion of the area where water enters into a treatment area usually by vegetation or armoring.

intensity–duration–frequency curves (IDF)

graphical representation of the intensity, duration, and frequency of a differing rainfalls over time.

intensive green roof

rooftop systems including earth-bermed structures which ar reliant on rich, deep substrates and may include shrubs or trees.

interflow

water that travels laterally or horizontally through the aeration zone during or immediately after a precipitation event and discharges into a stream or other body of water.

interstitial water

water in the pore spaces of soil or rock.

ion exchange

an exchange of ions between two electrolytes or between an electrolyte solution and a complex.

isopluvial

line on a map along which an equal percentage of the total annual precipitation falls in a given season or month.

Issue Paper

one in a series of nine decision papers on key topics developed by CWP and EOR during production of the manual.

K

Karst

A type of topography that results from dissolution and collapse of carbonate rocks such as limestone characterized by closed depressions, sinkholes, caves, and underground drainage.

L

Land disturbance (MIDS definition)

Any activity that results in a change or alteration in the existing ground cover (both vegetative and non-vegetative) and/or the existing soil topography. Land disturbing activities include, but are not limited to, development, redevelopment, demolition, construction, reconstruction, clearing, grading, filling, stockpiling, excavation and borrow pits.

Large storm hydrology

A 10–year or greater storm event.

Lattice blocks

A form of pervious pavers consisting of interlocking components having an open space in the middle for vegetation or gravel.

Linear project (MIDS definition)

Construction or reconstruction of roads, trails, sidewalks, and rail lines that are not part of a common plan of development or sale.

Live storage

The portion of a storage basin or reservoir that is at or above the outlet and used for temporary water storage.

Low density residential

A low concentration of housing units in a specific area or on a specific property, typical of rural areas.

Low impact development (LID)

Means an approach to stormwater management that mimics a site's natural hydrology as the landscape is developed. Using the LID approach, stormwater is managed on site and the rate and volume of predevelopment stormwater reaching receiving waters is unchanged. The calculation of predevelopment hydrology is based on native soil and vegetation.

M

Manning formula

an empirical estimating open channel flow, or free-surface flow driven by gravity.

Maximum Water Capacity (MWC)

terminology used in FLL guidelines for Maximum Media Water Retention (MMWR); the quantity of water held in a media at the maximum media density. This is a useful measure of the capacity of a media to hold water under drained conditions.

media filters

filtration of stormwater through a variety of different filtering materials whose purpose is to remove pollution from runoff.

medium density residential

a moderate concentration of housing units in a specific area or on a specific property, typical of suburban areas.

mesotrophic

waters containing an intermediate level of nutrients and biological production.

micropool

similar to wet ponds except there is a small micropool at the outlet to prevent resuspension of previously settled materials and prevents clogging of low–flow orifice.

minimum control measures

six required components of SWPPPs for MS4 communities. The six minimum control measures are public education/outreach; public participation/involvement; illicit discharge detection and elimination; construction site runoff control; post–construction site runoff control; and pollution prevention/ good housekeeping.

mobilization

the release and movement of bound chemicals, nutrients, or pollutants into the environment.

mottled

soil marked with irregular brown and gray/black colors indicative of poor drainage and routine saturation cycles.

municipal separate storm sewer systems (MS4)

A municipal separate storm sewer system is a conveyance or system of conveyances, owned or operated by a state, city, town, county, district, association, or other public body having jurisdiction over disposal of sewage, industrial wastes, stormwater, or other wastes that discharges to waters of the United States. There are three categories of regulated small MS4s: mandatory, discretionary and petition. MS4s are required to develop and implement a Stormwater Pollution Prevention Program (SWPPP which must cover six minimum control measures and identify best management practices (BMPs) and measurable goals associated with each of these minimum control measures.

N

native vegetation

plants that are adapted to and occur naturally in a specific location.

natural area conservation

the identification and protection of natural resources and features that maintain the pre-development hydrology at a site by reducing runoff, promoting infiltration, and preventing soil erosion. 90% capture rule the design of stormwater treatment practices to capture and treat 90% of the annual rainfall from runoff producing events.

New development (MIDS definition)

any development that results in the conversion of land that is currently prairie, agriculture, forest, or meadow and has less than 15 percent impervious surface. Land that was previously developed, but now razed and vacant, will not be considered new development.

nitrification

is the biological oxidation of ammonia or ammonium to nitrite followed by the oxidation of the nitrite to nitrate.

no exposure

all industrial materials or activities are protected by a storm resistant shelter to prevent exposure to rain, snow, snowmelt, or runoff.

