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 most commonly used to identify the elevation of flooding in a floodplain behind a levee when the hazard reaches the floodplain in a non-breach scenario. This is particularly useful for floodplains with considerable storage where the stage in the floodplain would not equal the stage in the channel when the stage in the channel reaches or exceeds the top of levee elevation. Without this relationship, HEC-FDA effectively assumes that the stage in the floodplain is equal to the stage in the channel for channel stages above the top of levee elevation.
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
- Right click on Exterior Interior Relationships under Stage Transforms in the Study Tree
- Enter a useful name and description
- Define the exterior-interior coordinates with the specification of the distribution of interior stages for a given exterior stage
- Click Save and Close. See below image for finished Exterior-Interior Relationship.
