Van Gelder, et.al. - 1 Coping with uncertainty in the economical optimization of a dike design P.H.A.J.M. VAN GELDER, J.K. VRIJLING, K.A.H. SLIJKHUIS Delft University of Technology, Faculty of Civil Engineering Delft, The Netherlands ABSTRACT A design philosophy for dikes based on an economical cost model is described and examined on its sensitivity with respect to statistical- and model uncertainty. The philosophy is applied on an example of a dike along the Dutch coast. 1 INTRODUCTION The Netherlands is a unique country by the fact that it can continue to exist thanks to its sea- and river dikes. Without these water retaining structures 2/3 of The Netherlands would be inundated quite regularly and most of its population should have to move elsewhere. In the past dikes were designed by building them as high as the highest known water level at that particular location. More recently other design philosophies have been developed. The approach where a dike has to withstand a certain water level with a fixed probability per year has become quite common. Probabilities of exceedances for water levels once per 10,000 years are adopted for the Dutch sea dikes and once per 1,250 years for the Dutch river dikes [Van Gelder, 1996a]. Other design philosophies have been suggested but didn=t become popular so far. However, the approach in which the dike design is determined by an economical optimization [Van Dantzig, 1956] has much advantages in comparison with other design philosophies. This approach will be explained in section 2 of this paper and applied to a location along the Dutch coast. It will be shown how the approach can deal with statistical- and model uncertainties in sections 3.1 and 3.2 respectively. In section 4 the influence of these uncertainties will be examined on the economical optimal dike height. Finally the conclusions are drawn in section 5. 2 ECONOMICAL OPTIMIZATION OF THE DIKE HEIGHT Taking account of the cost of dike building, of the material losses when a dike-break occurs, and of the frequency distribution of different sea levels, the optimal dike height can be determined by economical optimization [Van Dantzig, 1956]. Assume that H 0 is the current dike level and that we want to determine the amount X by which the dikes must be heightened to the height H (see figure 1).