Noncomputable pollutant

a pollutant for which there is not enough runoff concentration and BMP performance data available to perform a site–based pollutant load calculation documenting no increase in loading.

Non–point Education for Municipal Officials (NEMO)

a nationally recognized educational program for local elected and appointed decision makers addressing the relationship between land use and natural resource protection.

O

off-line system

a flow system where only stormwater runoff treated by a BMP enters the BMP, with remaining water bypassing the BMP.

oligotrophic

water bodies or habitats with low concentrations of nutrients.

one–half inch rule

based on the first flush concept stating that the majority of the pollutants in urban runoff are carried in the first one–half inch of runoff. The half–inch rule defines the water quality volume as one–half inch times the impervious area.

on-line system

a systems where all stormwater runoff enters and flows through a BMP.

orifice outlet

DEFINITION MISSING

Outstanding Resource Value Waters (ORVW)

defined in Minnesota Rule 7050.0180 as waters within the Boundary Waters Canoe Area Wilderness, Voyageur’s National Park, and Department of Natural Resources designated scientific and natural areas, wild, scenic, and recreational river segments, Lake Superior, those portions of the Mississippi River from Lake Itasca to the southerly boundary of Morrison County that are included in the Mississippi Headwaters Board comprehensive plan dated February 12, 1981, and other waters of the state with high water quality, wilderness characteristics, unique scientific or ecological significance, exceptional recreational value, or other special qualities which warrant stringent protection from pollution.

overbank flood protection

prevention of flood damage to conveyance systems and infrastructure and reduction of minor flooding caused by an increased frequency and magnitude of floods exceeding the bankful capacity of a channel and spilling out over the floodplain.

over–control

originally proposed by McCuen in 1979, the practice of designing a system with more controls in place than it is strictly computed to be necessary for the 2–year design storm to afford some measure of channel protection

P

peak flow control

controlling the timing and magnitude of the largest flow either leaving the site or flowing through the watershed utilizing stormwater management techniques to avoid flooding and damage downstream.

perimeter control

activities or practices designed to contain sediments on a project site.

permanent storage pool

the volume in a pond or reservoir below the lowest outlet level, designed for water quality purposes to settle out particles and nutrients.

permeable paver

a range of products that enable some fraction of rainfall to be infiltrated into a sub–base underneath the paver.

Phase II

A 1987 amendment to the federal Clean Water Act required implementation of a two–phase comprehensive national program to address stormwater runoff. Phase I regulated large construction sites, 10 categories of industrial facilities, and major metropolitan MS4s. On March 10, 2003 the program broadened to include smaller construction sites, municipally owned or operated industrial activity, and many more municipalities.

Pitt Method

means of calculating the treatment depth of rainfall based on Dr. Robert Pitt’s work on rainfall and pollutant distribution as part of the 1983 NURP program pollution load the product of flow volume times pollutant concentration.

pollution prevention practices

pro–active activities and strategies instituted to avoid introducing pollution into the environment.

pollutograph

graphical representation of pollution at a point in a drainage as a function of time.

polycyclic aromatic hydrocarbon (PAH)

organic compound resulting from combustion of petrochemical fuel.

PONDNET

an empirical model developed by William W. Walker in 1987 to evaluate flow and phosphorus routing in pond networks.

porosity

the ratio of the volume of pores in a soil to the total soil volume.

pretreatment

practices used to reduce, eliminate, or alter pollutants in stormwater, primarily sediment, before they are discharged into structural stormwater BMPs. Pretreatment practices include settling devices, screens, and pretreatment vegetated filter strips.

primary treatment

the first stage of wastewater treatment, including removal of floating debris and solids by screening, skimming and sedimentation.

Program for Predicting Polluting Particle Passage through Pits, Puddles and Ponds (P–8)

a physically–based model developed by William W. Walker to predict the generation and transport of stormwater runoff pollutants in urban watersheds.

proprietary devices

stormwater treatment devices which are privately developed and owned.

Q

Quiescent

Periods periods of rest or inactivity.

