Semi-quantitative evaluation of levees risk within the context of hydrology uncertainty

Abstract

Risk assessment is necessary to understand whether a levee is safe and able to prevent flood invasion. Most previous assessment methods attempted to quantitatively assess the risks posed by hydraulic structures using probabilistic analysis from a research perspective. The kind of method depends on the probability density function of factors but sometimes difficult to determine either a priori or posteriori in practice. On the other hand, current semi-quantitative methods despite the convenience are somewhat subjective in their implementation. This study sought to rectify the shortcomings of previous quantitative and semi-quantitative approaches to assessing the risk of levee failure wherein the uncertainty of risk factors is presented within a semi-quantitative framework, in accordance with ISO assessment procedures. The risk of levee failure discussed in this work is mainly related to overtopping and erosion of its foundation. This study proposed a semi-quantitative approach to maintain the exactness and objectivity of the risk assessment of levees and use Puzih river in Taiwan as a case study. In this study, we tried to add uncertainty facts and developed a risk assessment framework for levee management using a risk map according to our semi-quantitative methods. This study firstly defines the risk and associated factors within the context of government regulations, then implement a series of risk assessment tasks using the widely used hydraulic and sedimentation transport model, HEC-RAS, to simulate the risk of inundation, where hydrological events are used as a proxy for uncertainty. This study considers that consequence of levee failure and compute representative qualitative indices by which to evaluate hazards using a risk map and risk matrix in order to illustrate potential economic losses under given flood scenarios. After gaining the levee assessment results, the risk map is also built which can be applied on the practical engineering works. Our results reveal that the most secure levees along Puzih river should be able to withstand flood events with a 100-year return period; however, the levees in a several areas fail to provide suitable protection. Accordingly, this work developed a risk map and corresponding flood prevention strategies to decrease the risk in specific regions for the consideration of rive management administration.