Stormwater Infiltration in the Highway Environment: Guidance Manual (2019)

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111 The following glossary contains definitions of the key terms as used in the context of this Guidance Manual. Anisotropy: A soil structure property defined as the ratio between horizontal hydraulic conduc- tivity and vertical hydraulic conductivity. As a model parameter, anisotropy is a function of inherent microscale anisotropy (particle arrangement) and macroscale anisotropy (soil layering). Average Annual Capture Efficiency [also known as (aka) capture efficiency]: The estimated percentage of long-term average annual runoff volume that is managed or controlled by a BMP. Baseflow: The portion of streamflow that comes from the sum of deep subsurface flow and delayed shallow subsurface flow. Baseflow tends to dominate discharge during dry weather and small storm events. In contrast, elevated flows during large storm events tend to be derived primarily from overland flow or rapid shallow subsurface flow. Best Management Practice (BMP): A device, practice, or method for removing, reducing, retarding, or preventing targeted stormwater runoff quantity, constituents, pollutants, and contaminants from reaching receiving waters. This Guidance Manual uses the term BMP to refer to a stormwater control facility. BMP System: A system including the BMP and any related bypass or overflow. Bypass: Runoff that is routed around a BMP or passes through the BMP with minimal treat- ment. Bypass generally occurs when the inflow volume or flow rate has exceeded the capac- ity of the BMP. Catchment (aka subcatchments, drainage area, drainage basin, subwatershed): The land area that drains to a specific point of interest. A catchment is typically a portion of a watershed. Clean Water Act (CWA): Federal legislation (1972) that established the basic structure for regulating discharges of pollutants into the waters of the United States and regulating quality standards for surface waters. The CWA authorized the U.S. Environmental Protection Agency (EPA) to implement pollution control programs such as the National Pollutant Discharge Elimination System (NPDES). Coefficient of Uniformity: A soil characteristic that influences geotechnical properties. Defined as the ratio of D60/D10. D10 is the diameter at which 10% of the sample’s mass comprises particles with a diameter less than this value. D60 is the diameter at which 60% of the sample’s mass comprises particles with a diameter less than this value. Compaction: The densification, settlement, or packing of soil in such a way that the bulk density of the soil increases. Compaction tends to result in reduction in soil permeability. Compaction Glossary of Key Terms

112 Stormwater Infiltration in the Highway Environment: Guidance Manual may be intentional, as in the preparation of a site for construction, or incidental, as in the movement of machinery or foot traffic over an area. Confining Layer: A layer of low permeability soil or rock that limits the vertical movement of water. Bedrock is a colloquial term for a type of confining layer. Clay layers may also behave as confining layers. Continuous Simulation Modeling: A method of hydrological analysis in which a continuous timeseries (e.g., a period of years) of precipitation and other climatic data are used as input. Watershed processes including infiltration, evapotranspiration (ET), and runoff are calculated on a continuous basis. The outputs of continuous simulation models are typically continuous timeseries of watershed and BMP responses that can be analyzed sequentially or continuously. Cost-Effectiveness: Defined in general as the ratio of effectiveness of a control for a given metric versus the cost of the control. A higher cost-effectiveness results when the ratio of effectiveness to cost is higher. Depth Filtration: Refers to a process that occurs in granular media filtration in which larger par- ticles are retained at the surface and progressively finer particles penetrate the media matrix and are retained at various depths from the surface. [Contrast with Surface Filtration (also Cake Filtration)]. Design Criteria: In this context, design criteria refer to the set of requirements that serve as the basis for designing a BMP to achieve its intended performance. For example, design criteria for a filter strip may include the slope, length, vegetation density, amended soil thickness, maximum flow depth, and other criteria. Design Infiltration Rate (also Factored Infiltration Rate): An infiltration rate that has had appropriate factor of safety applied and is suitable for use in design. Design Parameters: The qualitative and quantitative physical characteristics that are used in the design process to describe and analyze a given BMP design. Design criteria are commonly expressed in terms of allowable bounds on design parameters. Design Storm: A prescribed precipitation distribution (hyetograph) and the total precipitation amount that is used as part of the design process of BMPs. Design storms may be statistically