**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

**Suggested Citation:**"Appendix A - Glossary." National Academies of Sciences, Engineering, and Medicine. 2013.

*Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction*. Washington, DC: The National Academies Press. doi: 10.17226/22477.

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

A-1 A P P E N D I X A Glossary I. Hydrologic, Hydraulic, and Geomorphic Terms* Aggradation: General and progressive buildup of the longitudinal profile of a channel bed due to sediment deposition. Alluvial Channel: Channel wholly in alluvium; no bedrock is exposed in channel at low flow or likely to be exposed by erosion. Alluvial Stream: A stream which has formed its channel in cohesive or noncohesive materials that have been and can be transported by the stream. Alluvium: Unconsolidated material deposited by a stream in a channel, floodplain, alluvial fan, or delta. Annual Flood: The maximum flow in one year (may be daily or instantaneous). Average Velocity: Velocity at a given cross section determined by dividing discharge by cross-sectional area. Backwater: The increase in water surface elevation relative to the elevation occurring under natural channel and floodplain conditions. It is induced by a bridge or other structure that obstructs or constricts the free flow of water in a channel. Backwater Area: The low-lying lands adjacent to a stream that may become flooded due to backwater. Bank: The sides of a channel between which the flow is normally confined. Bank, Left (Right): The side of a channel as viewed in a downstream direction. Bankfull Discharge: Discharge that, on the average, fills a channel to the point of overflowing. *Many of the entries in Part I of this Glossary are from HEC-18 (Arneson et al. 2012).

A-2 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Bed Material: Material found in and on the bed of a stream (may be transported as bed load or in suspension). Bed Shear (Tractive Force): Boulder: A rock fragment whose diameter is greater than 250 mm. Boundary Condition (Model): A specified hydraulic condition such as a water surface elevation or energy slope used as a starting point for a hydraulic model simulation. Bridge Opening: The cross-sectional area beneath a bridge that is available for conveyance of water. Bridge Substructure: Structural elements supporting a bridge in contact with the stream or channel bed, including bridge abutments, piers, and footings. Bridge Waterway: The area of a bridge opening available for flow, as measured below a specified stage and normal to the principal direction of flow. Catchment: See Drainage Basin. Channel: The bed and banks that confine the surface flow of a stream. Channel Pattern: The aspect of a stream channel in plan view, with particular reference to the degree of sinuosity, braiding, and anabranching. Channel Process: Behavior of a channel with respect to shifting, erosion, and sedimentation. Choking (of flow): Excessive constriction of flow which may cause severe backwater effect. Clay (Mineral): A particle whose diameter is in the range of 0.00024 to 0.004 mm. Base Floodplain: Floodplain associated with the flood with a 100-year recurrence interval. Bed: Bottom of a channel bounded by banks. Bed Form: A recognizable relief feature on the bed of a channel, such as a ripple, dune, plane bed, antidune, or bar. Bed forms are a consequence of the interaction between hydraulic forces (boundary shear stress) and the bed sediment. The force per unit area exerted by a fluid flowing past a stationary boundary.

Glossary A-3 Contraction Scour: Contraction scour, in a natural channel or at a bridge crossing, involves the removal of material from the bed and banks across all or most of the channel width. This component of scour results from a contraction of the flow area at the bridge which causes an increase in velocity and shear stress on the bed at the bridge. The contraction can be caused by the bridge or from a natural narrowing of the stream channel. Conveyance: A measure of the carrying capacity of a channel section. In the Manning equation, conveyance K is: S QAR n 486.1K 3/2 Critical Shear Stress: The minimum amount of shear stress required to initiate soil particle motion. Critical Velocity (Particle Motion): The velocity required to initiate motion of a particle of a specified size and weight. Cross Section: A section normal to the trend of a channel or flow. Daily Discharge: Discharge averaged over one day (24 hours). Debris: Floating or submerged material, such as logs, vegetation, or trash, transported by a stream (Drift). Degradation (Bed): A general and progressive (long-term) lowering of the channel bed due to erosion, over a relatively long channel length. Depth of Scour: The vertical distance a streambed is lowered by scour below a reference elevation. Design Flow (Design Flood): The discharge that is selected as the basis for the design or evaluation of a hydraulic structure including a hydraulic design flood, scour design flood, and scour design check flood. Clear-Water Scour: Scour at a pier or abutment (or contraction scour) when there is no movement of the bed material upstream of the bridge crossing at the flow causing bridge scour. Constriction: A natural or artificial control section, such as a bridge crossing, channel reach or dam, with limited flow capacity in which the upstream water surface elevation is related to discharge. Contraction: The effect of channel or bridge constriction on flow streamlines.

