Several changes have been made to HEC-FDA for Version 2.0. This page provides a comprehensive description of the study element changes and major improvements made to the HEC-FDA software for Version 2.0. This page focuses on the most important changes for the user. Take a look at the Version 2.0 Release Notes for detailed information on the latest changes to the computational procedures.

Differences in Results

A significant improvement to the aggregated stage-damage algorithm has been shown to cause consequential differences in mean expected annual damage.

The improvement removes bias from the mean expected annual damage estimate obtained from HEC-FDA Version 1.4.3. The bias was caused by a distributional assumption built into the HEC-FDA Version 1.4.3 implementation of the aggregated stage-damage algorithm. This algorithm is documented in Appendix E of the HEC-FDA Version 1.4.1 User's Manual. When computing aggregated stage-damage with uncertainty, HEC-FDA Version 1.4.3 computes damage for each stage many times, collecting a sample of damages for each stage until the sample satisfies convergence criteria. HEC-FDA Version 1.4.3 then calculates the mean and standard deviation of the sample, saves a Normal distribution based on that mean and standard deviation to the database, throws away the sample, and uses the Normal distribution of damage in the calculation of expected annual damage.

The HEC-FDA Version 2.0 development team designed an approach to implement the algorithm documented in Appendix E of the HEC-FDA Version 1.4.1 User's Manual without the need to assume that a sample of damage is Normally distributed. This approach takes the sample of damage to represent an empirical distribution. The team created tools to analyze an empirical distribution within the HEC-FDA Version 2.0 computational engine so that an empirical distribution has all of the same analytical capabilities as a Normal distribution. This technical innovation means that the software can save the sample as an empirical distribution to the database for use in the calculation of expected annual damage. In short, we do not throw away the sample.

It turns out that the shape of a sample of damage is very often very different from Normal. When you change the shape of the distribution by making a distributional assumption, the distribution becomes biased, biasing any estimate based on that distribution. The direction and magnitude will be different for every study because the sample of damage looks different for every study. The effect on differences in damage will also change from study to study as well as from plan to plan due to the strong heteroskedasticity in the aggregated stage-damage function. As a result, alternative ranking based on NED is subject to change.

The difference in EAD will occur when you need to re-compute stage-damage in HEC-FDA Version 2.0. If you transferred your Version 1.4.3 aggregated stage-damage functions into Version 2.0, then EAD calculated in Version 2.0 will match EAD calculated in Version 1.4.3 nicely. Once you use HEC-FDA Version 2.0 to compute stage-damage, this innovative approach comes into play, and your EAD estimate will be improved.

This approach is an improvement but is not without its weaknesses and requirements for assumptions, which we see as opportunity for further innovation. For HEC-FDA Version 2.0, we use very strict convergence criteria --- close to no variation in shape or 500,000 sample members, whichever comes first --- as a means of buying down model uncertainty. The point of contact for questions about this change is Dr. Richard J Nugent III, lead of the HEC-FDA development team: richard.j.nugent@usace.army.mil

Terminology Changes

Version 1.4.3 Term	Version 2.0 Term
Plan	Project condition
Water surface profiles	Hydraulics
Damage reach	Impact area
Geotechnical fragility function	System response curve
Levee	Lateral Structure
Equivalent annual damage	Average annual equivalent damage
Long-term risk	Long-term exceedance probability
Conditional non-exceedance probability	Assurance of Threshold
Probability Damage Reduced Exceeds Indicated Value (0.25, .5, 0.75)	Quartile of Damage Reduced Distribution

Review the Glossary for more HEC-FDA terminology information.

Study Data and Configuration Changes

Terrain

Terrain data (ground elevation) is used the very same way in HEC-FDA Version 2 as it was used in Version 1 - to calculate the depth of flooding. The only difference is the way that the data was imported. In HEC-FDA Version 1, ground elevations had to be imported as part of the structure inventory. In HEC-FDA Version 2, terrain data can be imported to identify the ground elevations at structures if the ground elevation data does not already exist in the structure inventory. Also, terrain data will be required if using the native HEC-RAS output format (HDF) for hydraulic data sets.

Impact Area Sets

Impact Areas are used the very same way in HEC-FDA Version 2 as they were used in Version 1 (in which impact areas were known as damage reaches). The only difference is the format of the data: in Version 1, impact areas were identified using the boundary (upper and lower) river stations. The use of river stations is how we tricked HEC-FDA Version 1 to consume geographic information. HEC-FDA Version 2 can natively process geospatial data. As such, impact areas are identified as polygons and are imported using the polygon shapefile.

Index Points

The primary difference between Versions 1 and 2 is that index points are optional. Index points will only be used to retrieve a graphical stage-frequency function from a hydraulics data set.
In Version 1, index points were identified using a river station. The use of river stations is how we tricked HEC-FDA Version 1 to consume geographic information. HEC-FDA Version 2 can natively process geospatial data. Hence, index points are identified as x-y coordinates and are imported using the point shapefile.
Sometimes, the best index points vary by alternative plan. In HEC-FDA Version 2, you can import multiple sets of index points to accommodate retrieval of graphical frequency functions at the correct locations by alternative plan.

Hydraulics

Hydraulics data is used the same way in HEC-FDA Version 2 as it was in Version 1 - to calculate stage-damage functions. The main difference in the data used by the two versions is the format. The data consumed by Version 1 consisted of a table of data organized by river mile - a 1-dimensional view of the world. In Version 2, the data is represented within a geospatial data set. In Version 2, the hydraulics modeling does not contain flow information like in Version 1, so data on flow-frequency functions and stage-discharge functions must be obtained separately.

