Dissipation of Waves over a Salt Marsh Meadow

Koetsier, J.O. (2015) Dissipation of Waves over a Salt Marsh Meadow.

Abstract:With an increasing awareness of ecological value and rising sea levels the use of vegetation in flood protection becomes widely accepted. To evaluate and quantify the benefits of a salt marsh meadow for flood protection, knowledge about the evolution of wave dissipation over the marsh is necessary. The objective of this research is to provide an empirical model which describes the development of wave dissipation over a salt marsh meadow under storm surge conditions. A method for the analysis of the data and the preparation of the model is also provided. The data is gathered in a large wave flume (Grosser Wellenkanal) in Hannover. A vegetated section of 39.44 meter length was built up of field excavated pieces of salt marsh turf. Elymus, Atriplex, and Puccinnellia are used as vegetation to represent a southern North Sea mars community. The data is obtained using 5 groups of 3 pressure transmitters and one group of 2 pressure transmitters. The pressure data is processed into surface elevation using linear wave theory. The surface elevation is calculated in frequency domain. It was necessary to use a low pass filter as the model was not appropriate for the higher frequencies. A high pass filter is applied to remove the offset of several spectrums. The method is limited as the pressure transmitters could not measure a signal of a wave with an amplitude of 0.1 meter. Several nonlinear tests were also excluded from the analysis. Nonlinear waves typically show more amplitude peaks in frequency domain. These peaks are determining for the wave shape in time domain. Some of the wave conditions showed peaks beyond the low pass filter. Both an analysis of the evolution of wave height dissipation as well as an analysis of the wave energy flux dissipation is considered. A time interval with no influence of reflected waves is used in which the mean wave height is calculated. As theory states that wave dissipation shows an exponential behaviour, exponential functions are fitted to the mean wave height. The average wave energy flux is calculated using the mean wave height. Exponential functions are also fitted to this average wave energy flux plotted against the distance into the marsh. Empirical functions are derived on the basis of the parameters of the fitted functions. Both approaches showed low coefficients of variation but the approach on the basis of wave height dissipation seemed most accurate. Both approaches are presented as a function of the initial value and a decrease parameter. As a suggestion, the decrease parameter is presented as a function of the water depth. A comparison of the empirical functions with corresponding data from tests with mowed vegetation shows a significant faster process of dissipation for the derived empirical functions. The salt marsh has a significant influence on wave height dissipation. This conclusion is supported by Möller et al. (2014) as they concluded that up 60% of wave height attenuation can be attributed to the vegetation. They used the same data for this conclusion. The evolution of wave dissipation shows a process that can be described as a function of the length of the salt marsh. This empirical relationship can be used as a quantitative basis for decisions concerning flood protection measurements. The present research showed a significant faster process of energy dissipation over a lower water depth. Recommendations include a further research to the dependency of the decrease parameter on other parameters than the water depth. Further research to the influence of the water depth on the dissipation process seems also necessary as the present research is only based on two water depths. Further research with the use of a longer salt marsh will benefit the quality of the results as the present data only seemed to describe the first section of the process of wave dissipation.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
Link to this item:http://purl.utwente.nl/essays/68815
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