University of Twente Student Theses


The uncertainty of discharge measurements and the water balance at river bifurcations

Wevers, Hidde (2022) The uncertainty of discharge measurements and the water balance at river bifurcations.

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Abstract:Discharge time series are mainly derived from rating curves using continuously measured water levels. Rating curves rely on periodic discharge and water level measurements. In the Netherlands, periodic discharge measurements are regularly performed during high discharge events often using the moving-boat Acoustic Doppler Current Profiler (ADCP) method. However, not every part of the river cross-section can be measured with an ADCP measuring instrument, which is often located near the near the water surface, the bed, the banks and at inundated floodplains. The discharge in these unmeasured zones must be estimated by extrapolation of the available ADCP measurements and this introduces uncertainty in the total discharge measurement, which directly translates into uncertainty in rating curves and the derived discharge data. Therefore, accurate discharge measurements are essential for flood risk management in the Dutch river area, especially during extreme events. This research focuses on the river branches at the Pannerdensche Kop, which is the bifurcation of the Bovenrijn in the Waal and the Pannerdensch Kanaal. Currently, rating curves as well as the underlying discharge data do not show a closing water balance at the Pannerdensche Kop. This is a direct indication of errors in the rating curves, which are partly due to uncertainties in the periodic discharge measurements. Therefore, the main aim of this research is to quantify the uncertainty of high discharge measurements and its influence on the water balance at the Pannerdensche Kop. Firstly, the uncertainties introduced in the discharge measurements are quantified and propagated to the overall uncertainty. The overall uncertainty is subdivided into the random uncertainty, edge uncertainty, extrapolation uncertainty, velocity uncertainty, bottom depth uncertainty and systematic uncertainty. Of the uncertainties, only the random and systematic uncertainty have a significant influence on the overall uncertainty. The systematic uncertainty is assumed constant for each measurement and the random uncertainty depends on the coefficient of variation of the total discharge for all transects and the number of transects. The overall uncertainties in the Bovenrijn are fairly constant over all measurements, ranging from 3.09% to 5.25%. In the Waal, there are larger fluctuations in the overall uncertainty over time from 3.66% to 9.75%. However, the largest uncertainties are probably overestimated compared to the lowest uncertainties due to a negative correlation with the number of transects. The Pannerdensch Kanaal has relatively large uncertainties over time, ranging from 3.74% to 9.90%. Secondly, the uncertainty was also estimated for all discharge components (i.e., edges, floodplains, top and bottom unmeasured zones and measured zone) individually. Although the measured discharge has the lowest degree of uncertainty, it has the greatest influence on the overall uncertainty as its discharge is responsible for a large fraction of the total discharge. After that, the top and bottom discharges account for the second and third largest fraction of total discharge but also have the second and third smallest degree of uncertainty. As a result, the fully inundated floodplains have a larger influence on the overall uncertainty. Although surrounded by uncertainty, the edge discharges generally have a small influence on the overall uncertainty. Finally, the comparison between the estimated ADCP discharges and the historical derived discharges showed no systematic over- or underestimation of the discharge by the rating curves for any of the branches. However, three ADCP measurements are slightly over- and underestimated by the rating curve, but this can be partly explained by the time delay in the discharge between the station and the ADCP measurement location. Additionally, it was found that by including the uncertainties in the river branches into the water balance error, the perfect water balance closure falls within the 95% confidence interval for all same-day ADCP measurements in the three branches. Therefore, the water balance error at the Pannerdensche Kop can be explained by the uncertainties in the three river branches and there seems to be no river branch that most likely caused the water balance error.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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