The following terms are provided for reference and use with this manual. They shall be superseded by any other definitions for these terms adopted by ordinance, unless they are defined in a Washington State WAC or RCW, or are used and defined as part of the Minimum Requirements for all new development and redevelopment.
A - B - C - D - E - F - G - H - I - L - M - N- O - P - R - S - T - U - V - W - Y
The official classification of soil materials and soil aggregate mixtures for highway construction, used by the American Association of State Highway and Transportation Officials. The penetration of a substance into or through another, such as when water fills the voids of soil. A variation in the application of a Minimum Requirement to a particular project. Adjustments provide equivalent environmental protection. The adhesion of a substance to the surface of a solid or liquid; often used to extract pollutants by causing them to be attached to such adsorbents as activated carbon or silica gel. Hydrophobic, or water-repulsing adsorbents, are used to extract oil from waterways when oil spills occur. Heavy metals such as zinc and lead often adsorb onto sediment particles. The process of being supplied or impregnated with air. In waste treatment, the process used to foster biological and chemical purification. In soils, the process by which air in the soil is replenished by air from the atmosphere. In a well aerated soil, the soil air is similar in composition to the atmosphere above the soil. Poorly aerated soils usually contain a much higher percentage of carbon dioxide and a correspondingly lower percentage of oxygen. Living or active only in the presence of free (dissolved or molecular) oxygen. Bacteria that require the presence of free oxygen for their metabolic processes. Primitive plants, many microscopic, containing chlorophyll and forming the base of the food chain in aquatic environments. Some species may create a nuisance when environmental conditions are suitable for prolific growth. Proliferation of living algae on the surface of lakes, streams or ponds; often stimulated by phosphate over-enrichment. Algal blooms reduce the oxygen available to other aquatic organisms. The Washington State Chapter of the American Public Works Association. Fish that grow to maturity in the ocean and return to rivers for spawning. Bacteria that do not require the presence of free or dissolved oxygen for metabolism. The degree of wetness of a watershed or within the soil at the beginning of a storm. The person who has applied for a City of Tacoma Permit or is submitting plans for approval (ex. when a Permit is not needed but plan review is required). Machinery, appliances, or auxiliary structures attached to a main structure, but not considered an integral part thereof, for the purpose of enabling it to function. Aquatic life uses are based upon the presence of, or intent to provide protection for: salmonid spawning, rearing, and migration, salmonid rearing and migration only, non-anadromous interior redband trout, indigenous warm water species, all indigenous fish and nonfish aquatic species. In the City of Tacoma, all freshwater bodies shall be considered to have an aquatic life use. See Volume 1, Minimum Requirement 6 - Stormwater Treatment to determine what treatment type applies to the project. A geologic stratum containing groundwater that can be withdrawn and used for human purposes. A road or street with the primary purpose of efficient movement of people and goods. See TMC 11 for classified arterials in the City of Tacoma. Engineering plans which have been revised to reflect all changes to the plans which occurred during construction. A description of pollutant levels arising from natural sources, and not due to human activities. Water upstream from an obstruction which is deeper than it would normally be without the obstruction. A flood having a one percent chance of being equaled or exceeded in any given year. This is also referred to as the 100-year flood. The water surface elevation of the base flood. It shall be referenced to the National Geodetic Vertical Datum of 1929 (NGVD). For information regarding conversion between the National Geodetic Vertical Datum of 1929 and the North American Vertical Datum of 1988, visit the National Geodetic Survey Website at http://www.ngs.noaa.gov/ A sample collected during dry-weather flow (i.e., it does not consist of runoff from a specific precipitation event). The more or less solid rock in place either on or beneath the surface of the earth. It may be soft, medium, or hard and have a smooth or irregular surface. A relatively level step excavated into earth material on which fill is to be placed. A constructed barrier of compacted earth, rock, or gravel. In a stormwater facility, a berm may serve as a vertical divider typically built up from the bottom. The schedules of activities, prohibitions of practices, maintenance procedures, and structural and/or managerial practices, that when used singly or in combination, prevent or reduce the release of pollutants and other adverse impacts to waters of Washington State. An indirect measure of the concentration of biologically degradable materials present in organic wastes. The amount of free oxygen utilized by aerobic organisms when allowed to attack the organic material in an aerobically maintained environment at a specified temperature (20°C) for a specific time period (5 days), and thus stated as BOD5. It is expressed in milligrams of oxygen utilized per liter of liquid waste volume (mg/l) or in milligrams of oxygen per kilogram of waste solution (mg/kg = ppm = parts per million parts). Also called biological oxygen demand. Capable of being readily broken down by biological means, especially by microbial action. Microbial action includes the combined effect of bacteria, fungus, flagellates, amoebae, ciliates, and nematodes. Degradation can be rapid or may take many years depending upon such factors as available oxygen and moisture. The combination of biological, mechanical, and ecological concepts (and methods) to control erosion and stabilize soil through the use of vegetation or in combination with construction materials. A designed treatment facility using a combined soil and vegetation system for filtration, infiltration, adsorption, and biological uptake of pollutants in stormwater when runoff flows over and through. Vegetation growing in these facilities acts as both a physical filter which causes gravity settling of particulates by regulating velocity of flow, and also as a biological sink when direct uptake of dissolved pollutants occurs. The former mechanism is probably the most important in western Washington where the period of major runoff coincides with the period of lowest biological activity. The process of reducing pollutant concentrations in water by filtering the polluted water through biological materials. A method of controlling pest organisms by means of introduced or naturally occurring predatory organisms, sterilization, the use of inhibiting hormones, or other means, rather than by mechanical or chemical means. Engineered facilities that store and treat stormwater by passing it through a specified soil profile, and either retain or detain the treated stormwater for full infiltration or flow attenuation. Municipal sewage sludge that is a primarily organic, semisolid product resulting from the wastewater treatment process, that can be beneficially recycled and meets all applicable requirements under Chapter 173-308 WAC. Biosolids include a material derived from biosolids and septic tank sludge, also known as septage, that can be beneficially recycled and meets all applicable requirements under Chapter 173-308 WAC. For the purposes of Chapter 173-308 WAC, semisolid products include biosolids or products derived from biosolids ranging in character from mostly liquid to fully dried solids. A temporary surface treatment provided to control dust and assist in the control of erosion. An unimproved roadway that receives this treatment does not meet the City of Tacoma Standard Roadway Design. When upgrading from bituminous surface treatment to asphalt or concrete, the surface is considered a new impervious surfaces. A post (may or may not be removable) used to prevent vehicular access. The zone contiguous with a sensitive area that is required for the continued maintenance, function, and structural stability of the sensitive area. The critical functions of a riparian buffer (those associated with an aquatic system) include shading, input of organic debris and coarse sediments, uptake of nutrients, stabilization of banks, interception of fine sediments, overflow during high water events, protection from disturbance by humans and domestic animals, maintenance of wildlife habitat, and room for variation of aquatic system boundaries over time due to hydrologic or climatic effects. The critical functions of terrestrial buffers include protection of slope stability, attenuation of surface water flows from stormwater runoff and precipitation, and erosion control. A line measured parallel to a property, easement, stormwater facility, or buffer boundary, that delineates the area (defined by the distance of separation) where buildings or other obstructions are prohibited (including decks, patios, outbuildings, or overhangs beyond 18 inches). Wooden or chain link fences and landscaping are allowable within a building setback line. In this manual the minimum building setback line shall be 5 feet. A chamber or well, usually built at the curb line of a street, for the admission of stormwater or surface water to the stormwater system, having at its base a sediment sump designed to retain grit and detritus below the point of overflow. The degree to which a soil can absorb and exchange cations, at a given pH. Typically defined in milliequivalents per 100 grams of soil (meq/100 g soil). Soil found to have a CEC of 5 meq at pH 7 will have CEC <5 meq when pH <7. An individual who is knowledgeable in the principles and practices of erosion and sediment control. The CESCL must have the skills to assess: the site conditions and construction activities that could impact the quality of the stormwater; and the effectiveness of erosion and sediment control measures used to control the quality of stormwater discharges. The CESCL must have current certification through an approved erosion and sediment control training program that meets the minimum training standards established by Ecology Certified Professional in Erosion and Sediment Control (CPESC) An individual who has educational training, expertise and experience in controlling erosion and sedimentation, and has met certification standards. The CPESC shall have a current certification through a program approved by Ecology. A feature that conveys stormwater and surface water and is open to the air. Channels constructed (or reconstructed natural channels) to convey stormwater and surface water. Channels that convey stormwater and surface water and have existed long enough to establish a stable route and/or biological community. Erosion prevention and stabilization of velocity distribution in a channel using vegetation, jetties, drops, revetments, and/or other measures. Alteration of a stream channel by widening, deepening, straightening, cleaning, or paving certain areas which changes characteristics. Small dam constructed in a gully, swale, or other small watercourse to decrease the streamflow velocity, minimize channel scour, and promote deposition of sediment. A measure of the amount of oxygen required to oxidize organic and oxidizable inorganic compounds in water. The COD test, like the BOD test, is used to determine the degree of pollution in water. In the City of Tacoma, chip sealing is a maintenance practice currently used to extend the life of existing asphalt or concrete roadways. The practice involves applying asphalt emulsion and chip rock to the existing asphalt or concrete section. On roadways that have received a chip seal maintenance application, when overlaying with asphalt or concrete, the overlain sections of roadway will not be considered new or replaced impervious surfaces and will not be counted toward the project thresholds. A City block shall be defined by existing arterial or residential roads. A City block shall not be defined by alleys. An area where vegetation has been removed by manual, mechanical, or chemical means. The destruction and removal of vegetation by manual, mechanical, or chemical methods. An area which is low-lying and either has no, or such a limited, outlet that during storm events the area acts as a retention basin. The capacity of a soil to resist shearing stress, exclusive of functional resistance. Microorganisms common in the intestinal tracts of man and other warm-blooded animals; all the aerobic and facultative anaerobic, gram-negative, nonspore-forming, rod-shaped bacteria which ferment lactose with gas formation within 48 hours at 35°C. Used as an indicator of bacterial pollution. Those activities conducted on lands defined in RCW 84.34.020(2), and activities involved in the production of crops or livestock for commercial trade. An activity ceases to be considered commercial agriculture when the area on which it is conducted is proposed for conversion to a nonagricultural use or has lain idle for more than five (5) years, unless the idle land is registered in a federal or state soils conservation program, or unless the activity is maintenance of irrigation ditches, laterals, canals, or conveyance ditches related to an existing and ongoing agricultural activity. Sites not considered residential, industrial or road-related. A project site where multiple separate and distinct construction activities may be taking place at different times on different schedules