PRUEBA DE TESTIGOS

High uncertainty in the prediction of output parameters in physically-based dam-break modelling is mainly due to the choice and formulation of the sediment transport equations. The application of different sediment transport equations can give model output parameters that differ by several orders of magnitude. Moreover, the uncertainty due to the choice of sediment transport equations is high in comparison to the uncertainty that is attributed to other input parameters such as bed roughness or resistance, eddy viscosity and angle of repose.

Dam-break floods are characterised by transport of sediment on very steep slopes up to 60%. Commonly used sediment transport equations in dam-break modelling were derived from data on mild and moderately steep slopes of up to 20%. The application of sediment transport equations that were calibrated on similar or comparative steep slopes has the potential to improve the prediction of sediment transport rates and reduce uncertainty in dam-break modelling.

The main focus of this dissertation was to develop empirical sediment transport equations for homogeneous earth dam-break analysis due to overtopping failure and to assess the levels of uncertainty that is associated with the sediment transport equations. The specific objectives were:  To derive new empirical sediment transport equations for application in dam-break

modelling on steep slopes

 To conduct an analytical comparison of the performance of the newly calibrated sediment transport equation against existing sediment transport equations from literature

 To evaluate the performance of the newly calibrated sediment transport equation in a dam- break numerical model and to compare the model output results with predictions by selected sediment transport equations from literature as well as to analyse the sensitivity of model output results to the application of the sediment transport equations.

The specific objectives were achieved and the main contribution of the research work that is presented in this dissertation is the determination of new calibrated sediment transport equations from steep slope data and the successful application of the equations in a physically-based dam- break numerical model.

Experimental studies were conducted in a laboratory flume and sediment transport rates were measured. In order to determine whether a sediment transport rate calculated by a selected sediment transport equation was practical or not, it was necessary to consider the corresponding flow. By converting sediment transport rates into concentrations, it was found that the applicability of a sediment transport equation could be judged by the practicality of the predicted concentrations at the unit discharges under consideration. The judgment of the applicability and sensitivity of the sediment transport equation should be done on a case by case basis with regard to the predicted concentration ranges in comparison to the amount of sediment that a particular

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unit discharge can reasonably transport. This approach was applied in the assessment of the reasonableness of the predicted sediment transport concentrations.

The data from the experimental study was used to derive sediment transport equations. Four sediment transport formulations were investigated. The proposed sediment transport equations predict sediment transport rates as a function of particle sizes of the sample for which 30 and 90% were finer (d30 and d90); shear velocity, average flow velocity, friction slope, dimensionless shear stress and critical dimensionless shear stress. The validity and statistical significance of the newly calibrated sediment transport equations was confirmed using statistical test methods and the degree of correlation between measured and predicted sediment transport rates.

The proposed sediment transport equations provided satisfactorily predictions with a deviation of less than 22% between the measured and predicted sediment transport rates. Two newly calibrated sediment transport equations (Equations 5.2-5 and 5.2-6) were found to have better predictive capabilities out of the four formulations that were investigated. The analytical comparison showed that the sediment transport predictions by the newly calibrated sediment transport equations (Equation 5.2-6) were within the same order of magnitude as those of Meyer- Peter Müller (1948) and Camenen & Larson (2005) sediment transport equations even though the Meyer-Peter Müller (1948) equation overestimated sediment transport rates for higher unit discharges. The Smart and Jäeggi (1983) sediment transport equation predicted higher sediment transport rates in comparison to the measured sediment transport rates and those predicted by Equation 5.2-6.

The MIKE 21C numerical model was appplied to evaluate and compare the performance of the newly calibrated sediment transport equations in dam-break modelling using five dam-break case studies. The simulated results using the newly calibrated sediment transport equations (Equations 5.2-5 and 5.2-6) were compared with those predicted by two selected sediment transport equations from literature, namely Camenen & Larson (2005) and Smart & Jäeggi (1983). The embankment slopes for three case studies (Case Studies 1, 2 and 5) were within the slope data calibration range of the newly calibrated sediment transport equations but outside the calibration data range of the selected sediment transport equations from literature, namely Camenen & Larson (2005) and Smart and Jäeggi (1983). Embankment slopes for two case studies (Case Studies 3 & 4) were outside the calibration range of all the four sediment transport equations (Equations 5.2-5 and 5.2-6, Camenen & Larson (2005) and Smart and Jäeggi (1983)). Case Study 1 was applied to analyse the performance of the sediment transport equations by analysing temporal bed level changes. Case Study 2 investigated the effect of the sediment transport equation on the breach shape. Case Studies 3 & 4 investigated the numerical modelling of dam- break outflow hydrographs for very steep slopes. Case Study 5 analysed the effect of the sediment transport on the simulated peak discharge and outflow volume. The newly calibrated sediment transport equations appeared to perform better in all the applicable case studies (Case Studies 1, 2 and 5). To some extent, Camenen & Larson (2005) and Smart & Jäeggi (1983) sediment transport equations performed better in Case Studies 1 and 5 respectively. Consistent

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and realistic predictions of numerical model output parameters were not possible when the sediment transport equations were applied to the two case studies with embankment slopes outside the recommended applicability range.

Model uncertainty to the input of sediment transport equations was assessed through sensitivity analysis by comparing model output results for the four sediment transport equations (Equation 5.2-5, Equation 5.2-6, Camenen & Larson (2005) and Smart and Jäeggi (1983)). The results from the numerical simulations can assist in understanding the level of variability of the model output results due to the application of different sediment transport equations. The results can be used to determine probable scenarios for output parameters such as peak discharge and flood water volumes.

Equations 5.2-5 and 5.1-6 are alternatives to the existing sediment transport equations in dam- break modelling considering that these two newly calibrated sediment transport equations were calibrated from steep slope data that are typical for on earth embankment dams. Nevertheless, any sediment transport equation that is to be applied to dam-break numerical modelling requires sensitivity analyses with the model output parameters in order to determine other possible scenarios. Professional judgment in the analysis of sensitivity, in particular with respect to the application of sediment transport equations, should always prevail. For instance, the predicted peak discharge by Equation 5.2-6 was in close agreement with the predicted peak discharge by Smart and Jäeggi (1983) for the same case study even though the two equations were derived from different slope range data. In that scenario, each sediment transport equation could have different predictive capabilities and limitations depending on the given input parameters and model configurations.

The study findings do not intend to invalidate the existing sediment transport equations but to highlight the importance of applying equations within their recommended applicability range and of comparing the results of two or three sediment transport equations. The comparison of the results can provide medium or worst case scenarios in the model output parameters such as outflow hydrograph peak, time to peak and outflow volumes. This information is crucial in dam safety planning, and for the development of early warning systems and disaster preparedness plans.

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