Glossary
This glossary is a collection of definitions from throughout the user manual plus definitions of other pertinent terms.
New to Version 2.0
List of new terms (highlighted in yellow) for HEC-FDA Version 2.0. Select the term to view the definition in the Terms and Definitions table provided.
Terminology Changes Between Versions
Terminology changes were introduced to bring the software up to date with the latest policy and guidance.
| Version 1.4.3 Term | Version 2.0 Term |
|---|---|
| Conditional non-exceedance probability | Assurance of Threshold |
| Damage reach | Impact area |
| Geotechnical fragility function | System response curve |
| Levee | Lateral Structure |
| Long-term risk | Long-term exceedance probability |
| Plan | Project condition |
| Probability Damage Exceeds Indicated Value (0.25, 0.5, 0.75) | Damage Quartile |
| Target Stage | Threshold Stage |
| Water surface profiles | Hydraulics |
Terms and Definitions used in HEC-FDA Version 2.0
This is a list of the terms used in the HEC-FDA Version 2.0 program. If the term changed between Version 1.4.3 and Version 2.0, then the Version 1.4.3 terminology is noted. Terminology changes were introduced to bring the software up to date with the latest policy and guidance.
| Current Term | Other Terms | Definition |
|---|---|---|
| Aggregated Stage-Damage Function with Uncertainty | Depth-Direct Dollar Damage Function | A relationship between damage in the floodplain for the range of possible elevations of water in the river. The aggregated stage-damage function is aggregated at the impact area-damage category-asset category level for a given set of hydraulics and H&H input functions (flow-frequency and stage-flow or just stage-frequency). Uncertainty about damage for each elevation of water in the river is computed through Monte Carlo simulation. Depth-direct dollar damage functions in HEC-FDA Version 1.4.3 must be specified in HEC-FDA Version 2.0 as manually entered aggregated stage-damage functions. |
Alternatives | Plans | An alternative represents the without-project condition or a with-project condition, for the entire period of analysis. In HEC-FDA Version 2.0, an alternative connects the results of two impact area scenarios for two analysis years (base year and most likely future year) and a given project condition. The purpose of an alternative is to collect the results necessary to compute average annual equivalent damage (formerly known as equivalent annual damage). An alternative should represent a given project condition and should include one Scenario for each analysis year. Alternative replaces the term Plan. Revised term in HEC-FDA Version 2.0. |
| Analysis Years | Analysis years are for determining static damage and project performance information for specific years, such as the base year or most likely future year. An analysis year represents an instance in time for which the hydrologic and hydraulic summary relationships, hydraulics modeling, and consequences modeling are simulated. If expected annual damage and system performance metrics are the sole interest, only one analysis year is required. If average annual equivalent damage is also of interest for calculation of benefits and comparison with cost, then two analysis years are required. Typically, the base year is the first year of the plan operation and the “Most Likely Future” condition is a development projection for a specific future year, say twenty years out from the base year. The most likely future year usually is based on the projections of local future development scenarios and future changes in hydrology or hydraulics. | |
| Analytical Discharge-Probability Function | A discharge-probability function that is fit with an analytical (statistical) distribution. The distribution is defined by statistics which are the mean, standard deviation, and skew for the Log Pearson Type III distribution. The function is developed by site specific, hydrologic engineering analysis procedures. An equivalent record length should be specified for parametrization of uncertainty about the analytical flow-frequency function. HEC-FDA uses procedures consistent with USGS Bulletin 17C for quantifying the uncertainty about an analytical discharge-probability function. | |
| Annual Exceedance Probability (AEP) | The annual probability that a variable’s magnitude exceeds a given threshold. For example, the per-year chance that the interior floodplain in an impact area will be inundated to some depth due to channel overflow, or in the case of a leveed area, due to levee overtopping or failure. In each case, the AEP can be computed with commonly available risk assessment software, considering the uncertainty in the hydrologic, hydraulic and if necessary levee system response curve inputs. | |
Assurance | Conditional Non-exceedance Probability | The term “assurance” is used to describe and account for the uncertainty that the river-infrastructure system contains the hazard at a given likelihood or stage, either the likelihood that a quantile (e.g., 1% stage) is below some target stage threshold, or the likelihood that the AEP of a target stage threshold is below some probability (e.g., 1%). Assurance is calculable only when risk assessment is carried out with uncertainty. Alternative term for conditional probability of non-exceedance (project has a 0.95 probability of containing the 0.01 exceedance probability event should it occur). In simple terms, assurance is cumulative probability. Assurance can be calculated for a threshold or for AEP. |