and/or by different contractors, but still under a single plan. Examples include: Phased projects and projects with multiple filings or lots, even if the separate phases or filings/lots will be constructed under separate contract or by separate owners (e.g., a plat or short plat where lots are sold to separate builders). A development plan that may be phased over multiple years, but is still under a consistent plan for long-term development. New development or redevelopment in contiguous areas that may be unrelated but still under the same contract, such as construction of a building extension and a new parking lot at the same facility. New development or redevelopment on contiguous lots that are not associated with a land use action, that are owned by a single entity, even if construction on the lots will not occur at the same time. New development or redevelopment on non-contiguous lots that are located on the same City block and discharge to the same threshold discharge area, owned by a single entity, even if construction will not occur at the same time. New development or redevelopment on linear projects such as roads, pipelines, or utilities. If the project is part of a common plan of development or sale, the disturbed area of the entire plan must be used to determine permit requirements. Conveyances into different ownership for the ostensible purpose of avoiding more comprehensive stormwater review and requirements, or where an innocent conveyance has this effect, may be considered to be part of a Common Plan of Development and reviewed by Environmental Services/Site Development Group for cumulative impacts. The densification, settlement, or packing of soil in such a way that permeability of the soil is reduced. Compaction effectively shifts the performance of a hydrologic group to a lower permeability hydrologic group. For example, a group B hydrologic soil can be compacted and be effectively converted to a group C hydrologic soil in the way it performs in regard to runoff. Compaction may also refer to the densification of a fill by mechanical means. Compensatory Mitigation (as defined in Tacoma Municipal Code) Replacing project-induced loses or impacts to a critical area, and includes, but is not limited to, the following: a. Restoration. Actions performed to reestablish wetland functional characteristics and processes that have been lost by alterations, activities, or catastrophic events within an area that no longer meets the definition of a wetland. b. Creation. Actions performed to intentionally establish a wetland at a location where it did not formerly exist. Tacoma Municipal Code City Clerk’s Office 13-45 (Published 03/2020) c. Enhancement. Actions performed to improve the condition of existing degraded wetlands so that the functions they provide are of a higher quality. d. Preservation actions taken to ensure the permanent protection of existing high quality wetlands. Organic material that has undergone biological degradation and transformation under controlled conditions designed to promote aerobic decomposition at a solid waste facility in compliance with the requirements of Chapter 173-350 WAC, or biosolids composted in compliance with Chapter 173-308 WAC. Composting is a form of organic material recycling. Natural decay of organic solid waste under uncontrolled conditions does not result in composted material. (Note: Various BMPs have restrictions on the percentage of biosolids in compost or do not allow biosolids in compost.) The biological degradation and transformation of organic solid waste under controlled conditions designed to promote aerobic decomposition. Natural decay of organic solid waste under uncontrolled conditions is not composting. A public organization created under state enabling law as a special-purpose district to develop and carry out a program of soil, water, and related resource conservation, use, and development within its boundaries, usually a subdivision of state government with a local governing body and always with limited authority. Often called a soil conservation district or a soil and water conservation district. Artificial wetlands created where wetlands do not currently exist for managing stormwater discharges. Constructed wetlands are considered part of the stormwater system. A document that describes the potential for pollution problems on a construction project and explains and illustrates the measures to be taken on the construction site to control those problems. An imaginary line on the surface of the earth connecting points of the same elevation. The geographical limits of an area that provides stormwater and surface water to a common location such as an outlet, discharge location, or stormwater facility. The surfaces on a project site where native vegetation, pasture, scrub/shrub, uncultivated vegetation, or unmaintained non-native vegetation (e.g., Himalayan blackberries, scotch broom) are converted to lawn or landscaped areas, or where native vegetation is converted to pasture. A mechanism for transporting water from one point to another, including pipes, ditches, and channels. The stormwater system components, both natural and artificial, which collect, contain, and provide for the flow of surface and stormwater from the highest points on the land down to a receiving waterbody. The natural elements of the conveyance system include natural channels, streams, rivers, lakes, and wetlands. The artificial elements of the conveyance system include gutters, ditches, pipes, channels, and most retention/detention facilities. At a minimum, areas which include wetlands, stream corridors, areas with a critical recharging effect on aquifers used for potable water, fish and wildlife habitat conservation areas, frequently flooded areas, geologically hazardous areas, including unstable slopes, and associated areas and ecosystems. An area equal to 1-foot radius from the base of the tree’s trunk for each 1-inch of the tree’s diameter at 4.5 feet above grade (diameter at breast height). The highest point of the internal surface of the transverse cross section of a pipe. The upper part of the tree, measured from the lowest branch, including all the branches and foliage. Pipe or concrete box structure that conveys stormwater or surface water under a roadway or embankment. Typically with no catchbasins or manholes along its length. Portion of land surface or area from which earth has been removed or will be removed by excavating; the depth below original ground surface to excavated surface. Process of earth moving by excavating part of an area and using the excavated material for adjacent embankments or fill areas. A slope formed by excavating overlying material to connect the original ground surface with a lower ground surface created by the excavation. A cut slope is distinguished from a bermed slope, which is constructed by importing soil to create the slope. The volume available in a depression below an elevation that could allow discharge from that depression. Refers to setting aside a portion of a property for a specific use or function. (Biological or chemical) The breakdown of complex organic or other chemical compounds into simpler substances, usually less harmful than the original compound, as with the degradation of a persistent pesticide. (Geological) Wearing down by erosion. (Water) The lowering of the water quality of a watercourse by an increase in the pollutant loading. A wetland (community) whose functions and values have been reduced as a result of human activities. For example, a wetland in which the vegetation, soils, and/or hydrology have been adversely altered, resulting in lost or reduced functions and values; generally, implies topographic isolation; hydrologic alterations such as hydroperiod alteration (increased or decreased quantity of water), diking, channelization, and/or outlet modification; soils alterations such as presence of fill, soil removal, and/or compaction; accumulation of toxicants in the biotic or abiotic components of the wetland; and/or low plant species richness with dominance by invasive weedy species. A wetland where the elevation of the surface within the wetland is lower than the surrounding landscape. These wetlands often pond water at the surface but they can also be saturated without surface ponding The Washington State Licensed Professional Engineer who prepares the analysis, design, and engineering plans for an applicant's permit or approval submittal. A prescribed hyetograph and total precipitation amount (for a specific duration recurrence frequency) used to estimate runoff for a hypothetical storm of interest or concern for the purposes of analyzing existing stormwater flow characteristics, designing stormwater systems or assessing other impacts of a proposed project on the flow of surface water. The release of stormwater runoff from the site at a slower rate than it is collected by the stormwater facility system, the difference being held in temporary storage. An above or below ground facility, such as a pond or tank, that temporarily stores stormwater runoff and subsequently releases it at a slower rate than it is collected by the drainage facility system. There is little or no infiltration of stored stormwater. The theoretical time required to displace the contents of a stormwater treatment facility at a given rate of discharge (volume divided by rate of discharge). The written decision by the responsible official of the lead agency that a proposal is not likely to have a significant adverse environmental impact, and therefore an EIS is not required. Means new development, redevelopment, or both. See definitions for each. Runoff leaving a site via overland flow, built conveyance systems, or infiltration facilities. A hydraulic rate of flow, specifically fluid flow; a volume of fluid passing a point per unit of time, commonly expressed as cubic feet per second, cubic meters per second, gallons per minute, gallons per day, or millions of gallons per day. The location where runoff leaves a site or stormwater system. The location where a discharge leaves the Permittee’s MS4 through the Permittee’s MS4 facilities/BMPs designed to infiltrate. Release of surface and stormwater runoff from a drainage facility system such that the flow spreads over a wide area and is located so as not to allow flow to concentrate anywhere upstream of a conveyance channel with erodible underlying granular soils. A natural or artificial channel used for the purposes of conveying stormwater and surface waters. A ditch top width is less than 10 feet at design flow. To provide channels, such as open ditches or closed drains, so that excess water can be removed by surface flow or by internal flow. To lose water (from the soil) by percolation. As a natural condition of the soil, soil drainage refers to the frequency and duration of periods when the soil is free of saturation; for example, in well-drained soils the water is removed readily but not rapidly; in poorly drained soils the root zone is waterlogged for long periods unless artificially drained, and the roots of ordinary crop plants cannot get enough oxygen; in excessively drained soils the water is removed so completely that most crop plants suffer from lack of water. Strictly speaking, excessively drained soils are a result of excessive runoff due to steep slopes or low available water-holding capacity due to small amounts of silt and clay in the soil material. The following classes are used to express soil drainage: Well drained - Excess water drains away rapidly and no mottling occurs within 36 inches of the surface. Moderately well drained - Water is removed from the soil somewhat slowly, resulting in small but significant periods of wetness. Mottling occurs between 18 and 36 inches. Somewhat poorly drained - Water is removed from the soil slowly enough to keep it wet for significant periods but not all of the time. Mottling occurs between 8 and 18 inches. Poorly drained - Water is removed so slowly that the soil is wet for a large part of the time. Mottling occurs between 0 and 8 inches. Very poorly drained - Water is removed so slowly that the water table remains at or near the surface for the greater part of the time. There may also be periods of surface ponding. The soil has a black to gray surface layer with mottles up to the surface. Lowering of the water surface (in open channel flow), water table or piezometric surface (in groundwater flow) resulting from a withdrawal of water. The area on the ground below the tree in which the boundary is designated by an imaginary line defined by the edge of the tree’s branch spread. An underground structure that disposes unwanted water. A drywell receives water from entry pipes located at the top of the facility. Typically, drywells are deeper than they are wide and they typically do not contain perforated pipe to distribute water evenly. Any rock, natural soil or fill and/or any combination thereof. Earth material shall not be considered topsoil used for landscape purposes. Topsoil used for landscaped purposes shall comply with ASTM D 5268 specifications. Engineered soil/landscape systems are also defined independently. The legal right to use a parcel or portion of land for a particular purpose. It does not include fee ownership, but may restrict the owners use of the land. Those impervious surfaces that are connected via sheet flow or discrete conveyance to a stormwater system. Impervious surfaces are considered ineffective if: • The runoff is dispersed through at least 100 feet of native vegetation in accordance with BMP L614 – Full Dispersion, or • Residential Roof Runoff is infiltrated in accordance with BMPL602: Downspout Full Infiltration Systems, or • Approved continuous runoff modeling methods indicate that the entire runoff file is