J.J.Caspers1*, P.E.Kindermann1*, C.P.M.Geerse1
1 HKV Lijn in Water
In recent years, research into hydraulic loads along the Dutch coast such as water levels and waves, has been boosted by the availability of long time series of meteorological data from the European Centre for Medium-Range Weather Forecasts (ECMWF), for which the Royal Dutch Meteorological Institute (KNMI) calculated the resulting water levels on the North Sea. As a result, a period of 8,000 years of simulated meteorological data of the current weather climate have become available for a large number of Dutch coastal stations. Compared to the limited availability of measurements from coastal stations (up to 50 or 100 years for a limited number of stations) these long time series are a great source of additional information. It provides the opportunity to study in greater detail how wind, water levels and waves evolve in time during extreme storm surges.
For the Dutch coastal regions, the safety standards of the primary flood defences range from 1/1,000 per year to 1/30,000 per year. In the currently applied approach, erosion of dunes and coastal dikes is computed for fixed assumptions with respect to the load evolution in time. With the insights of this study, the variability of storms can be included in safety assessments of dunes and dikes, by means of a time-dependent load model.
An hydraulic load model has been developed, yielding time series of water levels, wind and waves for Dutch coastal stations, with corresponding probabilities. The model is based on the 8,000 years of simulated data, from which storm evolutions are selected based on maximum wind speed. The developed load model is used to gain insight into relevant aspects for (time dependent) failure of dunes and coastal dikes, by linking the model to XBeach and to dike failure models. Another purpose of the model is to provide more consistency in the hydraulic loads that are used for the safety assessments of dunes and dikes, which currently utilizes different methods. Moreover, the impact of applying such a time-dependent load model on the failure probabilities along the Dutch shore can be investigated and compared to the currently applied approach with a single representative storm.
Based on the analysis, one of the conclusions is that wind durations and wind speed are strongly related, which is not included in the current load models. High wind speeds are often related to short wind durations. This plays an important role in the statistics of the wind, and it is recommended to add this correlation in future load models. Besides, a proof-of-concept was created in Python, to calculate exceedance frequencies of water levels, wave heights, and required dike heights. Currently, an extension is made to calculate time-dependent dune erosion by connecting XBeach to the hydraulic load model.
Figure 1: Comparison of the current implementation of the wind and surge evolution in load models for flood defence assessments (dashed lines) and the schematized wind and surge evolutions based on the ECMWF data analyses (coloured lines for different duration classes of wind and surge).