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Comparing the “Bathtub Method” with MIKE 21 HD Flow Model for Modelling Storm Surge Indundation. Case Study Kiel Fjord

Neumann, Tim; Ahrendt, Kai (2013): Comparing the “Bathtub Method” with MIKE 21 HD Flow Model for Modelling Storm Surge Indundation. Case Study Kiel Fjord. Geographisches Institut der Universität Kiel. RADOST-Berichtsreihe, Bericht Nr. 22.

Meteorological measurements in the past showed that the climate is changing rapidly over the last decades with an increasing tendency. It is expected that this trend will further increase in the future. Therefor there is a growing interest in understanding corresponding effects. One of the most challenging effects are floods. Due to the willing to evolve a sounder understanding of processes take place during an inundation event, there is an immense development of methods to analyse. This work deal with a comparison between the common "bathtub method" and a state-of-the-art hydrodynamic model, called MIKE21 HD Flow Model, for modelling storm surges. The aim of this study is to work out the differences between both approaches and to find out how probable differences look  like. There is the question if the "bathtub method" represents flooding adequate or, if the consideration of physics by hydrodynamic models makes a major difference and displays maybe the "real" risk of inundations. This work tries to underline the differences between those two approaches, where the strengths and weaknesses are and what influence those differences have for an inundation analysis. The investigation was made on a digital elevation model for the study area of Kiel, the capital city of the state Schleswig-Holstein in Germany. It is a midsize city of 242.041 inhabitants in the south-west of the Baltic Sea. The two approaches were made on data for a small storm surge on the basis of water-level-change and wind-regime data from 2010. The major aspect of investigation was the inundation extend under applying the different approaches. Water-level changes were implemented by developing different scenarios and additionally using time series from a surge for simulations. Further it was examined what influence surface resistance could have for the study area and how this influences the outcome of the two approaches as well as how inundation changes by taking physical behaviour of water-surface interaction into account. The results showed a difference between both approaches of average 11.25 % for a 2-D and 6.89 % for a 3-D surface analysis, where the "bathtub method" overestimates the hydrodynamic modelling (HDm). Further the outcome shows interesting behaviours when looking on different sea level rise scenarios what can be elucidated by changing resistances due to changing friction. Concluding the outcomes of the work, they show a distinct difference between both approaches. Taking the HDm as "real", the "bathtub method" overestimates the area of inundation for a storm surge. But it has to be differentiated by different scenarios. Generally a shallower inundation seems to be more influenced by forces like friction than an event of higher water ¬levels. By arguing so, it should always be considered to use HDMs, at least to get a comparable result for an analysis. The outcome of this work can be seen as a hint for inundation analysis and how to grade analysis of those two approaches.
To make a more reasonable statement about the error of the approaches further effort should be done in future, to repeat this work on different case-studies.