Abstract
This project presented the sensitivity analysis of dam breach parameters using Lang Creek Dam as a testing basis. Lang Creek Dam is an earthfill dam located in southern California with 62.8 feet height and 345 feet long. The dam was constructed to prevent the downstream residential area from flooding. In this study, the Lang Creek Dam breaching outflow hydrographs and downstream flood propagations were simulated applying computer programs to recognize the possible relationships among the peak flows, dam breach parameters, and downstream river parameters. The sensitivity analysis of dam breach parameters involved with reservoir routing and river routing techniques. The key inputs required in the flows routing processes include time to dam failure in hours, side slope of dam breach, shape of opening, downstream channel geometries, Manning roughness coefficients, inflows to reservoir, and existing channel bed slopes. The reservoir components of routing were performed applying HEC-1 computer program and resulted in multiple breaching outflow hydrographs for specified conditions at the dam site. Similarly, the HEC-RAS computer program was applied to conduct unsteady flows routing through the river components of vi the testing waterways and produced various peak flows for given conditions at specified downstream reach stations. The influences of dam breach parameters on maximum breaching outflow discharges were analyzed at the dam site under multiple scenarios. At the dam site, the relative effects of changes in breach parameters, time to dam failure in hours (TFH) and side slope of dam breach (SS) on maximum discharges, were analyzed to determine the controlling parameter. The sensitivity analyses results showed that the maximum discharge increased by 88.4% when the TFH decreased by 50% and SS decreased by 87.5%. The difference in percent decreased between the TFH and SS implied that the peak discharge was highly sensitive to changes in TFH than SS. The dam-break breaching outflow hydrographs generated by assuming five time to dam failure were routed through the downstream conveyances to conduct sensitivity analysis among river parameters. The relative influences of Manning coefficients, and time to dam failure in hours on peak flows for given conditions were analyzed at specified reach stations. For instance, the sensitivity analyses at reach station 4.74 revealed that a 50% decrease in time to dam failure and 12.5 % decrease in Manning coefficient value resulted in 8.2% increase in peak flow. Based on the test results, the peak flows in the channel were highly sensitive to changes in Manning coefficients than time to dam failure. Lastly, the sensitivity of peak flows for given changes in channel bed slopes were analyzed at all major river stations along the testing streamflow. Summarized results of the sensitivity analyses depicted that a 5% decrease in channel bed slopes resulted in vii change in peak flows ranging from 0% at the upstream to -24.8% at the downstream. Similarly, a 5% increase in channel bed slopes resulted in change in peak flows ranging from 0% at the upstream to 20.0% at the downstream. According to the findings, the peak flows in the channel were highly sensitive to a minor change in channel bed slopes, i.e. flatter channel bed slopes resulted in higher peak flows than steeper ones.