infiltrated. A structure of earth, gravel, or similar material raised to form a pond bank or foundation for a road. Aquatic plants that are rooted in the sediment but whose leaves are at or above the water surface. These wetland plants often have high habitat value for wildlife and waterfowl, and can aid in pollutant uptake. A channel used to safely convey flood discharges in excess of the capacity of the principal spillway. Stormwater treatment technologies that are currently being evaluated for performance or have been evaluated for performance and have achieved a use level designation following the Technology Assessment Protocol – Ecology (TAPE). Any means by which the total energy of flowing water is reduced. In stormwater design, they are usually mechanisms that reduce velocity prior to, or at, discharge from an outfall in order to prevent erosion. They include rock splash pads, drop manholes, concrete stilling basins or baffles, and check dams. The slope of the specific energy line (i.e., the sum of the potential and velocity heads). The Washington State Licensed Professional Engineer who seals the final engineering specifications, reports, drawings, plans, design information, and calculations. This is a self-sustaining soil and plant system that simultaneously supports plant growth, soil microbes, water infiltration, nutrient and pollutant adsorption, sediment and pollutant biofiltration, water interflow, and pollution decomposition. The system shall be protected from compaction and erosion. The system shall be planted and/or mulched as part of the installation. The engineered soil/plant system shall have the following characteristics: Be protected from compaction and erosion. Have a plant system to support a sustained soil quality. Possess permeability characteristics of not less than 6.0, 2.0, and 0.6 inches/hour for hydrologic soil groups A, B, and C, respectively (per ASTM D 3385). D is less than 0.6 inches/hour. Possess minimum percent organic matter of 12, 14, 16, and 18 percent (dry-weight basis) for hydrologic soil groups A, B, C, and D, respectively (per ASTM D 2974). A plan prepared and stamped by a Washington State Licensed Professional Engineer. To raise value, desirability, or attractiveness of an environment associated with surface water. Wetlands that temporarily hold water in the spring and early summer or after heavy rains. Periodically, these wetlands dry up, often in mid to late summer. Wastes, chemicals, or other substances that measurably alter the physical or chemical characteristics of runoff when exposed to rainfall. Examples include erodible soils that are stockpiled, uncovered process wastes, manure, fertilizers, oily substances, ashes, kiln dust, and materials that contribute to garbage dumpster leakage. The wearing away of the land surface by running water, wind, ice, or other geological agents, including such processes as gravitational creep. Also, detachment and movement of soil or rock fragments by water, wind, ice, or gravity. The following terms are used to describe different types of water erosion: Accelerated erosion - Erosion much more rapid than normal or geologic erosion, primarily as a result of the influence of the activities of man or, in some cases, of the animals or natural catastrophes that expose bare surfaces (e.g., overgrazing, fires). Geological erosion - The normal or natural erosion caused by geological processes acting over long geologic periods and resulting in the wearing away of mountains, the building up of floodplains, coastal plains, etc. Synonymous with natural erosion. Gully erosion - The erosion process whereby water accumulates in narrow channels and, over short periods, removes the soil from this narrow area to considerable depths, ranging from 1 to 2 feet to as much as 75 to 100 feet. Natural erosion - Wearing away of the earth's surface by water, ice, or other natural agents under natural environmental conditions of climate, vegetation, etc., undisturbed by man. Synonymous with geological erosion. Rill erosion - An erosion process in which numerous small channels only several inches deep are formed; occurs mainly on recently disturbed and exposed soils. See Rill. Sheet erosion - The removal of a fairly uniform layer of soil from the land surface by runoff. Splash erosion - The spattering of small soil particles caused by the impact of raindrops on wet soils. The loosened and spattered particles may or may not be subsequently removed by surface runoff. Any temporary or permanent measures taken to reduce erosion; control siltation and sedimentation; and ensure that sediment-laden water does not leave the site. A component of the stormwater system designed to hold water for a period of time to allow sediment contained in the surface and stormwater runoff directed to the facility to settle out , filter, or change chemically so as to improve the quality of the runoff. Wetlands where salt tolerant plant species dominate and the water regime is influenced by tidal action. The wetlands are usually partially enclosed by land with open, or partially obstructed access to open saline water. Salinity is greater than 0.5 ppt. The collective term for the processes of evaporation and plant transpiration by which water is returned to the atmosphere. A tree having foliage that persists and remains green throughout the year. Relief from the application of a Minimum Requirement to a project. Immediate pre-project legally permitted land cover condition of the project site at the time of the proposed new development or redevelopment project. Existing hard surfaces are hard surfaces that existed on the project site from the last City of Tacoma permitted development of the project site. Any material or mixture used to supply one or more of the essential plant nutrient elements. A woven or non-woven, water-permeable material generally made of synthetic products such as polypropylene and used in stormwater management and erosion and sediment control applications to trap sediment or prevent the clogging of aggregates by fine soil particles. A grassy area with gentle slopes that treats stormwater runoff from adjacent paved areas before it concentrates into a discrete channel or is discharged via sheet flow. The completion of all soil disturbing activities at the project site and the establishment of a permanent vegetative cover or equivalent permanent stabilization measure (pavement, riprap) which will prevent erosion. The process by which suspended colloidal or very fine particles are assembled into larger masses or floccules which eventually settle out of suspension. This process occurs naturally but can also be caused through the use of such chemicals as alum. An overflow or inundation that comes from a river or any other source, including (but not limited to) streams, tides, wave action, conveyance systems, or excess rainfall. Any relatively high stream flow overtopping the natural or artificial banks in any reach of a stream. Methods or facilities for reducing flood flows and the extent of flooding. That portion of the floodplain outside of the floodway which is covered by floodwaters during the base flood; it is generally associated with slower moving or standing water rather than rapidly flowing water. Those areas subject to inundation by the base flood. Includes, but is not limited to streams, lakes, wetlands, and closed depressions. The total area subject to inundation by a flood including the flood fringe and floodway. The channel of the river or stream and those portions of the adjoining floodplains that are reasonably required to carry and discharge the base flood flow. The portions of the adjoining floodplains which are considered to be "reasonably required" is defined by flood hazard regulations. A threshold discharge area (TDA) is flow control exempt if stormwater from that TDA discharges directly or indirectly to saltwater bodies or the Puyallup River and all the following restrictions are met: • Direct discharges to the saltwater body or Puyallup River does not result in the diversion of drainage from any perennial stream classified as Types 1, 2, 3, or 4 in the State of Washington Interim Water Typing System, or Types "S", "F", or "Np" in the Permanent Water Typing System, or from any category I, II, or III wetland. • If flow splitters or conveyance elements are applied to route natural runoff volumes from the TDA to any downstream Type 5 stream or category IV wetland, then: ◦ Design of the flow splitters or conveyance elements must be based on approved continuous simulation modeling analysis. The design must assure that flows delivered to Type 5 stream reaches will approximate, but in no case exceed, durations ranging from 50% of the 2-year to the 50-year peak flow. ◦ Flow splitters and conveyance elements that deliver flows to category IV wetlands must also be designed using approved continuous simulation modeling to preserve pre-project wetland hydrologic conditions unless specifically waived or exempted by regulatory agencies with permitting jurisdiction. • Stormwater and surface water from the TDA must discharge to a conveyance system that is comprised entirely of artificial conveyance elements (e.g., pipes, ditches, outfall protection) and extends to the ordinary high water line of the exempt receiving water. • The conveyance system between the TDA and the exempt receiving water shall have sufficient hydraulic capacity to convey discharges from future build-out conditions (under current zoning) from contributing areas of the Site, and the existing conditions from contributing off-site areas. • Any erodible elements of the artificial conveyance system must be adequately stabilized to prevent erosion under the conditions noted above. A project is flow control exempt if stormwater from that project discharges directly or indirectly to saltwater bodies, or the Puyallup River and all the following restrictions are met: Direct discharge to the saltwater body, or the Puyallup River does not result in the diversion of stormwater and surface water from any perennial stream classified as Types 1, 2, 3, or 4 in the State of Washington Interim Water Typing System, or Types "S", "F", or "Np" in the Permanent Water Typing System, or from any category I, II, or III wetland. If flow splitters or conveyance elements are applied to route natural runoff volumes from the project to any downstream Type 5 stream or category IV wetland, then: Design of the flow splitters or conveyance elements must be based on approved continuous simulation modeling analysis. The design must assure that flows delivered to Type 5 stream reaches will approximate, but in no case exceed, durations ranging from 50% of the 2-year to the 50-year peak flow. Flow splitters and conveyance elements that deliver flows to category IV wetlands must also be designed using approved continuous simulation modeling to preserve pre-project wetland hydrologic conditions unless specifically waived or exempted by regulatory agencies with permitting jurisdiction. Stormwater and surface water from the project (or TDA) must discharge to a conveyance system that is comprised entirely of artificial conveyance elements (e.g., pipes, ditches, outfall protection) and extends to the ordinary high water line of the exempt receiving water. The conveyance system between the project and the exempt receiving water shall have sufficient hydraulic capacity to convey discharges from future build-out conditions (under current zoning) from contributing areas of the Site, and the existing conditions from contributing off-site areas. Any erodible elements of the artificial conveyance system must be adequately stabilized to prevent erosion under the conditions noted above. A stormwater facility designed to mitigate the impacts of increased surface and stormwater flowrates generated by development. Flow control facilities are designed either to hold water for a considerable length of time and then release it by evaporation, plant transpiration, and/or infiltration into the ground, or to hold runoff for a short period of time, releasing it to the conveyance system at a controlled rate. The aggregate time that peak flows are at or above a particular flowrate of interest. For example, the amount of time that peak flows are at or above 50% of the 2-year peak flowrate for a period of record. The inverse of the probability that the flow will be equaled or exceeded in any given year (the exceedance probability). For example, if the exceedance probability is 0.01 or 1 in 100, that flow is referred to as the 100-year flow. The route that stormwater runoff follows between two points of interest. Also called a foundation drain, is a pipe or trench system located on the outer face of the building foundation wall installed to collect stormwater and surface waters away from the building. An easily maintained, extra storage area provided near an inlet of a BMP to trap incoming sediments before they accumulate in a pond or wetland BMP. Any activity conducted on or directly pertaining to forest land and relating to growing, harvesting, or processing timber, including but not limited to: Harvesting, final and intermediate. The vertical distance between the design water surface elevation and the elevation of the crest of the facility The 100-year floodplain designations of the Federal Emergency Management Agency and the National Flood Insurance Program or as defined by the local government. The anticipated period in years that will elapse, based on average probability of storms in the design region, before a storm of a given intensity and/or total volume will recur; thus a 10-year storm can be expected to occur on the average once every 10 years. Conveyance or facilities designed to handle flows that occur under such storm conditions would be expected to be surcharged