Abstract
This study comprises a detailed description of the individual overtopped bore impact processes against a vertical wall, situated on top of a dike. A twin peak force impact signal shape was observed with two distinct peaks during every impact. The two peaks were assigned consecutively to the dynamic components (thickness and velocity) or hydrostatic components (run-up of water at the wall) of the impacting bore. The two peaks were termed dynamic F1 and quasi-static F2 impact respectively. Based on available literature semi-empirical equations to describe either the dynamic F1 or quasi-static F2 impact force were investigated and the prediction accuracy evaluated using impact force data from large-scale experiments. The prediction accuracy of the dynamic F1 impacts was very low. The prediction accuracy of the quasi-static impact F2 was increased based on fitting the hydrostatic theory to the maximum run-up measurement at the wall. Based on these findings 80% of the maximum run-up height was effectively contributing to the maximum quasi-static force F2 on the wall. The results coincided well with previous small-scale studies (Chen et al. 2012). After deconstructing the process chain preceding an impact, using the physically most meaningful parameters to predict the impact force, evaluating on a range of existing approaches, and observing the scattered prediction results, it was concluded that the impact behavior is highly stochastic and statistical analysis would be more beneficial.
Highlights
The coastlines of Belgium, The Netherlands, and parts of Germany are comprised of a sandy or muddy, mildly sloping foreshore
The impact at the wall is the result of a deterministic process chain starting with the I) wave breaking in shallow waters, II) wave run-up at the dike, wave overtopping over the dike crest, III) transformation of the bore flow on the promenade and IV) impact of the bore at the wall (Figure 1)
This was in line with previous studies stating that not all of the total run-up height was effectively contributing to the hydrostatic impact force (Klammer et al 1996)
Summary
The coastlines of Belgium, The Netherlands, and parts of Germany are comprised of a sandy or muddy, mildly sloping foreshore (ranging between 1-to-10 and 1-to-250 slope angle). In Belgium the dike crest is constructed with a certain width, a promenade, to allow for an increasing variety of leisure activities on the dike and to facilitate construction of buildings as close as possible to the sea These buildings, and any additional hard structure protection, such as storm walls, have to withstand eventual wave impacts by overtopped water bores in case of high tides, storm events or sea level rise (IPCC 2014). For both buildings and storm walls, the bore impact force is a key parameter for a reliable and safe structural design. Existing formulae to predict the maximum impact force are evaluated on whether they rather describe the first peak or second peak in the impact force
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