HEC-SSP is designed to perform statistical analyses of hydrologic data. The following is a description of the major capabilities of HEC-SSP.
The user interacts with HEC-SSP through a graphical user interface (GUI). The main focus in the design of the interface was to make it easy to use the software, while still maintaining a high level of efficiency
for the user. The interface provides for the following functions:
- File Management
- Data entry, importing, and editing
- Statistical analyses
- Tabulation and graphical displays of results
- Reporting facilities
Statistical Analysis Components
Flow Frequency Analysis (Bulletin 17) – This component of the software allows the user to analytically perform annual peak flow frequency analyses. The software implements two algorithms for computing
annual peak flow frequencies. The first is contained within Bulletin 17B, which was published by the Interagency Advisory Committee on Water Data in 1982. The second is contained within Bulletin 17C, which was
released by the Subcommittee on Hydrology in 2018.
General Frequency Analysis – This component of the software allows the user to perform frequency analyses on hydrologic data using various methods. Types of data that can be used in this analysis
include (but aren’t limited to) flow, stage, and/or precipitation.
Volume Frequency Analysis – This component of the software allows the user to perform a volume frequency analysis on daily flow data.
Duration Analysis – This component of the software allows the user to perform a duration analysis on any type of data recorded at regular intervals. The duration analysis can be used to show the
percent of time that a hydrologic variable is likely to equal or exceed some specific value of interest.
Coincident Frequency Analysis – This component of the software assists the user in computing the exceedance frequency relationship for a variable that is a function of two other variables.
Curve Combination Analysis - This component of the software allows the user to define a “best fit” empirical distribution using two or more input datasets.
Balanced Hydrograph Analysis – This component of the software provides a tool for the creation of balanced hydrographs of multiple durations using existing flow and volume frequency analyses and input
Distribution Fitting Analysis - This component of the software provides a tool for fitting multiple different analytical distributions using two possible fitting methods. This tool allows the user to
assess the uncertainty in distribution/fitting method choice for their data set.
Mixed Population Analysis - This component of the software allows the user to compute a combined frequency distribution given multiple input frequency distributions using the probability of union concept.
Data Storage and Management
Data storage is accomplished through the use of "text" files (American Standard Code for Information Interchange; ASCII; and Extensible Markup Language; XML), as well as the HEC Data Storage System (HEC-DSS). User input data
are stored in flat files under separate categories of study, analyses, and a data storage list. Gage data are stored in a project HEC-DSS file as time series data. Output data is predominantly stored in HEC-DSS, while a
summary of the results is written to an XML file. Additionally, an analysis report file is generated whenever a computation is made. This report file is written to a standard ASCII text file.
Data management is accomplished through the user interface. The modeler is required to enter a Name and Description for each study under development. If necessary, a previously imported data set must also be selected.
Once the study name is entered and a data set has been selected (if required by the analysis), a directory with that name is created, as well as a study file. Additionally, a set of subdirectories is created with the
following names: Bulletin17Results, GeneralFrequencyResults, VolumeFrequencyAnalysisResults, DurationAnalysisResults, CoincidentFreqResults, CurveCombinationAnalysis, BalancedHydrograph, DistributionFitting, MixedPopulation,
Layouts, and Maps. As the user creates new analyses, an analysis file is created in the main project directory. The interface provides for renaming and deletion of files on a study-by-study basis.
Graphical and Tabular Output
Graphics include a map window, plots of the data, and plots of analysis results. The map window can be used to display background map layers. Locations of the data being analyzed can be displayed on top of the map layers. Once
data are brought into HEC-SSP, they can be plotted for visual inspection. The frequency curve plots show the results of the analyses, which include the analytically computed curve, the expected probability curve, confidence
limits, and the raw data points plotted based on the selected plotting position method. Tabular output consists of tables showing the computed frequency curves, confidence limits, and summary statistics. All graphical and
tabular output can be displayed on the screen, sent directly to a printer (or plotter), or passed through the Windows Clipboard to other software, such as a word-processor or spreadsheet.
A report file is available for each analysis. This report file includes options selected by the user, an echo of the input data, processed data, and results. The report file format is similar those generated by legacy
applications like HEC-FFA.