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HEC-SSP is designed to perform statistical analyses of hydrologic data. The following is a description of the major capabilities of HEC-SSP. User Interface

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
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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 in draft form by the Subcommittee on Hydrology in December 2015. The Bulletin 17C document is currently being reviewed.

General Frequency Analysis – This component of the software allows the user to perform peak flow frequency analyses by various methods. Additionally the user can perform frequency analysis of variables other than peak flows, such as stage and precipitation data.

Volume Frequency Analysis – This component of the software allows the user to perform a volume frequency analyses on daily flow or stage 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.

Balanced Hydrograph Analysis – This component provides a tool for the creation of balanced hydrographs of multiple durations using existing flow and volume frequency analyses and input hydrograph shapes.

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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 requested to enter a name and description for each study under development. Once the study name is entered, 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, BalancedHydrograph, 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.

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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 display 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 the input data, preliminary results, all of the statistical tests (Low and High Outliers, Broken Record, Zero Flows Years, Incomplete Record, Regional Skews, and Historic Information), and final results. This report file is similar to the FFA output file.

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