R

rain barrel

a container used to collect and store rainwater that is usually placed below the downspout of a roof gutter. The collected water is used to water the landscape.

rainfall distribution

describes how the rain fell in a 24–hour period, ie. whether the precipitation occurred over a 1–hour period or over the entire 24–hour period

rainfall frequency spectrum

describes the average frequency of the depth of precipitation events (adjusted for snowfall) that occur during a normal year

rain garden

a landscaping feature that is planted with native perennial plants and is used to manage stormwater runoff from impervious surfaces such as roofs, sidewalks, and parking lots rate control controlling the rate that stormwater is released from localized holding areas into larger conveyance systems

receiving water

a body of water such as a stream, river, lake, or ocean, which receives stormwater and wastewater

recessional limb

the portion of the hydrograph after the peak where flows are returning to lower or baseflow levels

recharge

the addition of water to an aquifer by natural infiltration or artificial means

recurrence interval

the inverse probability that a certain flow will occur. It represents a mean time interval based on the distribution of flows over a period of record

redevelopment

any construction, alteration, or improvement that disturbs greater than or equal to 5,000 square feet of existing impervious cover performed on sites where the existing land use is commercial, industrial, institutional, or residential

Redevelopment (MIDS definition)

any development that is not considered new development (see also the definition in the glossary of this manual).

removal rate

the rate at which a pollutant is removed from the water column.

retention

the permanent or temporary storage of stormwater to prevent it from leaving the development site retrofit the introduction of a new or improved stormwater management element where it either never existed or did not operate effectively.

return interval

the inverse probability that a certain flow will occur. It represents a mean time interval based on the distribution of flows over a period of record.

rill erosion

an erosion process in which numerous small channels several inches deep are formed.

riparian areas

areas adjacent to a water body acting as transition zones between terrestrial and aquatic systems.

riser

a vertical assembly of pipe and fittings that generally distributes water upward.

roof leader

a downspout or other conveyance for runoff that has been collected from roof tops routing stormwater down to the ground surface or to a sewer service.

rooftop runoff

storage installation of practices to capture and temporarily store rooftop runoff at confined sites and gradually release it over pervious areas for use for irrigation.

runoff

the portion of rainfall or snowmelt not immediately absorbed into the soil that drains or flows off the land and becomes surface flow.

runoff volume minimization

reducing as much as possible the amount of water running off surfaces or leaving a site.

runoff management techniques

practices and strategies for dealing with runoff and minimizing its impact to the greater environment.

S

saturated hydraulic conductivity

a property of soil that describes the ease with which a fluid (usually water) can move through pore spaces or fractures under saturated conditions.

saturated soil

the highest seasonal elevation in the soil that is evidenced by the presence of redoximorphic features or other parameters. It is recommended the determination of saturated soil be made by a soils specialist.

seasonal high water table

see saturated soil.

secondary treatment

biological and mechanical processes that remove dissolved or suspended material from wastewater.

sedge meadow

wetland vegetative communities dominated by sedges (Cyperaceae) growing on saturated soils.

sediment

any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water.

sediment control basins

a designed depression in the landscape utilized to settle out sediments from the water column before discharge into other drainages.

sedimentation/sediment removal

the removal, usually by settling or filtering, of suspended sediments from the water column.

sediment yield

the amount of sediment removed from a watershed over a specified period of time.

separation distance

the distance from the closest point of a Best Management Practice (BMP) to the particular feature being considered.

settling

a technique to remove sediment from wastewater by slowing the water flow velocity allowing the sediments to sink to the bottom

shrub–carr wetland

plant community composed of tall, deciduous shrubs growing on saturated or seasonally flooded soils

silt curtain

a natural or synthetic fabric suspended by floats and weighted at the bottom which is stretched across a water feature and used to trap and retain sediments on site

silt fence

fence constructed of wood or steel supports and either natural (e.g. burlap) or synthetic fabric stretched across an area of non–concentrated flow during site development to trap and retain on–site sediment due to rainfall runoff

Simple Method

a technique for estimating storm pollutant export delivered from urban development sites

site constraints

conditions unique to the site that that serve to restrain, restrict, or prevent the implementation of proposed or desired design features

site reforestation

reforestation of existing turf or barren ground at the development site with the explicit goal of establishing a mature forest canopy or prairie condition that intercepts rainfall, and maximizes infiltration and evapotranspiration.

skimmer

device used to take up or remove floating matter from the water’s surface.

slope stabilization

activities or techniques employed to maintain the integrity or stop the degradation of sloped areas.

