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Performance of GRADE in simulating flood wave characteristics in the Rhine basin

Trul, H. (2016) Performance of GRADE in simulating flood wave characteristics in the Rhine basin.

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Abstract:Hydraulic boundary conditions, with a low occurrence probability, are needed to carry out quality assessments of flood protection measures constructed in and around the Dutch Rhine. The physically based method, called Generator Rainfall and Discharge Extremes (GRADE), is used to determine these hydraulic boundary conditions. Within GRADE synthetic weather, generated by resampling of 56 years of historical precipitation and temperature data, is fed into the hydrological model HBV to simulate continuous daily discharge series. Extreme flood waves, selected from the continuous discharge series, will be used as the hydraulic boundary condition to assess the required stability of for example the dikes. Disapproved dike stretches should be reinforced, which might have large financial implications and can lead to public resistance. It is therefore important that the physical characteristics of flood waves simulated with GRADE are in accordance with reality. The objective of this research is to assess the performance of the hydrological model HBV and the combined performance of the weather generator (WG) with HBV, used within GRADE, in simulating the flood wave characteristics (peak discharge, peak timing, volume, duration and number of flood waves per year) and the contributions to flood waves at Lobith of 7 major sub-basins in the Rhine basin. The flood waves have been selected from the continuous observed and simulated discharge series by the use of a threshold value and a time window. Observed and simulated flood waves from the period 1951-2006 have been compared to each other, to assess the performance of the HBV model in simulating the flood wave characteristics. For each characteristic the ratio between observed and simulated is calculated to detect structural over- or underestimations, the mean absolute relative error from the maximum simulated or observed characteristic (MAREM) is calculated to quantify the difference and the coefficient of determination (R2) is calculated to assess the linear relation between the observed and simulated flood wave characteristic. The performance of HBV, HBV-WG and WG in simulating the flood wave characteristics has been evaluated by comparing the statistics of the flood characteristics, obtained from observed and simulated flood waves from the period 1951-2006 and simulated flood waves from 10.000 year synthetic discharge series. A t-test has been used to assess the equality of the means, a F-test has been used to assess the equality of the variances and a cumulative distribution function (CDF) has been used to visualize the differences between the observed and simulated flood wave characteristics. The results showed that the performance of HBV and HBV-WG in simulating the volumetric contributions of the 7 major Rhine sub-basins to flood waves detected at Lobith is good. Contributing discharges from the Moselle are a little underestimated by errors in the synthetic weather series. The results showed that the performance of HBV, HBV-WG and WG in simulating all flood wave characteristics of the whole Rhine basin at Lobith and Andernach is good. Also the characteristics of flood waves from the Main are simulated well. The simulated flood wave characteristics from the Moselle and Neckar differ slightly from the observed ones. For the Neckar this is mainly due to the HBV model. The errors detected in flood waves from the Moselle can be attributed to both the HBV model and WG. The flood wave characteristics from both Alpine sub-basins are poorly simulated due to the HBV model. The largest errors are found in flood waves from the East Alpine Rhine basin. The peak discharges and volumes of the winter flood waves are overestimated, whereas peak discharges of all flood waves from this basin are less overestimated. The volumes calculated from all flood waves are underestimated. Flood wave durations from winter flood waves are less underestimated than the durations of all simulated flood waves. Flood wave peaks of waves that contribute to Lobith waves are often simulated earlier than the observed ones, whereas assessing all flood waves reveals that flood wave peaks are simulated too late. The performance of HBV in simulating snow storage is probably responsible for the main errors. Presumably too little water is allocated to the snow storage, so that in winter there will be too much discharge, whereas in early summer there will be too little. All detected errors in this basin can be explained by this possible reason. Overall the peak discharge is the best simulated flood wave characteristic. Flood wave volumes, durations and number of flood waves are generally underestimated. The HBV model is in most cases responsible for the largest errors. Often the performance of HBV-WG is slightly worse than the performance of HBV only. The skill of the WG in reproducing comparable weather is good, however errors in flood wave characteristics simulated with HBV are often slightly increased due to extra uncertainty incorporated in the WG. It is recommended to do an in depth validation of HBV models for the Alpine region to assess the skill of these models in simulating the underlying hydrological processes that drive the discharge. It is furthermore recommended to be reserved in using GRADE in its current form for river flood applications in the Netherlands. Flood protection measures which will be disapproved, due to hydraulic boundary condition obtained from GRADE, should be ameliorated. People negatively affected by projects concerning the improvement of flood protection measures might use the large differences found for the Alpine region flood waves as argument against using GRADE. Because the Alpine region is responsible for 29% of the total Lobith wave discharge, those people have a point. It is therefore recommended to improve the HBV models for the Alpine region. Assessing possible GRADE extensions to simulate for example flooding in the Netherlands due to dike failure might be an interesting next step in GRADE’s development.
Item Type:Essay (Master)
Clients:
Deltares, Delft, The Netherlands
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/69719
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