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Data analysis of estuarine dunes: Linking estuarine sand dune characteristics to environmental parameters

Muurman, S.D. (2021) Data analysis of estuarine dunes: Linking estuarine sand dune characteristics to environmental parameters.

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Abstract:Estuaries form the transitional region between a river and the sea. Estuaries are important both from an economical and environmental standpoint. They are rich in intertidal area and biodiversity and form a gateway between the marine and the riverine environment, creating ideal harbor locations. In estuaries, dunes are often found. Dunes are rhythmic features that exist on the bed. They can limit navigation depths and alter the flow structure and sediment transport. To better understand the dynamics of estuarine dunes, this study analyses the relation between dune characteristics and environmental parameters. The goal of the study is to explain the dune length, height and asymmetry based on the water depth, tidal asymmetry, river discharge and sediment grain size. To this extent, bed level and environmental data is available of the Western Scheldt and the Elbe estuary. Furthermore, the results of a hydrodynamic model of the Scheldt estuary are available to characterize the flow properties in the Western Scheldt. In the Scheldt and Elbe estuary, the Antwerp and Hamburg port are located which are two of the three largest ports of Europe. The Western Scheldt is characterized by ebb and flood channels and a relatively weak influence of the river discharge. The Elbe estuary is characterized by a single channel and a significant discharge with a seasonal variation. Furthermore, the bed level data available for the Western Scheldt is extensive in space but scarce over time. The opposite holds for the Elbe where a smaller area of bed level data is available with a greater temporal resolution. The focus in the Western Scheldt is therefore set on the spatial variability of estuarine dunes and environmental parameters while the Elbe is mostly analysed for temporal correlations. In the Western Scheldt, three study sites are selected of which the bed level is mapped twice each in the period 2017-2019. In the Elbe, a single study location is selected where the bed level is measured 60 times in the period 2012-2014. The bed level of both estuaries is mapped using multibeam echosounders. Dune characteristics are extracted from the bed level data using a developed bedform tracking tool. Environmental data is processed in order to obtain the average water depth, river discharge and flow velocity asymmetry prior to the bed level measurements. The results of this study show a positive weak correlation between water depth and dune height in the Elbe study location, and water depth and dune length in the Western Scheldt. Since the dunes in the Elbe and Western Scheldt study locations exist in the same range of water depth, these obtained correlations were not able to explain the differences in dune length and height of the Elbe compared to the dunes of the Western Scheldt. Two study locations in the Western Scheldt showed differences in dune height and length. These dunes also exist in the same water depth range. Additionally, the median grain sizes of these study locations are very similar and can therefore not be used to explain differences in dune characteristics. The most prominent finding in this study is the relation between dune asymmetry and the environmental conditions. In the Western Scheldt, a strong correlation is observed between the peak current asymmetry and the dune asymmetry. Study locations in ebb channels, where the velocity asymmetry is ebb directed, also showed dunes which are ebb-directed, and vice versa. In the Elbe, the dunes fluctuate over time, being asymmetric in landward and seaward direction. This fluctuation is also observed in the velocity asymmetry of the Elbe. Furthermore, in the Elbe, a strong correlation is found between the river discharge and the dune asymmetry. High discharge events cause the dunes to deform and be asymmetric in seaward direction. It is hypothesized that during low discharge events, the salinity gradient shifts further land inward, causing gravitational circulation in the study area. The hypothesis is that this gravitational circulation causes the dunes to grow asymmetric in landward direction during low discharge events.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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