A-4 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Ephemeral Stream: A stream or reach of stream that does not flow for parts of the year. As used here, the term includes intermittent streams with flow less than perennial. Erosion: Displacement of soil particles due to water or wind action. FESWMS: A 2-dimensional open-channel flow model called the Finite Element Surface Water Modeling System developed and supported by the Federal Highway Administration (also referred to as FST-2DH). Fill Slope: Side or end slope of an earth-fill embankment. Where a fill-slope forms the streamward face of a spill-through abutment, it is regarded as part of the abutment. Flood: Large volumetric rate of discharge in a river or stream that occurs infrequently and is usually associated with inundation and economic damage. Flood Exceedance Probability: The statistical chance that a specified discharge rate will be equaled or exceeded in a given year. Flood Frequency: The average interval between floods exceeding a given magnitude. For example, a flood having an annual probability of exceedance of 1 percent has a 1/(0.01) = 100-year frequency of recurrence; a flood of this magnitude would be expected to occur on average about once every 100 years. Flood-Frequency Curve: A graph indicating the probability that the annual flood discharge will exceed a given magnitude, or the recurrence interval corresponding to a given magnitude. Floodplain: A nearly flat, alluvial lowland bordering a stream, which is subject to frequent inundation by floods. Flood Return Period/Recurrence Interval: See Flood Frequency. Flow Skew: The angle of incidence of flow on a rectangular or long wall pier. Flow aligned with the long axis of a structure has a skew of zero degrees. Discharge: Volume of water passing through a channel during a given time. Drainage Basin: An area confined by drainage divides, often having only one outlet for discharge (Catchment, Watershed). Drift: Alternative term for vegetative debris. Energy (Friction) Slope: Rate of energy loss with distance in the downstream flow direction: Sf = dH/dL where H is total energy and L is streamwise distance.

Glossary A-5 Freeboard: The vertical distance above a design stage that is allowed for waves, surges, drift, and other contingencies. Froude Number: A dimensionless number that represents the ratio of inertial to gravitational forces in open-channel flow. Gabion: A basket or compartmented rectangular container made of wire mesh. When filled with cobbles or other rock of suitable size, the gabion becomes a flexible and permeable unit with which flow- and erosion-control structures can be built. Gaging Station: Instrumentation on a stream or river that is used for measuring the volumetric rate of flow. Gaging stations exhibit a unique relationship between water surface elevation and flow rate which is periodically calibrated. Geomorphology/Morphology: That science that deals with the form of the Earth, the general configuration of its surface, and the changes that take place due to erosion and deposition. Graded Stream: A geomorphic term used for streams that have apparently achieved a state of equilibrium between the rate of sediment transport and the rate of sediment supply throughout long reaches. Gravel: A rock fragment whose diameter ranges from 2 to 64 mm. HEC-RAS: A 1-dimensional open-channel flow model developed and supported by the U.S. Army Corps of Engineers - Hydrologic Engineering Center. Headcutting: Channel degradation associated with abrupt changes in the bed elevation (headcut) that generally migrates in an upstream direction. Hydraulics: The applied science concerned with the behavior and flow of liquids, especially in pipes, channels, structures, and the ground. Hydraulic Model: A small-scale physical (or mathematical) representation of a flow situation. Hydrograph: The graph of stage or discharge against time. Fluvial Geomorphology: The science dealing with the morphology (form) and dynamics of streams and rivers. Fluvial System: The natural river system consisting of (1) the drainage basin, watershed, or sediment source area, (2) tributary and mainstem river channels or sediment transfer zone, and (3) alluvial fans, valley fills and deltas, or the sediment deposition zone.

A-6 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Instantaneous Discharge/Peak: The volumetric rate of flow passing a given cross section on a stream or river at a specific point in time. Invert: The lowest point in the channel cross section or at flow control devices such as weirs, culverts, or dams. Ineffective Flow: An area of flow where water is not being conveyed in a downstream direction (e.g., ponding above or below an embankment). Lateral Erosion (Migration): Erosion in which the removal of material is extended horizontally as contrasted with degradation and scour in a vertical direction. Live Flow: Area of flow where water is actively conveyed in a downstream direction (e.g., channel flow and unobstructed floodplain flow). Live-Bed Scour: Scour at a pier or abutment (or contraction scour) when the bed material in the channel upstream of the bridge is moving at the flow causing bridge scour. Local Scour: Removal of material from around piers, abutments, spurs, and embankments caused by an acceleration of flow and resulting vortices induced by obstructions to the flow. Longitudinal Profile: The profile of a stream or channel drawn along the length of its centerline. In drawing the profile, elevations of the water surface or the thalweg are plotted against distance as measured from the mouth or from an arbitrary initial point. Manning Equation: Relationship between discharge, channel geometry, and roughness: 2/1 f 3/2 SAR n 486.1Q Manning Roughness Coefficient (n): Parameter of the Manning equation that is a measure of the resistance to flow caused by the channel boundary. Mathematical Model: A numerical representation of a flow situation using mathematical equations (also computer model). Meandering Stream: A stream having a sinuosity greater than some arbitrary value. The term also implies a moderate degree of pattern Hydrology: The science concerned with the occurrence, distribution, and circulation of water on the earth. Incised Reach: A stretch of stream with an incised channel that only rarely overflows its banks. Incised Stream: A stream which has deepened its channel through the bed of the valley floor, so that the floodplain is a terrace.

Glossary A-7 Migration: Change in position of a channel by lateral erosion of one bank and simultaneous accretion of the opposite bank. Nonalluvial Channel: A channel whose boundary is in bedrock or non-erodible material. Normal Stage: The water stage prevailing during the greater part of the year. Obstructed Flow Area: Portion of the waterway and/or floodplain blocked by a structure such as a bridge pier or approach roadway embankment. Overbank Flow: Water movement that overtops the bank either due to stream stage or to overland surface water runoff. Overtopping Flow: Portion of the flood discharge that flows over a roadway embankment or bridge deck. Perennial Stream: A stream or reach of a stream that flows continuously for all or most of the year. Pile: An elongated member, usually made of timber, concrete, or steel, which serves as a structural component of a river-training structure or bridge. Pressure Flow/Scour: See Vertical Contraction Scour. Probable Maximum Flood: A very rare flood discharge value computed by hydro- meteorological methods, usually in connection with major hydraulic structures. Probability Distribution (Log-Pearson Type III): Statistical probability distribution used to estimate flood frequency characteristics, typically using historical flood peak flows from gaging station records. Reach: A segment of stream length that is arbitrarily bounded for purposes of study. Recurrence Interval: The reciprocal of the annual probability of exceedance of a hydrologic event (also return period, exceedance interval). symmetry, imparted by regularity of size and repetition of meander loops. The channel generally exhibits a characteristic process of bank erosion and point bar deposition associated with systematically shifting migrating meanders. Median Diameter: The particle diameter of the 50th percentile point on a size distribution curve such that half of the particles (by weight, number, or volume) are larger and half are smaller (D50.)