derived hypothetical events or real events that have been observed. Discharge Rate: Discharge rate refers to the rate at which water is discharged from a BMP. Disconnection: (aka dispersion, disconnected impervious area): A stormwater drainage pat- tern that routes flow from impervious areas across pervious surfaces prior to discharging to a storm drain or receiving water. There are various degrees of disconnection, such as dis- connection that attempts to fully mitigate hydrologic impacts, and disconnection that may attempt to provide only a portion of total control needed to mitigate impacts. Drawdown Rate: The rate at which the storage volume in a BMP is recovered as a result of water discharging from the BMP, making storage volume available for subsequent storm events. Drawdown Time: The time required for a BMP to drain and return to its dry-weather condition. For example, the drawdown time of an infiltration basin is the time it takes for the basin to drain from brim full to empty following the end of inflow. For detention facilities, drawdown time is a function of basin volume and outlet orifice size. For infiltration facilities, drawdown time is a function of basin volume and infiltration discharge rate. Effectiveness: A measure of how well a BMP system meets its goals for all stormwater flows reaching the BMP, including flow bypasses. For example, effectiveness is a function of capture

Glossary of Key Terms 113 efficiency, percentage of volume reduction, and effluent pollutant concentration. See Perfor- mance and Efficiency for complementary definitions. Efficiency: A measure of how well a BMP or BMP system removes pollutants. See Performance and Effectiveness for complementary definitions. Evaporation: The change of phase of a liquid into a vapor at a temperature below the boiling point, taking place at the liquid’s surface. Evapotranspiration (ET): The loss of water to the atmosphere by the combined processes of evaporation (from water, soil, and plant surfaces) and transpiration (from plant tissues). Factor of Safety (FS): A factor applied to a specific system design parameter that is intended to make the design of the system more robust in the event conditions are different than analyzed, conditions change with time, or other factors are present that are not explicitly considered or are not foreseen in the design process. Feasibility Criteria (aka infeasibility criteria): Specific qualitative or quantitative criteria that are used to identify conditions under which a given stormwater management approach is considered feasible or infeasible. Flood Control Regulations: In this Guidance Manual, Flood Control Regulations are requirements meant to reduce the risk of damage to public property or hazards to public safety as a result of runoff from large storm events. For example, flood control regulations may require peak runoff flowrates be matched for pre-project versus post-project for a specific large design storm event (e.g., 25-year, 24-hour event). In contrast, water quality regulations typically focus on smaller, more frequent events that are of specific interest to protection of receiving water quality. Full Infiltration BMP/Full Infiltration: A type of infiltration BMP that relies solely on infiltra- tion into underlying soils. Full Infiltration does not imply that all stormwater runoff is infil- trated. The amount of water infiltrated is a function of BMP size and design goals as well as site conditions. However, these BMPs do not have a design discharge to surface waters except when the system overflows or bypasses. The key distinguishing trait of these BMPs is that they depend on a certain minimum infiltration rate to meet their intended functions. Geotechnical Considerations: In this Guidance Manual, geotechnical considerations refer spe- cifically to factors related to geotechnical design and performance of soil structures when considering infiltration of stormwater. Considerations include landslides, liquefaction, settle- ment, and other factors. Groundwater Mounding: Refers to the development of a localized increase in the groundwater table below a BMP on a temporary basis in response to stormwater infiltration. The develop- ment of a mound is an inherent hydrogeologic phenomenon in response to point loading of stormwater infiltration. Groundwater Mounding Assessment Tool: Refers to the spreadsheet tool developed as part of NCHRP Project 25–51 and included as Appendix C. Groundwater Quality Criteria: Pursuant to the CWA, water quality criteria are numeric, nar- rative objectives, or limit used to determine when water has become unsafe for people and wildlife. The EPA developed water quality criteria as recommendations. State and tribal gov- ernments may use these criteria or use them as guidance in developing their own. Groundwater Quality Standard: Pursuant to the CWA, water quality standards are provisions of state, territorial, authorized tribal, or federal law approved by the EPA that describe the desired condition of a water body and the means by which that condition will be protected or achieved.

