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Discharge uncertainty in frequency analysis of Han River discharge

Klok, T. (2009) Discharge uncertainty in frequency analysis of Han River discharge.

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Abstract:Valuating the credibility of research is normally done by reviewing in which journals an article has been published or by how many times an article has been quoted, but still it doesn’t tell us exactly how credible the research is. When assessing the research we might look at the confidence intervals, the size of the confidence interval tell us something about the certainty of quantitative information. But not all the (un)certainty can be expressed in confidence intervals, some quality parts in a research can only be valued by a fellow researcher in the same research area. The problem is that results of research are not only red by other scientists, but also by politicians who are looking for grounds for their decisions. For them and other less informed readers Ravetz and Functowicz (1990) proposed the NUSAP method that assesses the uncertainties in a research model. NUSAP is an acronym for Numeral, Unit, Spread, Assessment and Pedigree. The numeral, unit and spread of a model give all quantitative information about the model. The assessment and pedigree part is more an assessment of the quality if the model. In this research the NUSAP method, proposed by, is used to assess the quality of input information for a frequency analysis of the Han River in China. The main question in this research was: What is the uncertainty of the propagated discharge with a given return period using a frequency analysis for the Baihe discharge station at the Han River? The identification of the different uncertainty sources in the frequency analysis is split up into three stages: Measurement (chapter 4), Time series (chapter 5) and Statistics (chapter 6). In each stage the uncertainty sources have been identified. In the measurement section different methods for the measurement of water level, river profile, velocity and discharge are assessed: for each the spread and Pedigree score have been estimated. The discharges at Baige station are measured according to the two depth velocity area method (ISO, 2007). The measurement error is computed by calculating the uncertainty in the velocity area method and the uncertainty was 3% (95% confidence) in the computed discharges. The NUSAP Pedigree scores are average to high, which means not so much uncertainty. The time series handles the assessment of the compilation of the peak discharge series. The selection of peak discharges from the time series is done by selecting the annual maximum discharges, but the Exponential distribution needed a threshold of 12.000m3/s, therefore the ‘peaks over threshold’ method is used to select peak discharges. The result was one series of AM discharges and one series of POT discharges. The discharge data has not been reviewed for stationary, because there was no information available. As a result the Pedigree score for the time series is low. The statics of the frequency analysis are assessed by fitting the Normal, Pearson type III and Gumbel distributions to the Annual Maximum and the Exponential distribution is fitted to the POT. The parameter estimation is done with the Method of Moments (MOM) and Maximum Likelihood Estimation (MLE). The goodness-of-fit is tested with the Chi-square test and the Kolmogorov-Smirnov test. A comparison of the distributions with plot-positions of discharges (visual), confidence intervals and the GOF-tests show that the normal distribution has a good fit for the discharges with a return period < 100 years. The Pearson III with MLE parameter estimation has a good fit for return period >100 years. Q100 Normal is 23089±1309 m3/s and PIII MLE is 25019±2258 m3/s. This fit is explained by the slight S-curve of the measured flows. The Pedigree scores for the different distributions are average to low. This is because the uncertainty of the fit. The different equations give different distributions with a wide range of possible discharges at a give return period. The main conclusion is that the uncertainty in the flood frequency analysis for the Han River is too large at this moment so that the frequency analysis in this research is not to be of any practical usage at this moment. In this research all the conclusions are drawn upon the differences in discharges. A significant difference in discharge could have relative small impact on the gauge height. Therefore more research on the effects of discharge changes is recommended.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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