A-8 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Riprap: Layer or facing of rock or broken concrete dumped or placed to protect a structure or embankment from erosion; also the rock or broken concrete suitable for such use. Riprap has also been applied to almost all kinds of armor, including wire-enclosed riprap, grouted riprap, sacked concrete, and concrete slabs. Roughness Coefficient: Numerical measure of the frictional resistance to flow in a channel, as in the Manning or Chezy's formulas. Sand: A rock fragment whose diameter is in the range of 0.062 to 2.0 mm. Scour: Erosion of streambed or bank material due to flowing water; often considered as being localized (see local scour, contraction scour, total scour). Scour Prism: Total volume of streambed material removed by scour in the bridge reach for design flood conditions. Sediment or Fluvial Sediment: Fragmental material transported, suspended, or deposited by water. Sediment Concentration: Weight or volume of sediment relative to the quantity of transporting (or suspending) fluid. Sediment Discharge: The quantity of sediment that is carried past any cross section of a stream in a unit of time. Discharge may be limited to certain sizes of sediment or to a specific part of the cross section. Sediment Load (Transport): Amount of sediment being moved (transported) by a stream. Sediment Yield: The total sediment outflow from a watershed or a drainage area at a point of reference and in a specified time period. This outflow is equal to the sediment discharge from the drainage area. Sediment Size (Median Diameter): The particle diameter of the 50th percentile point on a size distribution curve such that half of the particles (by weight, number, or volume) are larger and half are smaller (D50). Regression Relationship (Regional): A method for estimating the magnitude and frequency of floods using watershed characteristics such as drainage area, percent impervious surface, percent forest cover, etc. Relief Bridge: An opening in an embankment on a floodplain to permit passage of overbank flow. Riparian: Pertaining to anything connected with or adjacent to the banks of a stream (corridor, vegetation, zone, etc.).

Glossary A-9 Slope-Area Method: A method of estimating unmeasured flood discharges in a uniform channel reach using observed high-water levels. Spill-Through Abutment: A bridge abutment having a fill slope on the streamward side. The term originally referred to the "spill-through" of fill at an open abutment but is now applied to any abutment having such a slope. Spread Footing: A pier or abutment footing that transfers load directly to the earth. Stability: A condition of a channel when, though it may change slightly at different times of the year as the result of varying conditions of flow and sediment charge, there is no appreciable change from year to year; that is, accretion balances erosion over the years. Stable Channel: A condition that exists when a stream has a bed slope and cross section which allows its channel to transport the water and sediment delivered from the upstream watershed without aggradation, degradation, or bank erosion (a graded stream). Stage: Water-surface elevation of a stream with respect to a reference elevation. Stream: A body of water that may range in size from a large river to a small rill flowing in a channel. By extension, the term is sometimes applied to a natural channel or drainage course formed by flowing water whether it is occupied by water or not. Subcritical, Supercritical Flow: Open-channel flow conditions with Froude Number less than and greater than unity, respectively. Thalweg: The line extending down a channel that follows the lowest elevation of the bed. Toe of Bank: That portion of a stream cross section where the lower bank terminates and the channel bottom or the opposite lower bank begins. Shear Stress: See Unit Shear Force. Silt: A particle whose diameter is in the range of 0.004 to 0.062 mm. Sinuosity: The ratio between the thalweg length and the valley length of a stream. Slope (of Channel or Stream): Fall per unit length along the channel centerline or thalweg.

