am Breach Modelling and Flood mapping, a Case Study of Ribb Dam
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Date
2021-01
Authors
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Journal ISSN
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Publisher
Addis Ababa University
Abstract
The spontaneous dam breach phenomenon and the resultant flooding that happened in the
world history leads to the requirement of establishing a dam safety plans and hazard
management strategies. In this regard, the dam breach pre-event analysis will be the
prerequisite work.
This thesis addressed a pre-event analysis of a dam breach scenario for Ribb dam located in
Amhara regional state, Ethiopia. Deterministic and probabilistic approach for the modelling of
the dam breach is used. Overtopping and piping failure modes are assessed and the resulting
flood inundation is mapped. A 1.5 PMF inflow hydrograph and a base inflow hydrograph are
used as upstream boundary condition, while Lake Tana makes the downstream boundary
condition. Fourteen 2D simulations are carried out and of which ten are for different breach
parameters, two are for uncertainty analysis on breach parameters, and two are sensitivity
analysis on Manning’s roughness coefficient ‘n’.
HEC-RAS Ver 5.0.7 hydraulic model is employed, and McBreach is used for probabilistic dam
breach modelling. In this study, five deterministic non-physical empirical methods and
probabilistic breach modelling are assessed and compared.
The five deterministic non-physical empirical methods have resulted in peak flow values
between 67,570m
3
/s and 113,153m
3
/s for overtopping and between 22,269m
3
/s and 40,926m
3
/s
for piping modes of failure respectively. For both modes of failure, MacDonald and
langridge-Monopolis and Frohelich (1995a) produced the lowest and highest peak discharge
respectively. The 1% and 90% exceedance probability peak discharge for overtopping failure
mode is 104,379m
3
/s and 77,521m
3
/s respectively. The Manning roughness coefficient ‘n’
sensitivity analysis showed a 0.11 to 39.9 percentage increase in flood depth and 2.20 to 20.67
percentage decrease in velocity for an increase of the Manning roughness coefficient by 30%.
In addition, the Manning roughness coefficient ‘n’ sensitivity analysis showed a 0.00 to 15.32
percentage decrease in flood depth and 10.81 to 28.09 percentage increase in velocity for a
decrease of the Manning roughness coefficient by 30%.
The study highlighted the dam breach and its corresponding flooding could be potentially
catastrophic and high priority should be given to monitoring and surveillance of the dam.
Description
Keywords
EC-RAS, RAS Mapper, DEM, McBreach, Ribb Dam, Dam Breach Analysis, Scenario, 2D