114 Stormwater Infiltration in the Highway Environment: Guidance Manual Groundwater Recharge: The process by which surface water infiltrates into permeable soil and ultimately contributes additional water volume to groundwater sources. Head: In hydraulics, energy represented as a difference in elevation. In slow-flowing open sys- tems, the difference in water surface elevation (i.e., between an inlet and an outlet). Hydraulic Loading: The ratio of stormwater inflow (volume/time) to a BMP divided by the surface area of the BMP that receives flow; this can be a specific value, expressed in terms of a length per unit time (e.g., ft/s) or it can be used in a more qualitative sense to make a com- parison between BMP configurations. Hydraulic Loading Ratio: In this Guidance Manual, this term refers to the ratio between the impervious area tributary to a BMP and the infiltration or filtration surface area of the BMP. Hydrocollapse: A sudden collapse of granular soils caused by a rise in groundwater dissolving or deteriorating the inter-granular contacts between the sand particles. Hydrograph: A timeseries of flow discharge (e.g., runoff rate, inflow rate, outflow rate) versus time. Hydromodification: Changes in runoff and sediment yield caused by land use modifications. Impervious Surface: Surface area that allows little or no infiltration. Impervious surfaces include pavements, roofs, and similar surfaces. Highly compacted gravel and earth can behave as impervious surfaces. Incidental Infiltration: Infiltration that occurs within a BMP as an incidental and reasonably expected part of operation but is not a primary design goal or treatment process. Infiltration: The movement of water from the surface into the soil. Movement from shallow surface layers to deeper surface layers is referred to as “percolation.” Infiltration Approach: Refers to classes of infiltration BMPs. For example, Full Infiltration and Maximized Partial Infiltration are infiltration approaches. Infiltration BMP: A BMP designed to rely in whole or part on infiltration of stormwater into subsurface soils to meet stormwater management objectives. Infiltration BMPs vary in (1) the degree to which they rely on a certain minimum infiltration rate to remain operable, (2) the degree of infiltration provided, and (3) their design approach, relative to the speci- ficity of infiltration goals. See also Full Infiltration BMP, Partial Infiltration BMP, and Incidental Infiltration. Classes of approach involve infiltration BMPs that rely solely on infiltration into underlying soils. Full Infiltration does not imply that all stormwater runoff is infiltrated. The amount of water infiltrated is a function of BMP size and design goals as well as site conditions. However, these BMPs do not have a design discharge to surface waters except when the system overflows or bypasses. The key distinguishing trait of these BMPs is that they depend on a certain minimum infiltration rate to meet their intended functions. Infiltration Capacity: Refers to the overall ability of the infiltration receptor to accept infil- trated water, considering infiltration rates and soil permeability as well as hydrogeologic factors. This is a semi-quantitative metric. Infiltration Rate: The rate at which water moves into the soil, expressed as length per unit of time. Infiltration rate is a “bulk” measurement in that it describes the overall rate not the velocity of water through pores, which would tend to be faster. Infiltration Receptor: Refers broadly to the unsaturated soil layers, perched groundwater, or aquifer that will receive infiltrated stormwater.

Glossary of Key Terms 115 Interflow (aka shallow interflow): The flow of water through the upper soil zones into a stream. In comparison with baseflow, which tends to originate from lower soil zones, interflow tends to have a shorter travel time and quicker response. However, interflow tends to have a longer, more attenuated response than sheet flow and concentrated overland flow. Liquefaction: A seismically induced geological hazard that can result in damage to structures because of a reduction in bulk volume of saturated granular soils during shaking of the earth. Liquefaction results in the loss of a soil’s ability to support a structure. It is specifi- cally associated with saturated granular soils. Local Groundwater Protection Criteria: Criteria established by local groundwater protection agencies to protect the quality of groundwater. These may be at a state or local level. These criteria may be based on CWA water quality standards and/or SDWA maximum contaminant levels. Maximum Extent Practicable (MEP): Standard established by the 1987 amendments to the CWA, for the implementation of municipal stormwater pollution prevention programs. According to the Act, municipal stormwater NPDES permits “shall require controls to reduce the discharge of pollutants to the maximum extent practicable including manage- ment practices, control techniques and system, design and engineering methods, and such other provisions as the Administrator or the State determines appropriate for the control of such pollutants.” MEP is not defined by the CWA. Monitoring: Refers to observations and measurements used to assess condition, performance, maintenance needs, and impacts of an infiltration BMP. Municipal Separate Storm Sewer System (MS4): A conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, constructed channels, or storm drains) designed for collecting or conveying stormwater as defined in 40 CFR 122.26(b)(8). National Pollutant Discharge Elimination System (NPDES): A provision of the CWA that pro- hibits point-source discharges of pollutants into waters of the United States unless a special permit is issued and administered by states or the EPA. New Development Project: Refers to a project involving construction of a new segment of roadway in a previously undeveloped or much less developed ROW. (Contrast with Retrofit Project or Redevelopment Project.) Observed Infiltration Rate (also Raw Infiltration Rate): The estimated infiltration rate based on the results of field tests, prior to incorporating a factor of safety for design purposes. Off-Line BMP: Off-line BMP systems receive flow from a flow-splitter structure of some sort such that the maximum inflow to the system is restricted and peak flows are designed to bypass the system without treatment. On-Line BMP: On-line BMP systems receive all the stormwater runoff from a drainage area. Flows above the water quality design flow rate or volume are passed through the system, generally via an overflow device or structure. On-Site BMPs: BMPs that are implemented within the boundary of a project site. In contrast, see Regional BMPs. Operations and Maintenance (O&M): Refers to inspection of BMPs, operation of the BMPs (if actively operated), and implementation of preventative and corrective maintenance into perpetuity. O&M represents a continuing cost associated with the BMP after the initial capital cost of construction.