by any storms of greater amount or intensity. A rectangular or cylindrical wire mesh cage (a chicken wire basket) filled with rock and used as a protecting agent, revetment, etc., against erosion. Soft gabions, often used in streambank stabilization, are made of geotextiles filled with dirt, in between which cuttings are placed. A device for registering precipitation, water level, discharge, velocity, pressure, temperature, etc. Also, a measure of the thickness of metal; e.g., diameter of wire, wall thickness of steel pipe. Areas that, because of their susceptibility to erosion, sliding, earthquake, or other geological events, are not suited to the siting of commercial, residential, or industrial development consistent with public health or safety concerns. The slope of a road, channel, or natural ground. The finished surface of a canal bed, roadbed, top of embankment, or bottom of excavation; any surface prepared for the support of construction such as paving or the laying of a conduit. To finish the surface of a canal bed, roadbed, top of embankment or bottom of excavation. An earth embankment or a ridge-and-channel constructed with suitable spacing and an acceptable grade to reduce erosion damage by intercepting surface runoff and conducting it to a stable outlet at a stable nonerosive velocity. A natural or constructed waterway, usually broad and shallow, covered with erosion-resistant grasses, used to conduct surface water from an area at a reduced flowrate. See also biofilter. Water in a saturated zone or stratum beneath the land surface or below a surface waterbody. The area surrounding a drinking water source that includes the wellhead protection area and may also include aquifer sensitive areas, sole source aquifers, groundwater management areas, or critical aquifer recharge areas. The free surface of the groundwater, that surface subject to atmospheric pressure under the ground, generally rising and falling with the season, the rate of withdrawal, the rate of restoration, and other conditions. It is seldom static. A channel caused by the concentrated flow of surface and stormwater runoff over unprotected erodible land. The specific area or environment in which a particular type of plant or animal lives. An organism's habitat must provide all of the basic requirements for life and should be protected from harmful biological, chemical, and physical alterations. A cemented or compacted and often clay-like layer of soil that is impenetrable by roots. Also known as glacial till. An impervious surface, a permeable pavement or a vegetated roof. The height of water above any plane of reference. The energy, either kinetic or potential, possessed by each unit weight of a liquid, expressed as the vertical height through which a unit weight would have to fall to release the average energy possessed. Used in various compound terms such as pressure head, velocity head, and head loss. Energy loss due to friction, eddies, changes in velocity, or direction of flow. Metals of high specific gravity, present in municipal and industrial wastes, that pose long-term environmental hazards. Such metals include antimony, arsenic, beryllium, cadmium, chromium, cobalt, copper, lead, mercury, nickel, selenium, thallium, and zinc. High-use sites are those that typically generate high concentrations of oil due to high traffic turnover or the frequent transfer of oil. High-use sites include: An area of a commercial or industrial site subject to an expected average daily traffic (ADT) count equal to or greater than 100 vehicles per 1,000 square feet of gross building area; An area of a commercial or industrial site subject to petroleum storage and transfer in excess of 1,500 gallons per year, not including routinely delivered heating oil; An area of a commercial or industrial site subject to parking, storage or maintenance of 25 or more vehicles that are over 10 tons gross weight (trucks, buses, trains, heavy equipment, etc.); A road intersection with a measured ADT count of 25,000 vehicles or more on the main roadway and 15,000 vehicles or more on any intersecting roadway, excluding projects proposing primarily pedestrian or bicycle use. The quality of saturated soil that enables water or air to move through it. Also known as permeability coefficient. The science involving the energy and forces acting on water or other liquids and the resulting impact on the motion of the liquid. A graph of runoff rate, inflow rate or discharge rate, past a specific point over time. The circuit of water movement from the atmosphere to the earth and return to the atmosphere through various stages or processes as precipitation, interception, runoff, infiltration, percolation, storage, evaporation, and transpiration. A soil characteristic classification system defined by the U.S. Soil Conservation Service in which a soil may be categorized into one of four soil groups (A, B, C, or D) based upon infiltration rate and other properties. Type A: Low runoff potential. Soils having high infiltration rates, even when thoroughly wetted, and consisting chiefly of deep, well drained to excessively drained sands or gravels. These soils have a high rate of water transmission. Type B: Moderately low runoff potential. Soils having moderate infiltration rates when thoroughly wetted, and consisting chiefly of moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission. Type C: Moderately high runoff potential. Soils having slow infiltration rates when thoroughly wetted, and consisting chiefly of soils with a layer that impedes downward movement of water, or soils with moderately fine to fine textures. These soils have a slow rate of water transmission. Type D: High runoff potential. Soils having very slow infiltration rates when thoroughly wetted, and consisting chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a hardpan, till, or clay layer at or near the surface, soils with a compacted subgrade at or near the surface, and shallow soils or nearly impervious material. These soils have a very slow rate of water transmission.1 1 Vladimir Novotny and Harvey Olem. Water Quality Prevention, Identification, and Management of Diffuse Pollution, Van Nostrand Reinhold: New York, 1994, p. 109. The science of the behavior of water in the atmosphere, on the surface of the earth, and underground. A seasonal occurrence of flooding and/or soil saturation; it encompasses depth, frequency, duration, and seasonal pattern of inundation. A graph of percentages of total precipitation for a series of time steps representing the total time in which precipitation occurs. Any spill or direct or indirect discharge to the stormwater system, groundwater, or receiving waters that is not composed entirely of stormwater or of non-stormwater discharges as allowed in Tacoma Municipal Code 12.08. A non-vegetated surface area which either prevents or retards the entry of water into the soil mantle as under natural conditions prior to development. A non-vegetated surface area which causes water to run off the surface in greater quantities or at an increased rate of flow from the flow present under natural conditions prior to development. Common impervious surfaces include, but are not limited to, rooftops, walkways, patios, driveways, parking lots or storage areas, concrete or asphalt paving, gravel roads, packed earthen materials, and oiled, macadam or other surfaces which similarly impede the natural infiltration of stormwater. Open, uncovered retention/detention facilities shall not be considered as impervious surfaces for the purposes of determining whether the thresholds for application of minimum requirements are exceeded. Open, uncovered retention/detention facilities shall be considered impervious surfaces for purposes of runoff modeling. Streets (with or without curbs or gutters), sidewalks, crosswalks, parking lots, water mains, wastewater and stormwater pipes, stormwater facilities, street trees and other appropriate items. Stormwater discharges that do not discharge directly into a waterbody, but travel through other conveyance systems before reaching that waterbody. Material handling, transportation, or storage; manufacturing; maintenance; treatment; or disposal. Areas with industrial activities include plant yards, access roads and rail lines used by carriers of raw materials, manufactured products, waste material, or by-products; material handling sites; refuse sites; sites used for the application or disposal of process waste waters; sites used for the storage and maintenance of material handling equipment; sites used for residual treatment, storage, or disposal; shipping and receiving areas; manufacturing buildings; storage areas for raw materials, and intermediate and finished products; and areas where industrial activity has taken place in the past and significant materials remain and are exposed to stormwater. Those sites zoned industrial or with industrial activities. These include areas that manufacture, process, or store raw materials. Means the downward movement of water from the surface to the subsoil. A stormwater facility designed to use the hydrologic process of surface and stormwater runoff soaking into the ground, commonly referred to as a percolation, to dispose of surface and stormwater runoff. The rate, usually expressed in inches/hour, at which water moves downward (percolates) through the soil profile. Short-term infiltration rates may be inferred from soil analysis or texture or derived from field measurements. Long-term infiltration rates are affected by variability in soils and subsurface conditions at the site, the effectiveness of pretreatment or influent control, and the degree of long-term maintenance of the infiltration facility. A form of connection between surface of the ground and an above or below ground stormwater system for the admission of surface and stormwater runoff. A substance, usually chemical, that is used to kill insects. A natural, long-term ecologically based approach to controlling pest populations. The process by which precipitation is caught and held by foliage, twigs, and branches of trees, shrubs, and other vegetation. Often used for "interception loss" or the amount of water evaporated from the precipitation intercepted. That portion of rainfall that infiltrates into the soil and moves laterally through the upper soil horizons until intercepted by a stream channel or until it returns to the surface for example, in a roadside ditch, wetland, spring or seep. Interflow is a function of the soil system depth, permeability, and water-holding capacity. A stream or portion of a stream that flows only in direct response to precipitation. It receives little or no water from springs and no long-continued supply from melting snow or other sources. It is dry for a large part of the year, ordinarily more than three months. Nonnative organisms whose introduction causes or is likely to cause economic or environmental harm or harm to human, animal or plant health. The interval between the center of mass of the storm precipitation and the peak flow of the resultant runoff. A natural area permanently inundated by water in excess of two meters deep and greater than 20 acres in size as measured at the ordinary high water marks. Wetlands that are on the water side of the Ordinary High Water Mark (OHWM) of lakes where the area of open water next to a vegetated wetland is larger than 20 ac (8ha), and more than 6.6 feet deep over 30% of the open water areas. Any activity that results in a change in the existing soil cover (both vegetative and non-vegetative) and/or existing soil topography. Land disturbing activities include, but are not limited to clearing, grading, filling, and excavation. Compaction that is associated with stabilization of structures and road construction shall also be considered a land disturbing activity. Vegetation maintenance practices, including landscape maintenance and gardening, are not considered land disturbing activity if conducted according to established standards and procedures. Stormwater BMP/facility maintenance is not considered land disturbing activity if conducted according to established standards and procedures. An area of land that has a specified boundary used for planning purposes that defines an area of interrelated physical, chemical, and biological processes. A watershed or drainage basin is a common type of landscape unit. A ground water aquifer is another type of landscape unit. Vegetation, including but not limited to, annuals, woody and herbaceous perennials such as shrubs, vines, or trees that are regularly and/or systematically maintained through a combination of pruning, mowing, watering, trimming, fertilizing, and other activities. Landscaped areas are typically used for aesthetic purposes. Episodic downslope movement of a mass of soil or rock that includes but is not limited to rockfalls, slumps, mudflows, and earthflows. For the purpose of these rules, snow avalanches are considered to be a special case of landsliding. An area of land planted with grasses or other durable plants which are maintained at a short height and used for aesthetic and/or recreational purposes. The definition also includes turf surfaces, artificial lawn surfaces, and artificial turf surfaces. Those substances that, when exposed to rainfall, measurably alter the physical or chemical characteristics of the rainfall runoff. Examples include erodible soils, uncovered process wastes, manure, fertilizers, oil substances, ashes, kiln dust, and materials contributing to garbage dumpster leakage. Any liquid that in passing through matter, extracts solutes, suspended solids or any other component of the material through which it has passed. Removal of the more soluble materials from the soil or other material by percolating waters. A member