Fragility curves may be sampled for every event or once per lifecycle or realization.  In both sampling by event as well as the lifecycle/realization sampling, the uncertainty distribution for the fragility curve is sampled every realization to generate a fragility curve for each failure mode at each structure.

HEC-WAT Flood Risk Analysis (FRA) computes sample inputs in two loops: an outer loop where parameters linked to knowledge uncertainty are sampled once per realization, while an inner loop samples parameters linked to natural variability once per event.

(is there a figure from the general HEC-WAT manual I can use or link to)

Case 1: Randomize by Lifecycle or Realization

At the start of the Realization (Outer Loop)

  1. Randomly generate a fragility curve from the user input distribution for every potential failure mode
  2. Sample an elevation from the curve for every potential failure mode
  3. Sample a duration, if applicable, for every potential failure mode

At the start of each Event (Inner Loop)

  1. Choose a potential failure mode according to assigned probability.
  2. Save elevation and duration data to the Fragility Curve sampled object

Case 2: Randomize by Event

At the start of the Realization (Outer Loop)

  1. Randomly generate a fragility curve from the user input distribution for every potential failure mode

At the start of each Event (Inner Loop)

  1. Choose a potential failure mode according to assigned probability.
  2. Sample an elevation from the curve of the active potential failure mode
  3. Sample a duration, if applicable, for the active potential failure mode
  4. Save elevation and duration data to the Fragility Curve sampled object