116 Stormwater Infiltration in the Highway Environment: Guidance Manual Outlet Control: A design approach for bioretention BMPs in which the flow through the soil media bed is primarily controlled by an outlet control structure affixed to the underdrains of the system rather than limited by the hydraulic conductivity of the bioretention soil media. Overland Flow: Flow of water across the land surface in a downgradient direction. Sheet flow, shallow concentrated flow, and channelized flow are forms of overland flow. Partial Infiltration BMP/Maximized Partial Infiltration: A type of infiltration BMP that is designed specifically to maximize infiltration while also providing other treatment mecha- nisms. These BMP types are not wholly reliant on infiltration to maintain an operable con- dition and meet water quality and flow control requirements but are expected to result in significant levels of infiltration. Partially Feasible: The concept of “partial feasibility” refers to a condition in which it is feasible to achieve a portion of the established design goals, but in which it would be infeasible to achieve the entire design goal based on constraining factors. For example, if it is feasible to retain 0.3 in. of runoff, but the design goal is 1.0 in. of runoff, then it would be considered partially feasible to meet the design goal. Performance: A measure of how well a BMP meets its goals for the stormwater that flows through or is processed by it. In comparison to Effectiveness, assessment of BMP performance does not account for bypass of flows, because these flows are beyond the design goal of the system. See Effectiveness and Efficiency for complementary definitions. Performance Criteria: A specific measurable or verifiable set of requirements against which the performance of a system is compared to assess conformance with regulatory requirements. For example, reduction of a certain percentage of average annual runoff volume is a common form of a performance criterion established for volume reduction approaches. Pervious Surface: Surface area that allows infiltration of water. Physical Setting: The physical aspects of a project site that may impact project design and per- formance relative to volume reduction, including the site-specific climate, geology, soils, and vegetation. Precipitation: Water that falls to the earth in the form of rain, snow, hail, or sleet. Precipitation Event: A period of precipitation separated from other events by established inter- event criteria, such as a dry period of a certain length. Pretreatment: A system used to remove pollutants from stormwater before it enters the main part of a BMP. Project Attributes: The aspects of a project design that may impact performance relative to volume reduction, including planimetric geometry, topography, utilities, regulatory overlays, construction methods, and other factors. Redevelopment Project: Refers to a project involving re-alignment, lane addition, or other roadway construction work within an existing developed ROW. (Contrast with Retrofit Project or New Development Project.) Regional BMPs (aka watershed-scale BMPs): BMPs implemented within the local subwater- shed, typically outside and downstream of the project boundary or treating nearby areas. In contrast, see On-Site BMPs. Resiliency: In the context of stormwater BMPs, resiliency can be defined as the ability to tolerate, adapt to, or rapidly recover from adverse conditions, such as incomplete site investigations,

Glossary of Key Terms 117 construction impacts, elevated sediment loading, contaminant spills, extreme storm events, lack of maintenance, change in tributary area characteristics, and change in design goals. Retrofit: A type of project that principally involves retrofitting a roadway with a stormwater BMP for the purpose of providing treatment of existing paved surfaces. This may not be associated with a roadway. (Contrast with Redevelopment Project or New Development Project.) Right of Way (ROW): Is defined as the legal parcel within which the roadway project is constructed. Roadside BMP Clogging Risk Assessment Tool: Refers to the spreadsheet tool developed as part of NCHRP Project 25–51 and included as Appendix F. Roadway Design Regulations: Refers to regulations related to roadway geometrics, public safety, drainage, and other aspects of roadway design, inclusive of water quality and volume reduction as applicable. Root Zone: The depth to which the major vegetation draws water through a root system in soil. Runoff Volume: The volume of water that flows off a surface during a period of