To capture the likelihood of levee failure at specified levee locations, levee failure elevations will be sampled from the input fragility curves at each project location for each event or realization. HEC-WAT randomly selects a value between zero and one for either each event or each realization (user's choice), from the user-defined fragility curve(s) in the Fragility Curve Editor (Figure 17.1). The corresponding levee breach elevation is then passed to the RAS model alternative. HEC-RAS uses that elevation as the trigger failure elevation, which is defined in the HEC-RAS Levee (Lateral Structure) Breach Data Editor (Figure 17.2; for further details refer to the HEC-RAS User's Manual). If the HEC-RAS computed water surface elevation meets or exceeds the elevation sampled from the fragility curve, the levee is assumed to breach and the breach progresses according to the defined values within the HEC-RAS model.
Fragility is also an element of the project life cycle process. Commonly, without maintenance, structures (i.e., levees) deteriorate over time. This can be reflected within a fragility curve by increasing the likelihood of failure at a given water surface elevation (or a lower elevation for a given probability). Scheduled maintenance can be reflected within a fragility curve by decreasing the likelihood of failure at a given elevation or raising the elevation for a given probability. A levee failure and subsequent repair would also have an effect on the fragility curves. Figure 17.3 displays an example 50-year life cycle, and the hypothetical change in levee fragility with time as well as failure and repair. Also, the randomly sampled failure elevation


Figure 17.1 Fragility Curve Editor

Figure 17.2 HEC-RAS – Levee (Lateral Structure) Breach Data Editor
at that location (from random value 0.29) used for one realization of the Monte Carlo simulations is displayed in Figure 17.3. This sampled failure elevation would decrease as the fragility curve showed weakening over time and would further decrease after a failure, but would increase after subsequent repair. To capture these changing conditions, the user must enter a different fragility curve and perform a separate analysis for each condition within HEC-WAT.

Figure 17.3 Levee Fragility Curves at Different Points during a Fifty-Year Life Cycle

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