small storm hydrology

a less than 10–year event.

snowmelt

the sudden release of accumulated snow and ice with the advent of warm weather.

snowpack

a horizontally layered accumulation of snow from snowfall events which accumulates and persists through the winter and may be modified by meteorological conditions over time.

soakaway pit

small, excavated pits, backfilled with aggregate, used to infiltrate good quality stormwater runoff, such as uncontaminated roof runoff.

soil amendment

tilling and composting of new lawns and open spaces within a development site to recover soil porosity, bulk density, and reduce runoff.

sorbent

material which extracts one or more materials from the water via absorption or adsorption.

source water protection area

an identified area with restricted or modified land use practices designed to protect the public drinking water supply from the introduction of contaminants.

Special Waters

waters receiving special protections as defined in Minnesota Rules.

spring snowmelt event

large amount of melting of the winter’s accumulated snow over a short period of time (~2 weeks). Large flow volumes typical and may be the critical water quality design event.

standpipe

a vertical pipe or reservoir for water used to secure a uniform pressure.

stage

the height of a water surface above an established reference point.

storm distribution

a measure of how the intensity of rainfall varies over a given period of time.

stormwater

water that is generated by rainfall or snowmelt which causes runoff and is often routed into drain systems for treatment or conveyance.

stormwater credits

the quantity of stormwater or pollutant reduction achieved either by an individual BMP or cumulatively with multiple BMPs. In the previous version of this Manual, credit referred to activities that can be undertaken in order to reduce the sizing or requirements for stormwater management at a site.

Storm Water Management Model (SWMM)

a dynamic rainfall–runoff simulation model developed by the U.S. Environmental Protection Agency in 1971 for analysis of quantity and quality problems associated with urban runoff.

stormwater planter

self–contained landscaping areas which capture and temporarily store a fraction of rooftop runoff and filter it through the soil media.

stormwater pollution prevention plan (SWPPP)

a plan for preventing or minimizing pollution generated at construction sites.

stormwater pollution prevention program (SWPPP)

a program that is required to be developed by MS4 communities to incorporate applicable best management practices, measureable goals and which must include the six minimum control measures.

Stormwater retrofit

providing stormwater treatment on existing development that is currently untreated by any BMP or is inadequately treated by an existing BMP.

stormwater treatment train

a suite of stormwater management practices incorporating aspects of pollution prevention, volume control and water quality controls.

streambank stabilization

activities or techniques employed to maintain the integrity or stop the degradation of streambanks due to erosion and sedimentation.

structural stormwater BMP

is defined in the MS4 General permit as "a stationary and permanent BMP that is designed, constructed and operated to prevent or reduce the discharge of pollutants in stormwater". For the purposes of the TMDL Annual Reporting form, a structural BMP refers specifically to a constructed basin, filter, infiltrator, swale or strip.

sublimation

the process of transforming from a solid directly into a gas without passing through a liquid phase.

subwatershed

a subdivision based on hydrology corresponding to a smaller drainage area within a larger watershed.

swale

a wide, shallow, vegetated depression in the ground designed to channel drainage of water.

T

Technical Publication 40 (TP–40)

U.S. Weather Bureau publication that is the standard reference for frequency analysis in Minnesota

Technical Release Number 20 (TR–20)

a single–event rainfall–runoff computer model developed by the U.S. Soil Conservation Service in 1964.<.dd>

Technical Release Number 55 (TR–55)

a simplified procedure to calculate storm runoff, volume, peak rate of discharge, hydrographs and storage volumes developed by the U.S. Natural Resource Conservation Service in 1975.

temporary construction

sediment control techniques practices employed on an active construction site to control movement of sediment within or off of the site until permanent vegetation or sediment controls can be established.

ten percent rule

the downstream point where the development site represents 10% of the total contributing drainage area of a watershed. Downstream hydraulic and hydrologic analysis for the effects of coincident peaks should extend to this point.

thermal impact

the impact to streams and water bodies of stormwater runoff addition which are higher in temperature than the ambient stream or water body temperature. This causes stress or may result in the death of temperature sensitive organisms such as trout.

thermal protection

techniques and practices such as infiltration and shading which act to preserve and protect the ambient temperatures of streams and waterbodies from temperature–raising effects of stormwater runoff.

total maximum daily load (TMDL)

the amount of a pollutant from both point and nonpoint sources that a waterbody can receive and still meet water quality standards.

