Keywords: damage and reliability evaluation, fluid-structure interaction, material properties uncertainty, timber and masonry structures.

This work aims to set up an analytical method for the assessment, through proper indicators, of wave damages on both natural and artificial structures in urbanized lagoons. The final purpose is the development a reliable instrument to support the planning and the regulation of motor-boat navigation and the design of protection systems against the erosion of natural and artificial structures in the Venice lagoon.

Recent field studies (Carbognin [1], M.A.V.-C.V.N [2]) have revealed that wind and boat generated waves produce continuous actions on these structures that cause progressive damage (such as: loss of soil and vegetation from salt marshes and tidal flats; sedimentation in the channels; deterioration of timber piles and building foundations).

Load and resistance parameters, to be adopted for the calculation of the induced damage, are affected by great uncertainties due to the stochastic nature of the wave generation (see Umgiesser [5]) and the variability of the mechanical characteristics of the different structures (caused by material factors and environmental conditions, such as biochemical activity and the weather regime, see Thompson [4]).

A probabilistic method of analysis has been set up in which the uncertainty of the abovementioned factors has been assumed with the purpose to provide reliable estimates of wave damage in the Venice lagoon taking into account the random nature of both loads and resistance caused by their spatial and temporal variability.

The particular case of the erosion process for two low-depth mudflats caused by wind generated waves has been investigated. These sites are located in the Northern part and in the South-central zone of the lagoon respectively; each one is provided with an instrumented station recording the data relating to the significant wave height, spectral peak period, water depth and turbidity every fifteen minutes for a time period of six months.

The risk-analysis has been carried out starting with the definition of the "object of the analysis" (i.e. the mudflat area) and the stochastic variables affecting its strength (critical shear stress ) and the external load (wave-induced bottom friction ). At this early stage of the analysis bottom characteristics (i.e. critical shear stress and roughness) and water depth at the investigation sites have been assumed constant (mean values), while the spectral peak period has been related to the significant wave height through an empirical formula proposed for the Venice lagoon (M.A.V.-C.V.N [2]).

By processing significant wave height data recorded at these stations, the probability density function of this random variable has been estimated. The bottom velocity is then correlated to the significant wave height through a formula provided by the linear wave theory; the estimation of the induced bottom friction has been obtained as suggested by Soulsby [3].

Subsequently the failure mechanism has been described in order to define the reliability function .

The potential damage induced by wind generated waves in the two sites of interest could be estimated by means of the structure reliability: the probability of to be negative (failure) has been assumed as wave damage indicator.

In the time interval for which the experimental data are available, the frequency of the events in which turbidity exceeds a background value has been calculated and the results compared with the reliability estimation obtained for both sites in order to evaluate the model consistency.

1

L. Carbognin, G. Cecconi, V. Ardone, "Interventions to safeguard the environment of the Venice lagoon (Italy) against the effects of the land elevation loss", Sixth International Symposium on Land Subsidence, Ravenna, 24-29 September 2000.

2

Magistrato alle Acque di Venezia - Consorzio Venezia Nuova, "Studio C.2.5 Dati meteomarini per la gestione della laguna di Venezia- Processi ambientali e clima, rapporto finale di sintesi", Thetis S.p.A. (2004).

3

R.L. Soulsby, "Dynamics of Marine Sands. A Manual for Practical Applications", Thomas Telford London (1997) p. 248.

4

C.E.L. Thompson, C.L. Amos, G. Umgiesser, "A comparison between fluid shear stress reduction by halophytic plants in Venice Lagoon, Italy and Rustico Bay, Canada-analyses of in situ measurements", Journal of Marine Systems 51 (2004) 293- 308. doi:10.1016/j.jmarsys.2004.05.017

5

G. Umgiesser, M. Sclavo, S. Carniel, A. Bergamasco, "Exploring the bottom stress variability in the Venice Lagoon", Journal of Marine Systems 51 (2004) 161- 178. doi:10.1016/j.jmarsys.2004.05.023

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