Han Vrijling

Probabilistic Design of Berm Breakwaters

In mechanical engineering the approach of optimal maintenance is mostly based on the empirical failure rate. Due to the unique character of hydraulic engineering structures, empirical failure data are lacking and a fundamental approach is needed. For the modelling of the aging process the serviceability limit state models are applied to the initial resistance of the structure taking account of the uncertainties involved. The probability of failure as a function of time is calculated by confronting the diminishing resistance with the loading. Within this model the usefulness of optimal repair intervals will be shown for a berm breakwater. In the last decade a renewed interest is shown for the berm breakwater concept. In the design of a berm breakwater, many limit states have to be checked by the designer: the stability of the armour cross and longshore, the compliance of the subsequent layers with filter rules, the stability of the toe under wave action, the geotechnical stability, overtopping, etc. In this paper attention will be focussed on the stability of the outer armour. Special attention is given to the longshore transport case. It will be shown that only a probabilistic approach gives insight into the sensitivity of this failure mechanism.

A risk-informed approach to coastal zone management

Economic and population growth have led to an unprecedented increase in the value at risk in coastal zones over the last century. To avoid excessive future losses, particularly in the light of projected climate change impacts, coastal zone managers have various instruments at their disposal. These primarily concern land-use planning (establishing buffer zones) and engineering solutions (beach nourishment and coastal protection). In this paper, we focus on risk mitigation through the implementation of buffer zones (setback lines). Foregoing land-use opportunities in coastal regions and protecting coasts is costly, but so is damage caused by inundation and storm erosion. Defining appropriate setback lines for land-use planning purposes is a balancing act. It is however unclear what level of protection is facilitated by current approaches for defining setback lines, and whether this is, at least from an economic perspective, sufficient. In this paper, we present an economic model to determine which setback lines would be optimal from an economic perspective. The results provide a useful reference point in the political debate about the acceptability of risk in coastal zones. The main conclusions are (i) that it is useful to define setback lines on the basis of their exceedance probabilities, (ii) that the exceedance probability of an economically efficient setback line will typically be in the order of magnitude of 1/100 per year, (iii) that it is important to distinguish between situations in which morphological conditions are stationary and non-stationary, and (iv) that long-term uncertainties (e.g. due to climate change) influence the exceedance probability of efficient setback lines but only to a limited extent.

Reliability-Based Design of Flood Defenses and Flood Defense Systems

Abstract The combination of a quantitative reliability analysis with minimization of the construction cost is presented as a tool for the design of flood defenses. It is shown that the method can be used on the level of individual structures and on the level of systems of flood defenses. On the level of structures, the method provides the geometry of the structure as a function of the required safety level, the environmental conditions, and the cost of construction materials. On the level of systems, the method provides the optimal allocation of failure probabilities to individual structures to reach a required maximum flooding probability of the protected area.