| Base Year | The base year is the first analysis year, and is the first year the plan is implemented and operational. | |
| Beginning Damage Depth | A beginning damage depth is an optional structure parameter and reflects the depth in feet where damage begins, relative to the first floor elevation. Negative values reflect depths below the first floor elevation, positive values reflect depths above the first floor elevation. This parameter is crucial in analysis of structures with basements where flood waters enter above basement floor. Truncates damage function at below specified depth. This is an attribute of a Structure within the Structure Inventory. | |
Bulletin 17C | A U. S. Geological Survey publication entitled, “Guidelines for Determining Flood Flow Frequency, Bulletin 17C,” revised May 2019. The publication incorporates changes motivated by four of the items listed as future work within Bulletin 17B and more than 30 years of post-Bulletin 17B research on flood processes and statistical methods (England, et al., 2015). Approach used for analytical flow-frequency uncertainty in HEC-FDA Version 2.0 is consistent with USGS Bulletin 17C. | |
| Content to Structure Value Ratio | The numeric value in percent that represents the estimated content value of a structure as a percent of the structure value, which is specific to a particular structure occupancy type. | |
| Content/Structure Value Ratio Uncertainty | The standard deviation, min, or max of the uncertainty in estimating the content to structure value ratio, in the same units of the content-to-structure value ratio. Analytical options for characterizing the uncertainty include Normal, triangular, and uniform. | |
| Content Value | The value of the contents associated with the structure. Does not include the structure (building) value. | |
| Damage | Economic loss associated with specific flood events. Value may be at a structure, damage reach, or entire study area. | |
| Damage Category | Used to consolidate large number of structures into specific groups of similar characteristics for analysis and reporting purposes. Damage categories are defined for the inventory through the Occupancy Type data. | |
| Depth | Distance of the water surface straight down to the point of interest. Normally, refers to a value associated with corresponding damage from the depth of water above the first floor. | |
| Depth-Percent Damage Functions | Defines the percent of the maximum structure damage for a range of flood stages at a structure. The percent-damage is multiplied by the structure value to get a unique depth-damage function at the structure. Depth is taken to be relative to the first floor. | |
| Discharge | Flow | The volume of water passing a specific point for a given time interval. For example, 2,000 cubic feet per second. |
| Discharge-Exceedance Probability | Flow-Exceedance Probability Flow-Frequency | The relationship of peak discharge to the probability of that discharge being exceeded in any given year. This function can be specified analytically or graphically. |
| English Units | Data entry and output reports are English Units and not SI Units. Indicator is for labeling purposes only. No conversions are performed in this program version. | |
| Equivalent Annual Damage (EqAD) | The damage value associated with the without-or-with project condition over the analysis period considering changes in hydrology, hydraulics, and flood damage conditions. Equivalent annual damage with uncertainty is the result of an Alternatives analysis. | |
| Equivalent Record Length | Number of years of a systematic record of recorded peak discharges at a stream gage. For probability functions derived at ungaged locations using model or other data, the equivalent record length is estimated based on the overall “worth” or “quality” of the frequency function expressed as the number of years-of-record. This parameter is important in risk-based analysis because it relates directly to the uncertainty of the probability function. | |
| Exceedance Probability Event | The probability that a specific event will occur or be exceeded in any given year. For example, the .01 exceedance probability event has one chance in a hundred or a one percent chance of occurring or being exceeded in any given year. | |
| Expected Annual Damage (EAD) | The integral of the damage-probability function. In risk-based analysis it is equal to the average or mean of all possible values of damage determined by exhaustive Monte Carlo sampling of discharge-exceedance probability, stage-discharge, and stage-damage relationships and their associated uncertainties. Expected annual damage is specific to a given analysis year and cannot be compared to costs. | |
| Expected Value | The mean or average value. For normal probability density functions the expected, median, and mode are the same. For the empirical distributions that are produced through Monte Carlo simulation, the mean can be less than or greater than the median and mode, depending on the character of the skew of the empirical distribution. | |
| Exterior | The "exterior" side of the levee or flood wall is the river side where the stage-discharge function applies. | |
| First Floor | The normal entry floor of a building. The first finished floor to which the depth-percent damage functions are indexed. | |
| First Floor Stage | The stage/elevation of the first floor of the structure. First floor stage is an attribute of the structure inventory, and may be used instead of using ground elevation and foundation height. | |
