Evaluation of the allowance for seiches in the Hydraulic Boundary conditions for Europoort

Insights obtained over the last years have indicated that the generation of the sieches in the Port of Rotterdam might be less effective during extreme storm conditions than during more average situations. Previous studies could not provide suitable results to reliably verity this. This knowledge gap has been identified and taken up in the project ‘SBW Belastingen’. The goal of the present exploratory study is to use the existing insights on the genereation of seiches, particulary their meteorological origin, to make an evaluation of the present applied seiche allowances in the Hydraulic Boundary Conditions for the Europoort area and to determine which follow-up studies might be required to update them. The study started with an inventory of the background and history of the method that is presently uses to calculate the sieche statistics for the Hydraulic Boundary Conditions. Also the method that was used to determine seiche amplitudes for the design of the Storm Surge Barrier in the Nieuwe Waterweg (Rotterdam) has been described as part of this inventory. The consistency of the allowances computed with both methods was considered in a preliminary evaluation. This evaluation showed that the two methods have led to different allowances for seiches in the Europoort area. Furthermore, it indicate that the presently applied reference seiche allowances are most probably too low and would require updating as a short-term action; the present 1/10.000 years seiche amplitude is already exceeded in the available measurement data. The second part of this work involved making an inventory of the research that was done in the past on the meteorological forcing of seiches for the Port of Rotterdam. That inventory, in relation to the existing calculation methods, was evaluated in two expert discussion sessions. Those meetings particularly addressed the question whether (meteorological) conditions observed during moderate storms can be extrapolated to the return periods required for the Hydraulic Boundary Conditions. The results o the discussions indicated that the physics involved is very complex. A straightforward answer is at present not possible and will require extensive research. The evaluation of meteorological conditions during extreme storms, including numerical simulations with meteological models, should be made in a follow-up study. However, it is anticipated that it will require several years of research to arrive at practical conclusions for the Wettelijk Toetsinstrumentarium (WTI)