Evolution of geotechnical risk analysis in North American practice - Risk and Reliability in Geotechnical Engineering - page 470

Levee breach/
inundation
assessment

Flood hazards
analysis

Fragility
analysis

Systems
analysis

Consequence
assessment

Load Load Damage

Risk quantification
and uncertainty analysis

Figure 12.5 Overall risk analysis methodology. (Modified from IPET8. 2008. Performance Evaluation of the
New Orleans and Southeast Louisiana Hurricane Protection System, Final Report, v.8— Engineering and
Operational Risk and Reliability Analysis . Washington, DC: Interagency Performance Evaluation
Taskforce.)

and their effects is limited. Hurricane models can predict winds, waves, and surges only with
limited accuracy, and the reliability models used to predict levee performance when subjected
to hurricane forces are similarly limited. Hence, the risk profiles of hurricane-induced flood-
ing cannot be established with certainty. Risk analysis, therefore, had to include not just a
best estimate of risk, but also an estimate of the uncertainty in that best estimate. By identi-
fying the sources of uncertainty in the analysis, measures, such as gathering additional data,
could be taken to reduce the uncertainty and improve the risk estimates.
The analysis examined risks associated with the performance of the hurricane protection
system (HSDRRS). Probabilistic risk analysis used to develop the overall risk analysis meth-
odology of the hurricane protection system is presented in Figure 12.5 . The calculations
of risk were affected by large event-tree analyses. Much of this approach was informed by
earlier work on dam safety risk analysis, as discussed above.

12.8.2 California delta
The Delta Risk Management Strategy study (DRMS) in California undertook a similar risk
analysis of the historical and fragile levee system in the San Francisco Bay Delta, which is
subject to seismic hazard. The project was authorized by the Department of Water Resources
to perform a risk analysis of the Delta and Suisun Marsh and to develop a set of improve-
ment strategies to manage these risks (URS/JBA 2007).
The hazards considered in the DRMS study were (a) earthquakes that cause levees or their
foundations to fail, (b) high storm runoff that can overtop levees or increase seepage and
cause them to fail, (c) “sunny day events” caused by undetected flaws that fail levees during
non-flood-flow periods, and (d) wind waves and erosion that can weaken levees. The analy-
sis included the frequency that events of different magnitudes occur: Smaller earthquakes
and floods occur less often than the more extreme events, but all of them pose some risk to
the levee system. Small events may fail only one levee, and larger events may fail multiple
levees.