Am Breach Modelling And Flood Mapping A Case Study Of Ribb Dam

The spontaneous dam breach phenomenon and the resultant flooding that happened in thernworld history leads to the requirement of establishing a dam safety plans and hazardrnmanagement strategies. In this regard, the dam breach pre-event analysis will be thernprerequisite work. rnThis thesis addressed a pre-event analysis of a dam breach scenario for Ribb dam located inrnAmhara regional state, Ethiopia. Deterministic and probabilistic approach for the modelling ofrnthe dam breach is used. Overtopping and piping failure modes are assessed and the resultingrnflood inundation is mapped. A 1.5 PMF inflow hydrograph and a base inflow hydrograph arernused as upstream boundary condition, while Lake Tana makes the downstream boundaryrncondition. Fourteen 2D simulations are carried out and of which ten are for different breachrnparameters, two are for uncertainty analysis on breach parameters, and two are sensitivityrnanalysis on Manning’s roughness coefficient ‘n’. rnHEC-RAS Ver 5.0.7 hydraulic model is employed, and McBreach is used for probabilistic damrnbreach modelling. In this study, five deterministic non-physical empirical methods andrnprobabilistic breach modelling are assessed and compared. rnThe five deterministic non-physical empirical methods have resulted in peak flow valuesrnbetween 67,570mrn3rn/s and 113,153mrn3rn/s for overtopping and between 22,269mrn3rn/s and 40,926mrn3rn/srnfor piping modes of failure respectively. For both modes of failure, MacDonald andrnlangridge-Monopolis and Frohelich (1995a) produced the lowest and highest peak dischargernrespectively. The 1% and 90% exceedance probability peak discharge for overtopping failurernmode is 104,379mrn3rn/s and 77,521mrn3rn/s respectively. The Manning roughness coefficient ‘n’rnsensitivity analysis showed a 0.11 to 39.9 percentage increase in flood depth and 2.20 to 20.67rnpercentage decrease in velocity for an increase of the Manning roughness coefficient by 30%.rnIn addition, the Manning roughness coefficient ‘n’ sensitivity analysis showed a 0.00 to 15.32rnpercentage decrease in flood depth and 10.81 to 28.09 percentage increase in velocity for arndecrease of the Manning roughness coefficient by 30%. rnThe study highlighted the dam breach and its corresponding flooding could be potentiallyrncatastrophic and high priority should be given to monitoring and surveillance of the dam.

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