| First Floor Stage Uncertainty | The error of uncertainty in the first floor stage estimate of a particular structure occupancy type. Based on the procedures/type of surveys used to estimate the first floor stage. A normal, triangular, or uniform distribution may be used. | |
| Flood Inundation Damage | The damage that results to structures, contents, automobiles, traffic, infrastructure, etc. when flood waters covers or inundates them. | |
| Flood Risk | Generally, the probability and severity of undesirable consequences. In the context of flood risk, it includes the probability that an area will be flooded, resulting in undesirable consequences, as well as the consequences themselves. As used in this manual, risk is the function of five factors: hazard, performance, exposure, vulnerability and consequences. Risk involves exposure to a chance of injury or loss. | |
| Flood Risk Management | Flood Damage Reduction | Measures and actions taken to reduce flood risk. These may include implementation of reservoirs, detention storage, channel, diversions, levees and floodwalls, interior systems, flood proofing, raising, relocation, and flood warning-preparedness actions. |
| Flow Transform Relationship | A relationship that defines the outflow at a specific point on a stream or river based on the inflow at that point. These functions are used to modify the discharge during at a specific step in the risk-based analysis. An example would be a regulated versus unregulated function for a reservoir that could be used instead of a regulated discharge-probability function to represent the effect of a reservoir. | |
| Foundation Height | The vertical distance between the ground stage and first floor stage at the structure. Foundation height is often used alongside ground elevation as structure inventory attributes for identifying first floor elevation. | |
| Graphical Discharge (or Stage) Probability Function | A graphical or non-analytical discharge- or stage-probability function is one drawn graphically. For HEC-FDA, graphical functions should use Wiebull's plotting positions (not median). The reason is that Weibull positions are mean estimates of exceedance probability, and therefore unbiased. Uncertainty about a graphical frequency function is simulated using the Less Simple Method. | |
| Ground Elevation | Ground Stage | The stage or elevation of the ground at the structure in feet. |
| HEC-RAS | The Hydrologic Engineering Center’s River Analysis System computer program for computing hydraulic data sets for a series of flood events and stage-discharge functions with uncertainty. The program output may be designed to enable ready import of 2D hydraulic data sets into the HEC-FDA program as HDF. Gridded data is also handled. | |
| Historic Events | Observed and sometimes recorded flood events such as the 1973 Flood or 1993 Flood. | |
| Hydrology | Frequency Functions | Hydrology involves the estimation of the amount and shape of the runoff response throughout the study area. It also includes the frequency of the events. Hydrologic modeling takes the form of a Frequency Function in HEC-FDA Version 2.0. |
| Hydraulics | Water Surface Profiles | Hydraulics involves analysis of stream water surface profiles, flood inundation boundaries, and other technical studies of stream flow characteristics. Hydraulics data sets were previously known as water surface profiles. |
| Hypothetical Probability Events | A series of hydrographs derived from rainfall-runoff analysis using hypothetical frequency based rainfall patterns. Rainfall values are normally derived from National Weather Service Publications e.g., Technical Papers TP 40 and TP 49. | |
Impact Area Set | Damage Reaches | An impact area set is the collection of spatial floodplain areas, previously known as damage reaches. You use the impact area set to define consistent data for plan evaluations and to aggregate structure and other potential inundation damage information by stage of flooding. The impact area set is integral to both the hydrology and economic analyses. You must delineate the impact area set consistent with flood risk management measures. In other words, the spatial impact of a levee must exactly match the delineation of the impact area. Impact areas is a new term in HEC-FDA Version 2 which replaces the term damage reaches in Version 1. An impact area set is imported as a polygon shapefile. Revised term in HEC-FDA Version 2.0. |
| Interior | The “interior” side of the levee or flood wall is the floodplain or protected side where the stage-damage function applies. | |
| Less Simple Method | The "Less Simple" method is an approach to quantifying the uncertainty about a graphical frequency function that is based on an asymptotic approximation used to extrapolate the order statistics method. This approximation is based on the features of the order statistics method, and provides a result with the same characteristics, such as smaller uncertainty when the frequency curve is flatter; and larger uncertainty when it is steeper. | |
| Log Normal Distribution | A two-parameter probability distribution defined by the mean and standard deviation. A non-symmetrical distribution applicable to many kinds of data sets where the majority (more than half) of values are less than the mean but values greater than the mean can be extreme, such as with stream flow data. The distribution is truncated at three standard deviations. | |
