What is an Exterior-Interior Function?

An exterior-interior stage transform function relates the water surface elevation in the channel (exterior) to the water surface elevation in the floodplain (interior).

What is the Purpose of an Exterior-Interior Function?

An exterior-interior function reflects part of the hazard piece of the risk equation. An exterior-interior function is an uncommon way to identify the relationship between the water surface elevation on the floodplain side versus river side of a levee. Most commonly, this work is done using a Hydraulics data set that reflects the water levels in a floodplain in combination with a flow-frequency and stage-flow function for the river. If consideration is being placed on using an exterior-interior function, please consult with HEC on proper use. 

Where Do I Get an Exterior-Interior Function?

Hydraulic engineers use HEC-RAS or some other hydraulic modeling software to develop the relationship.

Working with Exterior-Interior Function in HEC-FDA

The exterior-interior function should be provided as paired data with uncertainty: a column of external stages and columns specifying the corresponding interior stage distributions. 

Differences Between HEC-FDA Versions 1 and 2

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

Steps to Define an Exterior-Interior Relationship

An exterior-interior stage transform function relates the water surface elevation in the channel (exterior) to the water surface elevation in the floodplain (interior). To create an exterior-interior stage transform function:

  1. From the Study Tree, under Stage Transform Functions, right-click on Exterior-Interior Functions and select the create new option.
    Exterior-Interior functions shortcut command options for creating or importing relationships.
  2. Enter the name for the function and a description (optional, but recommended). 
  3. Select the Distribution Type from the options: 
    • Deterministic (default selection)
    • Uniform
    • Normal
    • Triangular
    • LogNormal
  4. Enter the paired data with or without uncertainty. The X Value column should have external stages and Y Value column(s) should specify the corresponding interior stage.
    Example of the Create New Exterior-Interior Relationship dialog with the Distribution Type list.
  5. Click Save and Close to create the transform function.