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Wave Dissipation in Mangroves : Parameterization of the drag coefficient based on field data.

Hendriks, Jurjen (2014) Wave Dissipation in Mangroves : Parameterization of the drag coefficient based on field data.

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Abstract:Mangrove forests cover large parts of sheltered coastlines in the tropical and subtropical parts of the world. Over the past decades, mangrove forests have gained more interest because of their unique ecosystems, and the coastal defence function they provide, due to their attenuating effects on waves. Even though several theoretical and field studies have been performed, and a numerical model exists to simulate wave attenuation in mangroves, available models still require calibration of a certain coefficient. This coefficient cannot simply be calculated from observed vegetation and/or wave characteristics. The most extensive model available has been first developed by Dalrymple et al. (1984), and models the mangroves as a field of vertical cylinders. This field of cylinders affects the waves propagating through it. This formulation has been implemented in SWAN (a numerical wave model), by Suzuki et al. (2011b). Even though this model is a representation of reality based on the physical processes, there is no explicit formulation available for calculating the drag coefficient which is required in this model. Instead, the drag coefficient for a certain transect has to be resolved by a calibration procedure, resulting in a constant parameter value. Over the years, many studies have been performed looking into drag coefficients in general, and in more recent years, vegetation drag coefficients in mangroves gained interest (e.g. Mazda et al., 2006). In these studies, the effects of several parameters on the drag coefficient (such as the Reynolds number and the KC number) have been shown. Even though for some parameters the impact on wave attenuation has already been shown in studies, there is no explicit parameterization for the drag coefficient which takes into account all these effects and has been tested on field data as well. In this report, the data of a field campaign by Horstman et al. (2012) is used to further investigate the different variables determining the drag coefficient in mangrove vegetation. The aim was to develop a generic expression, describing this drag coefficient as a function of vegetation and wave characteristics within the mangroves. In order to do this, the field data has been used to calculate the drag coefficients required to obtain the observed effects, for numerous short datasets (bursts) of observed wave propagation through mangroves. The resulting large set of values for the drag coefficient CD, was analysed in relation to a number of variables, representing both the vegetation and the wave characteristics. From these variables the most significant relation was found with the KC-number with the Mazda length scale implemented instead of vegetation diameter. In which u is the maximum orbital flow velocity (m/s), T is the peak period (s), V is the control volume of water (m3), A is the projected surface of the vegetation within this control volume (m2), and VM is the volume of the mangroves within the control volume (m3). A multi variable analysis has been performed to analyse the combined impact of several parameters and to derive a definition for the drag coefficient. Due to the variation in the data, several formulations did show up with comparable results. However, based on literature and physical meaning, a basic parameterization only using the KC-number with Mazda length scale has been selected as the best representation of the data: In which CD is the drag coefficient (-), and KCM is the KC number with Mazda length scale (-) as defined before. When implementing this equation in the SWAN model, small improvements were obtained compared to the old situation with a single constant drag coefficient for each transect. This new equation slightly increased the R-squared and correlation values of predicted vs. observed energy dissipation rates. However, the greatest improvement lies in the fact that both transects available in the data are represented by the same equation, rather than both having a different value in the old situation. This is a first step towards more generic modelling of vegetation-induced wave dissipation. Even though the first results show some clear relations, due to much noise in the data, not all effects could be studied in detail. Improvements of the parameterization are likely to be possible. In order to do so, more field data and controlled laboratory tests are advised.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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