| Log Pearson Type III Distribution | Analytical discharge-probability functions can be defined (fit) with a statistical distribution such as Log Pearson Type III. The distribution has statistical parameters that define the moments of the data about the analytical curve. For a Log Pearson Type III distribution, these statistical parameters are mean (first moment), standard deviation (second moment), and weighted skew (third moment). Also, an there is an additional parameter, equivalent record length. | |
| Mean | The average value of a set of numbers, such as the annual peak discharges that have occurred over a period of time. | |
| Median | Value where there is a .5 probability that the actual value is less than that value. The middle value of an ordered list. | |
| Mode | The most frequently occurring value in a data set. | |
| Monte Carlo Simulation | A method for calculating an empirical probability distribution of a function of random variables by simulation of pseudorandom outcomes. Monte Carlo simulation seeks to simulate stochastic processes by random selection of values in proportion to their joint probability density function. | |
| Most Likely Future | The most likely future condition development projection for a specific future year. Used in equivalent analysis of project over it's life. Normally 25-30 years out from Base Year. Must be greater than Base Year. | |
| Nonstructural | Nonstructural measures include raising, relocating, flood proofing, and regulatory and emergency actions associated with structures and damageable property that modify the existing and/or future damage susceptibility. | |
| Normal Distribution | A two-parameter probability distribution defined by the mean and standard deviation. A symmetrical “bell shaped” curve applicable to many kinds of data sets where values are equally likely to be greater than or less than the mean. Also called a Gaussian distribution. The distribution is truncated at three standard deviations. | |
| Ordered Events | Ranked events or plotting positions that define a graphical discharge- or stage-probability function. Ordered events are used to determine the uncertainty about the discharge- or stage-probability function using order statistics. The Wiebull's plotting positions, which represent expected values, should be used. | |
| Order Statistics | Statistical procedure for defining the sample errors of events that define a graphical frequency function. Order statistics is the theory by which HEC-FDA defines the uncertainty (error limit curves) about a graphical frequency function defined by Weibull plotting positions (ordered events) for a specified equivalent record length. HEC-FDA uses an asymptotic approximation called the Less Simple method and ultimately relies on a normal distribution is used to define the errors. | |
| Plan Evaluation | The assessment of economic and plan performance to determine the impact of one or more plans versus the without-project condition. Results may be viewed by specific analysis years or as equivalent annual values for the project life. | |
| Plan Formulation | The development and evaluation of one or more measures and actions into a plan designed to reduce flood damage for one or more damage reaches. | |
| Ponding Area | Storage area usually a lake or pond normally requiring stage-frequency functions instead of discharge-frequency functions for analyses. | |
| Price Year | The price year associated with present updated economic (flood damage) values over the base price index. | |
| Probability | Likelihood | The chance of a random event occurring, often estimated by the ratio of past outcomes that satisfy the conditions of the event to the number of total past outcomes. Frequency or fraction; the ratio of outcome from a repetitive experiment of some kind, such as flipping coins. Such a number is called an "objective" probability because it exists in the real world and is in principle measurable by doing the experiment. A measure of the degree of confidence in a prediction, as dictated by the evidence, concerning the nature of an uncertain quantity or the occurrence of an uncertain future event. This measure has a value between zero and 1.0. Subjective probability; quantified measure of belief, judgment, or confidence in the likelihood of an outcome, obtained by considering all available information honestly, fairly, and with a minimum of bias. Subjective probability is affected by the state of understanding of a process, judgment regarding an evaluation, or the quality and quantity of information. It may change over time as the state of knowledge changes. |
| Probability Function | Frequency function | Input probability functions include a discharge- or stage-exceedance probability relationship developed by traditional, site specific, hydrologic engineering analysis procedures. Probability function type is either graphical (discharge- or stage-probability). An analytical discharge-exceedance probability function is one that can be fit with an analytical (statistical) distribution such as Log Person Type III. A graphical discharge- or stage- exceedance probability function is one that is best fit by a graphical, eye-fit curve. HEC-FDA produces damage probability functions through the Scenario analysis Monte Carlo simulation. |
