Peter Fröhle

Changes of 21st Century's average and extreme wave conditions at the German Baltic Sea Coast due to global climate change

ABSTRACT Dreier, N., Schlamkow, C., Fröhle, P. and Salecker, D., 2013. Changes of 21st Centurys average and extreme wave conditions at the German Baltic Sea Coast due to global climate change. On the basis of hourly simulated wind data from a regional circulation model (Cosmo-CLM) long-term time series (1960–2100) of wave parameters were compiled for two of the SRES scenarios A1B, B1 at three locations along the German Baltic Sea Coast. The time series of wave parameters were calculated with the help of statistical correlations between observed wind and wave data, which were derived for the three locations in the study area. Furthermore, we applied a wave model for the Western Baltic Sea to correct some of the calculated wave information. The key findings of this study are that: (i) significant changes of average wind conditions can directly be linked to significant changes of the average wave conditions, (ii) a spatial pattern for the changes of average wave conditions can be found, (iii) no general trend can be found for the changes of extreme wave conditions. Comparisons of wind conditions, as simulated by the regional circulation model, for the past and the future are showing two main changes: (a) increases of average wind velocities to the end of the 21st century up to +4% and (b) more wind events from westerly and less events from easterly directions. The changes of the average wind velocities are resulting in increases of the average significant wave heights at westerly wind exposed locations up to +7% and small decreases of the average significant wave heights at easterly wind exposed locations. The future changes of the wave directions, with more wave events from W-NW and fewer events from N-NE, can be connected to the future changes of wind directions. Analyses of extreme wave heights with a return period of 200 years are showing both increasing and decreasing changes of up/down to +/−14%. At one location of the study area (Warnemünde) a slight increasing trend for the change of extreme wave heights to the end of the 21st century exists.

Climate Change and Coastal Protection: Adaptation Strategies for the German Baltic Sea Coast

Consequences resulting from future climate change may be one of the most severe threats for people and economies in many countries of the world. With respect to coastal Protection, the resulting changed hydrodynamic impacts are discussed globally. At present, IPCC (2007) is estimating a world-wide average sea level rise of less than 1 m within the twenty-first century. Other sources (e.g. Rahmstorff and Schellnhuber 2007) which are taking into account possible melting of the two main continental ice covers (Greenland and Antarctica), estimate significantly higher values especially over long periods. Besides the problem of sea level rise, also possible general changes in the frequency and intensity of storms as well as general changes in the average wind and wave field are discussed. With respect to the protection of the coast against flooding and erosion, possible adaptation strategies and measure are described in brief.

What Determines the Change of Coastlines in the Baltic Sea

The change of coastline positions of the Baltic Sea is mainly determined by both the eustatic sea-level change and the glacio-isostatic adjustment (GIA). For changes on the Holocene time scale, the relative sea-level change can be reconstructed from paleo-coastline positions and correspondingly dated sediments and organic remains. On the decadal scale, tide gauge data are available. Both data sets display the relative value of sea-level change resulting from the superposition of climatically and meteorologically induced factors, vertical crustal displacement, and related gravitational forces. The isolation of the GIA signal from the compound relative sea-level change data plays a critical role for future projections of coastline changes within the frame of coastal zone management. To separate different components of sea-level data sets, statistical methods for the exploration of empirical water level, meteorological, and GPS data are combined with analytical methods to solve the sea-level equation. In the result, the pattern of vertical crustal movement can be displayed as maps covering the uplifting Fennoscandian Shield and its subsiding belt. Whereas along the uplifting coasts morphodynamic processes play a subordinated role, in the subsiding Southeast and South, Quaternary sediments are permanently exposed to coastal erosion, sediment transport, and re-deposition. This mainly wave-driven sediment dynamics together with aeolian processes depend on meteorological forcing of the in general west-east directed air-flow from the northern Atlantic Ocean to Eurasia. Regional coastal morphogenesis can generally be described by alongshore sediment transport pattern deduced from the integration of subregional to local models of transport capacities. For future projection, coastlines and the morphology of the adjacent zones have to be regarded a function of its position related to the vertical displacement of the Earths crust, the regional climatic and meteorological conditions, and the geological setting. Results of climate modelling, the Earth’s visco-elastic response to the deglaciation, geological data and regional sediment transport capacities have to be interpreted comprehensively.

On the Long-term Changes of Extreme Wave Heights at the German Baltic Sea Coast

ABSTRACT Xu, Z.S.; Dreier, N.; Chen, Y.P., Fröhle, P. and Xie, D.M., 2016. On the long-term changes of extreme wave heights at the German Baltic Sea Coast. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 962–966. Coconut Creek (Florida), ISSN 0749-0208. The assessment of extreme wave heights is of great importance for the design of coastal protection strucutres. This paper aims to present long-term changes of extreme wave heights at several selected locations at the German Baltic Sea Coast. Annual maximum significant wave heights were selected from the hourly wave data obtained from a hybrid approach (wind-wave-correlations combined with numerical simulations on the basis of the SWAN model) for a total period of 140 years, spanning from 1961 to 2100. The future projections of wind data were dreived from the regional COSMO-Climate Local Model for two emission scenarios A1B and B1. The extreme wave heights at each location were estimated from the respective best-fitting distribution, log-normal or Weibull distribution. The results indicate that, the long-term changes of extreme wave heights are related to the emission scenarios used in the regional climate model, the distribution chosen for the estimating (up to +9.6%) and also the sample size for the extreme value analysis (up to +8.7%). These factors should be carefully considered in the future coastal structure design.

