Overtopping Metrics and Coastal Safety: A Case of Study from the Catalan Coast

Corrado Altomare,Xavi Gironella,Tomohiro Suzuki,Giacomo Viccione,Alessandra Saponieri

doi:10.3390/jmse8080556

Abstract

Design criteria for coastal defenses exposed to wave overtopping are usually assessed by mean overtopping discharges and maximum individual overtopping volumes. However, it is often difficult to give clear and precise limits of tolerable overtopping for all kinds of layouts. A few studies analyzed the relationship between wave overtopping flows and hazard levels for people on sea dikes, confirming that one single value of admissible mean discharge or individual overtopping volume is not a sufficient indicator of the hazard, but detailed characterization of flow velocities and depths is required. This work presents the results of an experimental campaign aiming at analyzing the validity of the safety limits and design criteria for overtopping discharge applied to an urbanized stretch of the Catalan coast, exposed to significant overtopping events every stormy season. The work compares different safety criteria for pedestrians. The results prove that the safety of pedestrians on a sea dike can be still guaranteed, even for overtopping volumes larger than 1,000 L/m. Sea storms characterized by deep-water wave height between 3.6 and 4.5 m lead to overtopping flow depth values larger than 1 m and flow velocities up to 20 m/s. However, pedestrian hazard is proved to be linked to the combination of overtopping flow velocity and flow depth rather than to single maximum values of one of these parameters. The use of stability curves to assess people’s stability under overtopping waves is therefore advised.

Highlights

Wave overtopping assessment is a key procedure within the general design of any coastal defense
The groundwork for the assessment of wave overtopping was laid by [1], who chose the average discharge as a design value, stating that “there is no such thing as an absolute discharge: because the wave heights and periods exhibit a random distribution about a given mean, the discharge will vary randomly”
During the last few years, the increased storminess caused by climate change has raised concerns among researchers, engineers and decision-makers: is the average overtopping an appropriate design criterion? Or are the biggest waves the ones that cause the major damages and casualties? Allsop et al [5] highlighted that tests on the effects of overtopping flows on people indicate that the assessment of mean discharge is not enough to evaluate people’s safety; the authors proposed maximum individual overtopping volumes as more suitable hazard indicators

Summary

Introduction

Wave overtopping assessment is a key procedure within the general design of any coastal defense. Allsop et al [5] highlighted that tests on the effects of overtopping flows on people indicate that the assessment of mean discharge is not enough to evaluate people’s safety; the authors proposed maximum individual overtopping volumes as more suitable hazard indicators. The authors suggested that lower volumes may be required for violent overtopping processes with higher velocities. It was emphasized but not discussed further that other flow parameters might play an important role and be linked to overtopping hazards, namely overtopping flow velocity and overtopping flow depth. Several studies have been gathered and presented in EurOtop [6], where a tolerable limit for maximum individual volumes equal to 600 L/m was suggested, over which a single event cannot be tolerated by people (Table 1)

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Results