Probabilistic flood hazard mapping: effects of uncertain boundary conditions

A Domeneghetti,B Merz,A Brath,S Vorogushyn,A Castellarin

doi:10.5194/hess-17-3127-2013

Abstract

Abstract. Comprehensive flood risk assessment studies should quantify the global uncertainty in flood hazard estimation, for instance by mapping inundation extents together with their confidence intervals. This appears of particular importance in the case of flood hazard assessments along dike-protected reaches, where the possibility of occurrence of dike failures may considerably enhance the uncertainty. We present a methodology to derive probabilistic flood maps in dike-protected flood prone areas, where several sources of uncertainty are taken into account. In particular, this paper focuses on a 50 km reach of River Po (Italy) and three major sources of uncertainty in hydraulic modelling and flood mapping: uncertainties in the (i) upstream and (ii) downstream boundary conditions, and (iii) uncertainties in dike failures. Uncertainties in the definition of upstream boundary conditions (i.e. design-hydrographs) are assessed through a copula-based bivariate analysis of flood peaks and volumes. Uncertainties in the definition of downstream boundary conditions are characterised by uncertainty in the rating curve with confidence intervals which reflect discharge measurement and interpolation errors. The effects of uncertainties in boundary conditions and randomness of dike failures are assessed by means of the Inundation Hazard Assessment Model (IHAM), a recently proposed hybrid probabilistic-deterministic model that considers three different dike failure mechanisms: overtopping, piping and micro-instability due to seepage. The results of the study show that the IHAM-based analysis enables probabilistic flood hazard mapping and provides decision-makers with a fundamental piece of information for devising and implementing flood risk mitigation strategies in the presence of various sources of uncertainty.

Highlights

Many studies in the literature highlight how inundation hazard and risk assessments are affected by several sources of uncertainties which limit their reliability (e.g. Merz and Thieken, 2005; Apel et al, 2004, 2008; Most and Wehrung, 2005; Hall and Solomatine, 2008)
There is a consensus in the scientific community that a proper risk analysis should provide an indication of uncertainty, emphasising how the identification of the optimal flood risk management strategy can be pursued only if all major sources of uncertainty are adequately taken into consideration and a quantification of their impacts is provided (USACE, 1992)
Panel (a) of Fig. 7 reports the probabilistic flood hazard map related to a specific return period of 200 yr and obtained for the MedianT subset (∼ 2000 runs); the probability of inundation of each cell in the flood prone area is indicated through a blue colour scale

Summary

Introduction

Many studies in the literature highlight how inundation hazard and risk assessments are affected by several sources of uncertainties which limit their reliability (e.g. Merz and Thieken, 2005; Apel et al, 2004, 2008; Most and Wehrung, 2005; Hall and Solomatine, 2008). The unavoidable presence of uncertainty can be attributed to the fact that flood risk evaluations are usually carried out for extreme events that are seldom observed, which makes the calibration of flood risk assessment models difficult, if not impossible (Apel et al, 2004). Under such circumstances, the evaluation of uncertainty sources is a pragmatic extension to conventional validation. In a context where model calibration and validation is difficult due to consideration of extreme events or lack of data, Hall and Anderson (2002) and Hall (2003) suggest a transparent and Published by Copernicus Publications on behalf of the European Geosciences Union

Methods

Results

Discussion

Conclusion