Analytical Frequency Function

There are two differences between HEC-FDA Versions 1 and 2. First, the new procedure in HEC-FDA Version 2.0 for quantifying the uncertainty about flow-frequency is consistent with USGS Bulletin 17C - HEC-FDA Version 1.4.3 used USGS Bulletin 17B procedures. Second, synthetically deriving the LP3 parameters will not be supported for new HEC-FDA studies.

Graphical Frequency Function

Graphical flow- and stage- frequency functions are entered and used the same way in both versions.

Regulated-Unregulated Flow Function

Regulated-unregulated flow functions are entered as unique objects. That is, regulated-unregulated flow functions are no longer entered through the frequency function dialogue.

Exterior-Interior Functions

The primary difference between HEC-FDA Versions 1 and 2 is that uncertainty can be optionally defined around the interior stages.

Stage-Discharge Functions

There are no differences between the two versions in the way that stage-discharge functions are entered or used, except that stage-discharge functions cannot be retrieved from the hydraulics because the hydraulic data does not contain flow information.

Occupancy Types

There is a difference in HEC-FDA Version 2 when defining distributions about the content and other-to-value ratios. In HEC-FDA Version 1, the standard deviation, min, or max of the content (or other)-to-structure value ratio is given as a percent of the ratio, whereas HEC-FDA Version 2 directly uses the standard deviation, min, or max. In HEC-FDA Version 1 for example, for a content-to-structure value ratio of 50%, an entered standard deviation of 10% would mean that the plus or minus one standard deviation range is 45% to 55%. In HEC-FDA Version 2, the standard deviation would be entered as 5% to obtain the same range. Importing data into HEC-FDA Version 2 from HEC-FDA Version 1 corrects for this difference.
Uncertainty in HEC-FDA Version 1 could be defined about content-to-structure value ratio but not about content value in the case that content value is contained within the structure inventory (rather than being estimated using the content-to-structure value ratio). In HEC-FDA Version 2, uncertainty can be defined for either approach used.
Multiple groups of occupancy types are allowed. The group-occupancy type assignment is made upon import of a structure inventory.

Structure Inventory

HEC-FDA Version 2.0 does not handle flat file structure inventories (i.e., tab-delimited text files). A structure inventory must be in the form of a point shapefile.
The structure inventory no longer requires information about stations because the software can read the data set to understand where the structures are located. This means that revising impact area delineations to match structural alternatives throughout plan formulation is clean and easy.
Structure modules will not be used in HEC-FDA Version 2.0. To reflect differences in structure inventories between project conditions or analysis years, import separate structure inventories. Only one structure inventory will be used for a given stage-damage compute, so each imported structure inventory should contain all structures required for a given project condition - analysis year combination.
Stage-direct damage functions were entered into study data as part of the structure inventory in HEC-FDA Versions 1. In HEC-FDA Versions 2, a stage-direct damage function is entered into study data as a manually-entered stage-damage function.

Aggregated Stage-Damage Functions

In HEC-FDA Version 1.4.3, stage-direct damage functions were entered into study data through the structure inventory, and were saved, imported from/exported to tab delimited within the occupancy types. In HEC-FDA Version 2.0, a stage-direct damage function for a particular structure should be entered into a study using a manually-entered stage-damage function. The best way to track the structure of interest as it relates to its stage-direct damage function is to assign the structure to its own damage category. Stage-direct damage functions will not import into HEC-FDA Version 2.0 from HEC-FDA Version 1.4.3 - they will have to be manually entered. When using HEC-FDA Version 2.0 to compute stage-damage functions, create and manually enter a dedicated damage category for each structure that uses a stage-direct damage function and manually edit the function as needed.
Index points are no longer required for a stage-damage compute. The aggregation stages are identified using the H&H input functions.

Scenarios

A scenario is a new concept to the HEC-FDA software. A scenario reflects study configuration at the plan-analysis year level. This concept explicitly allows for a more nimble configuration through the ability to assign any of the input functions/data to a given plan-analysis year combination. A scenario contains the same information/configurations contained by the blue highlighted row in the image of the HEC-FDA Version 1 interface below. The default threshold in HEC-FDA Version 2 is calculated in the same the way that the default threshold is calculated in HEC-FDA Version 1. Expected annual damage and project performance results of a scenario are the same results obtained from evaluation of plans by analysis year for a given plan-year combination (a given row in the image below) in HEC-FDA Version 1.

Alternatives

An alternative reflects the project condition dimension in HEC-FDA Version 2. An alternative is a new concept in the HEC-FDA software, though not a new concept in planning, and is closely associated with the Plan in HEC-FDA Version 1. For example, the without-project alternative reflects the information contained in the blue highlighted row in the image of the HEC-FDA Version 1 user interface below. The results of a given alternative are the same as computing equivalent annual damage analysis for a given plan in HEC-FDA Version 1. The discount rate and period of analysis are set in the study properties under the File menu. The discount rate should be provided in decimal form, not percentage.

Alternative Comparison Reports

An alternative comparison report is a new concept to the HEC-FDA software. However, the methods and reporting are the same between the two versions. We have simply gathered the results of plan comparison in one dedicated place. The results of an alternative comparison report in HEC-FDA Version 2 are the same results found in the damage reduced distribution summary reports for expected annual damage and the reduced and distribution summary reports for average annual equivalent damage.

Differences Between Version 1.4.3 and Version 2.x