total phosphorus (TP)

a nutrient that can also be a contaminant because of its use by nuisance algae.

total suspended solids (TSS)

a measure of the amount of particulate material in suspension in a water column.

transmissivity

a measure of how much water can be transmitted horizontally. It is a function of the conductivity and saturated thickness of the media.

transpiration

the passage of water vapor into the atmosphere through the vascular system of plants.

trash rack

a structural device used to prevent debris from entering a pipe spillway or other hydraulic structure.

treatment

any method, technique, or practice used for management purposes.

trench

a long steep–sided depression in the ground used for drainage or infiltration.

turbidity

the cloudy appearance of water caused by the presence of suspended and colloidal matter.

U

Ultra-urban

Multi-family or mixed-use residential buildings with 50 or more dwelling units per acre, covering 75 percent or more of the lot, and parking is all or substantially all enclosed; or retail, office, civic or institutional buildings covering 75 percent or more of the lot and parking is all or substantially all enclosed.

Underdrain

An underground drain or trench with openings through which the water may percolate from the soil or ground above.

Unified sizing criteria

statewide criteria for the sizing of stormwater management systems.

V

Vadose zone

the zone between the soil surface and saturated soil zone (water table); also called the unsaturated zone

Vegetative filters

the removal of sediment, nutrients, or pollutants by plant structures

Volatilization

the conversion of a chemical substance from a liquid or solid state to a gaseous or vapor state by the application of heat, by reducing pressure, or by a combination of these processes

Volume control

controlling the overall volume or amount of stormwater that is released from a site or localized holding area into the larger conveyance system

W

Walker Method

a method for determining sizing for water detention ponds, developed in the upper Midwest to maximize phosphorus removal to protect sensitive lakes from eutrophication.

Wasteload Allocation (WLA)

is the portion of a receiving water's assimilative capacity that is allocated to one of its existing or future point sources of pollution.

water balance

A hydrological formula used by scientists and land managers to determine water surpluses and deficits in a given area. Includes inputs such as precipitation; outputs such as evapotranspiration, infiltration, and runoff; and storage within the system.

water holding capacity

the amount of water that can be held in a soil between saturation and a specific pressure. Field capacity (1/3 bar pressure) and wilting point (15 bar pressure) are commonly used to define a soil's water holding capacity.

water quality sizing

tied to the volume of stormwater runoff.

water quality volume

The volume of water that is treated by a BMP. To meet the requirements of the Construction Stormwater General Permit, the Water Quality Volume means one (1) inch of runoff from the new impervious surfaces created by the project (calculated as an instantaneous volume) and is the volume of water to be treated in the Permanent Stormwater Management System, as required by the permit.

watershed

a topographically defined area within which all water drains to a particular point.

watershed inch

a unit of measure corresponding to the volume of water spread out over the entire watershed area at a depth of one inch.

waters of the State

All streams, lakes, ponds, marshes, watercourses, waterways, wells, springs, aquifers, irrigation systems, drainage systems, and all other bodies or accumulations of water surface or underground, natural or artificial, public or private, which are contained within, flow through or under the state or any portion thereof.

waters of the United States

those waters coming under federal jurisdiction.

weir

a spillover dam–like device used to measure or control water flow.

Wellhead Protection Area (WHPA)

the surface and subsurface area surrounding a well or well field that supplies a public water system, through which contaminants are likely to move toward and reach the well or well field (Minnesota Statutes, section 103I.005, subdivision 24).

wetland

land that is transitional between aquatic and terrestrial ecosystems and must: have a predominance of hydric soils, be inundated or saturated by surface water or ground water at a frequency and duration sufficient to support a prevalence of hydrophytic vegetation typically adapted for life in saturated soil conditions, and under normal circumstances support a prevalence of hydrophytic vegetation. To be a wetland the area must meet wetland criteria for soils, vegetation, and hydrology as outlined in the 1987 U.S. Army Corps of Engineers Wetland Delineation Manual.

wetland systems

hydrologically interconnected series of wetlands which includes the interrelatedness of habitat, wetland functions, and biology.

wet pond

a permanent pool of water for treating incoming stormwater runoff.

wet vault

A wet vault is a vault stormwater management device with a permanent water pool, generally 3 to 5 feet deep.

wilting point

permanent wilting point or wilting point is the minimal point of soil water the plant requires not to wilt. This is often taken to be the water content at 15 bars pressure.

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