What is a Lateral Structure?

A lateral structure is a barrier against the hazard which is built between the channel and the people and assets exposed in a floodplain. Lateral structure is the general term for flood risk management measures like levees. The term levee can be used in this user manual interchangeably with the term lateral structure. The imperfect performance of the lateral structure system is represented using a relationship between the hazard loading (stage in the channel) and the probability of breach (water entering floodplain). The default failure function assumes 0% chance of breach for stages up to the top of levee elevation, after which the water level in the floodplain is equal to the water level in the channel with certainty. Avoid using the default failure function when possible because no levee should be assumed to be competent to the top of the levee without the benefit of some sort of geotechnical analysis. This is especially the case for non-federal levees or old levees that may have deteriorated and can no longer be assumed to hold water to the stage initially intended. 

What is the Purpose of a Lateral Structure?

A lateral structure reflects the performance piece of the risk equation. For a given stage in the channel up to the top elevation of the levee, the depth of flooding in the floodplain is reduced to zero in the non-breach scenario. A failure function is used to represent the chance of a breach scenario given the flood magnitude.  

What Data is Needed for a Lateral Structure?

At a minimum, the top of levee elevation is required. A failure function, referred to as a system response curve, with the probability of breach for each stage up to the top of levee elevation is desirable. This failure function should be developed by the geotechnical engineer on the PDT and should follow the procedures in ETL 1110-2-588 for guidance on developing composite fragility curves. 

Working with Lateral Structures in HEC-FDA

Steps to Specifying a Levee in HEC-FDA

Optionally, specify levees with a top of levee elevation minimally. Ideally, select user-defined and enter a system response curve in stages and failure probabilities. To create new lateral structure:

  1. From the Study Tree, right-click Lateral Structures and select the create new option. 
    Lateral Structures shortcut command options for creating or importing relationships.
  2. Give the levee a useful Name and Description (optional, but recommended). 

    Lateral Structure IconLateral Structure Failure Function IconWhen naming lateral structures it is recommended to include the impact area in the name. For example, "Left-bank" is included in the name for the levee in the example below because the levee in the Example Study Data is in the left-bank impact area.

  3. Specify the Top of Levee Elevation.
  4. The System Response Curve is set to Default. Alternatively, select User Defined and enter the failure function to specify the probability of breach for each stage. 
    Create New Lateral Structure dialog with the User Defined option selected and displaying an example geotechnical failure system response curve.
  5. Click Save and Close to add the lateral structure.