Glossary

A

• 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 - areas underlain by carbonate bedrock with less than 50 feet of sediment cover
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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
• 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
• 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
• 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 roof - a rooftop treatment practice where a thin planting media is established on roof surfaces and then planted with hardy, low–growing vegetation
• ground water - water occupying the sub–surface saturated zone
• ground water 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 - a surface in the landscape that impedes the infiltration of rainfall and results in an increased volume of surface runoff
• infiltration - flow of water from the land surface into the subsurface
• 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
• 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

• 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
• 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) - 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

M

• 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 resuspention 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
• 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 University of Connecticut educational program for land use decision makers that addresses the relationship of land use to natural resource protection nonpoint source pollution pollution that enters a water body from diffuse origins on the watershed and does not result from discernable, confined, or discrete conveyances

O

• 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.
• orifice outlet -
• 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
• pre–treatment - processes used to reduce, eliminate, or alter pollutants before they are discharged into publicly owned sewage treatment systems
• 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
• 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

• 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
• 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
• 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 (eg 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 conveyence
• stormwater credits - 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
• 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.
• 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 temperaturesensitive 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 transitional karst areas underlain by carbonate bedrock covered by 50- 100 feet of sediment
• 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 - highly developed urban land which has limited space and disturbed soils
• 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

• vegetative filters - the removal of sediment, nutrients, or pollutants by plant structures
• 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
• 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 quality sizing - tied to the volume of stormwater runoff
• water quality volume - the permanent pool in a water detention pond
• 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 - an identified area with restricted or modified land use practices designed to protect the well supply area from the introduction of contaminants
• 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