of the legume or pulse family, Leguminosae, one of the most important and widely distributed plant families. The fruit is a "legume" or pod. Includes many valuable food and forage species, such as peas, beans, clovers, alfalfas, sweet clovers, and vetches. Practically all legumes are nitrogen-fixing plants. A device used to spread out stormwater runoff uniformly over the ground surface as sheet flow (i.e., not through channels). The purpose of level spreaders is to prevent concentrated, erosive flows from occurring, and to enhance infiltration. Any county, city, town, or special purpose district having its own incorporated government for local affairs. A stormwater and land use management strategy that strives to mimic predisturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration by emphasizing conservation, use of onsite natural features, site planning, and distributed stormwater management practices that are integrated into a project design. Distributed stormwater management practices, integrated into a project design, that emphasize predisturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration. LID BMPs include, but are not limited to, bioretention, rain gardens, permeable pavements, roof downspout infiltration and dispersion, dispersion, soil quality and depth, minimal excavation foundations, vegetated roofs, and water reuse. Land use management strategies that emphasize conservation, use of onsite natural features, and site planning to minimize impervious surfaces, native vegetation loss and stormwater runoff. A layer of compacted till, compacted clay, concrete, or a geomembrane. Repair and maintenance includes activities conducted on currently serviceable structures, facilities, and equipment that involves no expansion or use beyond that previously existing and results in no significant adverse hydrologic impact. It includes those usual activities taken to prevent a decline, lapse, or cessation in the use of structures and systems and includes replacement of dysfunctioning facilities, including cases where environmental permits require replacing an existing structure with a different type structure, as long as the functioning characteristics of the original structure are not changed. For example, replacing a collapsed, fish blocking, round culvert with a new box culvert under the same span, or width, of roadway. For stormwater facilities, maintenance includes assessment to ensure ongoing proper operation, removal of built up pollutants (i.e., sediments), replacement of failed or failing treatment media, and other actions taken to correct deficits as identified in the maintenance standards, Volume 4. See exemptions for Pavement Maintenance, Volume 1. An equation used to predict the velocity of water flow in an open channel or pipelines: V is the mean velocity of flow in feet per second R is the hydraulic radius in feet S is the slope of the energy gradient or, for assumed uniform flow, the slope of the channel in feet per foot; and n is Manning's roughness coefficient or retardance factor of the channel lining. A tree that has achieved at least 75% of its full canopy growth or a tree that is over 15 years of age. Refers to paragraph 402(p)(3)(B)(iii) of the federal Clean Water Act which reads as follows: Permits for discharges from municipal storm sewers shall require controls to reduce the discharge of pollutants to the maximum extent practicable, including management practices, control techniques, and system design and engineering methods, and other such provisions as the Administrator or the State determines appropriate for the control of such pollutants. Measure the maximum water level (e.g., with a crest stage gage, Reinelt and Horner 1990) and the existing water level at the time of the site visit (e.g., with a staff gage) on at least eight occasions spread through a year. Take the difference of the maximum and existing water level on each occasion and divide by the number of occasions. Average depth; cross-sectional area of a stream or channel divided by its surface or top width. The average velocity of a stream flowing in a channel or conduit at a given cross-section or in a given reach. It is equal to the discharge divided by the cross-sectional area of the reach. Elements, such as mercury, lead, nickel, zinc and cadmium, which are of environmental concern because they do not degrade over time. Although many are necessary nutrients, they are sometimes magnified in the food chain, and they can be toxic to life in high enough concentrations. They are also referred to as heavy metals. The lower trophic levels of the soil food web. They are normally considered to include bacteria, fungi, flagellates, amoebae, ciliates, and nematodes. These in turn support the higher trophic levels, such as mites and earthworms. Together they are the basic life forms that are necessary for plant growth. Soil microbes also function to bioremediate pollutants such as petroleum, nutrients, and pathogens. To lessen known impacts to the environment. Mitigation should be conducted in the following order of preference: Avoiding the impact altogether by not taking a certain action or part of an action; Minimizing impacts by limiting the degree or magnitude of the action and its implementation, by using appropriate technology, or by taking affirmative steps to avoid or reduce impacts; Rectifying the impact by repairing, rehabilitating or restoring the affected environment; Reducing or eliminating the impact over time by preservation and maintenance operations during the life of the action; and Compensating for the impact by replacing, enhancing, or providing substitute resources or environments. A change to a design standard that is not considered an Exception or Adjustment. A wetland whose physical, hydrological, or water quality characteristics have been purposefully altered for a management purpose, such as by dredging, filling, forebay construction, and inlet or outlet control. To systematically and repeatedly measure something in order to track changes. The collection of data by various methods for the purposes of understanding natural systems and features, evaluating the impacts of development proposals on such systems, and assessing the performance of mitigation measures imposed as conditions of development. A layer of organic material or aggregate applied to the surface of soil. Its purpose is any or all of the following: To conserve soil moisture or temperature To improve the fertility and health of the soil To hold fertilizer, seed, and soil in place. To enhance aesthetics of the facility. Types of mulch include: chipped site vegetation, compost, hydromulch, wood-based mulch or wood straw, wood strand, straw, and aggregate. A parcel that contains 4 or more residential dwelling units. The part of the federal Clean Water Act, which requires point source dischargers to obtain permits. These permits are referred to as NPDES permits and, in Washington State, are administered by the Washington State Department of Ecology. An easement granted for the protection of native vegetation within a sensitive area or its associated buffer. The NGPE shall be recorded on the appropriate documents of title and filed with the County Assessor. The soil that is present before any construction associated with the project begins on the site. The infiltration rate of the soil that is present before construction associated with the project begins on the site. Vegetation comprised of plant species, other than noxious weeds, that are indigenous to the coastal region of the Pacific Northwest and which reasonably could have been expected to naturally occur on the site. Examples include trees such as Douglas fir, Western Hemlock, Western Red Cedar, Alder, Big-leaf Maple, and Vine Maple; shrubs such as willow, elderberry, salmonberry and salal; and herbaceous plants such as sword fern, foam flower, and fireweed. Land on which the existing plants arose naturally, or were planted intentionally to mimic natural processes. The location of those channels, swales, and other non-artificial conveyance systems as defined by the first documented topographic contours existing for the subject property, either from maps or photographs, or such other means as appropriate. Based upon site geology and the infiltration capability of the underlying soils, there may be no discernible surface discharge. Land Disturbing Activities, including Class IV – general forest practices that are conversions from timber land to other uses Structural development, including construction or installation of a building or other structure. Subdivision, short subdivision, and binding site plans as defined and applied in Chapter 58.17 RCW. Site Plan Approval per Tacoma Municipal Code 13. Projects meeting the definition of redevelopment shall not be considered new development A newly created impervious surface, permeable pavement, or vegetated roof. On redevelopment sites, the expansion of a structure over an existing impervious asphalt or concrete surface will not be considered replaced impervious surfaces and will not count toward project thresholds. See exemptions for Pavement Maintenance, Volume 1, section 1.2.2. A newly created impervious surface. Creation of an impervious surface shall include: Construction of new hard surfaces Extending the pavement edge without increasing the size of the road prism Upgrading from dirt to gravel, bituminous surface treatment, asphalt, or concrete Upgrading from gravel to bituminous surface treatment, asphalt or concrete Upgrading from bituminous surface treatment to asphalt or concrete. On redevelopment sites the following apply: For structures, the construction of a structure over an existing impervious asphalt or concrete surface will not be considered new or replaced impervious surfaces and will not be counted toward project thresholds if the existing underlying surface remains in place and the existing underlying surface is not a foundation. Construction of a structure over an existing foundation will be considered a replaced impervious surface per the definition of Replaced Impervious Surface. Asphalt or concrete overlays will not be considered a new or replaced impervious surface and will not be counted toward project thresholds. If non-porous asphalt or concrete is laid over existing permeable pavements, the asphalt or concrete will be considered a new impervious surface and the stormwater mitigation provided by the existing permeable surface shall be replaced. Usually ammonium, nitrite, and nitrate ions, and certain simple amines available for plant growth. A small fraction of organic or total nitrogen in the soil is available at any time. Pollution that enters a waterbody from diffuse origins on the watershed and does not result from discernible, confined, or discrete conveyances. Essential chemicals needed by plants or animals for growth. Excessive amounts of nutrients can lead to degradation of water quality and algal blooms. Some nutrients can be toxic at high concentrations. Stormwater facilities to which stormwater runoff is restricted to some maximum flowrate or volume by a flow-splitter. Any area lying upstream of the site that drains onto the site and any area lying downstream of the site to which the site drains. A surface treatment that creates a permanent unyielding non-skid roadway similar to asphalt or concrete and is considered a City of Tacoma Standard Roadway Design. When overlaying an oil mat surface with asphalt or concrete, the surface will not be considered a new or replaced surface and will not count toward the project thresholds. See Volume 1, section 1.2.2 for Pavement Maintenance Exemptions. A vault, usually underground, designed to provide a quiescent environment to separate oil from water. Stormwater facilities which receive all of the stormwater runoff from a contributing area. Flows above the water quality design flowrate or volume are passed through at a lower percent removal efficiency. Best management practices intended to manage stormwater runoff onsite where feasible. Operational BMPs are a type of Source Control BMP. They are schedules of activities, prohibition of practices, and other managerial practices to prevent or reduce pollutants from entering stormwater. Operational BMPs include formation of a pollution prevention team, good housekeeping, preventive maintenance procedures, spill prevention and clean-up, employee training, inspections of pollutant sources and BMPs, and record keeping. They can also include process changes, raw material/product changes, and recycling wastes. The term ordinary high water mark means the line on the shore established by the fluctuations of water and indicated by physical characteristics such as a clear, natural line impressed on the bank; shelving; changes in the character of soil destruction on terrestrial vegetation, or the presence of litter and debris; or other appropriate means that consider the characteristics of the surrounding area. The ordinary high water mark will be found by examining the bed and banks of a stream and ascertaining where the presence and action of waters are so common and usual, and so long maintained in all ordinary years, as to mark upon the soil a character distinct from that of the abutting upland, in respect to vegetation. In any area where the ordinary high water mark cannot be found, the line of mean high water shall substitute. In any area where neither can be found, the channel bank shall be substituted. In braided channels and alluvial fans, the ordinary high water mark or substitute shall be measured so as to include the entire stream feature. Organic matter as decomposed animal or vegetable matter. It is measured by ASTM D 2974. Organic matter is an important reservoir of carbon and a dynamic component of soil and the carbon cycle. It