Foreword

Abstract

Studying the impact of coastal water movement on the environment has been a geographical necessity for the Netherlands throughout its history, because most of the country lies below sea level and must be protected from flooding. Failure to predict and react to storm surges along this coast can have disastrous consequences. Many people still vividly remember the storm, surge disaster of 1 February 1953, when the dikes in southwestern part of the Netherlands broke, 136 000 ha of land were inundated and almost 2000 people were drowned.Accurate predictions of storm surges for the entire Dutch coast are therefore vital. They indicate whether dikes are at risk and whether the storm surge barrier in the Eastern Scheldt should be closed. Closing this barrier unncessarily can cause damage to the fragile environment in the Eastern Scheldt. In the case of the barrier currently being constructed in the Rotterdam Waterway, unnecessary closure has severe economic repercussions (Figure 1), and at least six hours' notice must be given of a decision to close it.The accurate forecasting of water levels is also vital as a service to shipping, because of water's major role in the transport of goods in the Netherlands. The forecasts indicate whether the navigable channels contain sufficient water for supertankers (Figure 2) and bulk carriers with a deep draught to safely enter or leave the ports of Rotterdam Ijmuiden and of the Western Scheldt. Rijkswaterstaat is responsible for the storm surge warning for the Dutch coast, for operating storm surge barriers and for forecasting the water levels to provide access to the Dutch North Sea ports. To fulfil this responsibility Rijkswaterstaat has developed a system to forecast the water levels and storm surges in the North Sea. This system is based on a numerical model that describes the tidal floww, and the effects of the meteorologically induced variation in water level in the entire Continental Shelf. The meteorological input of this hydrodynamic model is determined by predicted wind and pressure information provided by the Royal Dutch Meteorological Institute KNMI.The system's performance in predicting water levels and storm surges is influenced by how well the Continental Shelf Model (hereafter referred to as CSM) forecasts the, astronomical tide, and also by the accuracy of the meteorological input.Rijkswaterstaat collaborated with Delft University of Technology to develop advanced data assimilation procedures in order to obtain a computationally efficient procedure to improve model performance. Two types of data assimilation were developed: off-line and on-line. In the off-line data assimilation procedure several uncertain parameters in the model can be estimated simultaneously using long series of conventional water level measurements from the tide gauge or radar altimeter data from satellites. In the on-line data assimilation procedure the errors in operational water level forecasts introduced by inaccuracies in the meteorological forcing and meteorological effects outside the domain of the model (external surges) are corrected on a routine basis using the data available.In this paper an overview is given of the various data ssimilation applications in the Continental Shelf Model. Furthermore, one case study is described in detail.