Project Condition | A project condition reflects a given alternative plan, such as the no-action alternative (without-project condition), or one of the with-project alternative plans. If without-project condition damage is the sole interest, then only one project condition is required. If alternative plans are being evaluated, then one additional project condition is required to represent each additional alternative plan. New term in HEC-FDA Version 2.0. | |
| Period of Analysis | The period of time the plan is assumed in place and operational. Normally fifty years. The base year is the first year of the plan being operational. The most likely future year must lie within the period of analysis. | |
| Residual Risk | Flood risk that remains if a proposed flood damage reduction project is implemented. Includes the consequence of capacity exceedance. | |
Risk Analysis |
| Approach to evaluation and decision making that explicitly, and to the extent practical, analytically, incorporates consideration of risk and uncertainty. Includes risk assessment, risk communication, and risk management. |
Scenarios | A scenario collects the hydrologic, hydraulic, and economic data for each impact area and for a given project condition and analysis year. New term in HEC-FDA Version 2.0. | |
| SI Units | Metric units such as meters (stage, heights, etc.) and kilometers (distances). | |
| Skew | A statistic used as a measure of the dispersion of the data about the mean. The third moment statistic of a Log Person Type III distribution, calculated using the mean logarithm, the standard deviation of the logs, and the logarithms or peak discharge values. | |
| Stage | The vertical distance in feet (meters) above or below a local or national datum. | |
| Stage-Damage | The relationship of damage to a range of flood stages at a structure or the aggregated values of the structure damage by damage categories for a range of stages at the damage reach index location station. | |
| Stage-Damage Functions with Uncertainty | Stage-damage functions with uncertainty are computed at each structure and aggregated by damage category to damage reach index locations. Stage is elevation or locally referenced stage associated with the structure and index location. Damage is the median estimate of structure, content, or other inundation reduction damage associated with the stage of flood waters at the location. Uncertainty in the stage-damage function is due to errors in estimating the depth-damage function, first floor stage, structure value, and content to structure value ratio. | |
| Stage-Discharge Function | A graphical relationship that relates the uncertain stage of a river to a given discharge at a specific location on a stream or river. Referred to as a rating function or curve. These relationships are usually developed by computing water surface profiles for several discharges and plotting the stages vs. discharge relationship at a specific stream location. Uncertainty is the distribution of the errors of stage estimates about a specific discharge. The error probability density functions available are normal, log normal, and triangular. | |
| Stage-Probability Functions with Uncertainty | Relationship of water surface stage and exceedance probability of occurrence. Uncertainty is the distribution of the errors of stage estimates about a specific ordinate. The Less Simple Method is used to define the errors of the graphical relationship. | |
| Steady Flow | State where depth in flow is open channel does not change during the time interval under consideration, (Chow, Open Channel Hydraulics, 1959). | |
| Storage Area | Storage area is a water body such as a lake or pond that normally requires a stage-frequency instead of discharge-frequency function analysis. | |
| Structure Coordinates | The x, y spatial coordinates of the location of the structure. Not a required attribute; structure location is determined using modern GIS formats. | |
| Structure Occupancy Type | The name given to a similar set of structures that is used to define the depth-percent damage function and first floor, structure value, and content/structure value ratio uncertainty of the type of structures. Each structure is assigned to a structure occupancy type. Several structure occupancy types may be assigned to the specified study damage category. For example, single-story no basement, single-story with basement, duplexes, mobile homes are structure occupancy types assigned to a residential damage category. An existing structure occupancy type can be selected, updated, and deleted. | |
| Structure Value | The structure value is typically represented by the depreciated structure replacement value. | |
Study Data Elements | Components of the study, such as terrain data, impact area sets, index points, hydraulic data, frequency functions, and the like. New term in HEC-FDA Version 2.0. | |
| Surveyed Year | The year associated with the structure survey flood damage values used in the structure inventory. For reference only, not used in calculation. | |
| System of Units | Units of measure for study data and output reports. The units referenced in the user interface are for display only. The user must keep track of the agreement of units of measurement across all study data sets. | |