Ensembles of radar nowcasts and COSMO-DE-EPS for urban flood management

Sophisticated strategies are required for flood warning in urban areas regarding convective heavy rainfall events. An approach is presented to improve short-term precipitation forecasts by combining ensembles of radar nowcasts with the high-resolution numerical weather predictions COSMO-DE-EPS of the German Weather Service. The combined ensemble forecasts are evaluated and compared to deterministic precipitation forecasts of COSMO-DE. The results show a significantly improved quality of the short-term precipitation forecasts and great potential to improve flood warnings for urban catchments. The combined ensemble forecasts are produced operationally every 5 min. Applications involve the Flood Warning Service Hamburg (WaBiHa) and real-time hydrological simulations with the model KalypsoHydrology.

Modelling of Medium-Term (Decadal) Coastal Foredune Morphodynamics- Historical Hindcast and Future Scenarios of the Świna Gate Barrier Coast (Southern Baltic Sea)

Coastal foredunes are developed as a result of interplay among multi-scale land-sea processes. Natural foredune ridges along the Świna Gate barrier coast (southern Baltic Sea) developed since 6000 cal. year BP provide an excellent laboratory to study the land-sea interaction under a medium- to long-term climatic control. In this paper we investigate several basic driving mechanisms of coastal foredune morphodynamics as well as natural environmental factors involved in shaping the foredune geometry by a numerical model. The model couples a process-based module for subaqueous sediment transport and a probabilistic-type module for subaerial aeolian sand transport and vegetation growth. After an evaluation of the model performance for a 61-year (1951–2012 AD) historical hindcast of the foredune development along a 1 km-long section of the Świna Gate barrier coast, the model is applied for a future projection of the same area to 2050 AD based on three different climate change scenarios. The climate change scenarios represent three different impact levels with regard to their capacity to shape the coastal morphology. Simulation results demonstrate a remarkable variability in foredune development even along a small (1 km) coast section, implying that the medium-term land-sea interaction and foredune morphodynamics is quite sensitive to boundary conditions and various processes acting on multi-temporal and spatial scales. Foredune morphodynamics such as migration, bifurcation, destruction and separation are determined by different combinations of storm frequency, onshore sediment supply rate and relative sea-level change. In contrast to a low rate of relative sea-level rise during the last few decades, an accelerated sea level-rise over the twenty-first century predicted by existing literature, would result in a dramatic and non-linear response from the foredune development according to our simulations.

Operational wave now- and forecast in the German Bight as a basis for the assessment of wave-induced hydrodynamic loads on coastal dikes

The knowledge of the wave-induced hydrodynamic loads on coastal dikes including their temporal and spatial resolution on the dike in combination with actual water levels is of crucial importance of any risk-based early warning system. As a basis for the assessment of the wave-induced hydrodynamic loads, an operational wave now- and forecast system is set up that consists of i) available field measurements from the federal and local authorities and ii) data from numerical simulation of waves in the German Bight using the SWAN wave model. In this study, results of the hindcast of deep water wave conditions during the winter storm on 5–6 December, 2013 (German name ‘Xaver’) are shown and compared with available measurements. Moreover field measurements of wave run-up from the local authorities at a sea dike on the German North Sea Island of Pellworm are presented and compared against calculated wave run-up using the EurOtop (2016) approach.

ASSESSMENT OF CHANGES OF EXTREME WAVE CONDITIONS AT THE GERMAN BALTIC SEA COAST ON THE BASIS OF FUTURE CLIMATE CHANGE SCENARIOS

Information on possible changes in extreme wave heights is needed to determine the future effectiveness and safety of coastal and flood protection structures. In this study, an assessment of possible changes in the extreme wave heights at selected locations along the German Baltic Sea coast has been carried out on the basis of numerical simulations of waves in the western Baltic Sea and regional climate model data for the past and the future (1961-2100). The future climate change signal of significant wave heights with a return period of 200 years mainly depends on: (i) the location, (ii) the climate change scenario run, (iii) the time period of the comparison, and (iv) the approach adopted to calculate the wave climate. The results show increases of up to +0.5 m and decreases of up to -0.5 m of the extreme wave heights. The increases might have considerable effect on the constructional design of coastal and flood protection structures-such as breakwaters, sea dykes, and vertical walls-because the extreme wave heights are used as input parameters for the design of the structures.