A-10 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Ultimate Scour: The maximum depth of scour attained for a given flow condition. May require multiple flow events and in cemented or cohesive soils may be achieved over a long time period. Uniform Flow: Flow of constant cross section and velocity through a reach of channel at a given time. Both the energy slope and the water slope are equal to the bed slope under conditions of uniform flow. Unit Discharge: Discharge per unit width (may be average over a cross section, or local at a point). Unit Shear Force (Shear Stress): The force or drag developed at the channel bed by flowing water. For uniform flow, this force is equal to a component of the gravity force acting in a direction parallel to the channel bed on a unit wetted area. Usually in units of stress, Pa (N/m2) or (lb/ft2). Unsteady Flow: Flow of variable discharge and velocity through a cross section with respect to time. Velocity: The time rate of flow usually expressed in m/s (ft/sec). The average velocity is the velocity at a given cross section determined by dividing discharge by cross- sectional area. Vertical Contraction Scour: Scour resulting from flow impinging on bridge superstructure elements (e.g., low chord). Vortex: Turbulent eddy in the flow generally caused by an obstruction such as a bridge pier or abutment (e.g., horseshoe vortex). Watershed: See Drainage Basin. Waterway Opening Width (Area): Width (area) of bridge opening at (below) a specified stage, measured normal to the principal direction of flow. Total Scour: The sum of long-term degradation, contraction scour, and local scour. Tractive Force: The drag or shear on a streambed or bank caused by passing water which tends to move soil particles along with the streamflow. Turbulence: Motion of fluids in which local velocities and pressures fluctuate irregularly in a random manner as opposed to laminar flow where all particles of the fluid move in distinct and separate lines.

Glossary A-11 II. Probability and Statistical Terms Bias: A statistical measure of systematic difference between a predicted value and the population parameter of interest, typically showing as the symbol ; a measure of consistent overprediction or underprediction. Box-Muller Transform: A method for generating independent standard normally distributed random numbers given a source of uniformly distributed random numbers. Chi-Squared Test: A statistical test commonly used to compare observed data with data one would expect to obtain according to a specific hypothesis. Coefficient of Variation (COV): A measure of the dispersion of a probability distribution defined as the standard deviation divided by the mean: COV Confidence Limit: An interval estimator of a population parameter used to assess the reliability of an estimate, typically shown as the symbol Zc. For example, there is a 90% probability that the true value lies between the Upper and Lower 95% confidence limits. Confidence Limit Zc 90% 1.281 95% 1.645 98% 2.054 Cumulative Distribution (Density) Function (CDF): A mathematical expression that quantifies the likelihood (or percent chance) that a quantity will be exceeded. Data Set Outlier: An observation that is numerically distant from the rest of the data in a sample. Outliers are sometimes considered to be faulty data and are removed from the data set. Design Life (of Bridge): The useful life over which a structure is planned to perform its intended function without becoming damaged or obsolete. Typically this term refers to new structures. Deterministic Factor: A parameter which is not variable for a given structure; for example the width of a bridge pier. Equation (Design): A mathematical relationship that envelopes the observed data in such a way that the results are conservative in nature. Equation (Predictive): A mathematical relationship that tends to fit through the cloud of observed data points in such a way that overprediction and underprediction occur with relatively equal magnitude and frequency.

A-12 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Gaussian Distribution: The Standard Normal or âbell-shapedâ probability distribution function. Kolmogorov-Smirnov Test: A nonparametric goodness-of-fit test that compares a probability distribution obtained from a sample to a reference cumulative distribution function, or to a distribution from a second sample. Latin Hypercube Simulation (LHS): A statistical method of generating a sample using equally probable intervals, often used in uncertainty analysis. Level I Analysis/Approach: A method for accounting for uncertainty in bridge scour estimates that multiplies a scour estimate by a âscour factorâ to achieve a desired level of reliability that the resulting scour depth will not be exceeded during a design flood event. Level II Analysis/Approach: A method for accounting for uncertainty in bridge scour estimates that uses Monte Carlo simulation to develop scour estimates for a specific bridge using its unique characteristics. Load and Resistance Factor Design (LRFD): A structural design method that uses calibrated load factors and prescribed code values to achieve a desired level of reliability against structural failure. Load and Resistance Factor Rating (LRFR): A structural rating system used to evaluate bridges based upon calibrated load factors using principles of structural reliability. Log-Transform: The natural logarithms of a data series. Mean: The average value of a sample or a population, typically shown as the symbol . Monte Carlo Realization: One simulation out of many where certain variables are allowed to vary within prescribed limits in accordance with specified probability distributions. Monte Carlo Simulation (MCS): The net result of performing many individual realizations in order to obtain statistical information about the process or phenomenon being modeled. Poisson Process: A stochastic process which counts the number of events and the time that these events occur within a given time interval. Probability: A measure or estimate of the likelihood (or percent chance) that an event will occur or that a statement is true ranging from 0 (0% chance or will not happen) to 1 (100% chance, or will happen). Typically, P symbolizes probability (e.g., the term PF is the probability of failure).