improves soil and plant efficiency by improving soil physical properties including infiltration capability, aeration, and other structural characteristics. It contains the nutrients, microbes, and higher-form soil food web organisms necessary for plant growth. The maturity of organic matter is a measure of its beneficial properties. Raw organic matter can release water-soluble nutrients (similar to chemical fertilizer). Beneficial organic matter has undergone a humification process either naturally in the environment or through a composting process. An opening with closed perimeter, usually sharp-edged, and of regular form in a plate, wall, or partition through which water may flow, generally used for the purpose of measurement or control of water. A point source as defined by 40 CFR 122.2 at the point where a discharge leaves the Permittee’s MS4 and enters a surface receiving waterbody or surface receiving waters. Outfall does not include pipes, tunnels, or other conveyances which connect segments of the same stream or other surface waters and are used to convey primarily surface waters (i.e., culverts). Point of water disposal from a stream, river, lake, tidewater, or artificial conveyance system. A waterway constructed or altered primarily to carry water from artificial structures, such as terraces, tile lines, and diversions. A channel or device that allows that portion of flow above the design flowrate to discharge downstream. Placement of an additional layer of asphalt or concrete over existing asphalt or concrete. An overlay can be done when the existing asphalt or concrete is in overall good condition with minor problem areas (e.g, severe cracking, crumbling, sinking, or wavy). To flow over the limits of a containment or conveyance element. The effective diameter of a particle as measured by sedimentation, sieving, or micrometric methods. A program employing a network level, long-term strategy that enhances pavement performance by using an integrated, cost-effective set of practices that extend pavement life, improve safety and meet motorist expectations. Pavement preservation includes pavement maintenance activities (such as crack sealing, chip sealing, concrete joint sealing), minor rehabilitation activities (such as grind and overlay, overlay), and routine or corrective maintenance activities (such as pothole patching, square patching, overlays, maintenance of pavement markings). The maximum instantaneous rate of flow during a storm, usually in reference to a specific design storm event. Unique, irreplaceable bogs and fens that can exhibit pH in a wide range from highly acidic to alkaline. Wetlands where at least a portion of their area has permanent surface water (i.e., flooded or inundated throughout the year), in a normal water year or wetter. Pervious concrete, porous asphalt, permeable pavers, permeable interlocking concrete pavements, or other forms of pervious or porous paving material intended to allow passage of water through the pavement section. It often includes an aggregate base that provides structural support and acts as a stormwater reservoir. Soil materials with a sufficiently rapid infiltration rate so as to greatly reduce or eliminate surface and stormwater runoff. These soils are generally classified as SCS hydrologic soil types A and B. Any individual, partnership, co-partnership, firm, company, association, joint stock company, trust, estate, society, corporation, group, government, governmental agency or other legal entity, or their legal representatives, agents or assigns. This definition includes all federal, state, and local government entities. Any surface material that allows stormwater to infiltrate into the ground. Examples include lawn, landscape, pasture, native vegetation areas, and permeable pavements. A general term used to describe any substance - usually chemical - used to destroy or control organisms; includes herbicides, insecticides, algicides, fungicides, and others. Many of these substances are manufactured and are not naturally found in the environment. Others, such as pyrethrum, are natural toxins that are extracted from plants and animals. The negative logarithm of the effective hydrogen ion concentration or hydrogen activity in gram equivalents per liter used in expressing both acidity and alkalinity on a scale whose values run from 0 to 14, with 7 representing neutrality, numbers less than 7 increasing acidity and numbers greater than 7 increasing alkalinity. Characteristics of the natural physical environment (including hills). A map or representation of a subdivision showing the division of a tract or parcel of land into lots, blocks, streets, or other divisions and dedications. The release of collected and/or concentrated surface and stormwater runoff from a pipe, culvert, or channel. The location at which compliance with a discharge performance standard or a receiving water quality standard is measured. Additional treatment of a waste stream that has already received one or more stages of treatment by other means. Contamination or other alteration of the physical, chemical, or biological properties, of waters of the state, including change in temperature, taste, color, turbidity, or odor of the waters, or such discharge of any liquid, gaseous, solid, radioactive or other substance into any waters of the state as will or is likely to create a nuisance or render such waters harmful, detrimental or injurious to the public health, safety or welfare, or to domestic, commercial, industrial, agricultural, recreational, or other legitimate beneficial uses, or to livestock, wild animals, birds, fish or other aquatic life. Those hard surfaces considered to be a significant source of pollutants in stormwater runoff. PGHS includes permeable pavement subject to vehicular use. See the listing of surfaces under pollution generating impervious surfaces. Those impervious surfaces considered to be a significant source of pollutants in stormwater runoff. Such surfaces include: those which are subject to vehicular use (further defined in the glossary); those which are subject to industrial activities (further defined in the glossary); those which are subject to storage of erodible or leachable materials, wastes, or chemicals, and which receive direct rainfall or the runon or blow-in of rainfall; metal roofs unless they are coated with an inert, non-leachable material; roof that are subject to venting significant amounts of dusts, mists, or fumes from manufacturing, commercial, or other indoor activities. Any non-impervious surface subject to: industrial activities (as further defined in the glossary); storage of erodible or leachable materials, wastes, or chemicals, and which receive direct rainfall or the runon or blow-in of rainfall; use of pesticides and fertilizers; or Typical PGPS include lawns and landscaped areas including: golf courses, parks, cemeteries, and sports fields (natural and artificial turf). The native vegetation and soils that existed at a site prior to the influence of Euro-American settlement. Function performed by a BMP to remove at least 50% of total suspended solids. Unique, irreplaceable fens that can exhibit water pH in a wide range from highly acidic to alkaline, including fens typified by Sphagnum species, Ledum groenlandicum (Labrador tea), Drosera rotundifolia (sundew), and Vaccinium oxycoccos (bog cranberry); marl fens; estuarine peat deposits; and other moss peat systems with relatively diverse, undisturbed flora and fauna. Bog is the common name for peat systems having the Sphagnum association described, but this term applies strictly only to systems that receive water income from precipitation exclusively. Any proposed action to alter or develop a project site. The proposed action of a permit application or an approval which requires stormwater mitigation review. Common Plans of Development (definition in glossary) Land Use Actions, excluding Boundary Line Adjustments (BLAs) New development or redevelopment on contiguous or non-contiguous parcels that are permitted under a single permit number or that are part of a subdivision regardless of ownership. Other City departmental conditions and review (e.g. offsite improvements are imposed on multiple parcels) Single Family/Duplex Projects can be defined by: Land Use Actions, excluding Boundary Line Adjustments New development or redevelopment on contiguous or non-contiguous parcels that are permitted under a single permit number or that are part of a subdivision regardless of ownership. Other City departmental conditions and review (e.g. offsite improvements are imposed on multiple parcels) Land use actions that would affect if a proposed development is a project include, without limitation, plats, short plats, site specific rezones, wetland development permits, conditional use permits, shoreline development permits, and SEPA, if the intent of those land use actions is to develop the affected parcel, parcels or right of way. All other land use permits may create a project depending upon the project scope proposed in the land use action. Conveyances into different ownership for the ostensible purpose of avoiding more comprehensive stormwater review and requirements, or where an innocent conveyance has this effect, may be considered to be part of a project and reviewed by Environmental Services/Site Development Group for cumulative impacts. Per Joint Administrative Policy and Procedure Directive No. 2021-02-001, Environmental Services/Site Development Group reserves the right to make the final determination of what is considered to be the project. That portion of a property, properties, or right of way subject to land disturbing activities, new hard surfaces, or replaced hard surfaces. On-site and associated off-site improvements shall be added together when determining if a project site exceeds a threshold. Per Joint Administrative Policy and Procedure Directive No. 2021-02-001, Environmental Services/Site Development Group reserves the right to make the final determination of the project site. Equivalent to engineered soil/landscape system. This can also be a natural system that has not been disturbed or modified. Puget Sound south of Admiralty Inlet (including Hood Canal and Saratoga Passage); the waters north to the Canadian border, including portions of the Strait of Georgia; the Strait of Juan de Fuca south of the Canadian border; and all the lands draining into these waters as mapped in Water Resources Inventory Areas numbers 1 through 19, set forth in WAC 173-500-040. A non-engineered shallow landscaped depression, with compost-amended native soils and adapted plants. The depression is designed to pond and temporarily store stormwater runoff from adjacent areas, and to allow stormwater to pass through the amended soil profile. Refer to the Rain Garden Handbook for Western Washington Homeowners for additional information, http://raingarden.wsu.edu Plant or animal species that are regionally relatively uncommon, are nearing endangered status, or whose existence is in jeopardy and is usually restricted to highly specific habitats. Threatened, endangered, or sensitive species are officially listed by federal and state authorities, whereas rare species are unofficial species of concern that fit the above definitions. A means of computing stormwater flowrates (Q) by use of the formula Q = CIA, where C is a coefficient describing the physical contributing area, I is the rainfall intensity and A is the area. Naturally and/or reconstructed naturally occurring surface waterbodies, such as creeks, streams, rivers, lakes, wetlands, estuaries, and marine waters, or groundwater, to which a stormwater system discharges. The addition of water to the zone of saturation (i.e., an aquifer). As used for compliance with Minimum Requirement #3 - Source Control, recommended BMPs (also called Recommended Additional BMPs) are those BMPs that are not expected to be mandatory by local governments at new development and redevelopment sites. However, they may improve pollutant control efficiency, and may provide a more comprehensive and environmentally effective stormwater management program. On a site that is already substantially developed (has 35% or more of existing hard surface coverage - when determining percentage only include those areas that are buildable): • The creation or addition of hard surfaces • The expansion of a building footprint or addition or replacement of a structure • Structural development including construction, installation or expansion of a building or other structure • Replacement of hard surface that is not part of a routine maintenance activity An action (here, for stormwater management purposes) that involves more than one discrete property. A stormwater facility designed to provide stormwater treatment or flow control for a large region or portion of a basin or subbasin. The manipulation of the physical, chemical, or biological characteristics of a site with the goal of repairing natural or historic hydrologic functions and processes of a degraded wetland. Rehabilitation results in a gain in wetland function but does not result in a gain in wetland area. The computed peak rate of surface and stormwater runoff from a site. Areas where the impervious surface covering (i.e. building, pavement, gravel, etc.) has been removed, the soil has been amended in accordance with the BMP L613: Post Construction Soil Quality and Depth and the area is left as a permanent pervious surface (landscaping, natural areas) and planted with native vegetation including evergreen trees. Removed impervious surfaces are not required to be added to the impervious area totals to determine compliance with Minimum Requirements #6, #7 and #8. For structures, the removal and replacement of hard surfaces down to or including