An explicit risk-based approach for large-levee safety decisions
by Liu Yi
M.Phil. Civil and Environmental Engineering
xiii, 244 p. : ill., maps ; 30 cm
As one typical flood defense structure, a levee system may pose great risks to the safety of people protected by the system. Levee risk analysis is at the heart of levee hazard mitigation and engineering decision making. An explicit methodology of large-levee risk analysis is desirable....[ Read more ]
As one typical flood defense structure, a levee system may pose great risks to the safety of people protected by the system. Levee risk analysis is at the heart of levee hazard mitigation and engineering decision making. An explicit methodology of large-levee risk analysis is desirable.
This research starts with a literature review on risk definitions, steps involved in risk analysis and available methodologies for each step. Case studies on flood risk analysis are reviewed for dike ring South Holland, New Orleans hurricane protection systems, and levees in the middle and lower reaches of the Yangtze River. The case of New Orleans focuses on levee performance evaluation and flood scenario estimation. The other two cases focus on estimating the consequences, referring much to the previous flood disasters.
Within this study, risk analysis is accomplished in three steps. (1) Failure probability analysis, concerning piping and slope instability. Taylor’s series finite-difference method is employed to calculate the failure probability with SEEP/W and SLOPE/W serving as a deterministic analysis platform, where the effect of flood duration is considered. (2) Levee breach and flood routing analysis. The levee breach process and flood routing in the protected area are simulated using a one-dimensional model, HEC-RAS. Inundation maps are generated to provide flood characteristics: inundation depths, flow velocity and arrival time. (3) Estimation of loss-of-life. Graham’s model is refined based on results of human instability tests and an evacuation model. The refined model is used to estimate the loss of life within a GIS tool, MapInfo Professional. Recommended fatality rates are given in this study. With all components of risk analyzed, scenario trees are constructed to summarize the risks and analyze the effectiveness of measures for risk reduction. The procedure of the integrated levee risk analysis is summarized and illustrated in a flow chart.
Applications of quantitative levee risk analysis are still rare and mostly dependent on data of historic disasters. In this light, a detailed case study on the risks of the North Pearl River Levee System (NPRLS) in Guangdong Province is conducted to illustrate the proposed methods and procedure. Data required for risk analysis (levee historic record, soil parameters, topographic maps and population distribution) are first collected and analyzed. The performance of the levee upon a 100-year flood at Milestone 7+330 is evaluated. The failure probabilities are calculated for three failure modes: overtopping, piping and slope sliding. Two flood scenarios resulted by levee breaches at water level 14.7 m (50-year flood) and 15.53m (100-year flood) are simulated and analyzed. The loss of life is estimated based on the proposed model and the fatality rates suggested by the author. Risks of the levee at the two water levels are compared and analyzed. It is found that an early levee breach at the lower water level may bring higher risks. As indicated in the scenario tree, in order to mitigate the risk, three ways are available: making the system more reliable, improving the efficiency of emergency and evacuation actions, or reducing the fatality rates due to a failure.