Response of a shelf water to a traveling atmospheric pressure disturbance

Every harbour has its own uniques et of natural frequencies depending on its geometry. If waves with frequencies close to one of these natural frequencies continue to enter from offshore for some time, the harbour will resonate. This phenomenon is called harbour resonance or seiching and the characteristic oscillations seiches. A moredetailed description of seiches and longwaves is given in Chapter 2. The Port of Rotterdam consists of a number of harbours connected through long and narrow channels, see figure 1.1. This system of harbours and channels is very sensitive to the resonance of long waves. The construction of the storm surge barrier in the Nieuwe Waterweg at the entrance of the harbour,along with other major projects in the area, is likely to change this response significantly and to the worse [9]. In recent years a lot of research has been concentrated on determining the effects of these changes more accurately. Little is known about the origin and statistics of the long waves,and often this has been of little or no interest to these studies. Instead, due to the lack of information, the seaward boundary condition, representing the incoming waves,is simply modeled by a harmonic wave. Predictions of seiches are presently made based on the known statistics and by assuming the worst possible situation that seiching will occur every time there is a storm [13]. The aim of this study is to investigate the influence of an atmospheric pressure disturbance on the response of a shelf water, and to find out if such a disturbance could be responsible for the generation of long waves in the North Sea, later causing seiches in the Port of Rotterdam. To achieve this objective, the response of a shelf water to an atmospheric pressure disturbance was investigated. First analytically, then numerically. The approach followed in these two parts is given below. The analytical study was desired to determine which parameters are of special importance to the problem in general. The applicability of the shallow water equations was therefore analyzed and the equations subsequently simplified to enable a simple one-dimensional analysis. The resulting equation was first analyzed without a pressure field, corresponding to free waves. Then two types of pressure fields were considered, viz. a harmonic, propagating pressure wave and a moving pressure jump. The harmonic,propagating wave is a classical forcing term when analyzing linear differential equations, and the pressure jump should resemble the pressure before and after the passage of a steep cold front. The numerical study was split up in a sensitivity study and a study of a real meteorological event, the passage of a cold front that occurred in the period 18th to 23rd of April 1993. Later this event caused some seiche activity in the Port of Rotterdam [12]. In the sensitivity test a simple one-dimensional model was set up. With this model a number of experiments were carried out to test the sensitivity of parameters like time step, grid spacing and bottom friction. In these experiments a harmonic, propagating pressure field like in the analytical study was used with wave length, period, velocity and amplitude based on the (expected) characteristics of the pressure field of the event mentioned above. The primary aim of the sensitivity test was to find a setup of the model for the calculations of the real event. In the cold front simulation the measured air pressure was used to simulate the passing cold front. Again some aspects of modeling this event were investigated to finally choose an appropriate model setup. The results of the final simulations were then compared to observations presented by Veraart [12] and the analytical results.Hydraulic EngineeringCivil Engineering and Geoscience