the foundation. For other hard surfaces, the removal down to bare soil or top of base course layer and replacement, including replacement as required for repairing the base course layer. On redevelopment sites, the construction of a structure over an existing asphalt or concrete surface will not be considered new or replaced impervious surfaces and will not count toward project thresholds if the existing underlying asphalt or concrete surface remains in place and undisturbed. See exemptions for Pavement Maintenance, Volume 1, section 1.2.2. For structures, the removal and replacement of impervious surfaces down to or including the foundation. For other impervious surfaces, the removal down to bare soil or top of base course layer and replacement, including replacement as required for repairing the base course layer. On redevelopment sites, the construction of a structure over an existing asphalt or concrete surface will not be considered new or replaced impervious surfaces and will not count toward project thresholds if the existing underlying asphalt or concrete surface remains in place and undisturbed. See exemptions for Pavement Maintenance, Volume 1, section 1.2.2. Actions performed to reestablish wetland functional characteristics and processes that have been lost by alterations, activities, or catastrophic events in an area that no longer meets the definition of a wetland. Also known as a grind and overlay where the existing asphalt or concrete surface is milled or ground down to remove the damaged surface. An asphalt or concrete overlay is then placed. If material must be removed to the top of the base course layer, the activity is considered replacement of a hard surface and subject to the Minimum Requirements as applicable for replaced surfaces. The process of collecting and holding surface and stormwater runoff with no surface outflow. A type of stormwater facility designed either to hold water for a considerable length of time and then release it by evaporation, plant transpiration, and/or infiltration into the ground; or to hold surface and stormwater runoff for a short period of time and then release it to the surface and stormwater management system. The renovation of an existing structure or facility to meet changed conditions or to improve performance. A statistical term for the average time of expected interval that an event of some kind will equal or exceed given conditions (e.g., a stormwater flow that occurs every 2 years) A small intermittent watercourse with steep sides, usually only a few inches deep. Often rills are caused by an increase in surface water flow when soil is cleared of vegetation. A facing layer or protective mound of rocks placed to prevent erosion or sloughing of a structure or embankment due to flow of surface and stormwater runoff. Pertaining to the banks of streams, wetlands, lakes, or tidewater. A vertical pipe extending from the bottom of a pond BMP that is used to control the discharge rate from a BMP for a specified design storm. Wetlands that retain surface water significantly longer than the duration of the flood event. Riverine impounded wetlands tend to hold water for more than a week after a flood event. These wetlands are found in a topographic depression on the valley floor, or in areas where natural or human made barriers to downstream flow occur. Riverine wetlands occur in valleys associated with streams or river channels. They lie in the active floodplains of a river, and have important hydrologic links to the water dynamics of the river or stream. The distinguishing characteristic of riverine wetlands in Washington is that they are frequently flooded by overbank flow from the stream or river. A project whose objective is the construction or maintenance of elements within the roadway section or right-of-way including the driving surface, sidewalks, bike paths, and pedestrian paths. Sidewalks, bike paths, and pedestrian paths must be associated with an abutting or adjacent driving surface. Roadway elements built as a requirement for onsite actions and permit issuance are not included in this category. The area containing the road surface (including shoulders and curbs), cut slope and fill slope. Water originating from rainfall and other precipitation that is found in stormwater facilities, rivers, streams, springs, seeps, ponds, lakes and wetlands as well as shallow groundwater. As applied in this manual, it also means the portion of rainfall or other precipitation that becomes surface flow and interflow. A member of the fish family Salmonidae. Chinook, coho, chum, sockeye and pink salmon; cutthroat, brook, brown, rainbow, and steelhead trout; Dolly Varden, kokanee, and char are examples of salmonid species. An artificial depression or basin with a layer of sand or a sand layer within another facility that treats stormwater as it percolates through the sand and is discharged via a central collector pipe or allowed to infiltrate into the native soil. Erosion of channel banks due to excessive velocity of the flow of surface and stormwater runoff. A single event hydrologic analysis technique for estimating runoff based on the curve number approach developed by the USDA Soil Conservation Service (SCS) – now called Natural Resource Conservation Service (NCRS). The highest annual groundwater elevation as determined by a Washington State Licensed Professional Engineer, Washington State Licensed Professional Geologist, a Certified Professional Soil Scientist, or a Washington State Licensed Onsite Wastewater Treatment System Designer. Refer to specific BMPs for the method(s) and professional that can be used to determine the seasonal high groundwater level. A wetland that has water above the soil surface for a period of time (usually between two months to less than one year) during and/or after the wettest season but in typical years dries to or below the soil surface in warmer, drier weather. Fragmented material that originates from weathering and erosion of rocks or unconsolidated deposits, and is transported by, suspended in, or deposited by water. Those suspended solids in stormwater that separate by settling when the stormwater is held in a quiescent condition for a specified time. The relatively uniform removal of soil from an area without the development of conspicuous water channels. Runoff that flows over the ground surface as a thin, even layer, not concentrated in a channel. The proposed project as regulated by the Shoreline Management Act. Usually the construction over water or within a shoreline zone (generally 200 feet landward of the water) of structures such as buildings, piers, bulkheads, and breakwaters, including environmental alterations such as dredging and filling, or any project which interferes with public navigational rights on the surface waters. The passage of runoff through a BMP in less than the design treatment time. The process by which a river, lake, or other waterbody becomes clogged with sediment. Silt can clog gravel beds and prevent successful salmon spawning. The legal boundaries of a parcel or parcels of land that is (are) subject to new development or redevelopment. For road projects, the length of the project site and the right-of-way boundaries define the site. Also known as Low Impact Development Principles are land use management strategies that emphasize conservation, use of onsite natural features, and site planning to minimize impervious surfaces, native vegetation loss, and stormwater runoff. Site appropriate development principles include: Minimization of land disturbance by fitting development to the natural terrain Minimization of land disturbance by confining construction to the smallest area feasible and away from critical areas Preservation of natural vegetation Degree of deviation of a surface from the horizontal; measured as a numerical ratio, percent, or in degrees. Expressed as a ratio, the first number is the horizontal distance (run) and the second is the vertical distance (rise), as 2:1. A 2:1 slope is a 50 percent slope. Expressed in degrees, the slope is the angle from the horizontal plane, with a 90° slope being vertical (maximum) and 45° being a 1:1 or 100 percent slope. Slope wetlands occur on slopes where groundwater surfaces and begins running along the surface, or immediately below the surface. Water in these wetlands flows only in one direction (down the slope) and the gradient is steep enough that the water is not impounded. The downhill side of the wetland is always the point of lowest elevation in the wetland. The sliding of overlying material. It is the same effect as caving, but it usually occurs when the bank or an underlying stratum is saturated or scoured. The unconsolidated mineral and organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. See also topsoil, engineered soil/landscape system, and properly functioning soil system. Activities affected by construction activity including clearing, grading, excavation, and any other activity that disturbs the surface of the land including ingress/egress from the site. A classification of soils by the Soil Conservation Service into four runoff potential groups. The groups range from A soils, which are very permeable and produce little or no runoff, to D soils, which are not very permeable and produce much more runoff. A layer of soil, approximately parallel to the surface, which has distinct characteristics produced by soil-forming factors. A vertical section of the soil from the surface through all horizons, including C horizons. The relation of particles or groups of particles which impart to the whole soil a characteristic manner of breaking; some types are crumb structure, block structure, platy structure, and columnar structure. The ease with which gases, liquids, or plant roots penetrate or pass through a layer of soil. The use of measures such as rock lining, vegetation or other engineering structures to prevent the movement of soil when loads are applied to the soil. The physical or chemical binding of pollutants to sediment or organic particles. A structure or operation that is intended to prevent pollutants from coming into contact with stormwater through physical separation of areas or careful management of activities that are sources of pollutants. This manual separates source control BMPs into two types. Structural source control BMPs are physical, structural, or mechanical devices or facilities that are intended to prevent pollutants from entering stormwater. Operational BMPs are non-structural practices that prevent or reduce pollutants from entering stormwater. See Volume 6 for details. A Tee section or turn down elbow designed to retain a limited volume of pollutant that floats on water, such as oil or antifreeze. Spill control devices are passive and must be cleaned-out for the spilled pollutant to actually be removed. A passage such as a paved or otherwise stabilized apron or channel for surplus water over or around a dam or similar obstruction. An open or closed channel, or both, used to convey excess water from a reservoir. It may contain gates, either manually or automatically controlled, to regulate the discharge of excess water. A post-project departure, either higher or lower, from the water depth existing under a given set of conditions in the pre-development state. The time interval between major storms of predetermined intensity and volumes of runoff for which stormwater conveyance systems and other structures are designed and constructed to handle hydraulically without surcharging and backflooding, e.g., a 2-year, 10-year or 100-year storm. That portion of precipitation, including snowmelt, that does not naturally percolate into the ground or evaporate, but flows via overland flow, interflow, pipes and other features of a stormwater system into a receiving water or stormwater facility. Water originating from rainfall and other precipitation, including shallow groundwater that is found in the stormwater system, rivers, streams, springs, seeps, ponds, lakes, and wetlands. Constructed and natural features which function together as a system to collect, convey, channel, hold, inhibit, retain, detain, infiltrate, divert, treat or filter stormwater and surface waters. A component of the stormwater system constructed to perform a particular function such as onsite stormwater management, stormwater treatment, flow control, or conveyance. An opening that connects stormwater and surface water from the ground surface to an underground or above-ground conveyance system. Stormwater Management Manual for Western Washington (Stormwater Manual) The manual prepared by Ecology which contains BMPs to prevent, control or treat pollution in stormwater and reduce other stormwater-related impacts to waters of the State. The Stormwater Manual is intended to provide guidance on measures necessary in western Washington to control the quantity and quality of stormwater runoff from new development and redevelopment. This manual is equivalent to the Ecology manual and must be used for projects located within the City of Tacoma. A comprehensive report and drawing set containing all the technical information and analysis necessary to evaluate projects for compliance with the stormwater requirements. Those areas where surface waters flow sufficiently to produce a defined channel or bed. Lands and waters contained within a channel which supports hydrophytes and where the substrate is predominantly undrained hydric soils, nonsoil and/or is saturated with water or covered by water each growing season. The channel or bed need not contain water year-round. This definition is not meant to include irrigation ditches, canals, stormwater runoff devices or other entirely artificial watercourses unless they are used to convey streams naturally occurring prior to