What is a Stage-Discharge Function?

A stage-discharge function is also known as a stage-flow function and under certain circumstances, a rating curve. This function represents the relationship between a given flow and the expected distribution of stages at the index location of a given impact area. This function is required unless a stage-frequency function is used. See Chapter 5 of Engineering Manual 1110-2-1619, dated 1 August 1996, to learn more about rating curves. A stage-discharge function is typically unique to an impact area scenario, unless the stage-discharge function is the same for two analysis years of a given project condition or for multiple project conditions. Importantly, the range of stages must span the range of stages of the stage-damage function. HEC-FDA will not extrapolate the stage-discharge function beyond the highest coordinate.

What is the Purpose of a Stage-Discharge Function?

A stage-discharge function in part represents the hazard piece of the risk equation and transforms flows into stages and reflects the uncertainty about the water surface elevation for a given flow. The rating curve is the translator between the flow-frequency function and the stage-damage function.

Where Do I Get a Stage-Discharge Function?

Hydraulic engineers will evaluate stage versus flow data at a USGS gauge or use HEC-RAS or some other hydraulic modeling software to develop the relationship.

Working with a Stage-Discharge Function in HEC-FDA

 The stage-discharge function should be provided as paired data with uncertainty: a column of flows and a column of the corresponding stage distributions. 

Differences Between HEC-FDA Versions 1 and 2

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.  

Steps to Define a Stage-Discharge Function

  1. Right-click on Stage-Discharge Functions under Stage Transform Functions within the Study Tree and select Create New Stage-Discharge Relationship...
  2. Give the stage-discharge function a useful name and useful description.
  3. Define the stage-discharge function coordinates with the specification of the distribution of stages for a given flow. Be sure to include the zero flow-invert stage coordinate.
  4. Click save and close. See below image for finished Stage-Discharge Function.