Glossary A-13 Probability (Conditional): The likelihood (percent chance) that a quantity will be exceeded given the condition that another event has occurred or will occur. Probability Distribution (Density) Function (PDF): A mathematical expression that quantifies the likelihood that an event will occur or that a quantity will take on a value or fall within a range of values. Probability Distribution (Log- Normal): A mathematical expression of the Gaussian or âbell- shapedâ probability curve that fits the logarithms of the data points. Probability Distribution (Normal): A mathematical expression of the Gaussian or âbell- shapedâ probability curve that fits the values of the data points. Probability of Exceedance: The likelihood (percent chance) that a quantity will exceed a specified value, typically shown as PN for N years or Pa for annual probability of exceedance. Probability of Non-Exceedance: The likelihood (percent chance) that a quantity will not exceed a specified value. Probability (Unconditional): The likelihood (percent chance) that a design value will be exceeded over the entire remaining service life of a structure. Random Factor: A factor is random when the quantities under study are part of a larger population and the goal of the study is to make a statement or conclusion regarding the larger population. Random Number Generator (RNG): A computational or physical device designed to generate a sequence of numbers or symbols that lack any pattern. rasToolÂ© : The name given to the computer program which links the HEC-RAS hydraulic model to Monte Carlo simulation software. Reliability: A branch of statistics which seeks to quantify the ability of a system or component to perform its required functions under stated conditions for a specified period of time. Reliability Index: The probability of non-exceedance expressed as the number of standard deviations from the mean, typically shown as the symbol . For example, the standard Normal distribution has a probability of non-exceedance of 84.13% at = 1.0, 97.73% at = 2.0, and 99.87% at = 3.0.

A-14 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Risk: The potential that a chosen action or activity (including the choice of inaction) will lead to a loss (an undesirable outcome). In economic terms, risk is often defined as the product of probability of failure times the cost of failure, and is measured in dollars, typically shown as the symbol R. Scour Factor: A safety factor which multiplies a scour estimate to achieve a desired target Reliability Index . Service Life (of Bridge): Similar to Design Life. Refers to the remaining planned life of an existing structure. Typically this term refers to existing structures. Skew (Distribution): A measure of the asymmetry of a probability distribution. Standard Deviation (SD): In probability and statistics, a measure of the spread or dispersion that exists from the average value, typically shown as the symbol . Standard Error (SE): A measure of the accuracy of predictions. In hydrology, SE is often reported as the accuracy, in percent, of a discharge estimate developed using regional regression equations. Stochastic: A non-deterministic system or process which is characterized both by the system's predictable actions and by a random element. Target Reliability: The desired level of probability of non-exceedance. See Reliability Index. Uncertainty (Aleatory): Sources of uncertainty which reflect the natural randomness of a process and which cannot be suppressed by making more accurate measurements. Also referred to as statistical uncertainty. Uncertainty (Epistemic): Sources of uncertainty that reflect the inaccuracies in the modeling of a process. Also referred to as modeling uncertainty. Z Limit: The number of standard deviations from the mean.