| System Performance | Plan Performance | The system's response to the hazard. Performance refers to the system features and the capability to accommodate the flood hazard as a single event or load. In this manual, it is termed "system performance" (also termed "engineering performance"). Performance also refers to a metric that describes the uncertainty of the system accommodating a specific event (assurance) and the full range of events (AEP and LTEP). In that light, in addition to the levee failure probability functions, performance can also be described by the interior-exterior functions for leveed areas; unregulated-regulated transforms for reservoirs and diversions; and stage-discharge functions (rating curves) for channels. These too would be considered "system performance". When the structural integrity of a system or system component is discussed, such as the system response curve, the reference would be termed "structural performance". When the economics of a system is discussed, the reference would be termed "economic performance". The performance of a feature is described by various elements, such as flood risk management, reliability, capability, efficiency, and maintainability. Design and operation affect system performance. |
| System Response Curve | Geotechnical Failure Function Fragility Function | A single relationship of exterior levee stage versus probability of failure, developed external to the program and entered for analysis. |
| Threshold Stage | Target Stage | The threshold stage is the index by which system performance statistics are calculated. There are four different cases for determining the threshold value:
Revised term in HEC-FDA Version 2.0. |
| Top of Levee Stage | The stage of the top of the levee, flood wall, or tidal barrier that exists at the site or is being evaluated. | |
Total Risk | Total risk is the sum of failure risk and non-failure risk. Traditionally, when there is a levee assigned to an impact area (a.k.a. damage reach), HEC-FDA would calculate failure risk only, which is appropriate when there is no interior flooding expected if the levee does not fail. However, if there is any type of significant interior flooding despite the levee performing correctly (e.g., due to overtopping without failure or if a levee is outflanked), we need to include that non-fail risk. New term in HEC-FDA Version 2.0. | |
| Triangular Distribution | A three-parameter bounded probability distribution defined by the minimum, most likely (mode), and maximum. | |
| Uncertainty | Measure of imprecision of knowledge of parameters and functions used to describe hydraulic, hydrologic, geotechnical and economic aspects of a project plan. | |
| Uncertainty Distribution | Error distributions | A statistical relationship of possible outcomes that defines the dispersion or variation of errors about the median or “best estimate” of values along a function. Uncertainty distributions are used to quantify errors probabilistically when using risk-based analysis. The magnitude and range of input uncertainty distributions are defined by parameterizing an analytical distribution. Analytical options for characterizing the uncertainty include Normal, triangular, and uniform. Uncertainty distributions produced through Monte Carlo simulation in HEC-FDA are represented empirically. |
| Uncertainty Distribution Limit Curves | Error limit curves | Curves above and below the median or "best estimate" curve that describe the distribution of errors about the best estimate values. Error limit curves define the uncertainty of functions and are developed by fitting a statistical distribution to known data or the results of model sensitivity analyses. Error limit curves are for visualization and troubleshooting, the visualized curves are not used as bounds in Monte Carlo simulation. |
| Uniform Distribution | A two-parameter bounded probability distribution defined by the minimum and maximum that is used for data that varies uniformly between the minimum and maximum values. | |
| Unsteady Flow | The flow condition where depth changes with time (Chow, Open-channel Hydraulics, 1959). | |
| Updated Price Index | A value used to update all economic (structure damage) values to present (or other) values. The price index should be given as a ratio, so a price index of 2 would double values. | |
| Updated Year | The price year associated with the updated price index. Must be greater than or equal to the Surveyed Year. If left blank, the default will be the Surveyed Year. Not used in calculation, reference only. | |
| With-Project Condition | The economic, performance, and other conditions associated with implementing a flood damage reduction plan. It is compared to the without-project condition. It includes the base-year and static future analysis year periods to determine the equivalent values. | |
| Without-Project Condition | The condition of the study area for the project life analysis period that is anticipated to most likely occur if the flood damage reduction measures and actions are not implemented. It includes the base year and static future analysis year periods to determine the equivalent values. It is the basis or baseline condition for evaluating the benefits and performance of potential flood damage reduction plans. Emphasis is initially placed on defining the without-project base-year conditions. | |
| Year in Service | An attribute of a structure inventory, the year is service is the year that a particular structure was built or a future year when the structure is assumed to be in place. Used to designate which structures are to be used for the specified analysis years. Only structures with year in service dates equal or less than the designated analysis years are used. |
Retired Terms and Definitions
The below list of terms include retired terminology for changes in the term used for an existing concept, or concepts retired altogether.