interest. Sacrificial Soil Layer: A sacrificial soil or media layer consists of a layer of material (sand, soil, engineered media) placed over top of less permeable underlying soil to serve as an embed- ded pretreatment layer. Because of its higher permeability, more sediment can be loaded on this layer before it approaches the limiting rate of the underlying layer. Additionally, if this material is significantly coarser than incoming sediment, it is more likely that the depth filtration processes will prevail at the surface rather than cake filtration or surface filtration. Safe Drinking Water Act (SDWA): The federal law that protects public drinking water supplies throughout the nation. Under the SDWA, EPA sets standards for drinking water quality and with its partners implements various technical and financial programs to ensure drinking water safety. Sheet Flow: An overland flow, downslope movement of water taking the form of a thin continu- ous film over a generally smooth surface. Shoulder (of a roadway): A reserved open area located at the edge of a roadway consisting of pavement or pervious surface. Site Design: A stormwater management strategy that emphasizes conservation and use of exist- ing site features as well as incorporation of strategic drainage patterns to reduce the amount of runoff and pollutant loading that are generated from a project site. Sizing Criteria: Specific design criteria related to BMP sizes that serve as a presumptive basis for meeting performance criteria. Sole Source Aquifer (SSA): EPA defines an SSA as one in which (1) the aquifer supplies at least 50% of the drinking water for its service area, and (2) there are no reasonably available alter- native drinking water sources should the aquifer become contaminated. Spill Isolation and Containment: A system to capture and contain a contaminant spill before it reaches a BMP or a stormwater outfall. Surface Filtration (also Cake Filtration): Refers to a process that occurs in granular media filtration in which most particles are retained at the surface of the media and form a layer of material that predominantly comprises particles filtered from stormwater. (Contrast with Depth Filtration.)

118 Stormwater Infiltration in the Highway Environment: Guidance Manual Total Maximum Daily Load (TMDL): The calculation of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards and an allocation of that load among the various sources of that pollutant. Pollutant sources are characterized as either point sources that receive a wasteload allocation or nonpoint sources that receive a load allocation. Travel Lane: A portion of a road or highway that is primarily dedicated to conveying automobile travel. Vadose Zone (also Unsaturated Zone): Refers to soil layers that are unsaturated, extending from the ground surface to the water table. Volume Reduction: The process by which the volume of runoff that discharges directly to receiving waters is reduced through volume reduction approaches that include infiltration, ET, and/or harvest for beneficial use. Volume Reduction Tool: Refers to the spreadsheet tool developed as part of NCHRP Report 802. Water Balance (aka Water Budget): The accounting of a system’s state of water storage and flux, considering the total flow of water into and out of a system and the change in storage condi- tions in the system. For example, water balance can refer to the flux of water in and out of a specific BMP system, a local groundwater system, or a regional groundwater system. Water Balance Analysis: Water balance analysis refers to the consideration of the fate of retained stormwater (e.g., percolation, interflow, ET, or beneficial use) such that potential adverse effects on local systems can be evaluated. Water Table: Refers to the interface between saturated subsurface soil (phreatic zone) and unsaturated soil (vadose zone). Watershed Characteristics: Characteristics of the watershed in which a project is located that may influence goals for volume reduction or the amount of volume reduction that can be achieved (e.g., topography, regional groundwater table, regional water balance, and other factors). Whole Lifecycle Cost and Performance Tools: Refers to the spreadsheet tools developed as part of NCHRP Report 792. Whole Lifecycle Costs: An economic assessment, expressed in monetary value that considers all significant and relevant cost flows over a period of analysis (project life expectancy). Project costs include those needed to achieve defined levels of performance, including reliability, safety, and availability. Included are both capital and O&M costs.