Changes in the seasonal cycle of mean sea level (MSL) may affect the heights of storm surges and thereby flood risk in coastal areas. This study investigates the intra-and inter-annual variability of monthly MSL and its link to the North Atlantic Oscillation using records from 13 tide gauges located in the German Bight. The amplitudes of the seasonal MSL cycle are not regionally uniform and vary between 20 and 29 cm. Generally, the amplitudes are smaller at the southwestern stations, increasing as one travels to the northeastern part. The amplitudes, as well as the phase of the seasonal cycle, are characterized by a large inter-annual and inter-decadal variability, but no long-term trend could be detected. Nevertheless, in the last two decades annual maximum peaks more frequently occurred in January and February, whereas beforehand an accumulation was detected for the November and December period. These changes in phase in the various sea level time series are consistent with a shift in the annual cycle, which is, however, not significant. The changes are associated with strongly increasing trends in monthly MSL of the winter season (J-M), which are considerably higher compared to the remaining seasons. For the same season, the MSL and North Atlantic Oscillation (NAO) indices show strong similarities, resulting in statistically significant correlations (r ~ 0.7). Hence, these changes are linked with changing pressure conditions over the North Atlantic, which lead to a strong phase of positive values in the NAO index between the 1960's and 1990's.

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Physical processes in coastal waters and estuaries extend their influences on many economic and ecological processes in the coastal regions and affect the safety of the coastal defences. In a context with the global climate change, these physical processes underlie also inherent modifications. In order to win an impression of such future changes and of the probability of their occurrence, physically consistent simulations of these processes are used to describe how wind-waves and currents interact. This paper presents an offline-coupled simulation using the models HAMSOM (HAMburg Shelf Ocean Model) and SWAN (Simulating Waves Nearshore). These stateof-the-art models excel by high computing speed, so that they offer an opportunity to simulate hydrological conditions and physical processes over longer time periods, e.g. decades. For the influence of currents on the waves, we estimate less influence on tidal flats, but stronger influence in the tidal channels. Improvements in parameter estimation that were achieved by the interaction of currents and waves are described and discussed; we estimate new drag-coefficients for the hydrodynamic simulation. Because long-term simulations need to be simplified, a method is examined and presented that bypasses the direct online-coupling of models. For the aim of long term simulation improvements of the surface drag coefficient are useful, because online-coupled wind-wave models overcome the available machine time for climate runs. Our method yields an optimization regarding computing economy and physical consistency of simulations.

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In shallow waters the wave height distribution significantly differs from Rayleigh distribution during extreme wind conditions. The EurOtop manual (Pullen et al. 2007) recommends the use of a composite Rayleigh-Weibull distribution proposed by Battjes and Groenendijk (2000) in order to describe the wave statistics in shallow waters. A test of this recommendation by using wave measurements with continuously operated radar level gauges at three different sites at the German North Sea coast for comparison revealed the necessity for a change in the parameterization given in the EurOtop manual. References Barjenbruch, U., S. Mai, N. Ohle, and U. Mertinatis. 2002. Monitoring Water Level, Waves and Ice with Radar Gauges, Proceedings of the Hydro 2002 Conference, DHyG, 328-337. Barjenbruch, U., and J. Wilhelmi. 2008. Application of radar gauges to measure the water level and the sea state, Proceedings of 31st International Conference on Coastal Engineering, ASCE, 687-695. Battjes, J.A., and H.W. Groenendijk. 2000. Wave height distributions on shallow foreshores, Coastal Engineering, 40, 161-182. http://dx.doi.org/10.1016/S0378-3839(00)00007-7 Burcharth, H.F., P. Frigaard, J. Uzcanga, J.M. Berenguer, B.G. Madrigal, and J. Villanueva. 1996. Design of the Ciervana breakwater, Bilbao, Advances in coastal structures and breakwaters, Thomas Telford, London, 26-43. Forristall, G. 2008. Offshore LNG terminal designs must overcome complications of shallow water, Oil & Gas Journal, 106(43). IAHR Working Group on Wave Generation and Analysis. 1989. List of Sea-State Parameters, Journal of Waterway, Port, Coastal and Ocean Engineering, 115(6), pp. 793-80 http://dx.doi.org/10.1061/(ASCE)0733-950X(1989)115:6(793) Klopman, G., and M.J.F. Stive. 1989. Extreme waves and wave loading in shallow water, Proceedings of E&P Forum Workshop: Wave and current kinematics and loading, Paris, Oct. 25-26. Longuet-Higgins, M. S. 1952. On the Statistical Distribution of the Heights of Sea Waves. Journal of Marine Research, 11(3), 245–266. Mai, S. 2008. Statistics of Waves in the Estuaries of the Rivers Ems and Weser - Measurement vs. Numerical Wave Model, Proceedings of the 7th Int. Conf. on Coastal and Port Engineering in Developing Countries COPEDEC, CD-ROM. Nelson, R.C. 1994. Depth limited design wave heights in very flat regions, Coastal Engineering, 23, 43-59. http://dx.doi.org/10.1016/0378-3839(94)90014-0 Pullen, T., N.W.H. Allsop, T. Bruce, A. Kortenhaus, H. Schüttrumpf, and J.W. van der Meer. 2007. EurOtop – Wave Overtopping of Sea Defences and Related Structures: Assessment Manual, Die Küste, 73, 193 pp. (online: