University of Twente Student Theses


Model assimilation in river flow data processing : estimating dominant spatial scales in cross-sectional velocity data

Dongen, R.T.J. van (2018) Model assimilation in river flow data processing : estimating dominant spatial scales in cross-sectional velocity data.

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Abstract:Flow velocity measurements in rivers and coastal areas are increasingly carried out with Acoustic Doppler Current Profilers (ADCPs). This instrument collects velocity data at multiple locations in a cross-section over a certain period of time. Post-processing techniques of raw velocity data often involve temporal averaging and spatial smoothing. Smoothing and averaging windows are often chosen arbitrarily without a clear substantiation. The aim of this study is to identify dominant spatial patterns in river’s cross-sectional velocity data in order to average and smooth the data with more certainty. Spatial patterns are investigated by a method based on spectral analysis, which allows to identify the dominant scales. Higher order functions are progressively included to a base function for each velocity component, i.e., increasing the truncation number, and fitted to the measured velocity data. This process is repeated until the residuals have no spatial structure. Velocity locations are transformed into a normalised coordinate system in order to conduct the model fit for multiple cross-sections. The method is applied to velocity measurements collected in a sharp river bend. By analysing the available data set, weak flow is observed near the boundaries and strong flow in the center or slightly outwards of the center, but near the scour hole the flow recirculates at the outer sides causing an upstream flow locally. Water flows to the outer bend at the water surface and to the inner bend near the river bed. Dominant spatial scales can be observed from the computed amplitudes with Fourier transform, but the strongest amplitudes vary for different truncation scales. Most of the spatial structure in the residuals disappears after truncating at eight waveforms over local river width horizontally and almost eight waveforms over local water depth vertically. The method is particularly useful to represent the main flow pattern adequately with continuous functions, due to a relatively steady state during data collection. Fourier transform are applied generally to analyse the presence of dominant modes in river flow data, but only with respect to time. Investigating the cross-sectional spatial velocity distribution with the help of Fourier transform provides insights on the dominant spatial scales, which are, however, linked to the created model and the applied set of base functions for the three velocity components. Further research is recommended on validating the model by using different instruments to measure velocity data and on including different of base functions.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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