construction. Those topographic features that resemble streams but have no defined channels (i.e. swales) shall be considered streams when hydrologic and hydraulic analyses done pursuant to a development proposal predict formation of a defined channel after development. Any artificial item. Examples include catch basins, manholes, buildings, decks, etc. Physical, structural, or mechanical devices or facilities that are intended to prevent pollutants from entering stormwater. Structural source control BMPs typically include: Enclosing and/or covering the pollutant source (building or other enclosure, a roof over storage and working areas, temporary tarp, etc.). Segregating the pollutant source to prevent run-on of stormwater, and to direct only contaminated stormwater to appropriate treatment BMPs. A short length of pipe provided for future connection to the stormwater system. A drainage area that drains to a water-course or waterbody named and noted on common maps and which is contained within a basin. A layer of stone or soil used as the underlying base for a BMP or other facility. The B horizons of soils with distinct profiles. In soils with weak profile development, the subsoil can be defined as the soil below the plowed soil (or its equivalent of surface soil), in which roots normally grow. Although a common term, it cannot be defined accurately. It has been carried over from early days when "soil" was conceived only as the plowed soil and that under it as the "subsoil." The flow condition occurring in closed conduits when the hydraulic grade line is above the crown of the pipe. Waterbodies, such as creek, streams, rivers, lakes, wetlands, estuaries, and marine waters. Organic or inorganic particles that are suspended in and carried by the water. The term includes sand, mud, and clay particles (and associated pollutants) as well as solids in stormwater. A shallow conveyance with relatively gentle side slopes, generally with flow depths less than one foot. An embankment or combination of an embankment and channel across a slope to control erosion by diverting or storing surface runoff instead of permitting it to flow uninterrupted down the slope. An onsite area draining to a single natural or constructed discharge location or multiple natural or constructed discharge locations that converge within one-quarter mile downstream (as determined by the shortest flowpath). The examples in Figure G - 1: Threshold Discharge Area illustrate this definition. The purpose of this definition is to clarify how the thresholds of this manual are applied to project sites with multiple discharge locations. When determining which Minimum Requirements apply to a project, it shall be assumed that stormwater leaves the project site (ex. an area that infiltrates is not considered a separate TDA from another area of the project site). Figure G - 1: Threshold Discharge Area A continuous length of pipe that conveys water from one point to another with no inlets or collection points in between. A layer of poorly sorted soil deposited by glacial action that generally has very low infiltration rates. The time period necessary for surface runoff to reach the outlet of a subbasin from the hydraulically most remote point in the contributing area. General term to include characteristics of the ground surface such as plains, hills, mountains, degree of relief, steepness of slopes, and other physiographic features. The upper portion of a soil, usually dark colored and rich in organic material. It is more or less equivalent to the upper portion of an A horizon in an ABC soil. The dissolved salt loading in surface and subsurface waters. A large family of chemical compounds that come from crude oil. Two types important to stormwater include: TPH-Gx: The qualitative and quantitative method (extended) for volatile (“gasoline”) petroleum products in water; and TPH-Dx: The qualitative and quantitative method (extended) for semi-volatile (“diesel”) petroleum products in water. The solids in water, sewage, or other liquids, including the dissolved, filterable, and nonfilterable solids. The residue left when the moisture is evaporated and the remainder is dried at a specified temperature, usually 130°C. That portion of the solids carried by stormwater that can be captured on a standard glass filter. A calculation of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards, and an allocation of that amount to the pollutant’s sources. A TMDL (also known as a Water Cleanup Plan) is the sum of the allowable loads of a single pollutant from all contributing point and nonpoint sources. The calculation must include a margin of safety to ensure that the waterbody can be used for the purposes the State has designated. The calculation must also account for seasonable variation in water quality. Water quality standards are set by states, territories, and tribes. They identify the uses for each waterbody, for example, drinking water supply, contact recreation (swimming), and aquatic like support (fishing), and the scientific criteria to support that use. The Clean Water Act, Section 303, establishes the water quality standards and TMDL programs. Poisonous, carcinogenic, or otherwise directly harmful to life. Material such as dirt, mud, and other debris that is deposited outside the project site by vehicles exiting a construction site or a commercial or industrial facility. A legally created parcel of property designated for special nonresidential and noncommercial uses. The process of relocating and planting an existing plant into a landscape. Note: Trees are considered plants. A structural device used to prevent debris from entering a spillway or other hydraulic structure. The estimated time for surface water to flow between two points of interest. A BMP that is intended to remove pollutants from stormwater. A layer of soil that is designed to slow the rate of infiltration and provide sufficient pollutant removal so as to protect groundwater quality. A combination of two or more treatment facilities connected in series. A single or one direction vehicle movement with either the origin or destination being the subject site. (Ex. 250 vehicles entering a site and 250 vehicles leaving a site would be 500 trip ends) Publically owned stormwater pipes with inside diameters and minimum slopes as follows: 54" and greater at a slope greater than or equal to 0.1% 48" to less than 54" at a slope greater than or equal to 0.15% 42" to less than 48" at a slope greater than or equal to 0.25% 36" to less than 42" at a slope greater than or equal to 0.5% Dispersion or scattering of light in a liquid, caused by suspended solids and other factors; commonly used as a measure of suspended solids in a liquid. Refers to those soil strata that were not overconsolidated (compacted by the weight of the sediment and ice) by the last glacial event. Typically, Qvr (Vashion Recessional Outwash) soils are considered to be unconsolidated by glacial advance whereas Qva (Vashon Advance Outwash) Soils are considered to be overconsolidated by glacial advance. Vegetation, including but not limited to, all annuals, woody and herbaceous perennials such as shrubs, vines or trees that are not regularly or systematically maintained through any combination of the following: pruning, mowing, watering, trimming, fertilizing and any other activity intended to ensure public safety and assist vegetation to achieve full environmental and landscape function. Plastic pipes with holes drilled through the top, installed on the bottom of an infiltration BMP, which are used to collect and remove excess runoff. Underground lines which serve the public with various utilities. They are typically located in trenches. Dry utilities include such lines as telephone, cable, electric and gas lines. Wet utilities include stormwater conveyance pipes, fire lines, domestic water lines and wastewater pipes. Those sloping areas of land which have in the past exhibited, are currently exhibiting, or will likely in the future exhibit, mass movement of earth. A flowpath consisting of well-established lawn or pasture, landscaping with well-established groundcover, native vegetation with natural groundcover, or an area that meets BMP L613 - Post-Construction Soil Quality and Depth. The groundcover shall be dense enough to help disperse flows and to prevent erosion. A vegetated rooftop, also known as a green roof, is a rooftop that is partially or completely covered with vegetation and a growing medium planted over a waterproofing membrane. The green roof will also contain a root repelling membrane and stormwater system. As related to applicability of the Minimum Requirements, vegetation shall mean native vegetation, pasture, scrub/shrub, uncultivated vegetation, or unmaintained non-native vegetation (e.g., Himalayan blackberries, scotch broom). As related to pavement maintenance, vegetation maintenance includes caring for and controlling vegetation along roadways or paved surfaces to ensure vegetation does not damage the pavement surface or cause a safety concern. Vegetation maintenance may also include moss control and care and control of vegetation that is part of a permeable paver system. Regular use of an impervious or pervious surface by motor vehicles. The following are subject to regular vehicular use: bike lanes within the traveled lane of a roadway, driveways, parking lots, unrestricted access fire lanes, vehicular equipment storage yards, airport runways, and railroad tracks. The following are not considered subject to regular vehicular use: paved bike pathways separated from and not subject to stormwater and surface water discharges from roads for motor vehicles; Surface waters including rivers, streams, lakes, marine waters, estuaries, and wetlands. As related to Minimum Requirement #8 - Wetlands Protection, WLF is a defining characteristic of a wetland. WLF during a monitoring interval is as follows: Average base stage = (Instantaneous stage at beginning of interval + Instantaneous stage at end of interval)/2 A person who has met the necessary requirements to be licensed as a Professional Engineer in Washington State. Certain tasks may only be performed by a P.E. with a specialty license. It is the responsibility of every professional to know the limits of their licenses. A person who has met the necessary requirements to be licensed as a Professional Geologist in Washington State. Certain tasks may only be performed by a P.G. with a specialty license. It is the responsibility of every professional to know the limits of their licenses. A term used to describe the chemical, physical, and biological characteristics off water, usually in respect to its suitability for a particular purpose. Minimum requirements of purity of water for various uses; for example, water for agricultural use in irrigation systems should not exceed specific levels of sodium bicarbonate, pH, total dissolved salts, etc. In Washington, the Department of Ecology sets water quality standards. A geographic region within which water drains into a particular river, stream, or body of water. Watersheds can be as large as those identified and numbered by the State of Washington Water Resource Inventory Areas (WRIAs) as defined in Chapter 173-500 WAC. The upper surface or top of the saturated portion of the soil or bedrock layer, indicates the uppermost extent of groundwater. Delineation of the wetland edge shall be based on the U.S. Army Corps of Engineers Wetlands Delineation Manual, Technical Report Y-87-1, U.S. Army Engineers Waterways Experiment Station, Vicksburg, Miss. (1987) The physical components of a wetland, both the abiotic (physical and chemical) and biotic (living). Wetland processes or attributes that are valuable or beneficial to society (also see Functions). Wetland values include support of commercial and sport fish and wildlife species, protection of life and property from flooding, recreation, education, and aesthetic enhancement of human communities. Those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas. Wetlands do not include those artificial wetlands intentionally created from nonwetland sites, including, but not limited to, irrigation and conveyance ditches, grass-lined swales, canals, stormwater treatment wetlands, detention facilities, wastewater treatment facilities, farm ponds, and landscape amenities, or those wetlands created after July 1, 1990, that were unintentionally created as a result of the construction of a road, street, or highway. Wetlands may include those artificial wetlands intentionally created from nonwetland areas to mitigate the conversion of wetlands. (Waterbodies not included in the definition of wetlands as well as those mentioned in the definition are still waters of the state.) The ecological (physical, chemical, and biological) processes or attributes of a wetland without regard for their importance to society (see also values). Wetland functions include food chain support, provision of ecosystem diversity and fish and wildlife habitat, floodflow alteration, groundwater recharge and discharge, water quality improvement, and soil stabilization. Stormwater facilities for stormwater treatment that contain permanent pools of water that are filled during the initial runoff from a storm event. They are designed to optimize water quality by providing retention time in order to settle out particles of fine sediment to which pollutants such as heavy metals absorb, and to allow biologic activity to occur that metabolizes nutrients and organic pollutants. Western Washington Hydrology Model. A continuous simulation model approved by the Washington State Department of Ecology for use with this manual. A catch basin or similar structure used to collect and convey stormwater and surface water from lawn and landscaped areas of a site.
