Origin and prediction of seiches in Rotterdam harbour basins

Abstract

This thesis focuses on the harbour oscillations that occasionally occur in certain basins of the Port of Rotterdam. Such standing waves, called seiches, need to be taken into account in this harbour area for the design of water protection works (such as dykes) and for the closure management of a movable storm surge barrier, located along the Rotterdam Waterway, which can become susceptible to seiches under specific conditions. Previous studies and experiences indicated that the seiches that occur in the Port of Rotterdam have an atmospheric origin. A literature review indicated that the atmospheric generating mechanisms that were known from other ports do not apply to the Port of Rotterdam. Therefore, this study has been conducted with the main aim to identify the origin of the seiches that occur in this harbour and to eventually arrive at a prediction system for significant seiche episodes. Observations, combined with weather charts, showed that all significant seiche episodes coincided with the passage of a low pressure area and a cold front. The majority of these events (90%)occurred during the storm season. Following the front passages that coincided with seiche events in the storm season, significant wind speed fluctuations occurred with periods in the order of one hour. The records showed that enhanced low-frequency wave energy at sea in the vicinity of the harbour mouth, as well as the seiche events in the harbour, occur more or less simultaneously with these strong wind speed fluctuations. The oscillatory wind speed changes are due to atmospheric convection cells that arise in an unstable lower atmosphere in the area behind a cold front, where relatively cold air moves over the relatively warm sea surface. It has been shown that the moving system of a cold front and trailing convection cells generates forced low-frequency waves at sea that can cause seiche events inside the harbour. Based on these findings, it has now become possible to predict the occurrence of seiche episodes in the Port of Rotterdam with high accuracy. Such seiche prediction method could be used by the port authorities for ship traffic control and, after further development and testing, eventually could be incorporated into the closure- management system of the storm surge barrier in Rotterdam Waterway.