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

  1. Right click on Lateral Structures and select Create New Lateral Structure
  2. Give the levee a useful name and description. Be sure to include the impact area in the name and/or description
  3. Specify the top of levee elevation
  4. Optionally select user-defined failure function and specify the probability of breach for each stage. See below image for finished Lateral Structure with failure function.