| Term | Definition |
|---|---|
| Bank | Stream bank (looking downstream) where the damage reach or structure is located. The delineation of the bank should consider potential local protection flood damage reduction measures and jurisdictional boundaries. Some measures are typically implemented on only one bank, which include levees and walls, and various nonstructural actions. Channels and upstream storage projects reduce flooding for both banks. Bank is not a concept in 2.0 because the software calculates consequences geospatially. |
| Beginning Station | Beginning station number for downstream end of damage reach. Station is not a concept in HEC-FDA Version 2.0. |
Bulletin 17B | A U. S. Geological Survey publication entitled, “Guidelines for Determining Flood Flow Frequency,” by the Hydrology Subcommittee, Interagency Advisory Committee on Water Data, revised September 1981. The publication describes procedures for developing discharge-frequency functions using stream flow records. These procedures are recommended for all Federal agencies applications. |
| Compute Synthetic Statistics | The Log Pearson Type III statistics are computed based on the discharges associated with the 0.50, 0.10, and 0.01 probabilities of an adopted probability function. The synthetic statistics are based on equations given in Guidelines for Determining Flood Flow Frequency, Bulletin 17B, USGS, September 1981. This is not currently supported in HEC-FDA Version 2.0 but synthetically-derived functions in 1.4.3 can be imported into 2.0 using the ProbData.dbf file in the study directory. |
| Effective Wave Height | The vertical distance between the crest of a wave and the preceding wave trough. |
| Ending Station | Ending station number for upstream end of damage reach. The range of allowable values is from 0.00 to 999,999.99. Value must be greater than the Beginning Station. |
| HEC-2 | The Hydrologic Engineering Center's Water Surface Profile Computations computer program. The program output may be designed to enable ready import of water surface profiles data sets into the HEC-FDA program. |
| HEC-SID | The Hydrologic Engineering Center's Structure Inventory for Damage computer program. The structure inventory and depth-percent damage functions may be imported into the HEC-FDA program. |
| Index Location Station | A stream station location within a damage reach used to specify discharge-probability, stage-discharge, and stage-damage functions with uncertainty data for plan evaluations for that damage reach. Also, the index location station (corresponding stream station) is used for aggregation of stage-damage functions with uncertainty by damage category under economic analysis. The index location station is defined between the beginning and ending station values and normally where data is deemed most reliable, such as a streamgage location. The range of allowable values is from 0.00 to 999,999.99. Value must be greater than the Beginning Station and less than or equal to the Ending Station. |
| Invert | Stage associated with zero discharge or bottom of channel. The range of allowable values is from -300.00 to 30,000.00. |
| Left Bank | Indicates that the damage reach is only along the left bank of the stream. The left bank is defined looking downstream or with the current of the stream. |
| Right Bank | Indicator that a damage reach is only along the right bank of the stream. The right bank is defined looking downstream or with the current of the stream. |
| Still Water Stage | Stage that the water surface assumes if all wave action is absent. |
| Stream Station | Study adopted stations along the stream normally denoted as miles (kilometers) above the mouth of the stream. Must be consistent between damage reach boundaries, damage reach index location, water surface profiles, and structure location. The range of allowable values is from 9,999,999.99 to 9,999,999.99. |
| Streams | Streams are defined for the study and are therefore common for all plans and analyses. They include one or more damage reaches. They are used to aggregate data entry and output report information by the stream title. Streams are generalized to include rivers, creeks, bayous, channels, ditches, canals and even ponding areas. One or more streams may be specified as part of the study area. Several hydraulic routing reaches along the stream are normally combined by stream name for analysis and reporting purposes. Water surface profile data are entered by streams. |
| Total Overtopping Height (HW) | The difference between the levee or flood wall stage and the sum of the still water stage and the wave runup. Wave runup is assumed equal to wave height if the still water stage is below the levee or flood wall and two-thirds wave height if the still water elevation is above the levee or flood wall. These assumptions for wave runup are appropriate for non-coastal applications where waves are relatively small. |
| Wave Height | The vertical distance between the crest of a wave and the preceding wave trough. Wave height is dependent on still water level. |
| Wave Overtopping | The effects of water overtopping a levee or flood wall and the associated contribution to flood depth on the interior. |
| Weighted Skew | A statistic used as a measure of the dispersion of the data about the mean equal to the number of values times the sum of the cubes of the deviations from the mean divided by the number of values minus one, times the number of values minus two, times the standard deviation cubed. The third moment statistic of a Log Pearson Type III analytical discharge-probability function calculated using the mean logarithm, the standard deviation of the logs, and the logarithms of peak discharge values. |