University of Twente Student Theses


Exploratory process-based modelling of estuarine sand dunes: The influence of environmental and empirical model parameters on expected wave lengths, growth rates and migration rates

Maas, Dieuwke Saskia (2022) Exploratory process-based modelling of estuarine sand dunes: The influence of environmental and empirical model parameters on expected wave lengths, growth rates and migration rates.

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Abstract:Estuaries are large water bodies connecting river(s) with the seas. These areas form the transition between the fluvial situation in rivers to the marine processes at seas. These estuaries are important since the surroundings are highly populated and have both economical and ecological value. In these estuaries, we find large-scale rhythmic bed forms named sand dunes. The sand dunes are formed on top of the flat bed, develop into dunes under local environmental conditions over time and can migrate. This development of sand dunes affects the water depth and flow of the estuary. However, these sand dunes form only at some locations in estuaries. To better understand the formation of sand dunes, this study analysis the initial formation of a sand dune based on local environmental parameters in the process-based linear stability model developed by van der Sande et al. (2021a). This study analyses the effect of a change of one or several of the input parameters. These affect the expected fastest growing mode (FGM), being growth rate, wavelength and migration rate. This current model describes the formation of sand dunes including the estuarine processes, in the Gironde Estuary. However, this current model can adapt to different circumstances by changing the local environmental parameters and the empirical model parameters. Herein the environmental parameters are the water depth, depth-averaged M2 velocity amplitude, the depth-averaged river flow velocity, grain size and the salinity gradient. empirical model parameters consist of the drag coefficient, slip parameter, bed load exponent and slope correction factor. In this study, we analyse the effect of the local ranges in parameters on the expected FGM properties. Secondly, we look into the combined effect of combined changes in these parameters. The third step in this study is to validate the current model based on the three locations and their environmental parameters combined with measurements of local sand dunes. The individual parameter changes have a different impact on the growth rate, wave length and migration rate. Combing all changes in the environmental parameters, we find large variations in migration rates and directions. While the combination of ranges in the model parameter mainly influences the range in growth rates and wave lengths. The combination of two parameters will influence the FGM properties different since some parameters influence each other. So influence the combination of the bed load exponent and the slope correction the amount of sediment transport in the equation. The slope correction factor has a large influence on the wave length, however both factors influence the expected growth and migration rates. The current model helps to understand the formation and different sizes of sand dunes. But not all differences in wave lengths and migration direction could be explained by the local parameters in this model. The model is calibrated for the Gironde Estuary and the slip parameter needs to be adjusted to better present the wave lengths at other locations, however the current model can explain some difference between wavelengths between some locations. But there is still some difference which could not be explained. Also the migration direction is not always in the expected direction due to the type of model and the input parameters. So the current model is relatively robust, however it can not explain all the differences in wave lengths and migration directions.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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