A Spatial Structure Variable Approach to Characterize Storm Events for Coastal Flood Hazard Assessment

Chiara Favaretto,Piero Ruol,Luca Martinelli

doi:10.3390/w13182556

Abstract

Over the last decades, the evaluation of hazards and risks associated with coastal flooding has become increasingly more important in order to protect population and assets. The general purpose of this research was to assess reliable coastal flooding hazard maps due to overflow and wave overtopping. This paper addresses the problem of defining credible joint statistics of significant wave heights Hs and water levels ζ, focusing on the selection of the sample pair that characterizes each sea storm, to evaluate the occurrence probability of extreme events. The pair is selected maximizing a spatial structure variable, i.e., a linear combination of Hs and ζ, specific to each point of the area at risk. The structure variable is defined by the sensitivity of the flooding process to Hs and ζ, as found by analyzing a set of inundation maps produced through a Simplified Shallow-Water numerical model (SSW). The proposed methodology is applied to a coastal stretch in the Venetian littoral (Italy), by means of a 30 year-long time series recorded at the “Acqua Alta” oceanographic research tower, located in the Northern Adriatic Sea in front of the Venetian lagoon. The critical combination of Hs and ζ forming the structure variable is presented in a map, and it can be related to the topography and the presence of mitigation measures. The return period associated with the two recent large storms that occurred in this area in 2018 and 2019 is also investigated. The proposed procedure gives credible occurrence probabilities for these events, whereas other approaches would consider them extremely unlikely.

Highlights

Coastal flooding is frequently associated with two mechanisms occurring during a sea storm: overflow and wave overtopping
The main motivation of this paper is related to the challenges posed by the appropriate definition of sample pairs to be used for bivariate statistical analysis, in the framework of coastal flooding hazard assessment
In many low-lying coastal regions, flooding hazard assessment is critically affected by the correct evaluation of the simultaneous occurrence of the significant wave height Hs and the water level ζ

Summary

Introduction

Coastal flooding is frequently associated with two mechanisms occurring during a sea storm: overflow and wave overtopping. The freeboard of the local levee/defense structure is steadily below the water level, whereas in the second case the flood volumes depend on the oscillating nature of the waves. They are often threatened as separate mechanisms waves and water levels clearly affect each other. The main motivation of this paper is related to the challenges posed by the appropriate definition of sample pairs to be used for bivariate (significant wave heights Hs − water levels ζ) statistical analysis, in the framework of coastal flooding hazard assessment

Objectives

Methods

Results

Discussion

Conclusion