An empirical approach to detect an accelerated sea-level rise

Impacts of the climate change and with it the sea level rise are in the discussion of the scientific community for years. Numerical models are not yet able to regionalize the prediction of the expected sea-level rise on -for examplethe German Bight. The Hamburg Port Authority is working on the topic of sea-level rise and the effects on the Elbe estuary with empirical methods. A new approach for the detection of an accelerated sea-level rise is presented in this paper. A regression with the annual mean water levels over values of the wind direction and velocities was performed and the results offer a possibility to judge the changes of the mean water levels for the analysed gauge Cuxhaven. This method can also be applied to other gauges along the coastline with nearby wind measuring stations. 1 Background and Motivation Among all consequences of climate change the expected global sea-level rise will have the greatest impact on the planning and design of coastal protection. Also for the operation and maintenance of tidal influenced water ways and seaports (which handle more than 95 % of the intercontinental trade (Corinth 2009) the knowledge of the current and future sea-level rise is in the discussion about climate impacts essential. Background is the expected decreasing effect of the fresh water discharge with its flushing influence on sediments due to a reduced mean water level gradient between head water and estuary mouth (Fickert & Strotmann 2009). Figure 1: Map of the area. 2 Fickert & Strotmann 2 Misinterpretation of trends in analysing water levels A hind cast of the influence of recent climate change effects on the development of the global sealevel rise with the help of linked atmospheric and hydronumerical models is hardly to validate because time series of extra terrestrial global observations of the sea-level are not yet long enough. In addition to that the land based gauge recordings include not only regional varying tectonically and techno-genic influences but also due to their geographical sight in relation to the dominating wind direction more or less strong meteorological signals (as wind set-up and sunk-in). As a result of the relatively shallow water depths in the North Sea this wind set-up in the German Bight leads to particularly great variations in the annual mean values of the tidal parameters which does forbid conclusions about trends. y = ‐0,1708x + 351,25 R2 = 0,0113 y = 1,0366x ‐ 2060,2 R2 = 0,2783 ‐20 ‐15 ‐10 ‐5 0 5 10 15 20 25 3


Background and Motivation
Among all consequences of climate change the expected global sea-level rise will have the greatest impact on the planning and design of coastal protection.Also for the operation and maintenance of tidal influenced water ways and seaports (which handle more than 95 % of the intercontinental trade (Corinth 2009) the knowledge of the current and future sea-level rise is in the discussion about climate impacts essential.Background is the expected decreasing effect of the fresh water discharge with its flushing influence on sediments due to a reduced mean water level gradient between head water and estuary mouth (Fickert & Strotmann 2009).

Misinterpretation of trends in analysing water levels
A hind cast of the influence of recent climate change effects on the development of the global sealevel rise with the help of linked atmospheric and hydronumerical models is hardly to validate because time series of extra terrestrial global observations of the sea-level are not yet long enough.In addition to that the land based gauge recordings include not only regional varying tectonically and techno-genic influences but also due to their geographical sight in relation to the dominating wind direction more or less strong meteorological signals (as wind set-up and sunk-in).As a result of the relatively shallow water depths in the North Sea this wind set-up in the German Bight leads to particularly great variations in the annual mean values of the tidal parameters which does forbid conclusions about trends.

Multiple Regression
To eliminate the influence of the wind, the mean water levels are revised by wind data.These are available from a near-by offshore station since 1969.The hourly mean values are squared because the shear stress is influenced by the squared wind velocity (τ W =C d *ρ a *V 10 2 ) and these data are divided in 16 wind direction categories.A multiple regression over these values was performed, superimposed by an additional degree of freedom representing the linear trend in Sea Level Rise

Results
With the results of this regression the annual mean tidal water levels can be calculated and compared with the measured ones (figure 5, right diagram).The results have an excellent stability index and can be used for cutting down the water levels to their trend without further influence of the wind.
For the gauge Cuxhaven a trend of the mean tidal water levels of 3.1 mm/y is calculated (figure 6).

Discussion and Perspective
With this simple method it is possible to work with even shorter time series to figure out the trend or the changes of trends (like acceleration) in the water levels.To check the quality of this method a sensitivity test can be performed.The increase of a 20 years mean value is comparatively stable and can be used for further interpretations and as input data in hydrodynamical models.
So this method allows analyzing the hydrodynamics of the Elbe estuary and its changes due to the sealevel rise.Particularly with regard to the questions of sediment transportation and maintenance of the fairway and the port basins this will be crucial in the future.

Figure 1 :
Figure 1: Map of the area.

Figure 4 :
Figure 4: Regression coefficient for each wind direction.

Figure 5 :
Figure 5: Windstatistic (1996) and results of the correlation with the wind set-up.

Figure 6 :
Figure 6: Tidal mean water level of Cuxhaven -upper graph, prognosis of sea level rise -lower graph.